guile/doc/ref/api-data.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000-2004, 2006-2017, 2019-2020, 2022-2023
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Data Types
@section Data Types

Guile's data types form a powerful built-in library of representations
and functionality that you can apply to your problem domain.  This
chapter surveys the data types built-in to Guile, from the simple to the
complex.

@menu
* Booleans::                    True/false values.
* Numbers::                     Numerical data types.
* Characters::                  Single characters.
* Character Sets::              Sets of characters.
* Strings::                     Sequences of characters.
* Symbols::                     Symbols.
* Keywords::                    Self-quoting, customizable display keywords.
* Pairs::                       Scheme's basic building block.
* Lists::                       Special list functions supported by Guile.
* Vectors::                     One-dimensional arrays of Scheme objects.
* Bit Vectors::                 Vectors of bits.
* Bytevectors::                 Sequences of bytes.
* Arrays::                      Multidimensional matrices.
* VLists::                      Vector-like lists.
* Record Overview::             Walking through the maze of record APIs.
* SRFI-9 Records::              The standard, recommended record API.
* Records::                     Guile's historical record API.
* Structures::                  Low-level record representation.
* Dictionary Types::            About dictionary types in general.
* Association Lists::           List-based dictionaries.
* VHashes::                     VList-based dictionaries.   
* Hash Tables::                 Table-based dictionaries.
* Other Types::                 Other sections describe data types too.
@end menu


@node Booleans
@subsection Booleans
@tpindex Booleans

The two boolean values are @code{#t} for true and @code{#f} for false.
They can also be written as @code{#true} and @code{#false}, as per R7RS.

Boolean values are returned by predicate procedures, such as the general
equality predicates @code{eq?}, @code{eqv?} and @code{equal?}
(@pxref{Equality}) and numerical and string comparison operators like
@code{string=?} (@pxref{String Comparison}) and @code{<=}
(@pxref{Comparison}).

@lisp
(<= 3 8)
@result{} #t

(<= 3 -3)
@result{} #f

(equal? "house" "houses")
@result{} #f

(eq? #f #f)
@result{}
#t
@end lisp

In test condition contexts like @code{if} and @code{cond}
(@pxref{Conditionals}), where a group of subexpressions will be
evaluated only if a @var{condition} expression evaluates to ``true'',
``true'' means any value at all except @code{#f}.

@lisp
(if #t "yes" "no")
@result{} "yes"

(if 0 "yes" "no")
@result{} "yes"

(if #f "yes" "no")
@result{} "no"
@end lisp

A result of this asymmetry is that typical Scheme source code more often
uses @code{#f} explicitly than @code{#t}: @code{#f} is necessary to
represent an @code{if} or @code{cond} false value, whereas @code{#t} is
not necessary to represent an @code{if} or @code{cond} true value.

It is important to note that @code{#f} is @strong{not} equivalent to any
other Scheme value.  In particular, @code{#f} is not the same as the
number 0 (like in C and C++), and not the same as the ``empty list''
(like in some Lisp dialects).

In C, the two Scheme boolean values are available as the two constants
@code{SCM_BOOL_T} for @code{#t} and @code{SCM_BOOL_F} for @code{#f}.
Care must be taken with the false value @code{SCM_BOOL_F}: it is not
false when used in C conditionals.  In order to test for it, use
@code{scm_is_false} or @code{scm_is_true}.

@rnindex not
@deffn {Scheme Procedure} not x
@deffnx {C Function} scm_not (x)
Return @code{#t} if @var{x} is @code{#f}, else return @code{#f}.
@end deffn

@rnindex boolean?
@deffn {Scheme Procedure} boolean? obj
@deffnx {C Function} scm_boolean_p (obj)
Return @code{#t} if @var{obj} is either @code{#t} or @code{#f}, else
return @code{#f}.
@end deffn

@deftypevr {C Macro} SCM SCM_BOOL_T
The @code{SCM} representation of the Scheme object @code{#t}.
@end deftypevr

@deftypevr {C Macro} SCM SCM_BOOL_F
The @code{SCM} representation of the Scheme object @code{#f}.
@end deftypevr

@deftypefn {C Function} int scm_is_true (SCM obj)
Return @code{0} if @var{obj} is @code{#f}, else return @code{1}.
@end deftypefn

@deftypefn {C Function} int scm_is_false (SCM obj)
Return @code{1} if @var{obj} is @code{#f}, else return @code{0}.
@end deftypefn

@deftypefn {C Function} int scm_is_bool (SCM obj)
Return @code{1} if @var{obj} is either @code{#t} or @code{#f}, else
return @code{0}.
@end deftypefn

@deftypefn {C Function} SCM scm_from_bool (int val)
Return @code{#f} if @var{val} is @code{0}, else return @code{#t}.
@end deftypefn

@deftypefn {C Function} int scm_to_bool (SCM val)
Return @code{1} if @var{val} is @code{SCM_BOOL_T}, return @code{0}
when @var{val} is @code{SCM_BOOL_F}, else signal a `wrong type' error.

You should probably use @code{scm_is_true} instead of this function
when you just want to test a @code{SCM} value for trueness.
@end deftypefn

@node Numbers
@subsection Numerical data types
@tpindex Numbers

Guile supports a rich ``tower'' of numerical types --- integer,
rational, real and complex --- and provides an extensive set of
mathematical and scientific functions for operating on numerical
data.  This section of the manual documents those types and functions.

You may also find it illuminating to read R5RS's presentation of numbers
in Scheme, which is particularly clear and accessible: see
@ref{Numbers,,,r5rs,R5RS}.

@menu
* Numerical Tower::             Scheme's numerical "tower".
* Integers::                    Whole numbers.
* Reals and Rationals::         Real and rational numbers.
* Complex Numbers::             Complex numbers.
* Exactness::                   Exactness and inexactness.
* Number Syntax::               Read syntax for numerical data.
* Integer Operations::          Operations on integer values.
* Comparison::                  Comparison predicates.
* Conversion::                  Converting numbers to and from strings.
* Complex::                     Complex number operations.
* Arithmetic::                  Arithmetic functions.
* Scientific::                  Scientific functions.
* Bitwise Operations::          Logical AND, OR, NOT, and so on.
* Random::                      Random number generation.
@end menu


@node Numerical Tower
@subsubsection Scheme's Numerical ``Tower''
@rnindex number?

Scheme's numerical ``tower'' consists of the following categories of
numbers:

@table @dfn
@item integers
Whole numbers, positive or negative; e.g.@: --5, 0, 18.

@item rationals
The set of numbers that can be expressed as @math{@var{p}/@var{q}}
where @var{p} and @var{q} are integers; e.g.@: @math{9/16} works, but
pi (an irrational number) doesn't. These include integers
(@math{@var{n}/1}).

@item real numbers
The set of numbers that describes all possible positions along a
one-dimensional line. This includes rationals as well as irrational
numbers.

@item complex numbers
The set of numbers that describes all possible positions in a two
dimensional space. This includes real as well as imaginary numbers
(@math{@var{a}+@var{b}i}, where @var{a} is the @dfn{real part},
@var{b} is the @dfn{imaginary part}, and @math{i} is the square root of
@minus{}1.)
@end table

It is called a tower because each category ``sits on'' the one that
follows it, in the sense that every integer is also a rational, every
rational is also real, and every real number is also a complex number
(but with zero imaginary part).

In addition to the classification into integers, rationals, reals and
complex numbers, Scheme also distinguishes between whether a number is
represented exactly or not.  For example, the result of
@m{2\sin(\pi/4),2*sin(pi/4)} is exactly @m{\sqrt{2},2^(1/2)}, but Guile
can represent neither @m{\pi/4,pi/4} nor @m{\sqrt{2},2^(1/2)} exactly.
Instead, it stores an inexact approximation, using the C type
@code{double}.

Guile can represent exact rationals of any magnitude, inexact
rationals that fit into a C @code{double}, and inexact complex numbers
with @code{double} real and imaginary parts.

The @code{number?} predicate may be applied to any Scheme value to
discover whether the value is any of the supported numerical types.

@deffn {Scheme Procedure} number? obj
@deffnx {C Function} scm_number_p (obj)
Return @code{#t} if @var{obj} is any kind of number, else @code{#f}.
@end deffn

For example:

@lisp
(number? 3)
@result{} #t

(number? "hello there!")
@result{} #f

(define pi 3.141592654)
(number? pi)
@result{} #t
@end lisp

@deftypefn {C Function} int scm_is_number (SCM obj)
This is equivalent to @code{scm_is_true (scm_number_p (obj))}.
@end deftypefn

The next few subsections document each of Guile's numerical data types
in detail.

@node Integers
@subsubsection Integers

@tpindex Integer numbers

@rnindex integer?

Integers are whole numbers, that is numbers with no fractional part,
such as 2, 83, and @minus{}3789.

Integers in Guile can be arbitrarily big, as shown by the following
example.

@lisp
(define (factorial n)
  (let loop ((n n) (product 1))
    (if (= n 0)
        product
        (loop (- n 1) (* product n)))))

(factorial 3)
@result{} 6

(factorial 20)
@result{} 2432902008176640000

(- (factorial 45))
@result{} -119622220865480194561963161495657715064383733760000000000
@end lisp

Readers whose background is in programming languages where integers are
limited by the need to fit into just 4 or 8 bytes of memory may find
this surprising, or suspect that Guile's representation of integers is
inefficient.  In fact, Guile achieves a near optimal balance of
convenience and efficiency by using the host computer's native
representation of integers where possible, and a more general
representation where the required number does not fit in the native
form.  Conversion between these two representations is automatic and
completely invisible to the Scheme level programmer.

C has a host of different integer types, and Guile offers a host of
functions to convert between them and the @code{SCM} representation.
For example, a C @code{int} can be handled with @code{scm_to_int} and
@code{scm_from_int}.  Guile also defines a few C integer types of its
own, to help with differences between systems.

C integer types that are not covered can be handled with the generic
@code{scm_to_signed_integer} and @code{scm_from_signed_integer} for
signed types, or with @code{scm_to_unsigned_integer} and
@code{scm_from_unsigned_integer} for unsigned types.

Scheme integers can be exact and inexact.  For example, a number
written as @code{3.0} with an explicit decimal-point is inexact, but
it is also an integer.  The functions @code{integer?} and
@code{scm_is_integer} report true for such a number, but the functions
@code{exact-integer?}, @code{scm_is_exact_integer},
@code{scm_is_signed_integer}, and @code{scm_is_unsigned_integer} only
allow exact integers and thus report false.  Likewise, the conversion
functions like @code{scm_to_signed_integer} only accept exact
integers.

The motivation for this behavior is that the inexactness of a number
should not be lost silently.  If you want to allow inexact integers,
you can explicitly insert a call to @code{inexact->exact} or to its C
equivalent @code{scm_inexact_to_exact}.  (Only inexact integers will
be converted by this call into exact integers; inexact non-integers
will become exact fractions.)

@deffn {Scheme Procedure} integer? x
@deffnx {C Function} scm_integer_p (x)
Return @code{#t} if @var{x} is an exact or inexact integer number, else
return @code{#f}.

@lisp
(integer? 487)
@result{} #t

(integer? 3.0)
@result{} #t

(integer? -3.4)
@result{} #f

(integer? +inf.0)
@result{} #f
@end lisp
@end deffn

@deftypefn {C Function} int scm_is_integer (SCM x)
This is equivalent to @code{scm_is_true (scm_integer_p (x))}.
@end deftypefn

@deffn {Scheme Procedure} exact-integer? x
@deffnx {C Function} scm_exact_integer_p (x)
Return @code{#t} if @var{x} is an exact integer number, else
return @code{#f}.

@lisp
(exact-integer? 37)
@result{} #t

(exact-integer? 3.0)
@result{} #f
@end lisp
@end deffn

@deftypefn {C Function} int scm_is_exact_integer (SCM x)
This is equivalent to @code{scm_is_true (scm_exact_integer_p (x))}.
@end deftypefn

@defvr  {C Type} scm_t_int8
@defvrx {C Type} scm_t_uint8
@defvrx {C Type} scm_t_int16
@defvrx {C Type} scm_t_uint16
@defvrx {C Type} scm_t_int32
@defvrx {C Type} scm_t_uint32
@defvrx {C Type} scm_t_int64
@defvrx {C Type} scm_t_uint64
@defvrx {C Type} scm_t_intmax
@defvrx {C Type} scm_t_uintmax
The C types are equivalent to the corresponding ISO C types but are
defined on all platforms, with the exception of @code{scm_t_int64} and
@code{scm_t_uint64}, which are only defined when a 64-bit type is
available.  For example, @code{scm_t_int8} is equivalent to
@code{int8_t}.

You can regard these definitions as a stop-gap measure until all
platforms provide these types.  If you know that all the platforms
that you are interested in already provide these types, it is better
to use them directly instead of the types provided by Guile.
@end defvr

@deftypefn  {C Function} int scm_is_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
@deftypefnx {C Function} int scm_is_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
Return @code{1} when @var{x} represents an exact integer that is
between @var{min} and @var{max}, inclusive.

These functions can be used to check whether a @code{SCM} value will
fit into a given range, such as the range of a given C integer type.
If you just want to convert a @code{SCM} value to a given C integer
type, use one of the conversion functions directly.
@end deftypefn

@deftypefn  {C Function} scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
@deftypefnx {C Function} scm_t_uintmax scm_to_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
When @var{x} represents an exact integer that is between @var{min} and
@var{max} inclusive, return that integer.  Else signal an error,
either a `wrong-type' error when @var{x} is not an exact integer, or
an `out-of-range' error when it doesn't fit the given range.
@end deftypefn

@deftypefn  {C Function} SCM scm_from_signed_integer (scm_t_intmax x)
@deftypefnx {C Function} SCM scm_from_unsigned_integer (scm_t_uintmax x)
Return the @code{SCM} value that represents the integer @var{x}.  This
function will always succeed and will always return an exact number.
@end deftypefn

@deftypefn  {C Function} char scm_to_char (SCM x)
@deftypefnx {C Function} {signed char} scm_to_schar (SCM x)
@deftypefnx {C Function} {unsigned char} scm_to_uchar (SCM x)
@deftypefnx {C Function} short scm_to_short (SCM x)
@deftypefnx {C Function} {unsigned short} scm_to_ushort (SCM x)
@deftypefnx {C Function} int scm_to_int (SCM x)
@deftypefnx {C Function} {unsigned int} scm_to_uint (SCM x)
@deftypefnx {C Function} long scm_to_long (SCM x)
@deftypefnx {C Function} {unsigned long} scm_to_ulong (SCM x)
@deftypefnx {C Function} {long long} scm_to_long_long (SCM x)
@deftypefnx {C Function} {unsigned long long} scm_to_ulong_long (SCM x)
@deftypefnx {C Function} size_t scm_to_size_t (SCM x)
@deftypefnx {C Function} ssize_t scm_to_ssize_t (SCM x)
@deftypefnx {C Function} scm_t_uintptr scm_to_uintptr_t (SCM x)
@deftypefnx {C Function} scm_t_ptrdiff scm_to_ptrdiff_t (SCM x)
@deftypefnx {C Function} scm_t_int8 scm_to_int8 (SCM x)
@deftypefnx {C Function} scm_t_uint8 scm_to_uint8 (SCM x)
@deftypefnx {C Function} scm_t_int16 scm_to_int16 (SCM x)
@deftypefnx {C Function} scm_t_uint16 scm_to_uint16 (SCM x)
@deftypefnx {C Function} scm_t_int32 scm_to_int32 (SCM x)
@deftypefnx {C Function} scm_t_uint32 scm_to_uint32 (SCM x)
@deftypefnx {C Function} scm_t_int64 scm_to_int64 (SCM x)
@deftypefnx {C Function} scm_t_uint64 scm_to_uint64 (SCM x)
@deftypefnx {C Function} scm_t_intmax scm_to_intmax (SCM x)
@deftypefnx {C Function} scm_t_uintmax scm_to_uintmax (SCM x)
@deftypefnx {C Function} scm_t_intptr scm_to_intptr_t (SCM x)
@deftypefnx {C Function} scm_t_uintptr scm_to_uintptr_t (SCM x)
When @var{x} represents an exact integer that fits into the indicated
C type, return that integer.  Else signal an error, either a
`wrong-type' error when @var{x} is not an exact integer, or an
`out-of-range' error when it doesn't fit the given range.

The functions @code{scm_to_long_long}, @code{scm_to_ulong_long},
@code{scm_to_int64}, and @code{scm_to_uint64} are only available when
the corresponding types are.
@end deftypefn

@deftypefn  {C Function} SCM scm_from_char (char x)
@deftypefnx {C Function} SCM scm_from_schar (signed char x)
@deftypefnx {C Function} SCM scm_from_uchar (unsigned char x)
@deftypefnx {C Function} SCM scm_from_short (short x)
@deftypefnx {C Function} SCM scm_from_ushort (unsigned short x)
@deftypefnx {C Function} SCM scm_from_int (int  x)
@deftypefnx {C Function} SCM scm_from_uint (unsigned int x)
@deftypefnx {C Function} SCM scm_from_long (long x)
@deftypefnx {C Function} SCM scm_from_ulong (unsigned long x)
@deftypefnx {C Function} SCM scm_from_long_long (long long x)
@deftypefnx {C Function} SCM scm_from_ulong_long (unsigned long long x)
@deftypefnx {C Function} SCM scm_from_size_t (size_t x)
@deftypefnx {C Function} SCM scm_from_ssize_t (ssize_t x)
@deftypefnx {C Function} SCM scm_from_uintptr_t (uintptr_t x)
@deftypefnx {C Function} SCM scm_from_ptrdiff_t (scm_t_ptrdiff x)
@deftypefnx {C Function} SCM scm_from_int8 (scm_t_int8 x)
@deftypefnx {C Function} SCM scm_from_uint8 (scm_t_uint8 x)
@deftypefnx {C Function} SCM scm_from_int16 (scm_t_int16 x)
@deftypefnx {C Function} SCM scm_from_uint16 (scm_t_uint16 x)
@deftypefnx {C Function} SCM scm_from_int32 (scm_t_int32 x)
@deftypefnx {C Function} SCM scm_from_uint32 (scm_t_uint32 x)
@deftypefnx {C Function} SCM scm_from_int64 (scm_t_int64 x)
@deftypefnx {C Function} SCM scm_from_uint64 (scm_t_uint64 x)
@deftypefnx {C Function} SCM scm_from_intmax (scm_t_intmax x)
@deftypefnx {C Function} SCM scm_from_uintmax (scm_t_uintmax x)
@deftypefnx {C Function} SCM scm_from_intptr_t (scm_t_intptr x)
@deftypefnx {C Function} SCM scm_from_uintptr_t (scm_t_uintptr x)
Return the @code{SCM} value that represents the integer @var{x}.
These functions will always succeed and will always return an exact
number.
@end deftypefn

@deftypefn {C Function} void scm_to_mpz (SCM val, mpz_t rop)
Assign @var{val} to the multiple precision integer @var{rop}.
@var{val} must be an exact integer, otherwise an error will be
signaled.  @var{rop} must have been initialized with @code{mpz_init}
before this function is called.  When @var{rop} is no longer needed
the occupied space must be freed with @code{mpz_clear}.
@xref{Initializing Integers,,, gmp, GNU MP Manual}, for details.
@end deftypefn

@deftypefn {C Function} SCM scm_from_mpz (mpz_t val)
Return the @code{SCM} value that represents @var{val}.
@end deftypefn

@node Reals and Rationals
@subsubsection Real and Rational Numbers
@tpindex Real numbers
@tpindex Rational numbers

@rnindex real?
@rnindex rational?

Mathematically, the real numbers are the set of numbers that describe
all possible points along a continuous, infinite, one-dimensional line.
The rational numbers are the set of all numbers that can be written as
fractions @var{p}/@var{q}, where @var{p} and @var{q} are integers.
All rational numbers are also real, but there are real numbers that
are not rational, for example @m{\sqrt{2}, the square root of 2}, and
@m{\pi,pi}.

Guile can represent both exact and inexact rational numbers, but it
cannot represent precise finite irrational numbers.  Exact rationals are
represented by storing the numerator and denominator as two exact
integers.  Inexact rationals are stored as floating point numbers using
the C type @code{double}.

Exact rationals are written as a fraction of integers.  There must be
no whitespace around the slash:

@lisp
1/2
-22/7
@end lisp

Even though the actual encoding of inexact rationals is in binary, it
may be helpful to think of it as a decimal number with a limited
number of significant figures and a decimal point somewhere, since
this corresponds to the standard notation for non-whole numbers.  For
example:

@lisp
0.34
-0.00000142857931198
-5648394822220000000000.0
4.0
@end lisp

The limited precision of Guile's encoding means that any finite ``real''
number in Guile can be written in a rational form, by multiplying and
then dividing by sufficient powers of 10 (or in fact, 2).  For example,
@samp{-0.00000142857931198} is the same as @minus{}142857931198 divided
by 100000000000000000.  In Guile's current incarnation, therefore, the
@code{rational?} and @code{real?} predicates are equivalent for finite
numbers.


Dividing by an exact zero leads to a error message, as one might expect.
However, dividing by an inexact zero does not produce an error.
Instead, the result of the division is either plus or minus infinity,
depending on the sign of the divided number and the sign of the zero
divisor (some platforms support signed zeroes @samp{-0.0} and
@samp{+0.0}; @samp{0.0} is the same as @samp{+0.0}).

Dividing zero by an inexact zero yields a @acronym{NaN} (`not a number')
value, although they are actually considered numbers by Scheme.
Attempts to compare a @acronym{NaN} value with any number (including
itself) using @code{=}, @code{<}, @code{>}, @code{<=} or @code{>=}
always returns @code{#f}.  Although a @acronym{NaN} value is not
@code{=} to itself, it is both @code{eqv?} and @code{equal?} to itself
and other @acronym{NaN} values.  However, the preferred way to test for
them is by using @code{nan?}.

The real @acronym{NaN} values and infinities are written @samp{+nan.0},
@samp{+inf.0} and @samp{-inf.0}.  This syntax is also recognized by
@code{read} as an extension to the usual Scheme syntax.  These special
values are considered by Scheme to be inexact real numbers but not
rational.  Note that non-real complex numbers may also contain
infinities or @acronym{NaN} values in their real or imaginary parts.  To
test a real number to see if it is infinite, a @acronym{NaN} value, or
neither, use @code{inf?}, @code{nan?}, or @code{finite?}, respectively.
Every real number in Scheme belongs to precisely one of those three
classes.

On platforms that follow @acronym{IEEE} 754 for their floating point
arithmetic, the @samp{+inf.0}, @samp{-inf.0}, and @samp{+nan.0} values
are implemented using the corresponding @acronym{IEEE} 754 values.
They behave in arithmetic operations like @acronym{IEEE} 754 describes
it, i.e., @code{(= +nan.0 +nan.0)} @result{} @code{#f}.

@deffn {Scheme Procedure} real? obj
@deffnx {C Function} scm_real_p (obj)
Return @code{#t} if @var{obj} is a real number, else @code{#f}.  Note
that the sets of integer and rational values form subsets of the set
of real numbers, so the predicate will also be fulfilled if @var{obj}
is an integer number or a rational number.
@end deffn

@deffn {Scheme Procedure} rational? x
@deffnx {C Function} scm_rational_p (x)
Return @code{#t} if @var{x} is a rational number, @code{#f} otherwise.
Note that the set of integer values forms a subset of the set of
rational numbers, i.e.@: the predicate will also be fulfilled if
@var{x} is an integer number.
@end deffn

@deffn {Scheme Procedure} rationalize x eps
@deffnx {C Function} scm_rationalize (x, eps)
Returns the @emph{simplest} rational number differing
from @var{x} by no more than @var{eps}.  

As required by @acronym{R5RS}, @code{rationalize} only returns an
exact result when both its arguments are exact.  Thus, you might need
to use @code{inexact->exact} on the arguments.

@lisp
(rationalize (inexact->exact 1.2) 1/100)
@result{} 6/5
@end lisp

@end deffn

@deffn  {Scheme Procedure} inf? x
@deffnx {C Function} scm_inf_p (x)
Return @code{#t} if the real number @var{x} is @samp{+inf.0} or
@samp{-inf.0}.  Otherwise return @code{#f}.
@end deffn

@deffn {Scheme Procedure} nan? x
@deffnx {C Function} scm_nan_p (x)
Return @code{#t} if the real number @var{x} is @samp{+nan.0}, or
@code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} finite? x
@deffnx {C Function} scm_finite_p (x)
Return @code{#t} if the real number @var{x} is neither infinite nor a
NaN, @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} nan
@deffnx {C Function} scm_nan ()
Return @samp{+nan.0}, a @acronym{NaN} value.
@end deffn

@deffn {Scheme Procedure} inf
@deffnx {C Function} scm_inf ()
Return @samp{+inf.0}, positive infinity.
@end deffn

@deffn {Scheme Procedure} numerator x
@deffnx {C Function} scm_numerator (x)
Return the numerator of the rational number @var{x}.
@end deffn

@deffn {Scheme Procedure} denominator x
@deffnx {C Function} scm_denominator (x)
Return the denominator of the rational number @var{x}.
@end deffn

@deftypefn  {C Function} int scm_is_real (SCM val)
@deftypefnx {C Function} int scm_is_rational (SCM val)
Equivalent to @code{scm_is_true (scm_real_p (val))} and
@code{scm_is_true (scm_rational_p (val))}, respectively.
@end deftypefn

@deftypefn {C Function} double scm_to_double (SCM val)
Returns the number closest to @var{val} that is representable as a
@code{double}.  Returns infinity for a @var{val} that is too large in
magnitude.  The argument @var{val} must be a real number.
@end deftypefn

@deftypefn {C Function} SCM scm_from_double (double val)
Return the @code{SCM} value that represents @var{val}.  The returned
value is inexact according to the predicate @code{inexact?}, but it
will be exactly equal to @var{val}.
@end deftypefn

@node Complex Numbers
@subsubsection Complex Numbers
@tpindex Complex numbers

@rnindex complex?

Complex numbers are the set of numbers that describe all possible points
in a two-dimensional space.  The two coordinates of a particular point
in this space are known as the @dfn{real} and @dfn{imaginary} parts of
the complex number that describes that point.

In Guile, complex numbers are written in rectangular form as the sum of
their real and imaginary parts, using the symbol @code{i} to indicate
the imaginary part.

@lisp
3+4i
@result{}
3.0+4.0i

(* 3-8i 2.3+0.3i)
@result{}
9.3-17.5i
@end lisp

@cindex polar form
@noindent
Polar form can also be used, with an @samp{@@} between magnitude and
angle,

@lisp
1@@3.141592 @result{} -1.0      (approx)
-1@@1.57079 @result{} 0.0-1.0i  (approx)
@end lisp

Guile represents a complex number as a pair of inexact reals, so the
real and imaginary parts of a complex number have the same properties of
inexactness and limited precision as single inexact real numbers.

Note that each part of a complex number may contain any inexact real
value, including the special values @samp{+nan.0}, @samp{+inf.0} and
@samp{-inf.0}, as well as either of the signed zeroes @samp{0.0} or
@samp{-0.0}.


@deffn {Scheme Procedure} complex? z
@deffnx {C Function} scm_complex_p (z)
Return @code{#t} if @var{z} is a complex number, @code{#f}
otherwise.  Note that the sets of real, rational and integer
values form subsets of the set of complex numbers, i.e.@: the
predicate will also be fulfilled if @var{z} is a real,
rational or integer number.
@end deffn

@deftypefn {C Function} int scm_is_complex (SCM val)
Equivalent to @code{scm_is_true (scm_complex_p (val))}.
@end deftypefn

@node Exactness
@subsubsection Exact and Inexact Numbers
@tpindex Exact numbers
@tpindex Inexact numbers

@rnindex exact?
@rnindex inexact?
@rnindex exact->inexact
@rnindex inexact->exact

R5RS requires that, with few exceptions, a calculation involving inexact
numbers always produces an inexact result.  To meet this requirement,
Guile distinguishes between an exact integer value such as @samp{5} and
the corresponding inexact integer value which, to the limited precision
available, has no fractional part, and is printed as @samp{5.0}.  Guile
will only convert the latter value to the former when forced to do so by
an invocation of the @code{inexact->exact} procedure.

The only exception to the above requirement is when the values of the
inexact numbers do not affect the result.  For example @code{(expt n 0)}
is @samp{1} for any value of @code{n}, therefore @code{(expt 5.0 0)} is
permitted to return an exact @samp{1}.

@deffn {Scheme Procedure} exact? z
@deffnx {C Function} scm_exact_p (z)
Return @code{#t} if the number @var{z} is exact, @code{#f}
otherwise.

@lisp
(exact? 2)
@result{} #t

(exact? 0.5)
@result{} #f

(exact? (/ 2))
@result{} #t
@end lisp

@end deffn

@deftypefn {C Function} int scm_is_exact (SCM z)
Return a @code{1} if the number @var{z} is exact, and @code{0}
otherwise.  This is equivalent to @code{scm_is_true (scm_exact_p (z))}.

An alternate approch to testing the exactness of a number is to 
use @code{scm_is_signed_integer} or @code{scm_is_unsigned_integer}.
@end deftypefn

@deffn {Scheme Procedure} inexact? z
@deffnx {C Function} scm_inexact_p (z)
Return @code{#t} if the number @var{z} is inexact, @code{#f}
else.
@end deffn

@deftypefn {C Function} int scm_is_inexact (SCM z)
Return a @code{1} if the number @var{z} is inexact, and @code{0}
otherwise.  This is equivalent to @code{scm_is_true (scm_inexact_p (z))}.
@end deftypefn

@deffn {Scheme Procedure} inexact->exact z
@deffnx {C Function} scm_inexact_to_exact (z)
Return an exact number that is numerically closest to @var{z}, when
there is one.  For inexact rationals, Guile returns the exact rational
that is numerically equal to the inexact rational.  Inexact complex
numbers with a non-zero imaginary part can not be made exact.

@lisp
(inexact->exact 0.5)
@result{} 1/2
@end lisp

The following happens because 12/10 is not exactly representable as a
@code{double} (on most platforms).  However, when reading a decimal
number that has been marked exact with the ``#e'' prefix, Guile is
able to represent it correctly.

@lisp
(inexact->exact 1.2)  
@result{} 5404319552844595/4503599627370496

#e1.2
@result{} 6/5
@end lisp

@end deffn

@c begin (texi-doc-string "guile" "exact->inexact")
@deffn {Scheme Procedure} exact->inexact z
@deffnx {C Function} scm_exact_to_inexact (z)
Convert the number @var{z} to its inexact representation.
@end deffn


@node Number Syntax
@subsubsection Read Syntax for Numerical Data

The read syntax for integers is a string of digits, optionally
preceded by a minus or plus character, a code indicating the
base in which the integer is encoded, and a code indicating whether
the number is exact or inexact.  The supported base codes are:

@table @code
@item #b
@itemx #B
the integer is written in binary (base 2)

@item #o
@itemx #O
the integer is written in octal (base 8)

@item #d
@itemx #D
the integer is written in decimal (base 10)

@item #x
@itemx #X
the integer is written in hexadecimal (base 16)
@end table

If the base code is omitted, the integer is assumed to be decimal.  The
following examples show how these base codes are used.

@lisp
-13
@result{} -13

#d-13
@result{} -13

#x-13
@result{} -19

#b+1101
@result{} 13

#o377
@result{} 255
@end lisp

The codes for indicating exactness (which can, incidentally, be applied
to all numerical values) are:

@table @code
@item #e
@itemx #E
the number is exact

@item #i
@itemx #I
the number is inexact.
@end table

If the exactness indicator is omitted, the number is exact unless it
contains a radix point.  Since Guile can not represent exact complex
numbers, an error is signaled when asking for them.

@lisp
(exact? 1.2)
@result{} #f

(exact? #e1.2)
@result{} #t

(exact? #e+1i)
ERROR: Wrong type argument
@end lisp

Guile also understands the syntax @samp{+inf.0} and @samp{-inf.0} for
plus and minus infinity, respectively.  The value must be written
exactly as shown, that is, they always must have a sign and exactly
one zero digit after the decimal point.  It also understands
@samp{+nan.0} and @samp{-nan.0} for the special `not-a-number' value.
The sign is ignored for `not-a-number' and the value is always printed
as @samp{+nan.0}.

@node Integer Operations
@subsubsection Operations on Integer Values
@rnindex odd?
@rnindex even?
@rnindex quotient
@rnindex remainder
@rnindex modulo
@rnindex gcd
@rnindex lcm

@deffn {Scheme Procedure} odd? n
@deffnx {C Function} scm_odd_p (n)
Return @code{#t} if @var{n} is an odd number, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} even? n
@deffnx {C Function} scm_even_p (n)
Return @code{#t} if @var{n} is an even number, @code{#f}
otherwise.
@end deffn

@c begin (texi-doc-string "guile" "quotient")
@c begin (texi-doc-string "guile" "remainder")
@deffn {Scheme Procedure} quotient n d
@deffnx {Scheme Procedure} remainder n d
@deffnx {C Function} scm_quotient (n, d)
@deffnx {C Function} scm_remainder (n, d)
Return the quotient or remainder from @var{n} divided by @var{d}.  The
quotient is rounded towards zero, and the remainder will have the same
sign as @var{n}.  In all cases quotient and remainder satisfy
@math{@var{n} = @var{q}*@var{d} + @var{r}}.

@lisp
(remainder 13 4) @result{} 1
(remainder -13 4) @result{} -1
@end lisp

See also @code{truncate-quotient}, @code{truncate-remainder} and
related operations in @ref{Arithmetic}.
@end deffn

@c begin (texi-doc-string "guile" "modulo")
@deffn {Scheme Procedure} modulo n d
@deffnx {C Function} scm_modulo (n, d)
Return the remainder from @var{n} divided by @var{d}, with the same
sign as @var{d}.

@lisp
(modulo 13 4) @result{} 1
(modulo -13 4) @result{} 3
(modulo 13 -4) @result{} -3
(modulo -13 -4) @result{} -1
@end lisp

See also @code{floor-quotient}, @code{floor-remainder} and
related operations in @ref{Arithmetic}.
@end deffn

@c begin (texi-doc-string "guile" "gcd")
@deffn {Scheme Procedure} gcd x@dots{}
@deffnx {C Function} scm_gcd (x, y)
Return the greatest common divisor of all arguments.
If called without arguments, 0 is returned.

The C function @code{scm_gcd} always takes two arguments, while the
Scheme function can take an arbitrary number.
@end deffn

@c begin (texi-doc-string "guile" "lcm")
@deffn {Scheme Procedure} lcm x@dots{}
@deffnx {C Function} scm_lcm (x, y)
Return the least common multiple of the arguments.
If called without arguments, 1 is returned.

The C function @code{scm_lcm} always takes two arguments, while the
Scheme function can take an arbitrary number.
@end deffn

@deffn {Scheme Procedure} modulo-expt n k m
@deffnx {C Function} scm_modulo_expt (n, k, m)
Return @var{n} raised to the integer exponent
@var{k}, modulo @var{m}.

@lisp
(modulo-expt 2 3 5)
   @result{} 3
@end lisp
@end deffn

@deftypefn {Scheme Procedure} {} exact-integer-sqrt @var{k}
@deftypefnx {C Function} void scm_exact_integer_sqrt (SCM @var{k}, SCM *@var{s}, SCM *@var{r})
Return two exact non-negative integers @var{s} and @var{r}
such that @math{@var{k} = @var{s}^2 + @var{r}} and
@math{@var{s}^2 <= @var{k} < (@var{s} + 1)^2}.
An error is raised if @var{k} is not an exact non-negative integer.

@lisp
(exact-integer-sqrt 10) @result{} 3 and 1
@end lisp
@end deftypefn

@node Comparison
@subsubsection Comparison Predicates
@rnindex zero?
@rnindex positive?
@rnindex negative?

The C comparison functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  Also keep in mind that
the C functions return one of the Scheme boolean values
@code{SCM_BOOL_T} or @code{SCM_BOOL_F} which are both true as far as C
is concerned.  Thus, always write @code{scm_is_true (scm_num_eq_p (x,
y))} when testing the two Scheme numbers @code{x} and @code{y} for
equality, for example.

@c begin (texi-doc-string "guile" "=")
@deffn {Scheme Procedure} =
@deffnx {C Function} scm_num_eq_p (x, y)
Return @code{#t} if all parameters are numerically equal.
@end deffn

@c begin (texi-doc-string "guile" "<")
@deffn {Scheme Procedure} <
@deffnx {C Function} scm_less_p (x, y)
Return @code{#t} if the list of parameters is monotonically
increasing.
@end deffn

@c begin (texi-doc-string "guile" ">")
@deffn {Scheme Procedure} >
@deffnx {C Function} scm_gr_p (x, y)
Return @code{#t} if the list of parameters is monotonically
decreasing.
@end deffn

@c begin (texi-doc-string "guile" "<=")
@deffn {Scheme Procedure} <=
@deffnx {C Function} scm_leq_p (x, y)
Return @code{#t} if the list of parameters is monotonically
non-decreasing.
@end deffn

@c begin (texi-doc-string "guile" ">=")
@deffn {Scheme Procedure} >=
@deffnx {C Function} scm_geq_p (x, y)
Return @code{#t} if the list of parameters is monotonically
non-increasing.
@end deffn

@c begin (texi-doc-string "guile" "zero?")
@deffn {Scheme Procedure} zero? z
@deffnx {C Function} scm_zero_p (z)
Return @code{#t} if @var{z} is an exact or inexact number equal to
zero.
@end deffn

@c begin (texi-doc-string "guile" "positive?")
@deffn {Scheme Procedure} positive? x
@deffnx {C Function} scm_positive_p (x)
Return @code{#t} if @var{x} is an exact or inexact number greater than
zero.
@end deffn

@c begin (texi-doc-string "guile" "negative?")
@deffn {Scheme Procedure} negative? x
@deffnx {C Function} scm_negative_p (x)
Return @code{#t} if @var{x} is an exact or inexact number less than
zero.
@end deffn


@node Conversion
@subsubsection Converting Numbers To and From Strings
@rnindex number->string
@rnindex string->number

The following procedures read and write numbers according to their
external representation as defined by R5RS (@pxref{Lexical structure,
R5RS Lexical Structure,, r5rs, The Revised^5 Report on the Algorithmic
Language Scheme}).  @xref{Number Input and Output, the @code{(ice-9
i18n)} module}, for locale-dependent number parsing.

@deffn {Scheme Procedure} number->string n [radix]
@deffnx {C Function} scm_number_to_string (n, radix)
Return a string holding the external representation of the
number @var{n} in the given @var{radix}.  If @var{n} is
inexact, a radix of 10 will be used.
@end deffn

@deffn {Scheme Procedure} string->number string [radix]
@deffnx {C Function} scm_string_to_number (string, radix)
Return a number of the maximally precise representation
expressed by the given @var{string}. @var{radix} must be an
exact integer, either 2, 8, 10, or 16. If supplied, @var{radix}
is a default radix that may be overridden by an explicit radix
prefix in @var{string} (e.g.@: "#o177"). If @var{radix} is not
supplied, then the default radix is 10. If string is not a
syntactically valid notation for a number, then
@code{string->number} returns @code{#f}.
@end deffn

@deftypefn {C Function} SCM scm_c_locale_stringn_to_number (const char *string, size_t len, unsigned radix)
As per @code{string->number} above, but taking a C string, as pointer
and length.  The string characters should be in the current locale
encoding (@code{locale} in the name refers only to that, there's no
locale-dependent parsing).
@end deftypefn


@node Complex
@subsubsection Complex Number Operations
@rnindex make-rectangular
@rnindex make-polar
@rnindex real-part
@rnindex imag-part
@rnindex magnitude
@rnindex angle

@deffn {Scheme Procedure} make-rectangular real_part imaginary_part
@deffnx {C Function} scm_make_rectangular (real_part, imaginary_part)
Return a complex number constructed of the given @var{real-part} and @var{imaginary-part} parts.
@end deffn

@deffn {Scheme Procedure} make-polar mag ang
@deffnx {C Function} scm_make_polar (mag, ang)
@cindex polar form
Return the complex number @var{mag} * e^(i * @var{ang}).
@end deffn

@c begin (texi-doc-string "guile" "real-part")
@deffn {Scheme Procedure} real-part z
@deffnx {C Function} scm_real_part (z)
Return the real part of the number @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "imag-part")
@deffn {Scheme Procedure} imag-part z
@deffnx {C Function} scm_imag_part (z)
Return the imaginary part of the number @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "magnitude")
@deffn {Scheme Procedure} magnitude z
@deffnx {C Function} scm_magnitude (z)
Return the magnitude of the number @var{z}. This is the same as
@code{abs} for real arguments, but also allows complex numbers.
@end deffn

@c begin (texi-doc-string "guile" "angle")
@deffn {Scheme Procedure} angle z
@deffnx {C Function} scm_angle (z)
Return the angle of the complex number @var{z}.
@end deffn

@deftypefn  {C Function} SCM scm_c_make_rectangular (double re, double im)
@deftypefnx {C Function} SCM scm_c_make_polar (double x, double y)
Like @code{scm_make_rectangular} or @code{scm_make_polar},
respectively, but these functions take @code{double}s as their
arguments.
@end deftypefn

@deftypefn  {C Function} double scm_c_real_part (z)
@deftypefnx {C Function} double scm_c_imag_part (z)
Returns the real or imaginary part of @var{z} as a @code{double}.
@end deftypefn

@deftypefn  {C Function} double scm_c_magnitude (z)
@deftypefnx {C Function} double scm_c_angle (z)
Returns the magnitude or angle of @var{z} as a @code{double}.
@end deftypefn


@node Arithmetic
@subsubsection Arithmetic Functions
@rnindex max
@rnindex min
@rnindex +
@rnindex *
@rnindex -
@rnindex /
@findex 1+
@findex 1-
@rnindex abs
@rnindex floor
@rnindex ceiling
@rnindex truncate
@rnindex round
@rnindex euclidean/
@rnindex euclidean-quotient
@rnindex euclidean-remainder
@rnindex floor/
@rnindex floor-quotient
@rnindex floor-remainder
@rnindex ceiling/
@rnindex ceiling-quotient
@rnindex ceiling-remainder
@rnindex truncate/
@rnindex truncate-quotient
@rnindex truncate-remainder
@rnindex centered/
@rnindex centered-quotient
@rnindex centered-remainder
@rnindex round/
@rnindex round-quotient
@rnindex round-remainder

The C arithmetic functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  When you need to
invoke them with just one argument, for example to compute the
equivalent of @code{(- x)}, pass @code{SCM_UNDEFINED} as the second
one: @code{scm_difference (x, SCM_UNDEFINED)}.

@c begin (texi-doc-string "guile" "+")
@deffn {Scheme Procedure} + z1 @dots{}
@deffnx {C Function} scm_sum (z1, z2)
Return the sum of all parameter values.  Return 0 if called without any
parameters.
@end deffn

@c begin (texi-doc-string "guile" "-")
@deffn {Scheme Procedure} - z1 z2 @dots{}
@deffnx {C Function} scm_difference (z1, z2)
If called with one argument @var{z1}, -@var{z1} is returned. Otherwise
the sum of all but the first argument are subtracted from the first
argument.
@end deffn

@c begin (texi-doc-string "guile" "*")
@deffn {Scheme Procedure} * z1 @dots{}
@deffnx {C Function} scm_product (z1, z2)
Return the product of all arguments.  If called without arguments, 1 is
returned.
@end deffn

@c begin (texi-doc-string "guile" "/")
@deffn {Scheme Procedure} / z1 z2 @dots{}
@deffnx {C Function} scm_divide (z1, z2)
Divide the first argument by the product of the remaining arguments.  If
called with one argument @var{z1}, 1/@var{z1} is returned.
@end deffn

@deffn {Scheme Procedure} 1+ z
@deffnx {C Function} scm_oneplus (z)
Return @math{@var{z} + 1}.
@end deffn

@deffn {Scheme Procedure} 1- z
@deffnx {C function} scm_oneminus (z)
Return @math{@var{z} - 1}.
@end deffn

@c begin (texi-doc-string "guile" "abs")
@deffn {Scheme Procedure} abs x
@deffnx {C Function} scm_abs (x)
Return the absolute value of @var{x}.

@var{x} must be a number with zero imaginary part.  To calculate the
magnitude of a complex number, use @code{magnitude} instead.
@end deffn

@c begin (texi-doc-string "guile" "max")
@deffn {Scheme Procedure} max x1 x2 @dots{}
@deffnx {C Function} scm_max (x1, x2)
Return the maximum of all parameter values.
@end deffn

@c begin (texi-doc-string "guile" "min")
@deffn {Scheme Procedure} min x1 x2 @dots{}
@deffnx {C Function} scm_min (x1, x2)
Return the minimum of all parameter values.
@end deffn

@c begin (texi-doc-string "guile" "truncate")
@deffn {Scheme Procedure} truncate x
@deffnx {C Function} scm_truncate_number (x)
Round the inexact number @var{x} towards zero.
@end deffn

@c begin (texi-doc-string "guile" "round")
@deffn {Scheme Procedure} round x
@deffnx {C Function} scm_round_number (x)
Round the inexact number @var{x} to the nearest integer.  When exactly
halfway between two integers, round to the even one.
@end deffn

@c begin (texi-doc-string "guile" "floor")
@deffn {Scheme Procedure} floor x
@deffnx {C Function} scm_floor (x)
Round the number @var{x} towards minus infinity.
@end deffn

@c begin (texi-doc-string "guile" "ceiling")
@deffn {Scheme Procedure} ceiling x
@deffnx {C Function} scm_ceiling (x)
Round the number @var{x} towards infinity.
@end deffn

@deftypefn  {C Function} double scm_c_truncate (double x)
@deftypefnx {C Function} double scm_c_round (double x)
Like @code{scm_truncate_number} or @code{scm_round_number},
respectively, but these functions take and return @code{double}
values.
@end deftypefn

@deftypefn {Scheme Procedure} {} euclidean/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} euclidean-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} euclidean-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_euclidean_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_euclidean_quotient (SCM @var{x}, SCM @var{y})
@deftypefnx {C Function} SCM scm_euclidean_remainder (SCM @var{x}, SCM @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{euclidean-quotient} returns the
integer @var{q} and @code{euclidean-remainder} returns the real number
@var{r} such that @math{@var{x} = @var{q}*@var{y} + @var{r}} and
@math{0 <= @var{r} < |@var{y}|}.  @code{euclidean/} returns both @var{q} and
@var{r}, and is more efficient than computing each separately.  Note
that when @math{@var{y} > 0}, @code{euclidean-quotient} returns
@math{floor(@var{x}/@var{y})}, otherwise it returns
@math{ceiling(@var{x}/@var{y})}.

Note that these operators are equivalent to the R6RS operators
@code{div}, @code{mod}, and @code{div-and-mod}.

@lisp
(euclidean-quotient 123 10) @result{} 12
(euclidean-remainder 123 10) @result{} 3
(euclidean/ 123 10) @result{} 12 and 3
(euclidean/ 123 -10) @result{} -12 and 3
(euclidean/ -123 10) @result{} -13 and 7
(euclidean/ -123 -10) @result{} 13 and 7
(euclidean/ -123.2 -63.5) @result{} 2.0 and 3.8
(euclidean/ 16/3 -10/7) @result{} -3 and 22/21
@end lisp
@end deftypefn

@deftypefn {Scheme Procedure} {} floor/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} floor-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} floor-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_floor_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_floor_quotient (@var{x}, @var{y})
@deftypefnx {C Function} SCM scm_floor_remainder (@var{x}, @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{floor-quotient} returns the
integer @var{q} and @code{floor-remainder} returns the real number
@var{r} such that @math{@var{q} = floor(@var{x}/@var{y})} and
@math{@var{x} = @var{q}*@var{y} + @var{r}}.  @code{floor/} returns
both @var{q} and @var{r}, and is more efficient than computing each
separately.  Note that @var{r}, if non-zero, will have the same sign
as @var{y}.

When @var{x} and @var{y} are integers, @code{floor-remainder} is
equivalent to the R5RS integer-only operator @code{modulo}.

@lisp
(floor-quotient 123 10) @result{} 12
(floor-remainder 123 10) @result{} 3
(floor/ 123 10) @result{} 12 and 3
(floor/ 123 -10) @result{} -13 and -7
(floor/ -123 10) @result{} -13 and 7
(floor/ -123 -10) @result{} 12 and -3
(floor/ -123.2 -63.5) @result{} 1.0 and -59.7
(floor/ 16/3 -10/7) @result{} -4 and -8/21
@end lisp
@end deftypefn

@deftypefn {Scheme Procedure} {} ceiling/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} ceiling-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} ceiling-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_ceiling_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_ceiling_quotient (@var{x}, @var{y})
@deftypefnx {C Function} SCM scm_ceiling_remainder (@var{x}, @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{ceiling-quotient} returns the
integer @var{q} and @code{ceiling-remainder} returns the real number
@var{r} such that @math{@var{q} = ceiling(@var{x}/@var{y})} and
@math{@var{x} = @var{q}*@var{y} + @var{r}}.  @code{ceiling/} returns
both @var{q} and @var{r}, and is more efficient than computing each
separately.  Note that @var{r}, if non-zero, will have the opposite sign
of @var{y}.

@lisp
(ceiling-quotient 123 10) @result{} 13
(ceiling-remainder 123 10) @result{} -7
(ceiling/ 123 10) @result{} 13 and -7
(ceiling/ 123 -10) @result{} -12 and 3
(ceiling/ -123 10) @result{} -12 and -3
(ceiling/ -123 -10) @result{} 13 and 7
(ceiling/ -123.2 -63.5) @result{} 2.0 and 3.8
(ceiling/ 16/3 -10/7) @result{} -3 and 22/21
@end lisp
@end deftypefn

@deftypefn {Scheme Procedure} {} truncate/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} truncate-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} truncate-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_truncate_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_truncate_quotient (@var{x}, @var{y})
@deftypefnx {C Function} SCM scm_truncate_remainder (@var{x}, @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{truncate-quotient} returns the
integer @var{q} and @code{truncate-remainder} returns the real number
@var{r} such that @var{q} is @math{@var{x}/@var{y}} rounded toward zero,
and @math{@var{x} = @var{q}*@var{y} + @var{r}}.  @code{truncate/} returns
both @var{q} and @var{r}, and is more efficient than computing each
separately.  Note that @var{r}, if non-zero, will have the same sign
as @var{x}.

When @var{x} and @var{y} are integers, these operators are
equivalent to the R5RS integer-only operators @code{quotient} and
@code{remainder}.

@lisp
(truncate-quotient 123 10) @result{} 12
(truncate-remainder 123 10) @result{} 3
(truncate/ 123 10) @result{} 12 and 3
(truncate/ 123 -10) @result{} -12 and 3
(truncate/ -123 10) @result{} -12 and -3
(truncate/ -123 -10) @result{} 12 and -3
(truncate/ -123.2 -63.5) @result{} 1.0 and -59.7
(truncate/ 16/3 -10/7) @result{} -3 and 22/21
@end lisp
@end deftypefn

@deftypefn {Scheme Procedure} {} centered/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} centered-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} centered-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_centered_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_centered_quotient (SCM @var{x}, SCM @var{y})
@deftypefnx {C Function} SCM scm_centered_remainder (SCM @var{x}, SCM @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{centered-quotient} returns the
integer @var{q} and @code{centered-remainder} returns the real number
@var{r} such that @math{@var{x} = @var{q}*@var{y} + @var{r}} and
@math{-|@var{y}/2| <= @var{r} < |@var{y}/2|}.  @code{centered/}
returns both @var{q} and @var{r}, and is more efficient than computing
each separately.

Note that @code{centered-quotient} returns @math{@var{x}/@var{y}}
rounded to the nearest integer.  When @math{@var{x}/@var{y}} lies
exactly half-way between two integers, the tie is broken according to
the sign of @var{y}.  If @math{@var{y} > 0}, ties are rounded toward
positive infinity, otherwise they are rounded toward negative infinity.
This is a consequence of the requirement that
@math{-|@var{y}/2| <= @var{r} < |@var{y}/2|}.

Note that these operators are equivalent to the R6RS operators
@code{div0}, @code{mod0}, and @code{div0-and-mod0}.

@lisp
(centered-quotient 123 10) @result{} 12
(centered-remainder 123 10) @result{} 3
(centered/ 123 10) @result{} 12 and 3
(centered/ 123 -10) @result{} -12 and 3
(centered/ -123 10) @result{} -12 and -3
(centered/ -123 -10) @result{} 12 and -3
(centered/ 125 10) @result{} 13 and -5
(centered/ 127 10) @result{} 13 and -3
(centered/ 135 10) @result{} 14 and -5
(centered/ -123.2 -63.5) @result{} 2.0 and 3.8
(centered/ 16/3 -10/7) @result{} -4 and -8/21
@end lisp
@end deftypefn

@deftypefn {Scheme Procedure} {} round/ @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} round-quotient @var{x} @var{y}
@deftypefnx {Scheme Procedure} {} round-remainder @var{x} @var{y}
@deftypefnx {C Function} void scm_round_divide (SCM @var{x}, SCM @var{y}, SCM *@var{q}, SCM *@var{r})
@deftypefnx {C Function} SCM scm_round_quotient (@var{x}, @var{y})
@deftypefnx {C Function} SCM scm_round_remainder (@var{x}, @var{y})
These procedures accept two real numbers @var{x} and @var{y}, where the
divisor @var{y} must be non-zero.  @code{round-quotient} returns the
integer @var{q} and @code{round-remainder} returns the real number
@var{r} such that @math{@var{x} = @var{q}*@var{y} + @var{r}} and
@var{q} is @math{@var{x}/@var{y}} rounded to the nearest integer,
with ties going to the nearest even integer.  @code{round/}
returns both @var{q} and @var{r}, and is more efficient than computing
each separately.

Note that @code{round/} and @code{centered/} are almost equivalent, but
their behavior differs when @math{@var{x}/@var{y}} lies exactly half-way
between two integers.  In this case, @code{round/} chooses the nearest
even integer, whereas @code{centered/} chooses in such a way to satisfy
the constraint @math{-|@var{y}/2| <= @var{r} < |@var{y}/2|}, which
is stronger than the corresponding constraint for @code{round/},
@math{-|@var{y}/2| <= @var{r} <= |@var{y}/2|}.  In particular,
when @var{x} and @var{y} are integers, the number of possible remainders
returned by @code{centered/} is @math{|@var{y}|}, whereas the number of
possible remainders returned by @code{round/} is @math{|@var{y}|+1} when
@var{y} is even.

@lisp
(round-quotient 123 10) @result{} 12
(round-remainder 123 10) @result{} 3
(round/ 123 10) @result{} 12 and 3
(round/ 123 -10) @result{} -12 and 3
(round/ -123 10) @result{} -12 and -3
(round/ -123 -10) @result{} 12 and -3
(round/ 125 10) @result{} 12 and 5
(round/ 127 10) @result{} 13 and -3
(round/ 135 10) @result{} 14 and -5
(round/ -123.2 -63.5) @result{} 2.0 and 3.8
(round/ 16/3 -10/7) @result{} -4 and -8/21
@end lisp
@end deftypefn

@node Scientific
@subsubsection Scientific Functions

The following procedures accept any kind of number as arguments,
including complex numbers.

@rnindex sqrt
@c begin (texi-doc-string "guile" "sqrt")
@deffn {Scheme Procedure} sqrt z
Return the square root of @var{z}.  Of the two possible roots
(positive and negative), the one with a positive real part is
returned, or if that's zero then a positive imaginary part.  Thus,

@example
(sqrt 9.0)       @result{} 3.0
(sqrt -9.0)      @result{} 0.0+3.0i
(sqrt 1.0+1.0i)  @result{} 1.09868411346781+0.455089860562227i
(sqrt -1.0-1.0i) @result{} 0.455089860562227-1.09868411346781i
@end example
@end deffn

@rnindex expt
@c begin (texi-doc-string "guile" "expt")
@deffn {Scheme Procedure} expt z1 z2
Return @var{z1} raised to the power of @var{z2}.
@end deffn

@rnindex sin
@c begin (texi-doc-string "guile" "sin")
@deffn {Scheme Procedure} sin z
Return the sine of @var{z}.
@end deffn

@rnindex cos
@c begin (texi-doc-string "guile" "cos")
@deffn {Scheme Procedure} cos z
Return the cosine of @var{z}.
@end deffn

@rnindex tan
@c begin (texi-doc-string "guile" "tan")
@deffn {Scheme Procedure} tan z
Return the tangent of @var{z}.
@end deffn

@rnindex asin
@c begin (texi-doc-string "guile" "asin")
@deffn {Scheme Procedure} asin z
Return the arcsine of @var{z}.
@end deffn

@rnindex acos
@c begin (texi-doc-string "guile" "acos")
@deffn {Scheme Procedure} acos z
Return the arccosine of @var{z}.
@end deffn

@rnindex atan
@c begin (texi-doc-string "guile" "atan")
@deffn {Scheme Procedure} atan z
@deffnx {Scheme Procedure} atan y x
Return the arctangent of @var{z}, or of @math{@var{y}/@var{x}}.
@end deffn

@rnindex exp
@c begin (texi-doc-string "guile" "exp")
@deffn {Scheme Procedure} exp z
Return e to the power of @var{z}, where e is the base of natural
logarithms (2.71828@dots{}).
@end deffn

@rnindex log
@c begin (texi-doc-string "guile" "log")
@deffn {Scheme Procedure} log z
Return the natural logarithm of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "log10")
@deffn {Scheme Procedure} log10 z
Return the base 10 logarithm of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "sinh")
@deffn {Scheme Procedure} sinh z
Return the hyperbolic sine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "cosh")
@deffn {Scheme Procedure} cosh z
Return the hyperbolic cosine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "tanh")
@deffn {Scheme Procedure} tanh z
Return the hyperbolic tangent of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "asinh")
@deffn {Scheme Procedure} asinh z
Return the hyperbolic arcsine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "acosh")
@deffn {Scheme Procedure} acosh z
Return the hyperbolic arccosine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "atanh")
@deffn {Scheme Procedure} atanh z
Return the hyperbolic arctangent of @var{z}.
@end deffn


@node Bitwise Operations
@subsubsection Bitwise Operations

For the following bitwise functions, negative numbers are treated as
infinite precision twos-complements.  For instance @math{-6} is bits
@math{@dots{}111010}, with infinitely many ones on the left.  It can
be seen that adding 6 (binary 110) to such a bit pattern gives all
zeros.

@deffn {Scheme Procedure} logand n1 n2 @dots{}
@deffnx {C Function} scm_logand (n1, n2)
Return the bitwise @sc{and} of the integer arguments.

@lisp
(logand) @result{} -1
(logand 7) @result{} 7
(logand #b111 #b011 #b001) @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} logior n1 n2 @dots{}
@deffnx {C Function} scm_logior (n1, n2)
Return the bitwise @sc{or} of the integer arguments.

@lisp
(logior) @result{} 0
(logior 7) @result{} 7
(logior #b000 #b001 #b011) @result{} 3
@end lisp
@end deffn

@deffn {Scheme Procedure} logxor n1 n2 @dots{}
@deffnx {C Function} scm_loxor (n1, n2)
Return the bitwise @sc{xor} of the integer arguments.  A bit is
set in the result if it is set in an odd number of arguments.

@lisp
(logxor) @result{} 0
(logxor 7) @result{} 7
(logxor #b000 #b001 #b011) @result{} 2
(logxor #b000 #b001 #b011 #b011) @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} lognot n
@deffnx {C Function} scm_lognot (n)
Return the integer which is the ones-complement of the integer
argument, ie.@: each 0 bit is changed to 1 and each 1 bit to 0.

@lisp
(number->string (lognot #b10000000) 2)
   @result{} "-10000001"
(number->string (lognot #b0) 2)
   @result{} "-1"
@end lisp
@end deffn

@deffn {Scheme Procedure} logtest j k
@deffnx {C Function} scm_logtest (j, k)
Test whether @var{j} and @var{k} have any 1 bits in common.  This is
equivalent to @code{(not (zero? (logand j k)))}, but without actually
calculating the @code{logand}, just testing for non-zero.

@lisp
(logtest #b0100 #b1011) @result{} #f
(logtest #b0100 #b0111) @result{} #t
@end lisp
@end deffn

@deffn {Scheme Procedure} logbit? index j
@deffnx {C Function} scm_logbit_p (index, j)
Test whether bit number @var{index} in @var{j} is set.  @var{index}
starts from 0 for the least significant bit.

@lisp
(logbit? 0 #b1101) @result{} #t
(logbit? 1 #b1101) @result{} #f
(logbit? 2 #b1101) @result{} #t
(logbit? 3 #b1101) @result{} #t
(logbit? 4 #b1101) @result{} #f
@end lisp
@end deffn

@deffn {Scheme Procedure} ash n count
@deffnx {C Function} scm_ash (n, count)
Return @math{floor(n * 2^{count})}.
@var{n} and @var{count} must be exact integers.

With @var{n} viewed as an infinite-precision twos-complement
integer, @code{ash} means a left shift introducing zero bits
when @var{count} is positive, or a right shift dropping bits
when @var{count} is negative.  This is an ``arithmetic'' shift.

@lisp
(number->string (ash #b1 3) 2)     @result{} "1000"
(number->string (ash #b1010 -1) 2) @result{} "101"

;; -23 is bits ...11101001, -6 is bits ...111010
(ash -23 -2) @result{} -6
@end lisp
@end deffn

@deffn {Scheme Procedure} round-ash n count
@deffnx {C Function} scm_round_ash (n, count)
Return @math{round(n * 2^count)}.
@var{n} and @var{count} must be exact integers.

With @var{n} viewed as an infinite-precision twos-complement
integer, @code{round-ash} means a left shift introducing zero
bits when @var{count} is positive, or a right shift rounding
to the nearest integer (with ties going to the nearest even
integer) when @var{count} is negative.  This is a rounded
``arithmetic'' shift.

@lisp
(number->string (round-ash #b1 3) 2)     @result{} \"1000\"
(number->string (round-ash #b1010 -1) 2) @result{} \"101\"
(number->string (round-ash #b1010 -2) 2) @result{} \"10\"
(number->string (round-ash #b1011 -2) 2) @result{} \"11\"
(number->string (round-ash #b1101 -2) 2) @result{} \"11\"
(number->string (round-ash #b1110 -2) 2) @result{} \"100\"
@end lisp
@end deffn

@deffn {Scheme Procedure} logcount n
@deffnx {C Function} scm_logcount (n)
Return the number of bits in integer @var{n}.  If @var{n} is
positive, the 1-bits in its binary representation are counted.
If negative, the 0-bits in its two's-complement binary
representation are counted.  If zero, 0 is returned.

@lisp
(logcount #b10101010)
   @result{} 4
(logcount 0)
   @result{} 0
(logcount -2)
   @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} integer-length n
@deffnx {C Function} scm_integer_length (n)
Return the number of bits necessary to represent @var{n}.

For positive @var{n} this is how many bits to the most significant one
bit.  For negative @var{n} it's how many bits to the most significant
zero bit in twos complement form.

@lisp
(integer-length #b10101010) @result{} 8
(integer-length #b1111)     @result{} 4
(integer-length 0)          @result{} 0
(integer-length -1)         @result{} 0
(integer-length -256)       @result{} 8
(integer-length -257)       @result{} 9
@end lisp
@end deffn

@deffn {Scheme Procedure} integer-expt n k
@deffnx {C Function} scm_integer_expt (n, k)
Return @var{n} raised to the power @var{k}.  @var{k} must be an exact
integer, @var{n} can be any number.

Negative @var{k} is supported, and results in @m{1/n^|k|, 1/n^abs(k)}
in the usual way.  @math{@var{n}^0} is 1, as usual, and that includes
@math{0^0} is 1.

@lisp
(integer-expt 2 5)   @result{} 32
(integer-expt -3 3)  @result{} -27
(integer-expt 5 -3)  @result{} 1/125
(integer-expt 0 0)   @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} bit-extract n start end
@deffnx {C Function} scm_bit_extract (n, start, end)
Return the integer composed of the @var{start} (inclusive)
through @var{end} (exclusive) bits of @var{n}.  The
@var{start}th bit becomes the 0-th bit in the result.

@lisp
(number->string (bit-extract #b1101101010 0 4) 2)
   @result{} "1010"
(number->string (bit-extract #b1101101010 4 9) 2)
   @result{} "10110"
@end lisp
@end deffn


@node Random
@subsubsection Random Number Generation

Pseudo-random numbers are generated from a random state object, which
can be created with @code{seed->random-state} or
@code{datum->random-state}.  An external representation (i.e.@: one
which can written with @code{write} and read with @code{read}) of a
random state object can be obtained via
@code{random-state->datum}.  The @var{state} parameter to the
various functions below is optional, it defaults to the state object
in the @code{*random-state*} variable.

@deffn {Scheme Procedure} copy-random-state [state]
@deffnx {C Function} scm_copy_random_state (state)
Return a copy of the random state @var{state}.
@end deffn

@deffn {Scheme Procedure} random n [state]
@deffnx {C Function} scm_random (n, state)
Return a number in [0, @var{n}).

Accepts a positive integer or real n and returns a
number of the same type between zero (inclusive) and
@var{n} (exclusive). The values returned have a uniform
distribution.
@end deffn

@deffn {Scheme Procedure} random:exp [state]
@deffnx {C Function} scm_random_exp (state)
Return an inexact real in an exponential distribution with mean
1.  For an exponential distribution with mean @var{u} use @code{(*
@var{u} (random:exp))}.
@end deffn

@deffn {Scheme Procedure} random:hollow-sphere! vect [state]
@deffnx {C Function} scm_random_hollow_sphere_x (vect, state)
Fills @var{vect} with inexact real random numbers the sum of whose
squares is equal to 1.0.  Thinking of @var{vect} as coordinates in
space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
the coordinates are uniformly distributed over the surface of the unit
n-sphere.
@end deffn

@deffn {Scheme Procedure} random:normal [state]
@deffnx {C Function} scm_random_normal (state)
Return an inexact real in a normal distribution.  The distribution
used has mean 0 and standard deviation 1.  For a normal distribution
with mean @var{m} and standard deviation @var{d} use @code{(+ @var{m}
(* @var{d} (random:normal)))}.
@end deffn

@deffn {Scheme Procedure} random:normal-vector! vect [state]
@deffnx {C Function} scm_random_normal_vector_x (vect, state)
Fills @var{vect} with inexact real random numbers that are
independent and standard normally distributed
(i.e., with mean 0 and variance 1).
@end deffn

@deffn {Scheme Procedure} random:solid-sphere! vect [state]
@deffnx {C Function} scm_random_solid_sphere_x (vect, state)
Fills @var{vect} with inexact real random numbers the sum of whose
squares is less than 1.0.  Thinking of @var{vect} as coordinates in
space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
the coordinates are uniformly distributed within the unit
@var{n}-sphere.
@c FIXME: What does this mean, particularly the n-sphere part?
@end deffn

@deffn {Scheme Procedure} random:uniform [state]
@deffnx {C Function} scm_random_uniform (state)
Return a uniformly distributed inexact real random number in
[0,1).
@end deffn

@deffn {Scheme Procedure} seed->random-state seed
@deffnx {C Function} scm_seed_to_random_state (seed)
Return a new random state using @var{seed}.
@end deffn

@deffn {Scheme Procedure} datum->random-state datum
@deffnx {C Function} scm_datum_to_random_state (datum)
Return a new random state from @var{datum}, which should have been
obtained by @code{random-state->datum}.
@end deffn

@deffn {Scheme Procedure} random-state->datum state
@deffnx {C Function} scm_random_state_to_datum (state)
Return a datum representation of @var{state} that may be written out and
read back with the Scheme reader.
@end deffn

@deffn {Scheme Procedure} random-state-from-platform
@deffnx {C Function} scm_random_state_from_platform ()
Construct a new random state seeded from a platform-specific source of
entropy, appropriate for use in non-security-critical applications.
Currently @file{/dev/urandom} is tried first, or else the seed is based
on the time, date, process ID, an address from a freshly allocated heap
cell, an address from the local stack frame, and a high-resolution timer
if available.
@end deffn

@defvar *random-state*
The global random state used by the above functions when the
@var{state} parameter is not given.
@end defvar

Note that the initial value of @code{*random-state*} is the same every
time Guile starts up.  Therefore, if you don't pass a @var{state}
parameter to the above procedures, and you don't set
@code{*random-state*} to @code{(seed->random-state your-seed)}, where
@code{your-seed} is something that @emph{isn't} the same every time,
you'll get the same sequence of ``random'' numbers on every run.

For example, unless the relevant source code has changed, @code{(map
random (cdr (iota 30)))}, if the first use of random numbers since
Guile started up, will always give:

@lisp
(map random (cdr (iota 19)))
@result{}
(0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)
@end lisp

To seed the random state in a sensible way for non-security-critical
applications, do this during initialization of your program:

@lisp
(set! *random-state* (random-state-from-platform))
@end lisp


@node Characters
@subsection Characters
@tpindex Characters

In Scheme, there is a data type to describe a single character.  

Defining what exactly a character @emph{is} can be more complicated
than it seems.  Guile follows the advice of R6RS and uses The Unicode
Standard to help define what a character is.  So, for Guile, a
character is anything in the Unicode Character Database.

@cindex code point
@cindex Unicode code point

The Unicode Character Database is basically a table of characters
indexed using integers called 'code points'.  Valid code points are in
the ranges 0 to @code{#xD7FF} inclusive or @code{#xE000} to
@code{#x10FFFF} inclusive, which is about 1.1 million code points.

@cindex designated code point
@cindex code point, designated

Any code point that has been assigned to a character or that has
otherwise been given a meaning by Unicode is called a 'designated code
point'.  Most of the designated code points, about 200,000 of them,
indicate characters, accents or other combining marks that modify
other characters, symbols, whitespace, and control characters.  Some
are not characters but indicators that suggest how to format or
display neighboring characters.

@cindex reserved code point
@cindex code point, reserved

If a code point is not a designated code point -- if it has not been
assigned to a character by The Unicode Standard -- it is a 'reserved
code point', meaning that they are reserved for future use.  Most of
the code points, about 800,000, are 'reserved code points'.

By convention, a Unicode code point is written as
``U+XXXX'' where ``XXXX'' is a hexadecimal number.  Please note that
this convenient notation is not valid code.  Guile does not interpret
``U+XXXX'' as a character.

In Scheme, a character literal is written as @code{#\@var{name}} where
@var{name} is the name of the character that you want.  Printable
characters have their usual single character name; for example,
@code{#\a} is a lower case @code{a}.  

Some of the code points are 'combining characters' that are not meant
to be printed by themselves but are instead meant to modify the
appearance of the previous character.  For combining characters, an
alternate form of the character literal is @code{#\} followed by
U+25CC (a small, dotted circle), followed by the combining character.
This allows the combining character to be drawn on the circle, not on
the backslash of @code{#\}.

Many of the non-printing characters, such as whitespace characters and
control characters, also have names.

The most commonly used non-printing characters have long character
names, described in the table below.

@multitable {@code{#\backspace}} {Preferred}
@item Character Name @tab Codepoint
@item @code{#\nul} @tab U+0000
@item @code{#\alarm} @tab U+0007
@item @code{#\backspace} @tab U+0008
@item @code{#\tab} @tab U+0009
@item @code{#\linefeed} @tab U+000A
@item @code{#\newline} @tab U+000A
@item @code{#\vtab} @tab U+000B
@item @code{#\page} @tab U+000C
@item @code{#\return} @tab U+000D
@item @code{#\esc} @tab U+001B
@item @code{#\space} @tab U+0020
@item @code{#\delete} @tab U+007F
@end multitable

There are also short names for all of the ``C0 control characters''
(those with code points below 32).  The following table lists the short
name for each character.

@multitable @columnfractions .25 .25 .25 .25
@item 0 = @code{#\nul}
 @tab 1 = @code{#\soh}
 @tab 2 = @code{#\stx}
 @tab 3 = @code{#\etx}
@item 4 = @code{#\eot}
 @tab 5 = @code{#\enq}
 @tab 6 = @code{#\ack}
 @tab 7 = @code{#\bel}
@item 8 = @code{#\bs}
 @tab 9 = @code{#\ht}
 @tab 10 = @code{#\lf}
 @tab 11 = @code{#\vt}
@item 12 = @code{#\ff}
 @tab 13 = @code{#\cr}
 @tab 14 = @code{#\so}
 @tab 15 = @code{#\si}
@item 16 = @code{#\dle}
 @tab 17 = @code{#\dc1}
 @tab 18 = @code{#\dc2}
 @tab 19 = @code{#\dc3}
@item 20 = @code{#\dc4}
 @tab 21 = @code{#\nak}
 @tab 22 = @code{#\syn}
 @tab 23 = @code{#\etb}
@item 24 = @code{#\can}
 @tab 25 = @code{#\em}
 @tab 26 = @code{#\sub}
 @tab 27 = @code{#\esc}
@item 28 = @code{#\fs}
 @tab 29 = @code{#\gs}
 @tab 30 = @code{#\rs}
 @tab 31 = @code{#\us}
@item 32 = @code{#\sp}
@end multitable

The short name for the ``delete'' character (code point U+007F) is
@code{#\del}.

The R7RS name for the ``escape'' character (code point U+001B) is
@code{#\escape}.

There are also a few alternative names left over for compatibility with
previous versions of Guile.

@multitable {@code{#\backspace}} {Preferred}
@item Alternate @tab Standard
@item @code{#\nl} @tab @code{#\newline}
@item @code{#\np} @tab @code{#\page}
@item @code{#\null} @tab @code{#\nul}
@end multitable

Characters may also be written using their code point values.  They can
be written with as an octal number, such as @code{#\10} for
@code{#\bs} or @code{#\177} for @code{#\del}.

If one prefers hex to octal, there is an additional syntax for character
escapes: @code{#\xHHHH} -- the letter 'x' followed by a hexadecimal
number of one to eight digits.

@rnindex char?
@deffn {Scheme Procedure} char? x
@deffnx {C Function} scm_char_p (x)
Return @code{#t} if @var{x} is a character, else @code{#f}.
@end deffn

Fundamentally, the character comparison operations below are
numeric comparisons of the character's code points.

@rnindex char=?
@deffn {Scheme Procedure} char=? x y
Return @code{#t} if code point of @var{x} is equal to the code point
of @var{y}, else @code{#f}.
@end deffn

@rnindex char<?
@deffn {Scheme Procedure} char<? x y
Return @code{#t} if the code point of @var{x} is less than the code
point of @var{y}, else @code{#f}.
@end deffn

@rnindex char<=?
@deffn {Scheme Procedure} char<=? x y
Return @code{#t} if the code point of @var{x} is less than or equal
to the code point of @var{y}, else @code{#f}.
@end deffn

@rnindex char>?
@deffn {Scheme Procedure} char>? x y
Return @code{#t} if the code point of @var{x} is greater than the
code point of @var{y}, else @code{#f}.
@end deffn

@rnindex char>=?
@deffn {Scheme Procedure} char>=? x y
Return @code{#t} if the code point of @var{x} is greater than or
equal to the code point of @var{y}, else @code{#f}.
@end deffn

@cindex case folding

Case-insensitive character comparisons use @emph{Unicode case
folding}.  In case folding comparisons, if a character is lowercase
and has an uppercase form that can be expressed as a single character,
it is converted to uppercase before comparison.  All other characters
undergo no conversion before the comparison occurs.  This includes the
German sharp S (Eszett) which is not uppercased before conversion
because its uppercase form has two characters.  Unicode case folding
is language independent: it uses rules that are generally true, but,
it cannot cover all cases for all languages.

@rnindex char-ci=?
@deffn {Scheme Procedure} char-ci=? x y
Return @code{#t} if the case-folded code point of @var{x} is the same
as the case-folded code point of @var{y}, else @code{#f}.
@end deffn

@rnindex char-ci<?
@deffn {Scheme Procedure} char-ci<? x y
Return @code{#t} if the case-folded code point of @var{x} is less
than the case-folded code point of @var{y}, else @code{#f}.
@end deffn

@rnindex char-ci<=?
@deffn {Scheme Procedure} char-ci<=? x y
Return @code{#t} if the case-folded code point of @var{x} is less
than or equal to the case-folded code point of @var{y}, else
@code{#f}.
@end deffn

@rnindex char-ci>?
@deffn {Scheme Procedure} char-ci>? x y
Return @code{#t} if the case-folded code point of @var{x} is greater
than the case-folded code point of @var{y}, else @code{#f}.
@end deffn

@rnindex char-ci>=?
@deffn {Scheme Procedure} char-ci>=? x y
Return @code{#t} if the case-folded code point of @var{x} is greater
than or equal to the case-folded code point of @var{y}, else
@code{#f}.
@end deffn

@rnindex char-alphabetic?
@deffn {Scheme Procedure} char-alphabetic? chr
@deffnx {C Function} scm_char_alphabetic_p (chr)
Return @code{#t} if @var{chr} is alphabetic, else @code{#f}.
@end deffn

@rnindex char-numeric?
@deffn {Scheme Procedure} char-numeric? chr
@deffnx {C Function} scm_char_numeric_p (chr)
Return @code{#t} if @var{chr} is numeric, else @code{#f}.
@end deffn

@rnindex char-whitespace?
@deffn {Scheme Procedure} char-whitespace? chr
@deffnx {C Function} scm_char_whitespace_p (chr)
Return @code{#t} if @var{chr} is whitespace, else @code{#f}.
@end deffn

@rnindex char-upper-case?
@deffn {Scheme Procedure} char-upper-case? chr
@deffnx {C Function} scm_char_upper_case_p (chr)
Return @code{#t} if @var{chr} is uppercase, else @code{#f}.
@end deffn

@rnindex char-lower-case?
@deffn {Scheme Procedure} char-lower-case? chr
@deffnx {C Function} scm_char_lower_case_p (chr)
Return @code{#t} if @var{chr} is lowercase, else @code{#f}.
@end deffn

@deffn {Scheme Procedure} char-is-both? chr
@deffnx {C Function} scm_char_is_both_p (chr)
Return @code{#t} if @var{chr} is either uppercase or lowercase, else
@code{#f}.
@end deffn

@deffn {Scheme Procedure} char-general-category chr
@deffnx {C Function} scm_char_general_category (chr)
Return a symbol giving the two-letter name of the Unicode general 
category assigned to @var{chr} or @code{#f} if no named category is 
assigned.  The following table provides a list of category names along
with their meanings.

@multitable @columnfractions .1 .4 .1 .4
@item Lu
 @tab Uppercase letter
 @tab Pf
 @tab Final quote punctuation
@item Ll
 @tab Lowercase letter
 @tab Po
 @tab Other punctuation
@item Lt
 @tab Titlecase letter
 @tab Sm
 @tab Math symbol
@item Lm
 @tab Modifier letter
 @tab Sc
 @tab Currency symbol
@item Lo
 @tab Other letter
 @tab Sk
 @tab Modifier symbol
@item Mn
 @tab Non-spacing mark
 @tab So
 @tab Other symbol
@item Mc
 @tab Combining spacing mark
 @tab Zs
 @tab Space separator
@item Me
 @tab Enclosing mark
 @tab Zl
 @tab Line separator
@item Nd
 @tab Decimal digit number
 @tab Zp
 @tab Paragraph separator
@item Nl
 @tab Letter number
 @tab Cc
 @tab Control
@item No
 @tab Other number
 @tab Cf
 @tab Format
@item Pc
 @tab Connector punctuation
 @tab Cs
 @tab Surrogate
@item Pd
 @tab Dash punctuation
 @tab Co
 @tab Private use
@item Ps
 @tab Open punctuation
 @tab Cn
 @tab Unassigned
@item Pe
 @tab Close punctuation
 @tab
 @tab
@item Pi
 @tab Initial quote punctuation
 @tab
 @tab
@end multitable
@end deffn

@rnindex char->integer
@deffn {Scheme Procedure} char->integer chr
@deffnx {C Function} scm_char_to_integer (chr)
Return the code point of @var{chr}.
@end deffn

@rnindex integer->char
@deffn {Scheme Procedure} integer->char n
@deffnx {C Function} scm_integer_to_char (n)
Return the character that has code point @var{n}.  The integer @var{n}
must be a valid code point.  Valid code points are in the ranges 0 to
@code{#xD7FF} inclusive or @code{#xE000} to @code{#x10FFFF} inclusive.
@end deffn

@rnindex char-upcase
@deffn {Scheme Procedure} char-upcase chr
@deffnx {C Function} scm_char_upcase (chr)
Return the uppercase character version of @var{chr}.
@end deffn

@rnindex char-downcase
@deffn {Scheme Procedure} char-downcase chr
@deffnx {C Function} scm_char_downcase (chr)
Return the lowercase character version of @var{chr}.
@end deffn

@rnindex char-titlecase
@deffn {Scheme Procedure} char-titlecase chr
@deffnx {C Function} scm_char_titlecase (chr)
Return the titlecase character version of @var{chr} if one exists;
otherwise return the uppercase version.  

For most characters these will be the same, but the Unicode Standard 
includes certain digraph compatibility characters, such as @code{U+01F3}
``dz'', for which the uppercase and titlecase characters are different 
(@code{U+01F1} ``DZ'' and @code{U+01F2} ``Dz'' in this case, 
respectively).
@end deffn

@tindex scm_t_wchar
@deftypefn {C Function} scm_t_wchar scm_c_upcase (scm_t_wchar @var{c})
@deftypefnx {C Function} scm_t_wchar scm_c_downcase (scm_t_wchar @var{c})
@deftypefnx {C Function} scm_t_wchar scm_c_titlecase (scm_t_wchar @var{c})

These C functions take an integer representation of a Unicode
codepoint and return the codepoint corresponding to its uppercase,
lowercase, and titlecase forms respectively.  The type
@code{scm_t_wchar} is a signed, 32-bit integer.
@end deftypefn

Characters also have ``formal names'', which are defined by Unicode.
These names can be accessed in Guile from the @code{(ice-9 unicode)}
module:

@example
(use-modules (ice-9 unicode))
@end example

@deffn {Scheme Procedure} char->formal-name chr
Return the formal all-upper-case Unicode name of @var{ch},
as a string, or @code{#f} if the character has no name.
@end deffn

@deffn {Scheme Procedure} formal-name->char name
Return the character whose formal all-upper-case Unicode name is
@var{name}, or @code{#f} if no such character is known.
@end deffn

@node Character Sets
@subsection Character Sets

The features described in this section correspond directly to SRFI-14.

The data type @dfn{charset} implements sets of characters
(@pxref{Characters}).  Because the internal representation of
character sets is not visible to the user, a lot of procedures for
handling them are provided.

Character sets can be created, extended, tested for the membership of a
characters and be compared to other character sets.

@menu
* Character Set Predicates/Comparison::
* Iterating Over Character Sets::  Enumerate charset elements.
* Creating Character Sets::        Making new charsets.
* Querying Character Sets::        Test charsets for membership etc.
* Character-Set Algebra::          Calculating new charsets.
* Standard Character Sets::        Variables containing predefined charsets.
@end menu

@node Character Set Predicates/Comparison
@subsubsection Character Set Predicates/Comparison

Use these procedures for testing whether an object is a character set,
or whether several character sets are equal or subsets of each other.
@code{char-set-hash} can be used for calculating a hash value, maybe for
usage in fast lookup procedures.

@deffn {Scheme Procedure} char-set? obj
@deffnx {C Function} scm_char_set_p (obj)
Return @code{#t} if @var{obj} is a character set, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} char-set= char_set @dots{}
@deffnx {C Function} scm_char_set_eq (char_sets)
Return @code{#t} if all given character sets are equal.
@end deffn

@deffn {Scheme Procedure} char-set<= char_set @dots{}
@deffnx {C Function} scm_char_set_leq (char_sets)
Return @code{#t} if every character set @var{char_set}i is a subset
of character set @var{char_set}i+1.
@end deffn

@deffn {Scheme Procedure} char-set-hash cs [bound]
@deffnx {C Function} scm_char_set_hash (cs, bound)
Compute a hash value for the character set @var{cs}.  If
@var{bound} is given and non-zero, it restricts the
returned value to the range 0 @dots{} @var{bound} - 1.
@end deffn

@c ===================================================================

@node Iterating Over Character Sets
@subsubsection Iterating Over Character Sets

Character set cursors are a means for iterating over the members of a
character sets.  After creating a character set cursor with
@code{char-set-cursor}, a cursor can be dereferenced with
@code{char-set-ref}, advanced to the next member with
@code{char-set-cursor-next}.  Whether a cursor has passed past the last
element of the set can be checked with @code{end-of-char-set?}.

Additionally, mapping and (un-)folding procedures for character sets are
provided.

@deffn {Scheme Procedure} char-set-cursor cs
@deffnx {C Function} scm_char_set_cursor (cs)
Return a cursor into the character set @var{cs}.
@end deffn

@deffn {Scheme Procedure} char-set-ref cs cursor
@deffnx {C Function} scm_char_set_ref (cs, cursor)
Return the character at the current cursor position
@var{cursor} in the character set @var{cs}.  It is an error to
pass a cursor for which @code{end-of-char-set?} returns true.
@end deffn

@deffn {Scheme Procedure} char-set-cursor-next cs cursor
@deffnx {C Function} scm_char_set_cursor_next (cs, cursor)
Advance the character set cursor @var{cursor} to the next
character in the character set @var{cs}.  It is an error if the
cursor given satisfies @code{end-of-char-set?}.
@end deffn

@deffn {Scheme Procedure} end-of-char-set? cursor
@deffnx {C Function} scm_end_of_char_set_p (cursor)
Return @code{#t} if @var{cursor} has reached the end of a
character set, @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} char-set-fold kons knil cs
@deffnx {C Function} scm_char_set_fold (kons, knil, cs)
Fold the procedure @var{kons} over the character set @var{cs},
initializing it with @var{knil}.
@end deffn

@deffn {Scheme Procedure} char-set-unfold p f g seed [base_cs]
@deffnx {C Function} scm_char_set_unfold (p, f, g, seed, base_cs)
This is a fundamental constructor for character sets.
@itemize @bullet
@item @var{g} is used to generate a series of ``seed'' values
from the initial seed: @var{seed}, (@var{g} @var{seed}),
(@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
@item @var{p} tells us when to stop -- when it returns true
when applied to one of the seed values.
@item @var{f} maps each seed value to a character. These
characters are added to the base character set @var{base_cs} to
form the result; @var{base_cs} defaults to the empty set.
@end itemize
@end deffn

@deffn {Scheme Procedure} char-set-unfold! p f g seed base_cs
@deffnx {C Function} scm_char_set_unfold_x (p, f, g, seed, base_cs)
This is a fundamental constructor for character sets.
@itemize @bullet
@item @var{g} is used to generate a series of ``seed'' values
from the initial seed: @var{seed}, (@var{g} @var{seed}),
(@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
@item @var{p} tells us when to stop -- when it returns true
when applied to one of the seed values.
@item @var{f} maps each seed value to a character. These
characters are added to the base character set @var{base_cs} to
form the result; @var{base_cs} defaults to the empty set.
@end itemize
@end deffn

@deffn {Scheme Procedure} char-set-for-each proc cs
@deffnx {C Function} scm_char_set_for_each (proc, cs)
Apply @var{proc} to every character in the character set
@var{cs}.  The return value is not specified.
@end deffn

@deffn {Scheme Procedure} char-set-map proc cs
@deffnx {C Function} scm_char_set_map (proc, cs)
Map the procedure @var{proc} over every character in @var{cs}.
@var{proc} must be a character -> character procedure.
@end deffn

@c ===================================================================

@node Creating Character Sets
@subsubsection Creating Character Sets

New character sets are produced with these procedures.

@deffn {Scheme Procedure} char-set-copy cs
@deffnx {C Function} scm_char_set_copy (cs)
Return a newly allocated character set containing all
characters in @var{cs}.
@end deffn

@deffn {Scheme Procedure} char-set chr @dots{}
@deffnx {C Function} scm_char_set (chrs)
Return a character set containing all given characters.
@end deffn

@deffn {Scheme Procedure} list->char-set list [base_cs]
@deffnx {C Function} scm_list_to_char_set (list, base_cs)
Convert the character list @var{list} to a character set.  If
the character set @var{base_cs} is given, the character in this
set are also included in the result.
@end deffn

@deffn {Scheme Procedure} list->char-set! list base_cs
@deffnx {C Function} scm_list_to_char_set_x (list, base_cs)
Convert the character list @var{list} to a character set.  The
characters are added to @var{base_cs} and @var{base_cs} is
returned.
@end deffn

@deffn {Scheme Procedure} string->char-set str [base_cs]
@deffnx {C Function} scm_string_to_char_set (str, base_cs)
Convert the string @var{str} to a character set.  If the
character set @var{base_cs} is given, the characters in this
set are also included in the result.
@end deffn

@deffn {Scheme Procedure} string->char-set! str base_cs
@deffnx {C Function} scm_string_to_char_set_x (str, base_cs)
Convert the string @var{str} to a character set.  The
characters from the string are added to @var{base_cs}, and
@var{base_cs} is returned.
@end deffn

@deffn {Scheme Procedure} char-set-filter pred cs [base_cs]
@deffnx {C Function} scm_char_set_filter (pred, cs, base_cs)
Return a character set containing every character from @var{cs}
so that it satisfies @var{pred}.  If provided, the characters
from @var{base_cs} are added to the result.
@end deffn

@deffn {Scheme Procedure} char-set-filter! pred cs base_cs
@deffnx {C Function} scm_char_set_filter_x (pred, cs, base_cs)
Return a character set containing every character from @var{cs}
so that it satisfies @var{pred}.  The characters are added to
@var{base_cs} and @var{base_cs} is returned.
@end deffn

@deffn {Scheme Procedure} ucs-range->char-set lower upper [error [base_cs]]
@deffnx {C Function} scm_ucs_range_to_char_set (lower, upper, error, base_cs)
Return a character set containing all characters whose
character codes lie in the half-open range
[@var{lower},@var{upper}).

If @var{error} is a true value, an error is signaled if the
specified range contains characters which are not contained in
the implemented character range.  If @var{error} is @code{#f},
these characters are silently left out of the resulting
character set.

The characters in @var{base_cs} are added to the result, if
given.
@end deffn

@deffn {Scheme Procedure} ucs-range->char-set! lower upper error base_cs
@deffnx {C Function} scm_ucs_range_to_char_set_x (lower, upper, error, base_cs)
Return a character set containing all characters whose
character codes lie in the half-open range
[@var{lower},@var{upper}).

If @var{error} is a true value, an error is signaled if the
specified range contains characters which are not contained in
the implemented character range.  If @var{error} is @code{#f},
these characters are silently left out of the resulting
character set.

The characters are added to @var{base_cs} and @var{base_cs} is
returned.
@end deffn

@deffn {Scheme Procedure} ->char-set x
@deffnx {C Function} scm_to_char_set (x)
Coerces x into a char-set. @var{x} may be a string, character or
char-set. A string is converted to the set of its constituent
characters; a character is converted to a singleton set; a char-set is
returned as-is.
@end deffn

@c ===================================================================

@node Querying Character Sets
@subsubsection Querying Character Sets

Access the elements and other information of a character set with these
procedures.

@deffn {Scheme Procedure} %char-set-dump cs
Returns an association list containing debugging information
for @var{cs}. The association list has the following entries.
@table @code
@item char-set
The char-set itself
@item len
The number of groups of contiguous code points the char-set
contains
@item ranges
A list of lists where each sublist is a range of code points
and their associated characters
@end table
The return value of this function cannot be relied upon to be
consistent between versions of Guile and should not be used in code.
@end deffn

@deffn {Scheme Procedure} char-set-size cs
@deffnx {C Function} scm_char_set_size (cs)
Return the number of elements in character set @var{cs}.
@end deffn

@deffn {Scheme Procedure} char-set-count pred cs
@deffnx {C Function} scm_char_set_count (pred, cs)
Return the number of the elements int the character set
@var{cs} which satisfy the predicate @var{pred}.
@end deffn

@deffn {Scheme Procedure} char-set->list cs
@deffnx {C Function} scm_char_set_to_list (cs)
Return a list containing the elements of the character set
@var{cs}.
@end deffn

@deffn {Scheme Procedure} char-set->string cs
@deffnx {C Function} scm_char_set_to_string (cs)
Return a string containing the elements of the character set
@var{cs}.  The order in which the characters are placed in the
string is not defined.
@end deffn

@deffn {Scheme Procedure} char-set-contains? cs ch
@deffnx {C Function} scm_char_set_contains_p (cs, ch)
Return @code{#t} if the character @var{ch} is contained in the
character set @var{cs}, or @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} char-set-every pred cs
@deffnx {C Function} scm_char_set_every (pred, cs)
Return a true value if every character in the character set
@var{cs} satisfies the predicate @var{pred}.
@end deffn

@deffn {Scheme Procedure} char-set-any pred cs
@deffnx {C Function} scm_char_set_any (pred, cs)
Return a true value if any character in the character set
@var{cs} satisfies the predicate @var{pred}.
@end deffn

@c ===================================================================

@node Character-Set Algebra
@subsubsection Character-Set Algebra

Character sets can be manipulated with the common set algebra operation,
such as union, complement, intersection etc.  All of these procedures
provide side-effecting variants, which modify their character set
argument(s).

@deffn {Scheme Procedure} char-set-adjoin cs chr @dots{}
@deffnx {C Function} scm_char_set_adjoin (cs, chrs)
Add all character arguments to the first argument, which must
be a character set.
@end deffn

@deffn {Scheme Procedure} char-set-delete cs chr @dots{}
@deffnx {C Function} scm_char_set_delete (cs, chrs)
Delete all character arguments from the first argument, which
must be a character set.
@end deffn

@deffn {Scheme Procedure} char-set-adjoin! cs chr @dots{}
@deffnx {C Function} scm_char_set_adjoin_x (cs, chrs)
Add all character arguments to the first argument, which must
be a character set.
@end deffn

@deffn {Scheme Procedure} char-set-delete! cs chr @dots{}
@deffnx {C Function} scm_char_set_delete_x (cs, chrs)
Delete all character arguments from the first argument, which
must be a character set.
@end deffn

@deffn {Scheme Procedure} char-set-complement cs
@deffnx {C Function} scm_char_set_complement (cs)
Return the complement of the character set @var{cs}.
@end deffn

Note that the complement of a character set is likely to contain many
reserved code points (code points that are not associated with
characters).  It may be helpful to modify the output of
@code{char-set-complement} by computing its intersection with the set
of designated code points, @code{char-set:designated}.

@deffn {Scheme Procedure} char-set-union cs @dots{}
@deffnx {C Function} scm_char_set_union (char_sets)
Return the union of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-intersection cs @dots{}
@deffnx {C Function} scm_char_set_intersection (char_sets)
Return the intersection of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-difference cs1 cs @dots{}
@deffnx {C Function} scm_char_set_difference (cs1, char_sets)
Return the difference of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-xor cs @dots{}
@deffnx {C Function} scm_char_set_xor (char_sets)
Return the exclusive-or of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-diff+intersection cs1 cs @dots{}
@deffnx {C Function} scm_char_set_diff_plus_intersection (cs1, char_sets)
Return the difference and the intersection of all argument
character sets.
@end deffn

@deffn {Scheme Procedure} char-set-complement! cs
@deffnx {C Function} scm_char_set_complement_x (cs)
Return the complement of the character set @var{cs}.
@end deffn

@deffn {Scheme Procedure} char-set-union! cs1 cs @dots{}
@deffnx {C Function} scm_char_set_union_x (cs1, char_sets)
Return the union of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-intersection! cs1 cs @dots{}
@deffnx {C Function} scm_char_set_intersection_x (cs1, char_sets)
Return the intersection of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-difference! cs1 cs @dots{}
@deffnx {C Function} scm_char_set_difference_x (cs1, char_sets)
Return the difference of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-xor! cs1 cs @dots{}
@deffnx {C Function} scm_char_set_xor_x (cs1, char_sets)
Return the exclusive-or of all argument character sets.
@end deffn

@deffn {Scheme Procedure} char-set-diff+intersection! cs1 cs2 cs @dots{}
@deffnx {C Function} scm_char_set_diff_plus_intersection_x (cs1, cs2, char_sets)
Return the difference and the intersection of all argument
character sets.
@end deffn

@c ===================================================================

@node Standard Character Sets
@subsubsection Standard Character Sets

In order to make the use of the character set data type and procedures
useful, several predefined character set variables exist.

@cindex codeset
@cindex charset
@cindex locale

These character sets are locale independent and are not recomputed
upon a @code{setlocale} call.  They contain characters from the whole
range of Unicode code points. For instance, @code{char-set:letter}
contains about 100,000 characters.

@defvr {Scheme Variable} char-set:lower-case
@defvrx {C Variable} scm_char_set_lower_case
All lower-case characters.
@end defvr

@defvr {Scheme Variable} char-set:upper-case
@defvrx {C Variable} scm_char_set_upper_case
All upper-case characters.
@end defvr

@defvr {Scheme Variable} char-set:title-case
@defvrx {C Variable} scm_char_set_title_case
All single characters that function as if they were an upper-case
letter followed by a lower-case letter.
@end defvr

@defvr {Scheme Variable} char-set:letter
@defvrx {C Variable} scm_char_set_letter
All letters.  This includes @code{char-set:lower-case},
@code{char-set:upper-case}, @code{char-set:title-case}, and many
letters that have no case at all.  For example, Chinese and Japanese
characters typically have no concept of case.
@end defvr

@defvr {Scheme Variable} char-set:digit
@defvrx {C Variable} scm_char_set_digit
All digits.
@end defvr

@defvr {Scheme Variable} char-set:letter+digit
@defvrx {C Variable} scm_char_set_letter_and_digit
The union of @code{char-set:letter} and @code{char-set:digit}.
@end defvr

@defvr {Scheme Variable} char-set:graphic
@defvrx {C Variable} scm_char_set_graphic
All characters which would put ink on the paper.
@end defvr

@defvr {Scheme Variable} char-set:printing
@defvrx {C Variable} scm_char_set_printing
The union of @code{char-set:graphic} and @code{char-set:whitespace}.
@end defvr

@defvr {Scheme Variable} char-set:whitespace
@defvrx {C Variable} scm_char_set_whitespace
All whitespace characters.
@end defvr

@defvr {Scheme Variable} char-set:blank
@defvrx {C Variable} scm_char_set_blank
All horizontal whitespace characters, which notably includes
@code{#\space} and @code{#\tab}.
@end defvr

@defvr {Scheme Variable} char-set:iso-control
@defvrx {C Variable} scm_char_set_iso_control
The ISO control characters are the C0 control characters (U+0000 to
U+001F), delete (U+007F), and the C1 control characters (U+0080 to
U+009F).
@end defvr

@defvr {Scheme Variable} char-set:punctuation
@defvrx {C Variable} scm_char_set_punctuation
All punctuation characters, such as the characters
@code{!"#%&'()*,-./:;?@@[\\]_@{@}}
@end defvr

@defvr {Scheme Variable} char-set:symbol
@defvrx {C Variable} scm_char_set_symbol
All symbol characters, such as the characters @code{$+<=>^`|~}.
@end defvr

@defvr {Scheme Variable} char-set:hex-digit
@defvrx {C Variable} scm_char_set_hex_digit
The hexadecimal digits @code{0123456789abcdefABCDEF}.
@end defvr

@defvr {Scheme Variable} char-set:ascii
@defvrx {C Variable} scm_char_set_ascii
All ASCII characters.
@end defvr

@defvr {Scheme Variable} char-set:empty
@defvrx {C Variable} scm_char_set_empty
The empty character set.
@end defvr

@defvr {Scheme Variable} char-set:designated
@defvrx {C Variable} scm_char_set_designated
This character set contains all designated code points.  This includes
all the code points to which Unicode has assigned a character or other
meaning.
@end defvr

@defvr {Scheme Variable} char-set:full
@defvrx {C Variable} scm_char_set_full
This character set contains all possible code points.  This includes
both designated and reserved code points.
@end defvr

@node Strings
@subsection Strings
@tpindex Strings

Strings are fixed-length sequences of characters.  They can be created
by calling constructor procedures, but they can also literally get
entered at the @acronym{REPL} or in Scheme source files.

@c Guile provides a rich set of string processing procedures, because text
@c handling is very important when Guile is used as a scripting language.

Strings always carry the information about how many characters they are
composed of with them, so there is no special end-of-string character,
like in C.  That means that Scheme strings can contain any character,
even the @samp{#\nul} character @samp{\0}.

To use strings efficiently, you need to know a bit about how Guile
implements them.  In Guile, a string consists of two parts, a head and
the actual memory where the characters are stored.  When a string (or
a substring of it) is copied, only a new head gets created, the memory
is usually not copied.  The two heads start out pointing to the same
memory.

When one of these two strings is modified, as with @code{string-set!},
their common memory does get copied so that each string has its own
memory and modifying one does not accidentally modify the other as well.
Thus, Guile's strings are `copy on write'; the actual copying of their
memory is delayed until one string is written to.

This implementation makes functions like @code{substring} very
efficient in the common case that no modifications are done to the
involved strings.

If you do know that your strings are getting modified right away, you
can use @code{substring/copy} instead of @code{substring}.  This
function performs the copy immediately at the time of creation.  This
is more efficient, especially in a multi-threaded program.  Also,
@code{substring/copy} can avoid the problem that a short substring
holds on to the memory of a very large original string that could
otherwise be recycled.

If you want to avoid the copy altogether, so that modifications of one
string show up in the other, you can use @code{substring/shared}.  The
strings created by this procedure are called @dfn{mutation sharing
substrings} since the substring and the original string share
modifications to each other.

If you want to prevent modifications, use @code{substring/read-only}.

Guile provides all procedures of SRFI-13 and a few more.

@menu
* String Syntax::                   Read syntax for strings.
* String Predicates::               Testing strings for certain properties.
* String Constructors::             Creating new string objects.
* List/String Conversion::          Converting from/to lists of characters.
* String Selection::                Select portions from strings.
* String Modification::             Modify parts or whole strings.
* String Comparison::               Lexicographic ordering predicates.
* String Searching::                Searching in strings.
* Alphabetic Case Mapping::         Convert the alphabetic case of strings.
* Reversing and Appending Strings:: Appending strings to form a new string.
* Mapping Folding and Unfolding::   Iterating over strings.
* Miscellaneous String Operations:: Replicating, insertion, parsing, ...
* Representing Strings as Bytes::   Encoding and decoding strings.
* Conversion to/from C::
* String Internals::                The storage strategy for strings.
@end menu

@node String Syntax
@subsubsection String Read Syntax

@c  In the following @code is used to get a good font in TeX etc, but
@c  is omitted for Info format, so as not to risk any confusion over
@c  whether surrounding ` ' quotes are part of the escape or are
@c  special in a string (they're not).

The read syntax for strings is an arbitrarily long sequence of
characters enclosed in double quotes (@nicode{"}).

Backslash is an escape character and can be used to insert the following
special characters.  @nicode{\"} and @nicode{\\} are R5RS standard,
@nicode{\|} is R7RS standard, the next seven are R6RS standard ---
notice they follow C syntax --- and the remaining four are Guile
extensions.

@table @asis
@item @nicode{\\}
Backslash character.

@item @nicode{\"}
Double quote character (an unescaped @nicode{"} is otherwise the end
of the string).

@item @nicode{\|}
Vertical bar character.

@item @nicode{\a}
Bell character (ASCII 7).

@item @nicode{\f}
Formfeed character (ASCII 12).

@item @nicode{\n}
Newline character (ASCII 10).

@item @nicode{\r}
Carriage return character (ASCII 13).

@item @nicode{\t}
Tab character (ASCII 9).

@item @nicode{\v}
Vertical tab character (ASCII 11).

@item @nicode{\b}
Backspace character (ASCII 8).

@item @nicode{\0}
NUL character (ASCII 0).

@item @nicode{\(}
Open parenthesis.  This is intended for use at the beginning of lines in
multiline strings to avoid confusing Emacs lisp modes.

@item @nicode{\} followed by newline (ASCII 10)
Nothing.  This way if @nicode{\} is the last character in a line, the
string will continue with the first character from the next line,
without a line break.

If the @code{hungry-eol-escapes} reader option is enabled, which is not
the case by default, leading whitespace on the next line is discarded.

@lisp
"foo\
  bar"
@result{} "foo  bar"
(read-enable 'hungry-eol-escapes)
"foo\
  bar"
@result{} "foobar"
@end lisp
@item @nicode{\xHH}
Character code given by two hexadecimal digits.  For example
@nicode{\x7f} for an ASCII DEL (127).

@item @nicode{\uHHHH}
Character code given by four hexadecimal digits.  For example
@nicode{\u0100} for a capital A with macron (U+0100).

@item @nicode{\UHHHHHH}
Character code given by six hexadecimal digits.  For example
@nicode{\U010402}.
@end table

@noindent
The following are examples of string literals:

@lisp
"foo"
"bar plonk"
"Hello World"
"\"Hi\", he said."
@end lisp

The three escape sequences @code{\xHH}, @code{\uHHHH} and @code{\UHHHHHH} were
chosen to not break compatibility with code written for previous versions of
Guile.  The R6RS specification suggests a different, incompatible syntax for hex
escapes: @code{\xHHHH;} -- a character code followed by one to eight hexadecimal
digits terminated with a semicolon.  If this escape format is desired instead,
it can be enabled with the reader option @code{r6rs-hex-escapes}.

@lisp
(read-enable 'r6rs-hex-escapes)
@end lisp

For more on reader options, @xref{Scheme Read}.

@node String Predicates
@subsubsection String Predicates

The following procedures can be used to check whether a given string
fulfills some specified property.

@rnindex string?
@deffn {Scheme Procedure} string? obj
@deffnx {C Function} scm_string_p (obj)
Return @code{#t} if @var{obj} is a string, else @code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_string (SCM obj)
Returns @code{1} if @var{obj} is a string, @code{0} otherwise.
@end deftypefn

@deffn {Scheme Procedure} string-null? str
@deffnx {C Function} scm_string_null_p (str)
Return @code{#t} if @var{str}'s length is zero, and
@code{#f} otherwise.
@lisp
(string-null? "")  @result{} #t
y                    @result{} "foo"
(string-null? y)     @result{} #f
@end lisp
@end deffn

@deffn {Scheme Procedure} string-any char_pred s [start [end]]
@deffnx {C Function} scm_string_any (char_pred, s, start, end)
Check if @var{char_pred} is true for any character in string @var{s}.

@var{char_pred} can be a character to check for any equal to that, or
a character set (@pxref{Character Sets}) to check for any in that set,
or a predicate procedure to call.

For a procedure, calls @code{(@var{char_pred} c)} are made
successively on the characters from @var{start} to @var{end}.  If
@var{char_pred} returns true (ie.@: non-@code{#f}), @code{string-any}
stops and that return value is the return from @code{string-any}.  The
call on the last character (ie.@: at @math{@var{end}-1}), if that
point is reached, is a tail call.

If there are no characters in @var{s} (ie.@: @var{start} equals
@var{end}) then the return is @code{#f}.
@end deffn

@deffn {Scheme Procedure} string-every char_pred s [start [end]]
@deffnx {C Function} scm_string_every (char_pred, s, start, end)
Check if @var{char_pred} is true for every character in string
@var{s}.

@var{char_pred} can be a character to check for every character equal
to that, or a character set (@pxref{Character Sets}) to check for
every character being in that set, or a predicate procedure to call.

For a procedure, calls @code{(@var{char_pred} c)} are made
successively on the characters from @var{start} to @var{end}.  If
@var{char_pred} returns @code{#f}, @code{string-every} stops and
returns @code{#f}.  The call on the last character (ie.@: at
@math{@var{end}-1}), if that point is reached, is a tail call and the
return from that call is the return from @code{string-every}.

If there are no characters in @var{s} (ie.@: @var{start} equals
@var{end}) then the return is @code{#t}.
@end deffn

@node String Constructors
@subsubsection String Constructors

The string constructor procedures create new string objects, possibly
initializing them with some specified character data.  See also
@xref{String Selection}, for ways to create strings from existing
strings.

@c FIXME::martin: list->string belongs into `List/String Conversion'

@deffn {Scheme Procedure} string char@dots{}
@rnindex string
Return a newly allocated string made from the given character
arguments.

@example
(string #\x #\y #\z) @result{} "xyz"
(string)             @result{} ""
@end example
@end deffn

@deffn {Scheme Procedure} list->string lst
@deffnx {C Function} scm_string (lst)
@rnindex list->string
Return a newly allocated string made from a list of characters.

@example
(list->string '(#\a #\b #\c)) @result{} "abc"
@end example
@end deffn

@deffn {Scheme Procedure} reverse-list->string lst
@deffnx {C Function} scm_reverse_list_to_string (lst)
Return a newly allocated string made from a list of characters, in
reverse order.

@example
(reverse-list->string '(#\a #\B #\c)) @result{} "cBa"
@end example
@end deffn

@rnindex make-string
@deffn {Scheme Procedure} make-string k [chr]
@deffnx {C Function} scm_make_string (k, chr)
Return a newly allocated string of
length @var{k}.  If @var{chr} is given, then all elements of
the string are initialized to @var{chr}, otherwise the contents
of the string are unspecified.
@end deffn

@deftypefn {C Function} SCM scm_c_make_string (size_t len, SCM chr)
Like @code{scm_make_string}, but expects the length as a
@code{size_t}.
@end deftypefn

@deffn {Scheme Procedure} string-tabulate proc len
@deffnx {C Function} scm_string_tabulate (proc, len)
@var{proc} is an integer->char procedure.  Construct a string
of size @var{len} by applying @var{proc} to each index to
produce the corresponding string element.  The order in which
@var{proc} is applied to the indices is not specified.
@end deffn

@deffn {Scheme Procedure} string-join ls [delimiter [grammar]]
@deffnx {C Function} scm_string_join (ls, delimiter, grammar)
Append the string in the string list @var{ls}, using the string
@var{delimiter} as a delimiter between the elements of @var{ls}.
@var{delimiter} defaults to @w{@samp{ }}, that is, strings in @var{ls}
are appended with the space character in between them.  @var{grammar} is
a symbol which specifies how the delimiter is placed between the
strings, and defaults to the symbol @code{infix}.

@table @code
@item infix
Insert the separator between list elements.  An empty string
will produce an empty list.
@item strict-infix
Like @code{infix}, but will raise an error if given the empty
list.
@item suffix
Insert the separator after every list element.
@item prefix
Insert the separator before each list element.
@end table
@end deffn

@node List/String Conversion
@subsubsection List/String conversion

When processing strings, it is often convenient to first convert them
into a list representation by using the procedure @code{string->list},
work with the resulting list, and then convert it back into a string.
These procedures are useful for similar tasks.

@rnindex string->list
@deffn {Scheme Procedure} string->list str [start [end]]
@deffnx {C Function} scm_substring_to_list (str, start, end)
@deffnx {C Function} scm_string_to_list (str)
Convert the string @var{str} into a list of characters.
@end deffn

@deffn {Scheme Procedure} string-split str char_pred
@deffnx {C Function} scm_string_split (str, char_pred)
Split the string @var{str} into a list of substrings delimited
by appearances of characters that

@itemize @bullet
@item
equal @var{char_pred}, if it is a character,

@item
satisfy the predicate @var{char_pred}, if it is a procedure,

@item
are in the set @var{char_pred}, if it is a character set.
@end itemize

Note that an empty substring between separator characters will result in
an empty string in the result list.

@lisp
(string-split "root:x:0:0:root:/root:/bin/bash" #\:)
@result{}
("root" "x" "0" "0" "root" "/root" "/bin/bash")

(string-split "::" #\:)
@result{}
("" "" "")

(string-split "" #\:)
@result{}
("")
@end lisp
@end deffn


@node String Selection
@subsubsection String Selection

Portions of strings can be extracted by these procedures.
@code{string-ref} delivers individual characters whereas
@code{substring} can be used to extract substrings from longer strings.

@rnindex string-length
@deffn {Scheme Procedure} string-length string
@deffnx {C Function} scm_string_length (string)
Return the number of characters in @var{string}.
@end deffn

@deftypefn {C Function} size_t scm_c_string_length (SCM str)
Return the number of characters in @var{str} as a @code{size_t}.
@end deftypefn

@rnindex string-ref
@deffn {Scheme Procedure} string-ref str k
@deffnx {C Function} scm_string_ref (str, k)
Return character @var{k} of @var{str} using zero-origin
indexing. @var{k} must be a valid index of @var{str}.
@end deffn

@deftypefn {C Function} SCM scm_c_string_ref (SCM str, size_t k)
Return character @var{k} of @var{str} using zero-origin
indexing. @var{k} must be a valid index of @var{str}.
@end deftypefn

@rnindex string-copy
@deffn {Scheme Procedure} string-copy str [start [end]]
@deffnx {C Function} scm_substring_copy (str, start, end)
@deffnx {C Function} scm_string_copy (str)
Return a copy of the given string @var{str}.

The returned string shares storage with @var{str} initially, but it is
copied as soon as one of the two strings is modified.
@end deffn

@rnindex substring
@deffn {Scheme Procedure} substring str start [end]
@deffnx {C Function} scm_substring (str, start, end)
Return a new string formed from the characters
of @var{str} beginning with index @var{start} (inclusive) and
ending with index @var{end} (exclusive).
@var{str} must be a string, @var{start} and @var{end} must be
exact integers satisfying:

0 <= @var{start} <= @var{end} <= @code{(string-length @var{str})}.

The returned string shares storage with @var{str} initially, but it is
copied as soon as one of the two strings is modified.
@end deffn

@deffn {Scheme Procedure} substring/shared str start [end]
@deffnx {C Function} scm_substring_shared (str, start, end)
Like @code{substring}, but the strings continue to share their storage
even if they are modified.  Thus, modifications to @var{str} show up
in the new string, and vice versa.
@end deffn

@deffn {Scheme Procedure} substring/copy str start [end]
@deffnx {C Function} scm_substring_copy (str, start, end)
Like @code{substring}, but the storage for the new string is copied
immediately.
@end deffn

@deffn {Scheme Procedure} substring/read-only str start [end]
@deffnx {C Function} scm_substring_read_only (str, start, end)
Like @code{substring}, but the resulting string can not be modified.
@end deffn

@deftypefn  {C Function} SCM scm_c_substring (SCM str, size_t start, size_t end)
@deftypefnx {C Function} SCM scm_c_substring_shared (SCM str, size_t start, size_t end)
@deftypefnx {C Function} SCM scm_c_substring_copy (SCM str, size_t start, size_t end)
@deftypefnx {C Function} SCM scm_c_substring_read_only (SCM str, size_t start, size_t end)
Like @code{scm_substring}, etc. but the bounds are given as a @code{size_t}.
@end deftypefn

@deffn {Scheme Procedure} string-take s n
@deffnx {C Function} scm_string_take (s, n)
Return the @var{n} first characters of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-drop s n
@deffnx {C Function} scm_string_drop (s, n)
Return all but the first @var{n} characters of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-take-right s n
@deffnx {C Function} scm_string_take_right (s, n)
Return the @var{n} last characters of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-drop-right s n
@deffnx {C Function} scm_string_drop_right (s, n)
Return all but the last @var{n} characters of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-pad s len [chr [start [end]]]
@deffnx {Scheme Procedure} string-pad-right s len [chr [start [end]]]
@deffnx {C Function} scm_string_pad (s, len, chr, start, end)
@deffnx {C Function} scm_string_pad_right (s, len, chr, start, end)
Take characters @var{start} to @var{end} from the string @var{s} and
either pad with @var{chr} or truncate them to give @var{len}
characters.

@code{string-pad} pads or truncates on the left, so for example

@example
(string-pad "x" 3)     @result{} "  x"
(string-pad "abcde" 3) @result{} "cde"
@end example

@code{string-pad-right} pads or truncates on the right, so for example

@example
(string-pad-right "x" 3)     @result{} "x  "
(string-pad-right "abcde" 3) @result{} "abc"
@end example
@end deffn

@deffn {Scheme Procedure} string-trim s [char_pred [start [end]]]
@deffnx {Scheme Procedure} string-trim-right s [char_pred [start [end]]]
@deffnx {Scheme Procedure} string-trim-both s [char_pred [start [end]]]
@deffnx {C Function} scm_string_trim (s, char_pred, start, end)
@deffnx {C Function} scm_string_trim_right (s, char_pred, start, end)
@deffnx {C Function} scm_string_trim_both (s, char_pred, start, end)
Trim occurrences of @var{char_pred} from the ends of @var{s}.

@code{string-trim} trims @var{char_pred} characters from the left
(start) of the string, @code{string-trim-right} trims them from the
right (end) of the string, @code{string-trim-both} trims from both
ends.

@var{char_pred} can be a character, a character set, or a predicate
procedure to call on each character.  If @var{char_pred} is not given
the default is whitespace as per @code{char-set:whitespace}
(@pxref{Standard Character Sets}).

@example
(string-trim " x ")              @result{} "x "
(string-trim-right "banana" #\a) @result{} "banan"
(string-trim-both ".,xy:;" char-set:punctuation)
                  @result{} "xy"
(string-trim-both "xyzzy" (lambda (c)
                             (or (eqv? c #\x)
                                 (eqv? c #\y))))
                  @result{} "zz"
@end example
@end deffn

@node String Modification
@subsubsection String Modification

These procedures are for modifying strings in-place.  This means that the
result of the operation is not a new string; instead, the original string's
memory representation is modified.

@rnindex string-set!
@deffn {Scheme Procedure} string-set! str k chr
@deffnx {C Function} scm_string_set_x (str, k, chr)
Store @var{chr} in element @var{k} of @var{str} and return
an unspecified value. @var{k} must be a valid index of
@var{str}.
@end deffn

@deftypefn {C Function} void scm_c_string_set_x (SCM str, size_t k, SCM chr)
Like @code{scm_string_set_x}, but the index is given as a @code{size_t}.
@end deftypefn

@rnindex string-fill!
@anchor{x-string-fill!}
@deffn {Scheme Procedure} string-fill! str chr [start [end]]
@deffnx {C Function} scm_substring_fill_x (str, chr, start, end)
@deffnx {C Function} scm_string_fill_x (str, chr)
Stores @var{chr} in every element of the given @var{str} and
returns an unspecified value.
@end deffn

@deffn {Scheme Procedure} substring-fill! str start end fill
@deffnx {C Function} scm_substring_fill_x (str, start, end, fill)
Change every character in @var{str} between @var{start} and
@var{end} to @var{fill}.

@lisp
(define y (string-copy "abcdefg"))
(substring-fill! y 1 3 #\r)
y
@result{} "arrdefg"
@end lisp
@end deffn

@deffn {Scheme Procedure} substring-move! str1 start1 end1 str2 start2
@deffnx {C Function} scm_substring_move_x (str1, start1, end1, str2, start2)
Copy the substring of @var{str1} bounded by @var{start1} and @var{end1}
into @var{str2} beginning at position @var{start2}.
@var{str1} and @var{str2} can be the same string.
@end deffn

@deffn {Scheme Procedure} string-copy! target tstart s [start [end]]
@deffnx {C Function} scm_string_copy_x (target, tstart, s, start, end)
Copy the sequence of characters from index range [@var{start},
@var{end}) in string @var{s} to string @var{target}, beginning
at index @var{tstart}.  The characters are copied left-to-right
or right-to-left as needed -- the copy is guaranteed to work,
even if @var{target} and @var{s} are the same string.  It is an
error if the copy operation runs off the end of the target
string.
@end deffn


@node String Comparison
@subsubsection String Comparison

The procedures in this section are similar to the character ordering
predicates (@pxref{Characters}), but are defined on character sequences.

The first set is specified in R5RS and has names that end in @code{?}.
The second set is specified in SRFI-13 and the names have not ending
@code{?}.

The predicates ending in @code{-ci} ignore the character case
when comparing strings.  For now, case-insensitive comparison is done
using the R5RS rules, where every lower-case character that has a
single character upper-case form is converted to uppercase before
comparison.  See @xref{Text Collation, the @code{(ice-9
i18n)} module}, for locale-dependent string comparison.

@rnindex string=?
@deffn {Scheme Procedure} string=? s1 s2 s3 @dots{}
Lexicographic equality predicate; return @code{#t} if all strings are
the same length and contain the same characters in the same positions,
otherwise return @code{#f}.

The procedure @code{string-ci=?} treats upper and lower case
letters as though they were the same character, but
@code{string=?} treats upper and lower case as distinct
characters.
@end deffn

@rnindex string<?
@deffn {Scheme Procedure} string<? s1 s2 s3 @dots{}
Lexicographic ordering predicate; return @code{#t} if, for every pair of
consecutive string arguments @var{str_i} and @var{str_i+1}, @var{str_i} is
lexicographically less than @var{str_i+1}.
@end deffn

@rnindex string<=?
@deffn {Scheme Procedure} string<=? s1 s2 s3 @dots{}
Lexicographic ordering predicate; return @code{#t} if, for every pair of
consecutive string arguments @var{str_i} and @var{str_i+1}, @var{str_i} is
lexicographically less than or equal to @var{str_i+1}.
@end deffn

@rnindex string>?
@deffn {Scheme Procedure} string>? s1 s2 s3 @dots{}
Lexicographic ordering predicate; return @code{#t} if, for every pair of
consecutive string arguments @var{str_i} and @var{str_i+1}, @var{str_i} is
lexicographically greater than @var{str_i+1}.
@end deffn

@rnindex string>=?
@deffn {Scheme Procedure} string>=? s1 s2 s3 @dots{}
Lexicographic ordering predicate; return @code{#t} if, for every pair of
consecutive string arguments @var{str_i} and @var{str_i+1}, @var{str_i} is
lexicographically greater than or equal to @var{str_i+1}.
@end deffn

@rnindex string-ci=?
@deffn {Scheme Procedure} string-ci=? s1 s2 s3 @dots{}
Case-insensitive string equality predicate; return @code{#t} if
all strings are the same length and their component
characters match (ignoring case) at each position; otherwise
return @code{#f}.
@end deffn

@rnindex string-ci<?
@deffn {Scheme Procedure} string-ci<? s1 s2 s3 @dots{}
Case insensitive lexicographic ordering predicate; return @code{#t} if,
for every pair of consecutive string arguments @var{str_i} and
@var{str_i+1}, @var{str_i} is lexicographically less than @var{str_i+1}
regardless of case.
@end deffn

@rnindex string<=?
@deffn {Scheme Procedure} string-ci<=? s1 s2 s3 @dots{}
Case insensitive lexicographic ordering predicate; return @code{#t} if,
for every pair of consecutive string arguments @var{str_i} and
@var{str_i+1}, @var{str_i} is lexicographically less than or equal to
@var{str_i+1} regardless of case.
@end deffn

@rnindex string-ci>?
@deffn {Scheme Procedure} string-ci>? s1 s2 s3 @dots{}
Case insensitive lexicographic ordering predicate; return @code{#t} if,
for every pair of consecutive string arguments @var{str_i} and
@var{str_i+1}, @var{str_i} is lexicographically greater than
@var{str_i+1} regardless of case.
@end deffn

@rnindex string-ci>=?
@deffn {Scheme Procedure} string-ci>=? s1 s2 s3 @dots{}
Case insensitive lexicographic ordering predicate; return @code{#t} if,
for every pair of consecutive string arguments @var{str_i} and
@var{str_i+1}, @var{str_i} is lexicographically greater than or equal to
@var{str_i+1} regardless of case.
@end deffn

@deffn {Scheme Procedure} string-compare s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
mismatch index, depending upon whether @var{s1} is less than,
equal to, or greater than @var{s2}.  The mismatch index is the
largest index @var{i} such that for every 0 <= @var{j} <
@var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
@var{i} is the first position that does not match.
@end deffn

@deffn {Scheme Procedure} string-compare-ci s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_compare_ci (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
mismatch index, depending upon whether @var{s1} is less than,
equal to, or greater than @var{s2}.  The mismatch index is the
largest index @var{i} such that for every 0 <= @var{j} <
@var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
@var{i} is the first position where the lowercased letters 
do not match.

@end deffn

@deffn {Scheme Procedure} string= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_eq (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
value otherwise.
@end deffn

@deffn {Scheme Procedure} string<> s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_neq (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} and @var{s2} are equal, a true
value otherwise.
@end deffn

@deffn {Scheme Procedure} string< s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_lt (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
true value otherwise.
@end deffn

@deffn {Scheme Procedure} string> s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_gt (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
true value otherwise.
@end deffn

@deffn {Scheme Procedure} string<= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_le (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is greater to @var{s2}, a true
value otherwise.
@end deffn

@deffn {Scheme Procedure} string>= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ge (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is less to @var{s2}, a true value
otherwise.
@end deffn

@deffn {Scheme Procedure} string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
value otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} and @var{s2} are equal, a true
value otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
true value otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
true value otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_le (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is greater to @var{s2}, a true
value otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
Return @code{#f} if @var{s1} is less to @var{s2}, a true value
otherwise.  The character comparison is done
case-insensitively.
@end deffn

@deffn {Scheme Procedure} string-hash s [bound [start [end]]]
@deffnx {C Function} scm_substring_hash (s, bound, start, end)
Compute a hash value for @var{s}.  The optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
@end deffn

@deffn {Scheme Procedure} string-hash-ci s [bound [start [end]]]
@deffnx {C Function} scm_substring_hash_ci (s, bound, start, end)
Compute a hash value for @var{s}.  The optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
@end deffn

Because the same visual appearance of an abstract Unicode character can 
be obtained via multiple sequences of Unicode characters, even the 
case-insensitive string comparison functions described above may return
@code{#f} when presented with strings containing different 
representations of the same character.  For example, the Unicode 
character ``LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE'' can be 
represented with a single character (U+1E69) or by the character ``LATIN
SMALL LETTER S'' (U+0073) followed by the combining marks ``COMBINING 
DOT BELOW'' (U+0323) and ``COMBINING DOT ABOVE'' (U+0307).

For this reason, it is often desirable to ensure that the strings
to be compared are using a mutually consistent representation for every 
character.  The Unicode standard defines two methods of normalizing the
contents of strings: Decomposition, which breaks composite characters 
into a set of constituent characters with an ordering defined by the
Unicode Standard; and composition, which performs the converse.

There are two decomposition operations.  ``Canonical decomposition'' 
produces character sequences that share the same visual appearance as
the original characters, while ``compatibility decomposition'' produces
ones whose visual appearances may differ from the originals but which
represent the same abstract character.

These operations are encapsulated in the following set of normalization
forms:

@table @dfn
@item NFD
Characters are decomposed to their canonical forms.

@item NFKD
Characters are decomposed to their compatibility forms.

@item NFC
Characters are decomposed to their canonical forms, then composed.

@item NFKC
Characters are decomposed to their compatibility forms, then composed.

@end table

The functions below put their arguments into one of the forms described
above.

@deffn {Scheme Procedure} string-normalize-nfd s
@deffnx {C Function} scm_string_normalize_nfd (s)
Return the @code{NFD} normalized form of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-normalize-nfkd s
@deffnx {C Function} scm_string_normalize_nfkd (s)
Return the @code{NFKD} normalized form of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-normalize-nfc s
@deffnx {C Function} scm_string_normalize_nfc (s)
Return the @code{NFC} normalized form of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-normalize-nfkc s
@deffnx {C Function} scm_string_normalize_nfkc (s)
Return the @code{NFKC} normalized form of @var{s}.
@end deffn

@node String Searching
@subsubsection String Searching

@deffn {Scheme Procedure} string-index s char_pred [start [end]]
@deffnx {C Function} scm_string_index (s, char_pred, start, end)
Search through the string @var{s} from left to right, returning
the index of the first occurrence of a character which

@itemize @bullet
@item
equals @var{char_pred}, if it is character,

@item
satisfies the predicate @var{char_pred}, if it is a procedure,

@item
is in the set @var{char_pred}, if it is a character set.
@end itemize

Return @code{#f} if no match is found.
@end deffn

@deffn {Scheme Procedure} string-rindex s char_pred [start [end]]
@deffnx {C Function} scm_string_rindex (s, char_pred, start, end)
Search through the string @var{s} from right to left, returning
the index of the last occurrence of a character which

@itemize @bullet
@item
equals @var{char_pred}, if it is character,

@item
satisfies the predicate @var{char_pred}, if it is a procedure,

@item
is in the set if @var{char_pred} is a character set.
@end itemize

Return @code{#f} if no match is found.
@end deffn

@deffn {Scheme Procedure} string-prefix-length s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_prefix_length (s1, s2, start1, end1, start2, end2)
Return the length of the longest common prefix of the two
strings.
@end deffn

@deffn {Scheme Procedure} string-prefix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_prefix_length_ci (s1, s2, start1, end1, start2, end2)
Return the length of the longest common prefix of the two
strings, ignoring character case.
@end deffn

@deffn {Scheme Procedure} string-suffix-length s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_suffix_length (s1, s2, start1, end1, start2, end2)
Return the length of the longest common suffix of the two
strings.
@end deffn

@deffn {Scheme Procedure} string-suffix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_suffix_length_ci (s1, s2, start1, end1, start2, end2)
Return the length of the longest common suffix of the two
strings, ignoring character case.
@end deffn

@deffn {Scheme Procedure} string-prefix? s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
Is @var{s1} a prefix of @var{s2}?
@end deffn

@deffn {Scheme Procedure} string-prefix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_prefix_ci_p (s1, s2, start1, end1, start2, end2)
Is @var{s1} a prefix of @var{s2}, ignoring character case?
@end deffn

@deffn {Scheme Procedure} string-suffix? s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
Is @var{s1} a suffix of @var{s2}?
@end deffn

@deffn {Scheme Procedure} string-suffix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_suffix_ci_p (s1, s2, start1, end1, start2, end2)
Is @var{s1} a suffix of @var{s2}, ignoring character case?
@end deffn

@deffn {Scheme Procedure} string-index-right s char_pred [start [end]]
@deffnx {C Function} scm_string_index_right (s, char_pred, start, end)
Search through the string @var{s} from right to left, returning
the index of the last occurrence of a character which

@itemize @bullet
@item
equals @var{char_pred}, if it is character,

@item
satisfies the predicate @var{char_pred}, if it is a procedure,

@item
is in the set if @var{char_pred} is a character set.
@end itemize

Return @code{#f} if no match is found.
@end deffn

@deffn {Scheme Procedure} string-skip s char_pred [start [end]]
@deffnx {C Function} scm_string_skip (s, char_pred, start, end)
Search through the string @var{s} from left to right, returning
the index of the first occurrence of a character which

@itemize @bullet
@item
does not equal @var{char_pred}, if it is character,

@item
does not satisfy the predicate @var{char_pred}, if it is a
procedure,

@item
is not in the set if @var{char_pred} is a character set.
@end itemize
@end deffn

@deffn {Scheme Procedure} string-skip-right s char_pred [start [end]]
@deffnx {C Function} scm_string_skip_right (s, char_pred, start, end)
Search through the string @var{s} from right to left, returning
the index of the last occurrence of a character which

@itemize @bullet
@item
does not equal @var{char_pred}, if it is character,

@item
does not satisfy the predicate @var{char_pred}, if it is a
procedure,

@item
is not in the set if @var{char_pred} is a character set.
@end itemize
@end deffn

@deffn {Scheme Procedure} string-count s char_pred [start [end]]
@deffnx {C Function} scm_string_count (s, char_pred, start, end)
Return the count of the number of characters in the string
@var{s} which

@itemize @bullet
@item
equals @var{char_pred}, if it is character,

@item
satisfies the predicate @var{char_pred}, if it is a procedure.

@item
is in the set @var{char_pred}, if it is a character set.
@end itemize
@end deffn

@deffn {Scheme Procedure} string-contains s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_contains (s1, s2, start1, end1, start2, end2)
Does string @var{s1} contain string @var{s2}?  Return the index
in @var{s1} where @var{s2} occurs as a substring, or false.
The optional start/end indices restrict the operation to the
indicated substrings.
@end deffn

@deffn {Scheme Procedure} string-contains-ci s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_contains_ci (s1, s2, start1, end1, start2, end2)
Does string @var{s1} contain string @var{s2}?  Return the index
in @var{s1} where @var{s2} occurs as a substring, or false.
The optional start/end indices restrict the operation to the
indicated substrings.  Character comparison is done
case-insensitively.
@end deffn

@node Alphabetic Case Mapping
@subsubsection Alphabetic Case Mapping

These are procedures for mapping strings to their upper- or lower-case
equivalents, respectively, or for capitalizing strings.

They use the basic case mapping rules for Unicode characters.  No
special language or context rules are considered.  The resulting strings
are guaranteed to be the same length as the input strings.

@xref{Character Case Mapping, the @code{(ice-9
i18n)} module}, for locale-dependent case conversions.

@deffn {Scheme Procedure} string-upcase str [start [end]]
@deffnx {C Function} scm_substring_upcase (str, start, end)
@deffnx {C Function} scm_string_upcase (str)
Upcase every character in @code{str}.
@end deffn

@deffn {Scheme Procedure} string-upcase! str [start [end]]
@deffnx {C Function} scm_substring_upcase_x (str, start, end)
@deffnx {C Function} scm_string_upcase_x (str)
Destructively upcase every character in @code{str}.

@lisp
(string-upcase! y)
@result{} "ARRDEFG"
y
@result{} "ARRDEFG"
@end lisp
@end deffn

@deffn {Scheme Procedure} string-downcase str [start [end]]
@deffnx {C Function} scm_substring_downcase (str, start, end)
@deffnx {C Function} scm_string_downcase (str)
Downcase every character in @var{str}.
@end deffn

@deffn {Scheme Procedure} string-downcase! str [start [end]]
@deffnx {C Function} scm_substring_downcase_x (str, start, end)
@deffnx {C Function} scm_string_downcase_x (str)
Destructively downcase every character in @var{str}.

@lisp
y
@result{} "ARRDEFG"
(string-downcase! y)
@result{} "arrdefg"
y
@result{} "arrdefg"
@end lisp
@end deffn

@deffn {Scheme Procedure} string-capitalize str
@deffnx {C Function} scm_string_capitalize (str)
Return a freshly allocated string with the characters in
@var{str}, where the first character of every word is
capitalized.
@end deffn

@deffn {Scheme Procedure} string-capitalize! str
@deffnx {C Function} scm_string_capitalize_x (str)
Upcase the first character of every word in @var{str}
destructively and return @var{str}.

@lisp
y                      @result{} "hello world"
(string-capitalize! y) @result{} "Hello World"
y                      @result{} "Hello World"
@end lisp
@end deffn

@deffn {Scheme Procedure} string-titlecase str [start [end]]
@deffnx {C Function} scm_string_titlecase (str, start, end)
Titlecase every first character in a word in @var{str}.
@end deffn

@deffn {Scheme Procedure} string-titlecase! str [start [end]]
@deffnx {C Function} scm_string_titlecase_x (str, start, end)
Destructively titlecase every first character in a word in
@var{str}.
@end deffn

@node Reversing and Appending Strings
@subsubsection Reversing and Appending Strings

@deffn {Scheme Procedure} string-reverse str [start [end]]
@deffnx {C Function} scm_string_reverse (str, start, end)
Reverse the string @var{str}.  The optional arguments
@var{start} and @var{end} delimit the region of @var{str} to
operate on.
@end deffn

@deffn {Scheme Procedure} string-reverse! str [start [end]]
@deffnx {C Function} scm_string_reverse_x (str, start, end)
Reverse the string @var{str} in-place.  The optional arguments
@var{start} and @var{end} delimit the region of @var{str} to
operate on.  The return value is unspecified.
@end deffn

@rnindex string-append
@deffn {Scheme Procedure} string-append arg @dots{}
@deffnx {C Function} scm_string_append (args)
Return a newly allocated string whose characters form the
concatenation of the given strings, @var{arg} @enddots{}.

@example
(let ((h "hello "))
  (string-append h "world"))
@result{} "hello world"
@end example
@end deffn

@deffn {Scheme Procedure} string-append/shared arg @dots{}
@deffnx {C Function} scm_string_append_shared (args)
Like @code{string-append}, but the result may share memory
with the argument strings.
@end deffn

@deffn {Scheme Procedure} string-concatenate ls
@deffnx {C Function} scm_string_concatenate (ls)
Append the elements (which must be strings) of @var{ls} together into a
single string.  Guaranteed to return a freshly allocated string.
@end deffn

@deffn {Scheme Procedure} string-concatenate-reverse ls [final_string [end]]
@deffnx {C Function} scm_string_concatenate_reverse (ls, final_string, end)
Without optional arguments, this procedure is equivalent to

@lisp
(string-concatenate (reverse ls))
@end lisp

If the optional argument @var{final_string} is specified, it is
consed onto the beginning to @var{ls} before performing the
list-reverse and string-concatenate operations.  If @var{end}
is given, only the characters of @var{final_string} up to index
@var{end} are used.

Guaranteed to return a freshly allocated string.
@end deffn

@deffn {Scheme Procedure} string-concatenate/shared ls
@deffnx {C Function} scm_string_concatenate_shared (ls)
Like @code{string-concatenate}, but the result may share memory
with the strings in the list @var{ls}.
@end deffn

@deffn {Scheme Procedure} string-concatenate-reverse/shared ls [final_string [end]]
@deffnx {C Function} scm_string_concatenate_reverse_shared (ls, final_string, end)
Like @code{string-concatenate-reverse}, but the result may
share memory with the strings in the @var{ls} arguments.
@end deffn

@node Mapping Folding and Unfolding
@subsubsection Mapping, Folding, and Unfolding

@deffn {Scheme Procedure} string-map proc s [start [end]]
@deffnx {C Function} scm_string_map (proc, s, start, end)
@var{proc} is a char->char procedure, it is mapped over
@var{s}.  The order in which the procedure is applied to the
string elements is not specified.
@end deffn

@deffn {Scheme Procedure} string-map! proc s [start [end]]
@deffnx {C Function} scm_string_map_x (proc, s, start, end)
@var{proc} is a char->char procedure, it is mapped over
@var{s}.  The order in which the procedure is applied to the
string elements is not specified.  The string @var{s} is
modified in-place, the return value is not specified.
@end deffn

@deffn {Scheme Procedure} string-for-each proc s [start [end]]
@deffnx {C Function} scm_string_for_each (proc, s, start, end)
@var{proc} is mapped over @var{s} in left-to-right order.  The
return value is not specified.
@end deffn

@deffn {Scheme Procedure} string-for-each-index proc s [start [end]]
@deffnx {C Function} scm_string_for_each_index (proc, s, start, end)
Call @code{(@var{proc} i)} for each index i in @var{s}, from left to
right.

For example, to change characters to alternately upper and lower case,

@example
(define str (string-copy "studly"))
(string-for-each-index
    (lambda (i)
      (string-set! str i
        ((if (even? i) char-upcase char-downcase)
         (string-ref str i))))
    str)
str @result{} "StUdLy"
@end example
@end deffn

@deffn {Scheme Procedure} string-fold kons knil s [start [end]]
@deffnx {C Function} scm_string_fold (kons, knil, s, start, end)
Fold @var{kons} over the characters of @var{s}, with @var{knil}
as the terminating element, from left to right.  @var{kons}
must expect two arguments: The actual character and the last
result of @var{kons}' application.
@end deffn

@deffn {Scheme Procedure} string-fold-right kons knil s [start [end]]
@deffnx {C Function} scm_string_fold_right (kons, knil, s, start, end)
Fold @var{kons} over the characters of @var{s}, with @var{knil}
as the terminating element, from right to left.  @var{kons}
must expect two arguments: The actual character and the last
result of @var{kons}' application.
@end deffn

@deffn {Scheme Procedure} string-unfold p f g seed [base [make_final]]
@deffnx {C Function} scm_string_unfold (p, f, g, seed, base, make_final)
@itemize @bullet
@item @var{g} is used to generate a series of @emph{seed}
values from the initial @var{seed}: @var{seed}, (@var{g}
@var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
@dots{}
@item @var{p} tells us when to stop -- when it returns true
when applied to one of these seed values.
@item @var{f} maps each seed value to the corresponding
character in the result string.  These chars are assembled
into the string in a left-to-right order.
@item @var{base} is the optional initial/leftmost portion
of the constructed string; it default to the empty
string.
@item @var{make_final} is applied to the terminal seed
value (on which @var{p} returns true) to produce
the final/rightmost portion of the constructed string.
The default is nothing extra.
@end itemize
@end deffn

@deffn {Scheme Procedure} string-unfold-right p f g seed [base [make_final]]
@deffnx {C Function} scm_string_unfold_right (p, f, g, seed, base, make_final)
@itemize @bullet
@item @var{g} is used to generate a series of @emph{seed}
values from the initial @var{seed}: @var{seed}, (@var{g}
@var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
@dots{}
@item @var{p} tells us when to stop -- when it returns true
when applied to one of these seed values.
@item @var{f} maps each seed value to the corresponding
character in the result string.  These chars are assembled
into the string in a right-to-left order.
@item @var{base} is the optional initial/rightmost portion
of the constructed string; it default to the empty
string.
@item @var{make_final} is applied to the terminal seed
value (on which @var{p} returns true) to produce
the final/leftmost portion of the constructed string.
It defaults to @code{(lambda (x) )}.
@end itemize
@end deffn

@node Miscellaneous String Operations
@subsubsection Miscellaneous String Operations

@deffn {Scheme Procedure} xsubstring s from [to [start [end]]]
@deffnx {C Function} scm_xsubstring (s, from, to, start, end)
This is the @emph{extended substring} procedure that implements
replicated copying of a substring of some string.

@var{s} is a string, @var{start} and @var{end} are optional
arguments that demarcate a substring of @var{s}, defaulting to
0 and the length of @var{s}.  Replicate this substring up and
down index space, in both the positive and negative directions.
@code{xsubstring} returns the substring of this string
beginning at index @var{from}, and ending at @var{to}, which
defaults to @var{from} + (@var{end} - @var{start}).
@end deffn

@deffn {Scheme Procedure} string-xcopy! target tstart s sfrom [sto [start [end]]]
@deffnx {C Function} scm_string_xcopy_x (target, tstart, s, sfrom, sto, start, end)
Exactly the same as @code{xsubstring}, but the extracted text
is written into the string @var{target} starting at index
@var{tstart}.  The operation is not defined if @code{(eq?
@var{target} @var{s})} or these arguments share storage -- you
cannot copy a string on top of itself.
@end deffn

@deffn {Scheme Procedure} string-replace s1 s2 [start1 [end1 [start2 [end2]]]]
@deffnx {C Function} scm_string_replace (s1, s2, start1, end1, start2, end2)
Return the string @var{s1}, but with the characters
@var{start1} @dots{} @var{end1} replaced by the characters
@var{start2} @dots{} @var{end2} from @var{s2}.
@end deffn

@deffn {Scheme Procedure} string-tokenize s [token_set [start [end]]]
@deffnx {C Function} scm_string_tokenize (s, token_set, start, end)
Split the string @var{s} into a list of substrings, where each
substring is a maximal non-empty contiguous sequence of
characters from the character set @var{token_set}, which
defaults to @code{char-set:graphic}.
If @var{start} or @var{end} indices are provided, they restrict
@code{string-tokenize} to operating on the indicated substring
of @var{s}.
@end deffn

@deffn {Scheme Procedure} string-filter char_pred s [start [end]]
@deffnx {C Function} scm_string_filter (char_pred, s, start, end)
Filter the string @var{s}, retaining only those characters which
satisfy @var{char_pred}.

If @var{char_pred} is a procedure, it is applied to each character as
a predicate, if it is a character, it is tested for equality and if it
is a character set, it is tested for membership.
@end deffn

@deffn {Scheme Procedure} string-delete char_pred s [start [end]]
@deffnx {C Function} scm_string_delete (char_pred, s, start, end)
Delete characters satisfying @var{char_pred} from @var{s}.

If @var{char_pred} is a procedure, it is applied to each character as
a predicate, if it is a character, it is tested for equality and if it
is a character set, it is tested for membership.
@end deffn

The following additional functions are available in the module @code{(ice-9 string-fun)}. They can be used with:

@example
(use-modules (ice-9 string-fun))
@end example

@deffn {Scheme Procedure} string-replace-substring str substring replacement
Return a new string where every instance of @var{substring} in string
@var{str} has been replaced by @var{replacement}. For example:

@lisp
(string-replace-substring "a ring of strings" "ring" "rut")
@result{} "a rut of struts"
@end lisp
@end deffn

@node Representing Strings as Bytes
@subsubsection Representing Strings as Bytes

Out in the cold world outside of Guile, not all strings are treated in
the same way.  Out there there are only bytes, and there are many ways
of representing a strings (sequences of characters) as binary data
(sequences of bytes).

As a user, usually you don't have to think about this very much.  When
you type on your keyboard, your system encodes your keystrokes as bytes
according to the locale that you have configured on your computer.
Guile uses the locale to decode those bytes back into characters --
hopefully the same characters that you typed in.

All is not so clear when dealing with a system with multiple users, such
as a web server.  Your web server might get a request from one user for
data encoded in the ISO-8859-1 character set, and then another request
from a different user for UTF-8 data.

@cindex iconv
@cindex character encoding
Guile provides an @dfn{iconv} module for converting between strings and
sequences of bytes.  @xref{Bytevectors}, for more on how Guile
represents raw byte sequences.  This module gets its name from the
common @sc{unix} command of the same name.

Note that often it is sufficient to just read and write strings from
ports instead of using these functions.  To do this, specify the port
encoding using @code{set-port-encoding!}.  @xref{Ports}, for more on
ports and character encodings.

Unlike the rest of the procedures in this section, you have to load the
@code{iconv} module before having access to these procedures:

@example
(use-modules (ice-9 iconv))
@end example

@deffn {Scheme Procedure} string->bytevector string encoding [conversion-strategy]
Encode @var{string} as a sequence of bytes.

The string will be encoded in the character set specified by the
@var{encoding} string.  If the string has characters that cannot be
represented in the encoding, by default this procedure raises an
@code{encoding-error}.  Pass a @var{conversion-strategy} argument to
specify other behaviors.

The return value is a bytevector.  @xref{Bytevectors}, for more on
bytevectors.  @xref{Ports}, for more on character encodings and
conversion strategies.
@end deffn

@deffn {Scheme Procedure} bytevector->string bytevector encoding [conversion-strategy]
Decode @var{bytevector} into a string.

The bytes will be decoded from the character set by the @var{encoding}
string.  If the bytes do not form a valid encoding, by default this
procedure raises an @code{decoding-error}.  As with
@code{string->bytevector}, pass the optional @var{conversion-strategy}
argument to modify this behavior.  @xref{Ports}, for more on character
encodings and conversion strategies.
@end deffn

@deffn {Scheme Procedure} call-with-output-encoded-string encoding proc [conversion-strategy]
Like @code{call-with-output-string}, but instead of returning a string,
returns a encoding of the string according to @var{encoding}, as a
bytevector.  This procedure can be more efficient than collecting a
string and then converting it via @code{string->bytevector}.
@end deffn

@node Conversion to/from C
@subsubsection Conversion to/from C

When creating a Scheme string from a C string or when converting a
Scheme string to a C string, the concept of character encoding becomes
important.

In C, a string is just a sequence of bytes, and the character encoding
describes the relation between these bytes and the actual characters
that make up the string.  For Scheme strings, character encoding is not
an issue (most of the time), since in Scheme you usually treat strings
as character sequences, not byte sequences.

Converting to C and converting from C each have their own challenges.

When converting from C to Scheme, it is important that the sequence of
bytes in the C string be valid with respect to its encoding.  ASCII
strings, for example, can't have any bytes greater than 127.  An ASCII
byte greater than 127 is considered @emph{ill-formed} and cannot be
converted into a Scheme character.

Problems can occur in the reverse operation as well.  Not all character
encodings can hold all possible Scheme characters.  Some encodings, like
ASCII for example, can only describe a small subset of all possible
characters.  So, when converting to C, one must first decide what to do
with Scheme characters that can't be represented in the C string.

Converting a Scheme string to a C string will often allocate fresh
memory to hold the result.  You must take care that this memory is
properly freed eventually.  In many cases, this can be achieved by
using @code{scm_dynwind_free} inside an appropriate dynwind context,
@xref{Dynamic Wind}.

@deftypefn  {C Function} SCM scm_from_locale_string (const char *str)
@deftypefnx {C Function} SCM scm_from_locale_stringn (const char *str, size_t len)
Creates a new Scheme string that has the same contents as @var{str} when
interpreted in the character encoding of the current locale.

For @code{scm_from_locale_string}, @var{str} must be null-terminated.

For @code{scm_from_locale_stringn}, @var{len} specifies the length of
@var{str} in bytes, and @var{str} does not need to be null-terminated.
If @var{len} is @code{(size_t)-1}, then @var{str} does need to be
null-terminated and the real length will be found with @code{strlen}.

If the C string is ill-formed, an error will be raised.

Note that these functions should @emph{not} be used to convert C string
constants, because there is no guarantee that the current locale will
match that of the execution character set, used for string and character
constants.  Most modern C compilers use UTF-8 by default, so to convert
C string constants we recommend @code{scm_from_utf8_string}.
@end deftypefn

@deftypefn  {C Function} SCM scm_take_locale_string (char *str)
@deftypefnx {C Function} SCM scm_take_locale_stringn (char *str, size_t len)
Like @code{scm_from_locale_string} and @code{scm_from_locale_stringn},
respectively, but also frees @var{str} with @code{free} eventually.
Thus, you can use this function when you would free @var{str} anyway
immediately after creating the Scheme string.  In certain cases, Guile
can then use @var{str} directly as its internal representation.
@end deftypefn

@deftypefn  {C Function} {char *} scm_to_locale_string (SCM str)
@deftypefnx {C Function} {char *} scm_to_locale_stringn (SCM str, size_t *lenp)
Returns a C string with the same contents as @var{str} in the character
encoding of the current locale.  The C string must be freed with
@code{free} eventually, maybe by using @code{scm_dynwind_free},
@xref{Dynamic Wind}.

For @code{scm_to_locale_string}, the returned string is
null-terminated and an error is signaled when @var{str} contains
@code{#\nul} characters.

For @code{scm_to_locale_stringn} and @var{lenp} not @code{NULL},
@var{str} might contain @code{#\nul} characters and the length of the
returned string in bytes is stored in @code{*@var{lenp}}.  The
returned string will not be null-terminated in this case.  If
@var{lenp} is @code{NULL}, @code{scm_to_locale_stringn} behaves like
@code{scm_to_locale_string}.

If a character in @var{str} cannot be represented in the character
encoding of the current locale, the default port conversion strategy is
used.  @xref{Ports}, for more on conversion strategies.

If the conversion strategy is @code{error}, an error will be raised.  If
it is @code{substitute}, a replacement character, such as a question
mark, will be inserted in its place.  If it is @code{escape}, a hex
escape will be inserted in its place.
@end deftypefn

@deftypefn {C Function} size_t scm_to_locale_stringbuf (SCM str, char *buf, size_t max_len)
Puts @var{str} as a C string in the current locale encoding into the
memory pointed to by @var{buf}.  The buffer at @var{buf} has room for
@var{max_len} bytes and @code{scm_to_local_stringbuf} will never store
more than that.  No terminating @code{'\0'} will be stored.

The return value of @code{scm_to_locale_stringbuf} is the number of
bytes that are needed for all of @var{str}, regardless of whether
@var{buf} was large enough to hold them.  Thus, when the return value
is larger than @var{max_len}, only @var{max_len} bytes have been
stored and you probably need to try again with a larger buffer.
@end deftypefn

For most situations, string conversion should occur using the current
locale, such as with the functions above.  But there may be cases where
one wants to convert strings from a character encoding other than the
locale's character encoding.  For these cases, the lower-level functions
@code{scm_to_stringn} and @code{scm_from_stringn} are provided.  These
functions should seldom be necessary if one is properly using locales.

@deftp {C Type} scm_t_string_failed_conversion_handler
This is an enumerated type that can take one of three values:
@code{SCM_FAILED_CONVERSION_ERROR},
@code{SCM_FAILED_CONVERSION_QUESTION_MARK}, and
@code{SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE}.  They are used to indicate
a strategy for handling characters that cannot be converted to or from a
given character encoding.  @code{SCM_FAILED_CONVERSION_ERROR} indicates
that a conversion should throw an error if some characters cannot be
converted.  @code{SCM_FAILED_CONVERSION_QUESTION_MARK} indicates that a
conversion should replace unconvertable characters with the question
mark character.  And, @code{SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE}
requests that a conversion should replace an unconvertable character
with an escape sequence.

While all three strategies apply when converting Scheme strings to C,
only @code{SCM_FAILED_CONVERSION_ERROR} and
@code{SCM_FAILED_CONVERSION_QUESTION_MARK} can be used when converting C
strings to Scheme.
@end deftp

@deftypefn {C Function} char *scm_to_stringn (SCM str, size_t *lenp, const char *encoding, scm_t_string_failed_conversion_handler handler)
This function returns a newly allocated C string from the Guile string
@var{str}.  The length of the returned string in bytes will be returned in
@var{lenp}.  The character encoding of the C string is passed as the ASCII,
null-terminated C string @var{encoding}.  The @var{handler} parameter
gives a strategy for dealing with characters that cannot be converted
into @var{encoding}.

If @var{lenp} is @code{NULL}, this function will return a null-terminated C
string.  It will throw an error if the string contains a null
character.

The Scheme interface to this function is @code{string->bytevector}, from the
@code{ice-9 iconv} module.  @xref{Representing Strings as Bytes}.
@end deftypefn

@deftypefn {C Function} SCM scm_from_stringn (const char *str, size_t len, const char *encoding, scm_t_string_failed_conversion_handler handler)
This function returns a scheme string from the C string @var{str}.  The
length in bytes of the C string is input as @var{len}.  The encoding of the C
string is passed as the ASCII, null-terminated C string @code{encoding}.
The @var{handler} parameters suggests a strategy for dealing with
unconvertable characters.

The Scheme interface to this function is @code{bytevector->string}.
@xref{Representing Strings as Bytes}.
@end deftypefn

The following conversion functions are provided as a convenience for the
most commonly used encodings.

@deftypefn {C Function} SCM scm_from_latin1_string (const char *str)
@deftypefnx {C Function} SCM scm_from_utf8_string (const char *str)
@deftypefnx {C Function} SCM scm_from_utf32_string (const scm_t_wchar *str)
Return a scheme string from the null-terminated C string @var{str},
which is ISO-8859-1-, UTF-8-, or UTF-32-encoded.  These functions should
be used to convert hard-coded C string constants into Scheme strings.
@end deftypefn

@deftypefn {C Function} SCM scm_from_latin1_stringn (const char *str, size_t len)
@deftypefnx {C Function} SCM scm_from_utf8_stringn (const char *str, size_t len)
@deftypefnx {C Function} SCM scm_from_utf32_stringn (const scm_t_wchar *str, size_t len)
Return a scheme string from C string @var{str}, which is ISO-8859-1-,
UTF-8-, or UTF-32-encoded, of length @var{len}.  @var{len} is the number
of bytes pointed to by @var{str} for @code{scm_from_latin1_stringn} and
@code{scm_from_utf8_stringn}; it is the number of elements (code points)
in @var{str} in the case of @code{scm_from_utf32_stringn}.
@end deftypefn

@deftypefn {C function} char *scm_to_latin1_stringn (SCM str, size_t *lenp)
@deftypefnx {C function} char *scm_to_utf8_stringn (SCM str, size_t *lenp)
@deftypefnx {C function} scm_t_wchar *scm_to_utf32_stringn (SCM str, size_t *lenp)
Return a newly allocated, ISO-8859-1-, UTF-8-, or UTF-32-encoded C string
from Scheme string @var{str}.  An error is thrown when @var{str}
cannot be converted to the specified encoding.  If @var{lenp} is
@code{NULL}, the returned C string will be null terminated, and an error
will be thrown if the C string would otherwise contain null
characters.  If @var{lenp} is not @code{NULL}, the string is not null terminated,
and the length of the returned string is returned in @var{lenp}.  The length
returned is the number of bytes for @code{scm_to_latin1_stringn} and
@code{scm_to_utf8_stringn}; it is the number of elements (code points)
for @code{scm_to_utf32_stringn}.
@end deftypefn

It is not often the case, but sometimes when you are dealing with the
implementation details of a port, you need to encode and decode strings
according to the encoding and conversion strategy of the port.  There
are some convenience functions for that purpose as well.

@deftypefn {C Function} SCM scm_from_port_string (const char *str, SCM port)
@deftypefnx {C Function} SCM scm_from_port_stringn (const char *str, size_t len, SCM port)
@deftypefnx {C Function} char* scm_to_port_string (SCM str, SCM port)
@deftypefnx {C Function} char* scm_to_port_stringn (SCM str, size_t *lenp, SCM port)
Like @code{scm_from_stringn} and friends, except they take their
encoding and conversion strategy from a given port object.
@end deftypefn

@node String Internals
@subsubsection String Internals

Guile stores each string in memory as a contiguous array of Unicode code
points along with an associated set of attributes.  If all of the code
points of a string have an integer range between 0 and 255 inclusive,
the code point array is stored as one byte per code point: it is stored
as an ISO-8859-1 (aka Latin-1) string.  If any of the code points of the
string has an integer value greater that 255, the code point array is
stored as four bytes per code point: it is stored as a UTF-32 string.

Conversion between the one-byte-per-code-point and
four-bytes-per-code-point representations happens automatically as
necessary.

No API is provided to set the internal representation of strings;
however, there are pair of procedures available to query it.  These are
debugging procedures.  Using them in production code is discouraged,
since the details of Guile's internal representation of strings may
change from release to release.

@deffn {Scheme Procedure} string-bytes-per-char str
@deffnx {C Function} scm_string_bytes_per_char (str)
Return the number of bytes used to encode a Unicode code point in string
@var{str}.  The result is one or four.
@end deffn

@deffn {Scheme Procedure} %string-dump str
@deffnx {C Function} scm_sys_string_dump (str)
Returns an association list containing debugging information for
@var{str}. The association list has the following entries.
@table @code

@item string
The string itself.

@item start
The start index of the string into its stringbuf

@item length
The length of the string

@item shared
If this string is a substring, it returns its
parent string.  Otherwise, it returns @code{#f}

@item read-only
@code{#t} if the string is read-only

@item stringbuf-chars
A new string containing this string's stringbuf's characters

@item stringbuf-length
The number of characters in this stringbuf

@item stringbuf-shared
@code{#t} if this stringbuf is shared

@item stringbuf-wide
@code{#t} if this stringbuf's characters are stored in a 32-bit buffer,
or @code{#f} if they are stored in an 8-bit buffer
@end table
@end deffn


@node Symbols
@subsection Symbols
@tpindex Symbols

Symbols in Scheme are widely used in three ways: as items of discrete
data, as lookup keys for alists and hash tables, and to denote variable
references.

A @dfn{symbol} is similar to a string in that it is defined by a
sequence of characters.  The sequence of characters is known as the
symbol's @dfn{name}.  In the usual case --- that is, where the symbol's
name doesn't include any characters that could be confused with other
elements of Scheme syntax --- a symbol is written in a Scheme program by
writing the sequence of characters that make up the name, @emph{without}
any quotation marks or other special syntax.  For example, the symbol
whose name is ``multiply-by-2'' is written, simply:

@lisp
multiply-by-2
@end lisp

Notice how this differs from a @emph{string} with contents
``multiply-by-2'', which is written with double quotation marks, like
this:

@lisp
"multiply-by-2"
@end lisp

Looking beyond how they are written, symbols are different from strings
in two important respects.

The first important difference is uniqueness.  If the same-looking
string is read twice from two different places in a program, the result
is two @emph{different} string objects whose contents just happen to be
the same.  If, on the other hand, the same-looking symbol is read twice
from two different places in a program, the result is the @emph{same}
symbol object both times.

Given two read symbols, you can use @code{eq?} to test whether they are
the same (that is, have the same name).  @code{eq?} is the most
efficient comparison operator in Scheme, and comparing two symbols like
this is as fast as comparing, for example, two numbers.  Given two
strings, on the other hand, you must use @code{equal?} or
@code{string=?}, which are much slower comparison operators, to
determine whether the strings have the same contents.

@lisp
(define sym1 (quote hello))
(define sym2 (quote hello))
(eq? sym1 sym2) @result{} #t

(define str1 "hello")
(define str2 "hello")
(eq? str1 str2) @result{} #f
(equal? str1 str2) @result{} #t
@end lisp

The second important difference is that symbols, unlike strings, are not
self-evaluating.  This is why we need the @code{(quote @dots{})}s in the
example above: @code{(quote hello)} evaluates to the symbol named
"hello" itself, whereas an unquoted @code{hello} is @emph{read} as the
symbol named "hello" and evaluated as a variable reference @dots{} about
which more below (@pxref{Symbol Variables}).

@menu
* Symbol Data::                 Symbols as discrete data.
* Symbol Keys::                 Symbols as lookup keys.
* Symbol Variables::            Symbols as denoting variables.
* Symbol Primitives::           Operations related to symbols.
* Symbol Read Syntax::          Extended read syntax for symbols.
* Symbol Uninterned::           Uninterned symbols.
@end menu


@node Symbol Data
@subsubsection Symbols as Discrete Data

Numbers and symbols are similar to the extent that they both lend
themselves to @code{eq?} comparison.  But symbols are more descriptive
than numbers, because a symbol's name can be used directly to describe
the concept for which that symbol stands.

For example, imagine that you need to represent some colours in a
computer program.  Using numbers, you would have to choose arbitrarily
some mapping between numbers and colours, and then take care to use that
mapping consistently:

@lisp
;; 1=red, 2=green, 3=purple

(if (eq? (colour-of vehicle) 1)
    ...)
@end lisp

@noindent
You can make the mapping more explicit and the code more readable by
defining constants:

@lisp
(define red 1)
(define green 2)
(define purple 3)

(if (eq? (colour-of vehicle) red)
    ...)
@end lisp

@noindent
But the simplest and clearest approach is not to use numbers at all, but
symbols whose names specify the colours that they refer to:

@lisp
(if (eq? (colour-of vehicle) 'red)
    ...)
@end lisp

The descriptive advantages of symbols over numbers increase as the set
of concepts that you want to describe grows.  Suppose that a car object
can have other properties as well, such as whether it has or uses:

@itemize @bullet
@item
automatic or manual transmission
@item
leaded or unleaded fuel
@item
power steering (or not).
@end itemize

@noindent
Then a car's combined property set could be naturally represented and
manipulated as a list of symbols:

@lisp
(properties-of vehicle1)
@result{}
(red manual unleaded power-steering)

(if (memq 'power-steering (properties-of vehicle1))
    (display "Unfit people can drive this vehicle.\n")
    (display "You'll need strong arms to drive this vehicle!\n"))
@print{}
Unfit people can drive this vehicle.
@end lisp

Remember, the fundamental property of symbols that we are relying on
here is that an occurrence of @code{'red} in one part of a program is an
@emph{indistinguishable} symbol from an occurrence of @code{'red} in
another part of a program; this means that symbols can usefully be
compared using @code{eq?}.  At the same time, symbols have naturally
descriptive names.  This combination of efficiency and descriptive power
makes them ideal for use as discrete data.


@node Symbol Keys
@subsubsection Symbols as Lookup Keys

Given their efficiency and descriptive power, it is natural to use
symbols as the keys in an association list or hash table.

To illustrate this, consider a more structured representation of the car
properties example from the preceding subsection.  Rather than
mixing all the properties up together in a flat list, we could use an
association list like this:

@lisp
(define car1-properties '((colour . red)
                          (transmission . manual)
                          (fuel . unleaded)
                          (steering . power-assisted)))
@end lisp

Notice how this structure is more explicit and extensible than the flat
list.  For example it makes clear that @code{manual} refers to the
transmission rather than, say, the windows or the locking of the car.
It also allows further properties to use the same symbols among their
possible values without becoming ambiguous:

@lisp
(define car1-properties '((colour . red)
                          (transmission . manual)
                          (fuel . unleaded)
                          (steering . power-assisted)
                          (seat-colour . red)
                          (locking . manual)))
@end lisp

With a representation like this, it is easy to use the efficient
@code{assq-XXX} family of procedures (@pxref{Association Lists}) to
extract or change individual pieces of information:

@lisp
(assq-ref car1-properties 'fuel) @result{} unleaded
(assq-ref car1-properties 'transmission) @result{} manual

(assq-set! car1-properties 'seat-colour 'black)
@result{}
((colour . red)
 (transmission . manual)
 (fuel . unleaded)
 (steering . power-assisted)
 (seat-colour . black)
 (locking . manual)))
@end lisp

Hash tables also have keys, and exactly the same arguments apply to the
use of symbols in hash tables as in association lists.  The hash value
that Guile uses to decide where to add a symbol-keyed entry to a hash
table can be obtained by calling the @code{symbol-hash} procedure:

@deffn {Scheme Procedure} symbol-hash symbol
@deffnx {C Function} scm_symbol_hash (symbol)
Return a hash value for @var{symbol}.
@end deffn

See @ref{Hash Tables} for information about hash tables in general, and
for why you might choose to use a hash table rather than an association
list.


@node Symbol Variables
@subsubsection Symbols as Denoting Variables

When an unquoted symbol in a Scheme program is evaluated, it is
interpreted as a variable reference, and the result of the evaluation is
the appropriate variable's value.

For example, when the expression @code{(string-length "abcd")} is read
and evaluated, the sequence of characters @code{string-length} is read
as the symbol whose name is "string-length".  This symbol is associated
with a variable whose value is the procedure that implements string
length calculation.  Therefore evaluation of the @code{string-length}
symbol results in that procedure.

The details of the connection between an unquoted symbol and the
variable to which it refers are explained elsewhere.  See @ref{Binding
Constructs}, for how associations between symbols and variables are
created, and @ref{Modules}, for how those associations are affected by
Guile's module system.


@node Symbol Primitives
@subsubsection Operations Related to Symbols

Given any Scheme value, you can determine whether it is a symbol using
the @code{symbol?} primitive:

@rnindex symbol?
@deffn {Scheme Procedure} symbol? obj
@deffnx {C Function} scm_symbol_p (obj)
Return @code{#t} if @var{obj} is a symbol, otherwise return
@code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_symbol (SCM val)
Equivalent to @code{scm_is_true (scm_symbol_p (val))}.
@end deftypefn

Once you know that you have a symbol, you can obtain its name as a
string by calling @code{symbol->string}.  Note that Guile differs by
default from R5RS on the details of @code{symbol->string} as regards
case-sensitivity:

@rnindex symbol->string
@deffn {Scheme Procedure} symbol->string s
@deffnx {C Function} scm_symbol_to_string (s)
Return the name of symbol @var{s} as a string.  By default, Guile reads
symbols case-sensitively, so the string returned will have the same case
variation as the sequence of characters that caused @var{s} to be
created.

If Guile is set to read symbols case-insensitively (as specified by
R5RS), and @var{s} comes into being as part of a literal expression
(@pxref{Literal expressions,,,r5rs, The Revised^5 Report on Scheme}) or
by a call to the @code{read} or @code{string-ci->symbol} procedures,
Guile converts any alphabetic characters in the symbol's name to
lower case before creating the symbol object, so the string returned
here will be in lower case.

If @var{s} was created by @code{string->symbol}, the case of characters
in the string returned will be the same as that in the string that was
passed to @code{string->symbol}, regardless of Guile's case-sensitivity
setting at the time @var{s} was created.

It is an error to apply mutation procedures like @code{string-set!} to
strings returned by this procedure.
@end deffn

Most symbols are created by writing them literally in code.  However it
is also possible to create symbols programmatically using the following
procedures:

@deffn {Scheme Procedure} symbol char@dots{}
@rnindex symbol
Return a newly allocated symbol made from the given character arguments.

@example
(symbol #\x #\y #\z) @result{} xyz
@end example
@end deffn

@deffn {Scheme Procedure} list->symbol lst
@rnindex list->symbol
Return a newly allocated symbol made from a list of characters.

@example
(list->symbol '(#\a #\b #\c)) @result{} abc
@end example
@end deffn

@rnindex symbol-append
@deffn {Scheme Procedure} symbol-append arg @dots{}
Return a newly allocated symbol whose characters form the
concatenation of the given symbols, @var{arg} @enddots{}.

@example
(let ((h 'hello))
  (symbol-append h 'world))
@result{} helloworld
@end example
@end deffn

@rnindex string->symbol
@deffn {Scheme Procedure} string->symbol string
@deffnx {C Function} scm_string_to_symbol (string)
Return the symbol whose name is @var{string}.  This procedure can create
symbols with names containing special characters or letters in the
non-standard case, but it is usually a bad idea to create such symbols
because in some implementations of Scheme they cannot be read as
themselves.
@end deffn

@deffn {Scheme Procedure} string-ci->symbol str
@deffnx {C Function} scm_string_ci_to_symbol (str)
Return the symbol whose name is @var{str}.  If Guile is currently
reading symbols case-insensitively, @var{str} is converted to lowercase
before the returned symbol is looked up or created.
@end deffn

The following examples illustrate Guile's detailed behaviour as regards
the case-sensitivity of symbols:

@lisp
(read-enable 'case-insensitive)   ; R5RS compliant behaviour

(symbol->string 'flying-fish)    @result{} "flying-fish"
(symbol->string 'Martin)         @result{} "martin"
(symbol->string
   (string->symbol "Malvina"))   @result{} "Malvina"

(eq? 'mISSISSIppi 'mississippi)  @result{} #t
(string->symbol "mISSISSIppi")   @result{} mISSISSIppi
(eq? 'bitBlt (string->symbol "bitBlt")) @result{} #f
(eq? 'LolliPop
  (string->symbol (symbol->string 'LolliPop))) @result{} #t
(string=? "K. Harper, M.D."
  (symbol->string
    (string->symbol "K. Harper, M.D."))) @result{} #t

(read-disable 'case-insensitive)   ; Guile default behaviour

(symbol->string 'flying-fish)    @result{} "flying-fish"
(symbol->string 'Martin)         @result{} "Martin"
(symbol->string
   (string->symbol "Malvina"))   @result{} "Malvina"

(eq? 'mISSISSIppi 'mississippi)  @result{} #f
(string->symbol "mISSISSIppi")   @result{} mISSISSIppi
(eq? 'bitBlt (string->symbol "bitBlt")) @result{} #t
(eq? 'LolliPop
  (string->symbol (symbol->string 'LolliPop))) @result{} #t
(string=? "K. Harper, M.D."
  (symbol->string
    (string->symbol "K. Harper, M.D."))) @result{} #t
@end lisp

From C, there are lower level functions that construct a Scheme symbol
from a C string in the current locale encoding.

When you want to do more from C, you should convert between symbols
and strings using @code{scm_symbol_to_string} and
@code{scm_string_to_symbol} and work with the strings.

@deftypefn {C Function} SCM scm_from_latin1_symbol (const char *name)
@deftypefnx {C Function} SCM scm_from_utf8_symbol (const char *name)
Construct and return a Scheme symbol whose name is specified by the
null-terminated C string @var{name}.  These are appropriate when
the C string is hard-coded in the source code.
@end deftypefn

@deftypefn {C Function} SCM scm_from_locale_symbol (const char *name)
@deftypefnx {C Function} SCM scm_from_locale_symboln (const char *name, size_t len)
Construct and return a Scheme symbol whose name is specified by
@var{name}.  For @code{scm_from_locale_symbol}, @var{name} must be null
terminated; for @code{scm_from_locale_symboln} the length of @var{name} is
specified explicitly by @var{len}.

Note that these functions should @emph{not} be used when @var{name} is a
C string constant, because there is no guarantee that the current locale
will match that of the execution character set, used for string and
character constants.  Most modern C compilers use UTF-8 by default, so
in such cases we recommend @code{scm_from_utf8_symbol}.
@end deftypefn

@deftypefn  {C Function} SCM scm_take_locale_symbol (char *str)
@deftypefnx {C Function} SCM scm_take_locale_symboln (char *str, size_t len)
Like @code{scm_from_locale_symbol} and @code{scm_from_locale_symboln},
respectively, but also frees @var{str} with @code{free} eventually.
Thus, you can use this function when you would free @var{str} anyway
immediately after creating the Scheme string.  In certain cases, Guile
can then use @var{str} directly as its internal representation.
@end deftypefn

The size of a symbol can also be obtained from C:

@deftypefn {C Function} size_t scm_c_symbol_length (SCM sym)
Return the number of characters in @var{sym}.
@end deftypefn

Finally, some applications, especially those that generate new Scheme
code dynamically, need to generate symbols for use in the generated
code.  The @code{gensym} primitive meets this need:

@deffn {Scheme Procedure} gensym [prefix]
@deffnx {C Function} scm_gensym (prefix)
Create a new symbol with a name constructed from a prefix and a counter
value.  The string @var{prefix} can be specified as an optional
argument.  Default prefix is @samp{@w{ g}}.  The counter is increased by 1
at each call.  There is no provision for resetting the counter.
@end deffn

The symbols generated by @code{gensym} are @emph{likely} to be unique,
since their names begin with a space and it is only otherwise possible
to generate such symbols if a programmer goes out of their way to do
so.  Uniqueness can be guaranteed by instead using uninterned symbols
(@pxref{Symbol Uninterned}), though they can't be usefully written out
and read back in.


@node Symbol Read Syntax
@subsubsection Extended Read Syntax for Symbols

@cindex r7rs-symbols

The read syntax for a symbol is a sequence of letters, digits, and
@dfn{extended alphabetic characters}, beginning with a character that
cannot begin a number.  In addition, the special cases of @code{+},
@code{-}, and @code{...} are read as symbols even though numbers can
begin with @code{+}, @code{-} or @code{.}.

Extended alphabetic characters may be used within identifiers as if
they were letters.  The set of extended alphabetic characters is:

@example
! $ % & * + - . / : < = > ? @@ ^ _ ~
@end example

In addition to the standard read syntax defined above (which is taken
from R5RS (@pxref{Formal syntax,,,r5rs,The Revised^5 Report on
Scheme})), Guile provides an extended symbol read syntax that allows the
inclusion of unusual characters such as space characters, newlines and
parentheses.  If (for whatever reason) you need to write a symbol
containing characters not mentioned above, you can do so as follows.

@itemize @bullet
@item
Begin the symbol with the characters @code{#@{},

@item
write the characters of the symbol and

@item
finish the symbol with the characters @code{@}#}.
@end itemize

Here are a few examples of this form of read syntax.  The first symbol
needs to use extended syntax because it contains a space character, the
second because it contains a line break, and the last because it looks
like a number.

@lisp
#@{foo bar@}#

#@{what
ever@}#

#@{4242@}#
@end lisp

Although Guile provides this extended read syntax for symbols,
widespread usage of it is discouraged because it is not portable and not
very readable.

Alternatively, if you enable the @code{r7rs-symbols} read option (see
@pxref{Scheme Read}), you can write arbitrary symbols using the same
notation used for strings, except delimited by vertical bars instead of
double quotes.

@example
|foo bar|
|\x3BB; is a greek lambda|
|\| is a vertical bar|
@end example

Note that there's also an @code{r7rs-symbols} print option
(@pxref{Scheme Write}).  To enable the use of this notation, evaluate
one or both of the following expressions:

@example
(read-enable  'r7rs-symbols)
(print-enable 'r7rs-symbols)
@end example


@node Symbol Uninterned
@subsubsection Uninterned Symbols

What makes symbols useful is that they are automatically kept unique.
There are no two symbols that are distinct objects but have the same
name.  But of course, there is no rule without exception.  In addition
to the normal symbols that have been discussed up to now, you can also
create special @dfn{uninterned} symbols that behave slightly
differently.

To understand what is different about them and why they might be useful,
we look at how normal symbols are actually kept unique.

Whenever Guile wants to find the symbol with a specific name, for
example during @code{read} or when executing @code{string->symbol}, it
first looks into a table of all existing symbols to find out whether a
symbol with the given name already exists.  When this is the case, Guile
just returns that symbol.  When not, a new symbol with the name is
created and entered into the table so that it can be found later.

Sometimes you might want to create a symbol that is guaranteed `fresh',
i.e.@: a symbol that did not exist previously.  You might also want to
somehow guarantee that no one else will ever unintentionally stumble
across your symbol in the future.  These properties of a symbol are
often needed when generating code during macro expansion.  When
introducing new temporary variables, you want to guarantee that they
don't conflict with variables in other people's code.

The simplest way to arrange for this is to create a new symbol but
not enter it into the global table of all symbols.  That way, no one
will ever get access to your symbol by chance.  Symbols that are not in
the table are called @dfn{uninterned}.  Of course, symbols that
@emph{are} in the table are called @dfn{interned}.

You create new uninterned symbols with the function @code{make-symbol}.
You can test whether a symbol is interned or not with
@code{symbol-interned?}.

Uninterned symbols break the rule that the name of a symbol uniquely
identifies the symbol object.  Because of this, they can not be written
out and read back in like interned symbols.  Currently, Guile has no
support for reading uninterned symbols.  Note that the function
@code{gensym} does not return uninterned symbols for this reason.

@deffn {Scheme Procedure} make-symbol name
@deffnx {C Function} scm_make_symbol (name)
Return a new uninterned symbol with the name @var{name}.  The returned
symbol is guaranteed to be unique and future calls to
@code{string->symbol} will not return it.
@end deffn

@deffn {Scheme Procedure} symbol-interned? symbol
@deffnx {C Function} scm_symbol_interned_p (symbol)
Return @code{#t} if @var{symbol} is interned, otherwise return
@code{#f}.
@end deffn

For example:

@lisp
(define foo-1 (string->symbol "foo"))
(define foo-2 (string->symbol "foo"))
(define foo-3 (make-symbol "foo"))
(define foo-4 (make-symbol "foo"))

(eq? foo-1 foo-2)
@result{} #t
; Two interned symbols with the same name are the same object,

(eq? foo-1 foo-3)
@result{} #f
; but a call to make-symbol with the same name returns a
; distinct object.

(eq? foo-3 foo-4)
@result{} #f
; A call to make-symbol always returns a new object, even for
; the same name.

foo-3
@result{} #<uninterned-symbol foo 8085290>
; Uninterned symbols print differently from interned symbols,

(symbol? foo-3)
@result{} #t
; but they are still symbols,

(symbol-interned? foo-3)
@result{} #f
; just not interned.
@end lisp


@node Keywords
@subsection Keywords
@tpindex Keywords

Keywords are self-evaluating objects with a convenient read syntax that
makes them easy to type.

Guile's keyword support conforms to R5RS, and adds a (switchable) read
syntax extension to permit keywords to begin with @code{:} as well as
@code{#:}, or to end with @code{:}.

@menu
* Why Use Keywords?::           Motivation for keyword usage.
* Coding With Keywords::        How to use keywords.
* Keyword Read Syntax::         Read syntax for keywords.
* Keyword Procedures::          Procedures for dealing with keywords.
@end menu

@node Why Use Keywords?
@subsubsection Why Use Keywords?

Keywords are useful in contexts where a program or procedure wants to be
able to accept a large number of optional arguments without making its
interface unmanageable.

To illustrate this, consider a hypothetical @code{make-window}
procedure, which creates a new window on the screen for drawing into
using some graphical toolkit.  There are many parameters that the caller
might like to specify, but which could also be sensibly defaulted, for
example:

@itemize @bullet
@item
color depth -- Default: the color depth for the screen

@item
background color -- Default: white

@item
width -- Default: 600

@item
height -- Default: 400
@end itemize

If @code{make-window} did not use keywords, the caller would have to
pass in a value for each possible argument, remembering the correct
argument order and using a special value to indicate the default value
for that argument:

@lisp
(make-window 'default              ;; Color depth
             'default              ;; Background color
             800                   ;; Width
             100                   ;; Height
             @dots{})                  ;; More make-window arguments
@end lisp

With keywords, on the other hand, defaulted arguments are omitted, and
non-default arguments are clearly tagged by the appropriate keyword.  As
a result, the invocation becomes much clearer:

@lisp
(make-window #:width 800 #:height 100)
@end lisp

On the other hand, for a simpler procedure with few arguments, the use
of keywords would be a hindrance rather than a help.  The primitive
procedure @code{cons}, for example, would not be improved if it had to
be invoked as

@lisp
(cons #:car x #:cdr y)
@end lisp

So the decision whether to use keywords or not is purely pragmatic: use
them if they will clarify the procedure invocation at point of call.

@node Coding With Keywords
@subsubsection Coding With Keywords

If a procedure wants to support keywords, it should take a rest argument
and then use whatever means is convenient to extract keywords and their
corresponding arguments from the contents of that rest argument.

The following example illustrates the principle: the code for
@code{make-window} uses a helper procedure called
@code{get-keyword-value} to extract individual keyword arguments from
the rest argument.

@lisp
(define (get-keyword-value args keyword default)
  (let ((kv (memq keyword args)))
    (if (and kv (>= (length kv) 2))
        (cadr kv)
        default)))

(define (make-window . args)
  (let ((depth  (get-keyword-value args #:depth  screen-depth))
        (bg     (get-keyword-value args #:bg     "white"))
        (width  (get-keyword-value args #:width  800))
        (height (get-keyword-value args #:height 100))
        @dots{})
    @dots{}))
@end lisp

But you don't need to write @code{get-keyword-value}.  The @code{(ice-9
optargs)} module provides a set of powerful macros that you can use to
implement keyword-supporting procedures like this:

@lisp
(use-modules (ice-9 optargs))

(define (make-window . args)
  (let-keywords args #f ((depth  screen-depth)
                         (bg     "white")
                         (width  800)
                         (height 100))
    ...))
@end lisp

@noindent
Or, even more economically, like this:

@lisp
(use-modules (ice-9 optargs))

(define* (make-window #:key (depth  screen-depth)
                            (bg     "white")
                            (width  800)
                            (height 100))
  ...)
@end lisp

For further details on @code{let-keywords}, @code{define*} and other
facilities provided by the @code{(ice-9 optargs)} module, see
@ref{Optional Arguments}.

To handle keyword arguments from procedures implemented in C,
use @code{scm_c_bind_keyword_arguments} (@pxref{Keyword Procedures}).

@node Keyword Read Syntax
@subsubsection Keyword Read Syntax

Guile, by default, only recognizes a keyword syntax that is compatible
with R5RS.  A token of the form @code{#:NAME}, where @code{NAME} has the
same syntax as a Scheme symbol (@pxref{Symbol Read Syntax}), is the
external representation of the keyword named @code{NAME}.  Keyword
objects print using this syntax as well, so values containing keyword
objects can be read back into Guile.  When used in an expression,
keywords are self-quoting objects.

If the @code{keywords} read option is set to @code{'prefix}, Guile also
recognizes the alternative read syntax @code{:NAME}.  Otherwise, tokens
of the form @code{:NAME} are read as symbols, as required by R5RS.

@cindex SRFI-88 keyword syntax

If the @code{keywords} read option is set to @code{'postfix}, Guile
recognizes the SRFI-88 read syntax @code{NAME:} (@pxref{SRFI-88}).
Otherwise, tokens of this form are read as symbols.

To enable and disable the alternative non-R5RS keyword syntax, you use
the @code{read-set!} procedure documented @ref{Scheme Read}.  Note that
the @code{prefix} and @code{postfix} syntax are mutually exclusive.

@lisp
(read-set! keywords 'prefix)

#:type
@result{}
#:type

:type
@result{}
#:type

(read-set! keywords 'postfix)

type:
@result{}
#:type

:type
@result{}
:type

(read-set! keywords #f)

#:type
@result{}
#:type

:type
@print{}
ERROR: In expression :type:
ERROR: Unbound variable: :type
ABORT: (unbound-variable)
@end lisp

@node Keyword Procedures
@subsubsection Keyword Procedures

@deffn {Scheme Procedure} keyword? obj
@deffnx {C Function} scm_keyword_p (obj)
Return @code{#t} if the argument @var{obj} is a keyword, else
@code{#f}.
@end deffn

@deffn {Scheme Procedure} keyword->symbol keyword
@deffnx {C Function} scm_keyword_to_symbol (keyword)
Return the symbol with the same name as @var{keyword}.
@end deffn

@deffn {Scheme Procedure} symbol->keyword symbol
@deffnx {C Function} scm_symbol_to_keyword (symbol)
Return the keyword with the same name as @var{symbol}.
@end deffn

@deftypefn {C Function} int scm_is_keyword (SCM obj)
Equivalent to @code{scm_is_true (scm_keyword_p (@var{obj}))}.
@end deftypefn

@deftypefn {C Function} SCM scm_from_locale_keyword (const char *name)
@deftypefnx {C Function} SCM scm_from_locale_keywordn (const char *name, size_t len)
Equivalent to @code{scm_symbol_to_keyword (scm_from_locale_symbol
(@var{name}))} and @code{scm_symbol_to_keyword (scm_from_locale_symboln
(@var{name}, @var{len}))}, respectively.

Note that these functions should @emph{not} be used when @var{name} is a
C string constant, because there is no guarantee that the current locale
will match that of the execution character set, used for string and
character constants.  Most modern C compilers use UTF-8 by default, so
in such cases we recommend @code{scm_from_utf8_keyword}.
@end deftypefn

@deftypefn {C Function} SCM scm_from_latin1_keyword (const char *name)
@deftypefnx {C Function} SCM scm_from_utf8_keyword (const char *name)
Equivalent to @code{scm_symbol_to_keyword (scm_from_latin1_symbol
(@var{name}))} and @code{scm_symbol_to_keyword (scm_from_utf8_symbol
(@var{name}))}, respectively.
@end deftypefn

@deftypefn {C Function} void scm_c_bind_keyword_arguments (const char *subr, @
                             SCM rest, scm_t_keyword_arguments_flags flags, @
                             SCM keyword1, SCM *argp1, @
                             @dots{}, @
                             SCM keywordN, SCM *argpN, @
                             @nicode{SCM_UNDEFINED})

Extract the specified keyword arguments from @var{rest}, which is not
modified.  If the keyword argument @var{keyword1} is present in
@var{rest} with an associated value, that value is stored in the
variable pointed to by @var{argp1}, otherwise the variable is left
unchanged.  Similarly for the other keywords and argument pointers up to
@var{keywordN} and @var{argpN}.  The argument list to
@code{scm_c_bind_keyword_arguments} must be terminated by
@code{SCM_UNDEFINED}.

Note that since the variables pointed to by @var{argp1} through
@var{argpN} are left unchanged if the associated keyword argument is not
present, they should be initialized to their default values before
calling @code{scm_c_bind_keyword_arguments}.  Alternatively, you can
initialize them to @code{SCM_UNDEFINED} before the call, and then use
@code{SCM_UNBNDP} after the call to see which ones were provided.

If an unrecognized keyword argument is present in @var{rest} and
@var{flags} does not contain @code{SCM_ALLOW_OTHER_KEYS}, or if
non-keyword arguments are present and @var{flags} does not contain
@code{SCM_ALLOW_NON_KEYWORD_ARGUMENTS}, an exception is raised.
@var{subr} should be the name of the procedure receiving the keyword
arguments, for purposes of error reporting.

For example:

@example
SCM k_delimiter;
SCM k_grammar;
SCM sym_infix;

SCM my_string_join (SCM strings, SCM rest)
@{
  SCM delimiter = SCM_UNDEFINED;
  SCM grammar   = sym_infix;

  scm_c_bind_keyword_arguments ("my-string-join", rest, 0,
                                k_delimiter, &delimiter,
                                k_grammar, &grammar,
                                SCM_UNDEFINED);

  if (SCM_UNBNDP (delimiter))
    delimiter = scm_from_utf8_string (" ");

  return scm_string_join (strings, delimiter, grammar);
@}

void my_init ()
@{
  k_delimiter = scm_from_utf8_keyword ("delimiter");
  k_grammar   = scm_from_utf8_keyword ("grammar");
  sym_infix   = scm_from_utf8_symbol  ("infix");
  scm_c_define_gsubr ("my-string-join", 1, 0, 1, my_string_join);
@}
@end example
@end deftypefn


@node Pairs
@subsection Pairs
@tpindex Pairs

Pairs are used to combine two Scheme objects into one compound object.
Hence the name: A pair stores a pair of objects.

The data type @dfn{pair} is extremely important in Scheme, just like in
any other Lisp dialect.  The reason is that pairs are not only used to
make two values available as one object, but that pairs are used for
constructing lists of values.  Because lists are so important in Scheme,
they are described in a section of their own (@pxref{Lists}).

Pairs can literally get entered in source code or at the REPL, in the
so-called @dfn{dotted list} syntax.  This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses.  The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
@code{foo} and @code{bar} can be entered.  It is very important to write
the whitespace before and after the dot, because otherwise the Scheme
parser would not be able to figure out where to split the tokens.

@lisp
(1 . 2)
(foo . bar)
@end lisp

But beware, if you want to try out these examples, you have to
@dfn{quote} the expressions.  More information about quotation is
available in the section @ref{Expression Syntax}.  The correct way
to try these examples is as follows.

@lisp
'(1 . 2)
@result{}
(1 . 2)
'(foo . bar)
@result{}
(foo . bar)
@end lisp

A new pair is made by calling the procedure @code{cons} with two
arguments.  Then the argument values are stored into a newly allocated
pair, and the pair is returned.  The name @code{cons} stands for
"construct".  Use the procedure @code{pair?} to test whether a
given Scheme object is a pair or not.

@rnindex cons
@deffn {Scheme Procedure} cons x y
@deffnx {C Function} scm_cons (x, y)
Return a newly allocated pair whose car is @var{x} and whose
cdr is @var{y}.  The pair is guaranteed to be different (in the
sense of @code{eq?}) from every previously existing object.
@end deffn

@rnindex pair?
@deffn {Scheme Procedure} pair? x
@deffnx {C Function} scm_pair_p (x)
Return @code{#t} if @var{x} is a pair; otherwise return
@code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_pair (SCM x)
Return 1 when @var{x} is a pair; otherwise return 0.
@end deftypefn

The two parts of a pair are traditionally called @dfn{car} and
@dfn{cdr}.  They can be retrieved with procedures of the same name
(@code{car} and @code{cdr}), and can be modified with the procedures
@code{set-car!} and @code{set-cdr!}.

Since a very common operation in Scheme programs is to access the car of
a car of a pair, or the car of the cdr of a pair, etc., the procedures
called @code{caar}, @code{cadr} and so on are also predefined.  However,
using these procedures is often detrimental to readability, and
error-prone.  Thus, accessing the contents of a list is usually better
achieved using pattern matching techniques (@pxref{Pattern Matching}).

@rnindex car
@rnindex cdr
@deffn {Scheme Procedure} car pair
@deffnx {Scheme Procedure} cdr pair
@deffnx {C Function} scm_car (pair)
@deffnx {C Function} scm_cdr (pair)
Return the car or the cdr of @var{pair}, respectively.
@end deffn

@deftypefn  {C Macro} SCM SCM_CAR (SCM pair)
@deftypefnx {C Macro} SCM SCM_CDR (SCM pair)
These two macros are the fastest way to access the car or cdr of a
pair; they can be thought of as compiling into a single memory
reference.

These macros do no checking at all.  The argument @var{pair} must be a
valid pair.
@end deftypefn

@deffn  {Scheme Procedure} cddr pair
@deffnx {Scheme Procedure} cdar pair
@deffnx {Scheme Procedure} cadr pair
@deffnx {Scheme Procedure} caar pair
@deffnx {Scheme Procedure} cdddr pair
@deffnx {Scheme Procedure} cddar pair
@deffnx {Scheme Procedure} cdadr pair
@deffnx {Scheme Procedure} cdaar pair
@deffnx {Scheme Procedure} caddr pair
@deffnx {Scheme Procedure} cadar pair
@deffnx {Scheme Procedure} caadr pair
@deffnx {Scheme Procedure} caaar pair
@deffnx {Scheme Procedure} cddddr pair
@deffnx {Scheme Procedure} cdddar pair
@deffnx {Scheme Procedure} cddadr pair
@deffnx {Scheme Procedure} cddaar pair
@deffnx {Scheme Procedure} cdaddr pair
@deffnx {Scheme Procedure} cdadar pair
@deffnx {Scheme Procedure} cdaadr pair
@deffnx {Scheme Procedure} cdaaar pair
@deffnx {Scheme Procedure} cadddr pair
@deffnx {Scheme Procedure} caddar pair
@deffnx {Scheme Procedure} cadadr pair
@deffnx {Scheme Procedure} cadaar pair
@deffnx {Scheme Procedure} caaddr pair
@deffnx {Scheme Procedure} caadar pair
@deffnx {Scheme Procedure} caaadr pair
@deffnx {Scheme Procedure} caaaar pair
@deffnx {C Function} scm_cddr (pair)
@deffnx {C Function} scm_cdar (pair)
@deffnx {C Function} scm_cadr (pair)
@deffnx {C Function} scm_caar (pair)
@deffnx {C Function} scm_cdddr (pair)
@deffnx {C Function} scm_cddar (pair)
@deffnx {C Function} scm_cdadr (pair)
@deffnx {C Function} scm_cdaar (pair)
@deffnx {C Function} scm_caddr (pair)
@deffnx {C Function} scm_cadar (pair)
@deffnx {C Function} scm_caadr (pair)
@deffnx {C Function} scm_caaar (pair)
@deffnx {C Function} scm_cddddr (pair)
@deffnx {C Function} scm_cdddar (pair)
@deffnx {C Function} scm_cddadr (pair)
@deffnx {C Function} scm_cddaar (pair)
@deffnx {C Function} scm_cdaddr (pair)
@deffnx {C Function} scm_cdadar (pair)
@deffnx {C Function} scm_cdaadr (pair)
@deffnx {C Function} scm_cdaaar (pair)
@deffnx {C Function} scm_cadddr (pair)
@deffnx {C Function} scm_caddar (pair)
@deffnx {C Function} scm_cadadr (pair)
@deffnx {C Function} scm_cadaar (pair)
@deffnx {C Function} scm_caaddr (pair)
@deffnx {C Function} scm_caadar (pair)
@deffnx {C Function} scm_caaadr (pair)
@deffnx {C Function} scm_caaaar (pair)
These procedures are compositions of @code{car} and @code{cdr}, where
for example @code{caddr} could be defined by

@lisp
(define caddr (lambda (x) (car (cdr (cdr x)))))
@end lisp

@code{cadr}, @code{caddr} and @code{cadddr} pick out the second, third
or fourth elements of a list, respectively.  SRFI-1 provides the same
under the names @code{second}, @code{third} and @code{fourth}
(@pxref{SRFI-1 Selectors}).
@end deffn

@rnindex set-car!
@deffn {Scheme Procedure} set-car! pair value
@deffnx {C Function} scm_set_car_x (pair, value)
Stores @var{value} in the car field of @var{pair}.  The value returned
by @code{set-car!} is unspecified.
@end deffn

@rnindex set-cdr!
@deffn {Scheme Procedure} set-cdr! pair value
@deffnx {C Function} scm_set_cdr_x (pair, value)
Stores @var{value} in the cdr field of @var{pair}.  The value returned
by @code{set-cdr!} is unspecified.
@end deffn


@node Lists
@subsection Lists
@tpindex Lists

A very important data type in Scheme---as well as in all other Lisp
dialects---is the data type @dfn{list}.@footnote{Strictly speaking,
Scheme does not have a real datatype @dfn{list}.  Lists are made up of
@dfn{chained pairs}, and only exist by definition---a list is a chain
of pairs which looks like a list.}

This is the short definition of what a list is:

@itemize @bullet
@item
Either the empty list @code{()},

@item
or a pair which has a list in its cdr.
@end itemize

@c FIXME::martin: Describe the pair chaining in more detail.

@c FIXME::martin: What is a proper, what an improper list?
@c What is a circular list?

@c FIXME::martin: Maybe steal some graphics from the Elisp reference 
@c manual?

@menu
* List Syntax::                 Writing literal lists.
* List Predicates::             Testing lists.
* List Constructors::           Creating new lists.
* List Selection::              Selecting from lists, getting their length.
* Append/Reverse::              Appending and reversing lists.
* List Modification::           Modifying existing lists.
* List Searching::              Searching for list elements
* List Mapping::                Applying procedures to lists.
@end menu

@node List Syntax
@subsubsection List Read Syntax

The syntax for lists is an opening parentheses, then all the elements of
the list (separated by whitespace) and finally a closing
parentheses.@footnote{Note that there is no separation character between
the list elements, like a comma or a semicolon.}.

@lisp
(1 2 3)            ; @r{a list of the numbers 1, 2 and 3}
("foo" bar 3.1415) ; @r{a string, a symbol and a real number}
()                 ; @r{the empty list}
@end lisp

The last example needs a bit more explanation.  A list with no elements,
called the @dfn{empty list}, is special in some ways.  It is used for
terminating lists by storing it into the cdr of the last pair that makes
up a list.  An example will clear that up:

@lisp
(car '(1))
@result{}
1
(cdr '(1))
@result{}
()
@end lisp

This example also shows that lists have to be quoted when written
(@pxref{Expression Syntax}), because they would otherwise be
mistakingly taken as procedure applications (@pxref{Simple
Invocation}).


@node List Predicates
@subsubsection List Predicates

Often it is useful to test whether a given Scheme object is a list or
not.  List-processing procedures could use this information to test
whether their input is valid, or they could do different things
depending on the datatype of their arguments.

@rnindex list?
@deffn {Scheme Procedure} list? x
@deffnx {C Function} scm_list_p (x)
Return @code{#t} if @var{x} is a proper list, else @code{#f}.
@end deffn

The predicate @code{null?} is often used in list-processing code to
tell whether a given list has run out of elements.  That is, a loop
somehow deals with the elements of a list until the list satisfies
@code{null?}.  Then, the algorithm terminates.

@rnindex null?
@deffn {Scheme Procedure} null? x
@deffnx {C Function} scm_null_p (x)
Return @code{#t} if @var{x} is the empty list, else @code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_null (SCM x)
Return 1 when @var{x} is the empty list; otherwise return 0.
@end deftypefn


@node List Constructors
@subsubsection List Constructors

This section describes the procedures for constructing new lists.
@code{list} simply returns a list where the elements are the arguments,
@code{cons*} is similar, but the last argument is stored in the cdr of
the last pair of the list.

@c  C Function scm_list(rest) used to be documented here, but it's a
@c  no-op since it does nothing but return the list the caller must
@c  have already created.
@c
@deffn {Scheme Procedure} list elem @dots{}
@deffnx {C Function} scm_list_1 (elem1)
@deffnx {C Function} scm_list_2 (elem1, elem2)
@deffnx {C Function} scm_list_3 (elem1, elem2, elem3)
@deffnx {C Function} scm_list_4 (elem1, elem2, elem3, elem4)
@deffnx {C Function} scm_list_5 (elem1, elem2, elem3, elem4, elem5)
@deffnx {C Function} scm_list_n (elem1, @dots{}, elemN, @nicode{SCM_UNDEFINED})
@rnindex list
Return a new list containing elements @var{elem} @enddots{}.

@code{scm_list_n} takes a variable number of arguments, terminated by
the special @code{SCM_UNDEFINED}.  That final @code{SCM_UNDEFINED} is
not included in the list.  None of @var{elem} @dots{} can
themselves be @code{SCM_UNDEFINED}, or @code{scm_list_n} will
terminate at that point.
@end deffn

@c  C Function scm_cons_star(arg1,rest) used to be documented here,
@c  but it's not really a useful interface, since it expects the
@c  caller to have already consed up all but the first argument
@c  already.
@c
@deffn {Scheme Procedure} cons* arg1 arg2 @dots{}
Like @code{list}, but the last arg provides the tail of the
constructed list, returning @code{(cons @var{arg1} (cons
@var{arg2} (cons @dots{} @var{argn})))}.  Requires at least one
argument.  If given one argument, that argument is returned as
result.  This function is called @code{list*} in some other
Schemes and in Common LISP.
@end deffn

@deffn {Scheme Procedure} list-copy lst
@deffnx {C Function} scm_list_copy (lst)
Return a (newly-created) copy of @var{lst}.
@end deffn

@deffn {Scheme Procedure} make-list n [init]
Create a list containing of @var{n} elements, where each element is
initialized to @var{init}.  @var{init} defaults to the empty list
@code{()} if not given.
@end deffn

Note that @code{list-copy} only makes a copy of the pairs which make up
the spine of the lists.  The list elements are not copied, which means
that modifying the elements of the new list also modifies the elements
of the old list.  On the other hand, applying procedures like
@code{set-cdr!} or @code{delv!} to the new list will not alter the old
list.  If you also need to copy the list elements (making a deep copy),
use the procedure @code{copy-tree} from @code{(ice-9 copy-tree)}
(@pxref{Copying}).

@node List Selection
@subsubsection List Selection

These procedures are used to get some information about a list, or to
retrieve one or more elements of a list.

@rnindex length
@deffn {Scheme Procedure} length lst
@deffnx {C Function} scm_length (lst)
Return the number of elements in list @var{lst}.
@end deffn

@deffn {Scheme Procedure} last-pair lst
@deffnx {C Function} scm_last_pair (lst)
Return the last pair in @var{lst}, signalling an error if
@var{lst} is circular.
@end deffn

@rnindex list-ref
@deffn {Scheme Procedure} list-ref list k
@deffnx {C Function} scm_list_ref (list, k)
Return the @var{k}th element from @var{list}.
@end deffn

@rnindex list-tail
@deffn {Scheme Procedure} list-tail lst k
@deffnx {Scheme Procedure} list-cdr-ref lst k
@deffnx {C Function} scm_list_tail (lst, k)
Return the "tail" of @var{lst} beginning with its @var{k}th element.
The first element of the list is considered to be element 0.

@code{list-tail} and @code{list-cdr-ref} are identical.  It may help to
think of @code{list-cdr-ref} as accessing the @var{k}th cdr of the list,
or returning the results of cdring @var{k} times down @var{lst}.
@end deffn

@deffn {Scheme Procedure} list-head lst k
@deffnx {C Function} scm_list_head (lst, k)
Copy the first @var{k} elements from @var{lst} into a new list, and
return it.
@end deffn

@node Append/Reverse
@subsubsection Append and Reverse

@code{append} and @code{append!} are used to concatenate two or more
lists in order to form a new list.  @code{reverse} and @code{reverse!}
return lists with the same elements as their arguments, but in reverse
order.  The procedure variants with an @code{!} directly modify the
pairs which form the list, whereas the other procedures create new
pairs.  This is why you should be careful when using the side-effecting
variants.

@rnindex append
@deffn {Scheme Procedure} append lst @dots{} obj
@deffnx {Scheme Procedure} append
@deffnx {Scheme Procedure} append! lst @dots{} obj
@deffnx {Scheme Procedure} append!
@deffnx {C Function} scm_append (lstlst)
@deffnx {C Function} scm_append_x (lstlst)
Return a list comprising all the elements of lists @var{lst} @dots{}
@var{obj}.  If called with no arguments, return the empty list.

@lisp
(append '(x) '(y))          @result{}  (x y)
(append '(a) '(b c d))      @result{}  (a b c d)
(append '(a (b)) '((c)))    @result{}  (a (b) (c))
@end lisp

The last argument @var{obj} may actually be any object; an improper
list results if the last argument is not a proper list.

@lisp
(append '(a b) '(c . d))    @result{}  (a b c . d)
(append '() 'a)             @result{}  a
@end lisp

@code{append} doesn't modify the given lists, but the return may share
structure with the final @var{obj}.  @code{append!} is permitted, but
not required, to modify the given lists to form its return.

For @code{scm_append} and @code{scm_append_x}, @var{lstlst} is a list
of the list operands @var{lst} @dots{} @var{obj}.  That @var{lstlst}
itself is not modified or used in the return.
@end deffn

@rnindex reverse
@deffn {Scheme Procedure} reverse lst
@deffnx {Scheme Procedure} reverse! lst [newtail]
@deffnx {C Function} scm_reverse (lst)
@deffnx {C Function} scm_reverse_x (lst, newtail)
Return a list comprising the elements of @var{lst}, in reverse order.

@code{reverse} constructs a new list.  @code{reverse!} is permitted, but
not required, to modify @var{lst} in constructing its return.

For @code{reverse!}, the optional @var{newtail} is appended to the
result.  @var{newtail} isn't reversed, it simply becomes the list
tail.  For @code{scm_reverse_x}, the @var{newtail} parameter is
mandatory, but can be @code{SCM_EOL} if no further tail is required.
@end deffn

@node List Modification
@subsubsection List Modification

The following procedures modify an existing list, either by changing
elements of the list, or by changing the list structure itself.

@deffn {Scheme Procedure} list-set! list k val
@deffnx {C Function} scm_list_set_x (list, k, val)
Set the @var{k}th element of @var{list} to @var{val}.
@end deffn

@deffn {Scheme Procedure} list-cdr-set! list k val
@deffnx {C Function} scm_list_cdr_set_x (list, k, val)
Set the @var{k}th cdr of @var{list} to @var{val}.
@end deffn

@deffn {Scheme Procedure} delq item lst
@deffnx {C Function} scm_delq (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{eq?} to @var{item} removed.  This procedure mirrors
@code{memq}: @code{delq} compares elements of @var{lst} against
@var{item} with @code{eq?}.
@end deffn

@deffn {Scheme Procedure} delv item lst
@deffnx {C Function} scm_delv (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{eqv?} to @var{item} removed.  This procedure mirrors
@code{memv}: @code{delv} compares elements of @var{lst} against
@var{item} with @code{eqv?}.
@end deffn

@deffn {Scheme Procedure} delete item lst
@deffnx {C Function} scm_delete (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{equal?} to @var{item} removed.  This procedure mirrors
@code{member}: @code{delete} compares elements of @var{lst}
against @var{item} with @code{equal?}.

See also SRFI-1 which has an extended @code{delete} (@ref{SRFI-1
Deleting}), and also an @code{lset-difference} which can delete
multiple @var{item}s in one call (@ref{SRFI-1 Set Operations}).
@end deffn

@deffn {Scheme Procedure} delq! item lst
@deffnx {Scheme Procedure} delv! item lst
@deffnx {Scheme Procedure} delete! item lst
@deffnx {C Function} scm_delq_x (item, lst)
@deffnx {C Function} scm_delv_x (item, lst)
@deffnx {C Function} scm_delete_x (item, lst)
These procedures are destructive versions of @code{delq}, @code{delv}
and @code{delete}: they modify the pointers in the existing @var{lst}
rather than creating a new list.  Caveat evaluator: Like other
destructive list functions, these functions cannot modify the binding of
@var{lst}, and so cannot be used to delete the first element of
@var{lst} destructively.
@end deffn

@deffn {Scheme Procedure} delq1! item lst
@deffnx {C Function} scm_delq1_x (item, lst)
Like @code{delq!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{eq?}.  See also @code{delv1!} and @code{delete1!}.
@end deffn

@deffn {Scheme Procedure} delv1! item lst
@deffnx {C Function} scm_delv1_x (item, lst)
Like @code{delv!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{eqv?}.  See also @code{delq1!} and @code{delete1!}.
@end deffn

@deffn {Scheme Procedure} delete1! item lst
@deffnx {C Function} scm_delete1_x (item, lst)
Like @code{delete!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{equal?}.  See also @code{delq1!} and @code{delv1!}.
@end deffn

@deffn {Scheme Procedure} filter pred lst
@deffnx {Scheme Procedure} filter! pred lst
Return a list containing all elements from @var{lst} which satisfy the
predicate @var{pred}.  The elements in the result list have the same
order as in @var{lst}.  The order in which @var{pred} is applied to
the list elements is not specified.

@code{filter} does not change @var{lst}, but the result may share a
tail with it.  @code{filter!} may modify @var{lst} to construct its
return.
@end deffn

@node List Searching
@subsubsection List Searching

The following procedures search lists for particular elements.  They use
different comparison predicates for comparing list elements with the
object to be searched.  When they fail, they return @code{#f}, otherwise
they return the sublist whose car is equal to the search object, where
equality depends on the equality predicate used.

@rnindex memq
@deffn {Scheme Procedure} memq x lst
@deffnx {C Function} scm_memq (x, lst)
Return the first sublist of @var{lst} whose car is @code{eq?}
to @var{x} where the sublists of @var{lst} are the non-empty
lists returned by @code{(list-tail @var{lst} @var{k})} for
@var{k} less than the length of @var{lst}.  If @var{x} does not
occur in @var{lst}, then @code{#f} (not the empty list) is
returned.
@end deffn

@rnindex memv
@deffn {Scheme Procedure} memv x lst
@deffnx {C Function} scm_memv (x, lst)
Return the first sublist of @var{lst} whose car is @code{eqv?}
to @var{x} where the sublists of @var{lst} are the non-empty
lists returned by @code{(list-tail @var{lst} @var{k})} for
@var{k} less than the length of @var{lst}.  If @var{x} does not
occur in @var{lst}, then @code{#f} (not the empty list) is
returned.
@end deffn

@rnindex member
@deffn {Scheme Procedure} member x lst
@deffnx {C Function} scm_member (x, lst)
Return the first sublist of @var{lst} whose car is
@code{equal?} to @var{x} where the sublists of @var{lst} are
the non-empty lists returned by @code{(list-tail @var{lst}
@var{k})} for @var{k} less than the length of @var{lst}.  If
@var{x} does not occur in @var{lst}, then @code{#f} (not the
empty list) is returned.

See also SRFI-1 which has an extended @code{member} function
(@ref{SRFI-1 Searching}).
@end deffn


@node List Mapping
@subsubsection List Mapping

List processing is very convenient in Scheme because the process of
iterating over the elements of a list can be highly abstracted.  The
procedures in this section are the most basic iterating procedures for
lists.  They take a procedure and one or more lists as arguments, and
apply the procedure to each element of the list.  They differ in their
return value.

@rnindex map
@c begin (texi-doc-string "guile" "map")
@deffn {Scheme Procedure} map proc arg1 arg2 @dots{}
@deffnx {Scheme Procedure} map-in-order proc arg1 arg2 @dots{}
@deffnx {C Function} scm_map (proc, arg1, args)
Apply @var{proc} to each element of the list @var{arg1} (if only two
arguments are given), or to the corresponding elements of the argument
lists (if more than two arguments are given).  The result(s) of the
procedure applications are saved and returned in a list.  For
@code{map}, the order of procedure applications is not specified,
@code{map-in-order} applies the procedure from left to right to the list
elements.
@end deffn

@rnindex for-each
@c begin (texi-doc-string "guile" "for-each")
@deffn {Scheme Procedure} for-each proc arg1 arg2 @dots{}
Like @code{map}, but the procedure is always applied from left to right,
and the result(s) of the procedure applications are thrown away.  The
return value is not specified.
@end deffn

See also SRFI-1 which extends these functions to take lists of unequal
lengths (@ref{SRFI-1 Fold and Map}).

@node Vectors
@subsection Vectors
@tpindex Vectors

Vectors are sequences of Scheme objects.  Unlike lists, the length of a
vector, once the vector is created, cannot be changed.  The advantage of
vectors over lists is that the time required to access one element of a vector
given its @dfn{position} (synonymous with @dfn{index}), a zero-origin number,
is constant, whereas lists have an access time linear to the position of the
accessed element in the list.

Vectors can contain any kind of Scheme object; it is even possible to
have different types of objects in the same vector.  For vectors
containing vectors, you may wish to use @ref{Arrays} instead.
Note, too, that vectors are a special case of one dimensional
non-uniform arrays and that array procedures operate happily on vectors.

Also see @ref{SRFI-43}, @ref{R6RS Support}, or @ref{R7RS Support}, for
more comprehensive vector libraries.

@menu
* Vector Syntax::               Read syntax for vectors.
* Vector Creation::             Dynamic vector creation and validation.
* Vector Accessors::            Accessing and modifying vector contents.
* Vector Accessing from C::     Ways to work with vectors from C.
* Uniform Numeric Vectors::     Vectors of unboxed numeric values.
@end menu


@node Vector Syntax
@subsubsection Read Syntax for Vectors

Vectors can literally be entered in source code, just like strings,
characters or some of the other data types.  The read syntax for vectors
is as follows: A sharp sign (@code{#}), followed by an opening
parentheses, all elements of the vector in their respective read syntax,
and finally a closing parentheses.  Like strings, vectors do not have to
be quoted.

The following are examples of the read syntax for vectors; where the
first vector only contains numbers and the second three different object
types: a string, a symbol and a number in hexadecimal notation.

@lisp
#(1 2 3)
#("Hello" foo #xdeadbeef)
@end lisp

@node Vector Creation
@subsubsection Dynamic Vector Creation and Validation

Instead of creating a vector implicitly by using the read syntax just
described, you can create a vector dynamically by calling one of the
@code{vector} and @code{list->vector} primitives with the list of Scheme
values that you want to place into a vector.  The size of the vector
thus created is determined implicitly by the number of arguments given.

@rnindex vector
@rnindex list->vector
@deffn {Scheme Procedure} vector arg @dots{}
@deffnx {Scheme Procedure} list->vector l
@deffnx {C Function} scm_vector (l)
Return a newly allocated vector composed of the
given arguments.  Analogous to @code{list}.

@lisp
(vector 'a 'b 'c) @result{} #(a b c)
@end lisp
@end deffn

The inverse operation is @code{vector->list}:

@rnindex vector->list
@deffn {Scheme Procedure} vector->list v
@deffnx {C Function} scm_vector_to_list (v)
Return a newly allocated list composed of the elements of @var{v}.

@lisp
(vector->list #(dah dah didah)) @result{}  (dah dah didah)
(list->vector '(dididit dah)) @result{}  #(dididit dah)
@end lisp
@end deffn

To allocate a vector with an explicitly specified size, use
@code{make-vector}.  With this primitive you can also specify an initial
value for the vector elements (the same value for all elements, that
is):

@rnindex make-vector
@deffn {Scheme Procedure} make-vector len [fill]
@deffnx {C Function} scm_make_vector (len, fill)
Return a newly allocated vector of @var{len} elements.  If a
second argument is given, then each position is initialized to
@var{fill}.  Otherwise the initial contents of each position is
unspecified.
@end deffn

@deftypefn {C Function} SCM scm_c_make_vector (size_t k, SCM fill)
Like @code{scm_make_vector}, but the length is given as a @code{size_t}.
@end deftypefn

To check whether an arbitrary Scheme value @emph{is} a vector, use the
@code{vector?} primitive:

@rnindex vector?
@deffn {Scheme Procedure} vector? obj
@deffnx {C Function} scm_vector_p (obj)
Return @code{#t} if @var{obj} is a vector, otherwise return
@code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_vector (SCM obj)
Return non-zero when @var{obj} is a vector, otherwise return
@code{zero}.
@end deftypefn

@node Vector Accessors
@subsubsection Accessing and Modifying Vector Contents

@code{vector-length} and @code{vector-ref} return information about a
given vector, respectively its size and the elements that are contained
in the vector.

@rnindex vector-length
@deffn {Scheme Procedure} vector-length vector
@deffnx {C Function} scm_vector_length (vector)
Return the number of elements in @var{vector} as an exact integer.
@end deffn

@deftypefn {C Function} size_t scm_c_vector_length (SCM vec)
Return the number of elements in @var{vec} as a @code{size_t}.
@end deftypefn

@rnindex vector-ref
@deffn {Scheme Procedure} vector-ref vec k
@deffnx {C Function} scm_vector_ref (vec, k)
Return the contents of position @var{k} of @var{vec}.
@var{k} must be a valid index of @var{vec}.
@lisp
(vector-ref #(1 1 2 3 5 8 13 21) 5) @result{} 8
(vector-ref #(1 1 2 3 5 8 13 21)
    (let ((i (round (* 2 (acos -1)))))
      (if (inexact? i)
        (inexact->exact i)
           i))) @result{} 13
@end lisp
@end deffn

@anchor{x-scm_c_vector_ref}
@deftypefn {C Function} SCM scm_c_vector_ref (SCM vec, size_t k)
Return the contents of position @var{k} (a @code{size_t}) of
@var{vec}.
@end deftypefn

A vector created by one of the dynamic vector constructor procedures
(@pxref{Vector Creation}) can be modified using the following
procedures.

@emph{NOTE:} According to R5RS, it is an error to use any of these
procedures on a literally read vector, because such vectors should be
considered as constants.  Currently, however, Guile does not detect this
error.

@rnindex vector-set!
@deffn {Scheme Procedure} vector-set! vec k obj
@deffnx {C Function} scm_vector_set_x (vec, k, obj)
Store @var{obj} in position @var{k} of @var{vec}.
@var{k} must be a valid index of @var{vec}.
The value returned by @samp{vector-set!} is unspecified.
@lisp
(let ((vec (vector 0 '(2 2 2 2) "Anna")))
  (vector-set! vec 1 '("Sue" "Sue"))
  vec) @result{}  #(0 ("Sue" "Sue") "Anna")
@end lisp
@end deffn

@anchor{x-scm_c_vector_set_x}
@deftypefn {C Function} void scm_c_vector_set_x (SCM vec, size_t k, SCM obj)
Store @var{obj} in position @var{k} (a @code{size_t}) of @var{vec}.
@end deftypefn

@rnindex vector-fill!
@anchor{x-vector-fill!}
@deffn {Scheme Procedure} vector-fill! vec fill [start [end]]
@deffnx {C Function} scm_vector_fill_x (vec, fill)
Store @var{fill} in every position of @var{vec} in the range
[@var{start} ... @var{end}). @var{start} defaults to 0 and @var{end}
defaults to the length of @var{vec}.

The value returned by @code{vector-fill!} is unspecified.
@end deffn

@rnindex vector-copy
@anchor{x-vector-copy}
@deffn {Scheme Procedure} vector-copy vec [start [end]]
@deffnx {C Function} scm_vector_copy (vec)
Returns a freshly allocated vector containing the elements of @var{vec}
in the range [@var{start} ... @var{end}). @var{start} defaults to 0 and
@var{end} defaults to the length of @var{vec}.
@end deffn

@rnindex vector-copy!
@anchor{x-vector-copy!}
@deffn {Scheme Procedure} vector-copy! dst at src [start [end]]
Copy the block of elements from vector @var{src} in the range
[@var{start} ... @var{end}) into vector @var{dst}, starting at position
@var{at}. @var{at} and @var{start} default to 0 and @var{end} defaults
to the length of @var{src}.

It is an error for @var{dst} to have a length less than @var{at} +
(@var{end} - @var{start}).  

The order in which elements are copied is unspecified, except that if the
source and destination overlap, copying takes place as if the source is
first copied into a temporary vector and then into the destination.

The value returned by @code{vector-copy!} is unspecified.
@end deffn

@deffn {Scheme Procedure} vector-move-left! vec1 start1 end1 vec2 start2
@deffnx {C Function} scm_vector_move_left_x (vec1, start1, end1, vec2, start2)
Copy elements from @var{vec1}, positions @var{start1} to @var{end1},
to @var{vec2} starting at position @var{start2}.  @var{start1} and
@var{start2} are inclusive indices; @var{end1} is exclusive.

@code{vector-move-left!} copies elements in leftmost order.
Therefore, in the case where @var{vec1} and @var{vec2} refer to the
same vector, @code{vector-move-left!} is usually appropriate when
@var{start1} is greater than @var{start2}.

The value returned by @code{vector-move-left!} is unspecified.
@end deffn

@deffn {Scheme Procedure} vector-move-right! vec1 start1 end1 vec2 start2
@deffnx {C Function} scm_vector_move_right_x (vec1, start1, end1, vec2, start2)
Copy elements from @var{vec1}, positions @var{start1} to @var{end1},
to @var{vec2} starting at position @var{start2}.  @var{start1} and
@var{start2} are inclusive indices; @var{end1} is exclusive.

@code{vector-move-right!} copies elements in rightmost order.
Therefore, in the case where @var{vec1} and @var{vec2} refer to the
same vector, @code{vector-move-right!} is usually appropriate when
@var{start1} is less than @var{start2}.

The value returned by @code{vector-move-right!} is unspecified.
@end deffn

@node Vector Accessing from C
@subsubsection Vector Accessing from C

A vector can be read and modified from C with the functions
@ref{x-scm_c_vector_ref,@code{scm_c_vector_ref}} and
@ref{x-scm_c_vector_set_x,@code{scm_c_vector_set_x}}.  In addition to
these functions, there are two other ways to access vectors from C that
might be more efficient in certain situations: you can use the unsafe
@emph{vector macros}; or you can use the general framework for accessing
all kinds of arrays (@pxref{Accessing Arrays from C}), which is more
verbose, but can deal efficiently with all kinds of vectors (and
arrays).  For arrays of rank 1 whose backing store is a vector, you can
use the @code{scm_vector_elements} and
@code{scm_vector_writable_elements} functions as shortcuts.

@deftypefn {C Macro} size_t SCM_SIMPLE_VECTOR_LENGTH (SCM vec)
Evaluates to the length of the vector @var{vec}.  No type
checking is done.
@end deftypefn

@deftypefn {C Macro} SCM SCM_SIMPLE_VECTOR_REF (SCM vec, size_t idx)
Evaluates to the element at position @var{idx} in the vector @var{vec}.
No type or range checking is done.
@end deftypefn

@deftypefn {C Macro} void SCM_SIMPLE_VECTOR_SET (SCM vec, size_t idx, SCM val)
Sets the element at position @var{idx} in the vector @var{vec} to
@var{val}.  No type or range checking is done.
@end deftypefn

@deftypefn {C Function} {const SCM *} scm_vector_elements (SCM array, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Acquire a @ref{Accessing Arrays from C,handle} for @var{array} and
return a read-only pointer to its elements.  @var{array} must be either
a vector, or an array of rank 1 whose backing store is a vector;
otherwise an error is signaled.  The handle must eventually be released
with @ref{x-scm_array_handle_release,@code{scm_array_handle_release}}.

The variables pointed to by @var{lenp} and @var{incp} are filled with
the number of elements of the array and the increment (number of
elements) between successive elements, respectively.  Successive
elements of @var{array} need not be contiguous in their underlying
``root vector'' returned here; hence the increment is not necessarily
equal to 1 and may well be negative too (@pxref{Shared Arrays}).

The following example shows the typical way to use this function.  It
creates a list of all elements of @var{array} (in reverse order).

@example
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
const SCM *elt;
SCM list;

elt = scm_vector_elements (array, &handle, &len, &inc);
list = SCM_EOL;
for (i = 0; i < len; i++, elt += inc)
  list = scm_cons (*elt, list);
scm_array_handle_release (&handle);
@end example

@end deftypefn

@deftypefn {C Function} {SCM *} scm_vector_writable_elements (SCM array, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Like @code{scm_vector_elements} but the pointer can be used to modify
the array.

The following example shows the typical way to use this function.  It
fills an array with @code{#t}.

@example
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
SCM *elt;

elt = scm_vector_writable_elements (array, &handle, &len, &inc);
for (i = 0; i < len; i++, elt += inc)
  *elt = SCM_BOOL_T;
scm_array_handle_release (&handle);
@end example

@end deftypefn

@node Uniform Numeric Vectors
@subsubsection Uniform Numeric Vectors

A uniform numeric vector is a vector whose elements are all of a single
numeric type.  Guile offers uniform numeric vectors for signed and
unsigned 8-bit, 16-bit, 32-bit, and 64-bit integers, two sizes of
floating point values, and complex floating-point numbers of these two
sizes. @xref{SRFI-4}, for more information.

For many purposes, bytevectors work just as well as uniform vectors, and have
the advantage that they integrate well with binary input and output.
@xref{Bytevectors}, for more information on bytevectors.

@node Bit Vectors
@subsection Bit Vectors

@noindent
Bit vectors are zero-origin, one-dimensional arrays of booleans.  They
are displayed as a sequence of @code{0}s and @code{1}s prefixed by
@code{#*}, e.g.,

@example
(make-bitvector 8 #f) @result{}
#*00000000
@end example

Bit vectors are the special case of one dimensional bit arrays, and can
thus be used with the array procedures, @xref{Arrays}.

@deffn {Scheme Procedure} bitvector? obj
Return @code{#t} when @var{obj} is a bitvector, else
return @code{#f}.
@end deffn

@deffn {Scheme Procedure} make-bitvector len [fill]
Create a new bitvector of length @var{len} and
optionally initialize all elements to @var{fill}.
@end deffn

@deffn {Scheme Procedure} bitvector bit @dots{}
Create a new bitvector with the arguments as elements.
@end deffn

@deffn {Scheme Procedure} bitvector-length vec
Return the length of the bitvector @var{vec}.
@end deffn

@deffn {Scheme Procedure} bitvector-bit-set? vec idx
@deffnx {Scheme Procedure} bitvector-bit-clear? vec idx
Return @code{#t} if the bit at index @var{idx} of the bitvector
@var{vec} is set (for @code{bitvector-bit-set?}) or clear (for
@code{bitvector-bit-clear?}).
@end deffn

@deffn {Scheme Procedure} bitvector-set-bit! vec idx
@deffnx {Scheme Procedure} bitvector-clear-bit! vec idx
Set (for @code{bitvector-set-bit!}) or clear (for
@code{bitvector-clear-bit!}) the bit at index @var{idx} of the bitvector
@var{vec}.
@end deffn

@deffn {Scheme Procedure} bitvector-set-all-bits! vec
@deffnx {Scheme Procedure} bitvector-clear-all-bits! vec
@deffnx {Scheme Procedure} bitvector-flip-all-bits! vec
Set, clear, or flip all bits of @var{vec}.
@end deffn

@deffn {Scheme Procedure} list->bitvector list
@deffnx {C Function} scm_list_to_bitvector (list)
Return a new bitvector initialized with the elements
of @var{list}.
@end deffn

@deffn {Scheme Procedure} bitvector->list vec
@deffnx {C Function} scm_bitvector_to_list (vec)
Return a new list initialized with the elements
of the bitvector @var{vec}.
@end deffn

@deffn {Scheme Procedure} bitvector-copy bitvector [start [end]]
@deffnx {C Function} scm_bitvector_copy (bitvector, start, end)
Returns a freshly allocated bitvector containing the elements of @var{bitvector}
in the range [@var{start} ... @var{end}). @var{start} defaults to 0 and
@var{end} defaults to the length of @var{bitvector}.
@end deffn

@deffn {Scheme Procedure} bitvector-count bitvector
Return a count of how many entries in @var{bitvector} are set.

@example
(bitvector-count #*000111000)  @result{} 3
@end example
@end deffn

@deffn {Scheme Procedure} bitvector-count-bits bitvector bits
Return a count of how many entries in @var{bitvector} are set, with the
bitvector @var{bits} selecting the entries to consider.  @var{bitvector}
must be at least as long as @var{bits}.

For example,

@example
(bitvector-count-bits #*01110111 #*11001101) @result{} 3
@end example
@end deffn

@deffn {Scheme Procedure} bitvector-position bitvector bool start
@deffnx {C Function} scm_bitvector_position (bitvector, bool, start)
Return the index of the first occurrence of @var{bool} in
@var{bitvector}, starting from @var{start}.  If there is no @var{bool}
entry between @var{start} and the end of @var{bitvector}, then return
@code{#f}.  For example,

@example
(bitvector-position #*000101 #t 0)  @result{} 3
(bitvector-position #*0001111 #f 3) @result{} #f
@end example
@end deffn

@deffn {Scheme Procedure} bitvector-set-bits! bitvector bits
Set entries of @var{bitvector} to @code{#t}, with @var{bits} selecting
the bits to set.  The return value is unspecified.  @var{bitvector} must
be at least as long as @var{bits}.

@example
(define bv (bitvector-copy #*11000010))
(bitvector-set-bits! bv #*10010001)
bv
@result{} #*11010011
@end example
@end deffn

@deffn {Scheme Procedure} bitvector-clear-bits! bitvector bits
Set entries of @var{bitvector} to @code{#f}, with @var{bits} selecting
the bits to clear.  The return value is unspecified.  @var{bitvector}
must be at least as long as @var{bits}.

@example
(define bv (bitvector-copy #*11000010))
(bitvector-clear-bits! bv #*10010001)
bv
@result{} #*01000010
@end example
@end deffn

@deftypefn {C Function} int scm_is_bitvector (SCM obj)
@deftypefnx {C Function} SCM scm_c_make_bitvector (size_t len, SCM fill)
@deftypefnx {C Function} int scm_bitvector_bit_is_set (SCM vec, size_t idx)
@deftypefnx {C Function} int scm_bitvector_bit_is_clear (SCM vec, size_t idx)
@deftypefnx {C Function} void scm_c_bitvector_set_bit_x (SCM vec, size_t idx)
@deftypefnx {C Function} void scm_c_bitvector_clear_bit_x (SCM vec, size_t idx)
@deftypefnx {C Function} void scm_c_bitvector_set_bits_x (SCM vec, SCM bits)
@deftypefnx {C Function} void scm_c_bitvector_clear_bits_x (SCM vec, SCM bits)
@deftypefnx {C Function} void scm_c_bitvector_set_all_bits_x (SCM vec)
@deftypefnx {C Function} void scm_c_bitvector_clear_all_bits_x (SCM vec)
@deftypefnx {C Function} void scm_c_bitvector_flip_all_bits_x (SCM vec)
@deftypefnx {C Function} size_t scm_c_bitvector_length (SCM bitvector)
@deftypefnx {C Function} size_t scm_c_bitvector_count (SCM bitvector)
@deftypefnx {C Function} size_t scm_c_bitvector_count_bits (SCM bitvector, SCM bits)
C API for the corresponding Scheme bitvector interfaces.
@end deftypefn

@deftypefn {C Function} {const scm_t_uint32 *} scm_bitvector_elements (SCM vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp, ssize_t *incp)
Like @code{scm_vector_elements} (@pxref{Vector Accessing from C}), but
for bitvectors.  The variable pointed to by @var{offp} is set to the
value returned by @code{scm_array_handle_bit_elements_offset}.  See
@code{scm_array_handle_bit_elements} for how to use the returned
pointer and the offset.
@end deftypefn

@deftypefn {C Function} {scm_t_uint32 *} scm_bitvector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp, ssize_t *incp)
Like @code{scm_bitvector_elements}, but the pointer is good for reading
and writing.
@end deftypefn

@node Bytevectors
@subsection Bytevectors

@cindex bytevector
@cindex R6RS

A @dfn{bytevector} is a raw byte string.  The @code{(rnrs bytevectors)}
module provides the programming interface specified by the
@uref{http://www.r6rs.org/, Revised^6 Report on the Algorithmic Language
Scheme (R6RS)}.  It contains procedures to manipulate bytevectors and
interpret their contents in a number of ways: as signed or unsigned
integer of various sizes and endianness, as IEEE-754 floating point
numbers, or as strings.  It is a useful tool to encode and decode binary
data.  The @ref{R7RS Support,R7RS} offers its own set of bytevector
procedures (@pxref{Bytevector Procedures in R7RS}).

The R6RS (Section 4.3.4) specifies an external representation for
bytevectors, whereby the octets (integers in the range 0--255) contained
in the bytevector are represented as a list prefixed by @code{#vu8}:

@lisp
#vu8(1 53 204)
@end lisp

denotes a 3-byte bytevector containing the octets 1, 53, and 204.  Like
string literals, booleans, etc., bytevectors are ``self-quoting'', i.e.,
they do not need to be quoted:

@lisp
#vu8(1 53 204)
@result{} #vu8(1 53 204)
@end lisp

Bytevectors can be used with the binary input/output primitives
(@pxref{Binary I/O}).

@menu
* Bytevector Endianness::       Dealing with byte order.
* Bytevector Manipulation::     Creating, copying, manipulating bytevectors.
* Bytevectors as Integers::     Interpreting bytes as integers.
* Bytevectors and Integer Lists::  Converting to/from an integer list.
* Bytevectors as Floats::       Interpreting bytes as real numbers.
* Bytevectors as Strings::      Interpreting bytes as Unicode strings.
* Bytevectors as Arrays::       Guile extension to the bytevector API.
* Bytevectors as Uniform Vectors::  Bytevectors and SRFI-4.
* Bytevector Procedures in R7RS:: R7RS interface for bytevectors.
* Bytevector Slices::           Aliases for parts of a bytevector.
@end menu

@node Bytevector Endianness
@subsubsection Endianness

@cindex endianness
@cindex byte order
@cindex word order

Some of the following procedures take an @var{endianness} parameter.
The @dfn{endianness} is defined as the order of bytes in multi-byte
numbers: numbers encoded in @dfn{big endian} have their most
significant bytes written first, whereas numbers encoded in
@dfn{little endian} have their least significant bytes
first@footnote{Big-endian and little-endian are the most common
``endiannesses'', but others do exist. For instance, the GNU MP
library allows @dfn{word order} to be specified independently of
@dfn{byte order} (@pxref{Integer Import and Export,,, gmp, The GNU
Multiple Precision Arithmetic Library Manual}).}.

Little-endian is the native endianness of the IA32 architecture and
its derivatives, while big-endian is native to SPARC and PowerPC,
among others. The @code{native-endianness} procedure returns the
native endianness of the machine it runs on.

@deffn {Scheme Procedure} native-endianness
@deffnx {C Function} scm_native_endianness ()
Return a value denoting the native endianness of the host machine.
@end deffn

@deffn {Scheme Macro} endianness symbol
Return an object denoting the endianness specified by @var{symbol}.  If
@var{symbol} is neither @code{big} nor @code{little} then an error is
raised at expand-time.
@end deffn

@defvr {C Variable} scm_endianness_big
@defvrx {C Variable} scm_endianness_little
The objects denoting big- and little-endianness, respectively.
@end defvr


@node Bytevector Manipulation
@subsubsection Manipulating Bytevectors

Bytevectors can be created, copied, and analyzed with the following
procedures and C functions.

@anchor{x-make-bytevector}
@deffn {Scheme Procedure} make-bytevector len [fill]
@deffnx {C Function} scm_make_bytevector (len, fill)
@deffnx {C Function} scm_c_make_bytevector (size_t len)
Return a new bytevector of @var{len} bytes.  Optionally, if @var{fill}
is given, fill it with @var{fill}; @var{fill} must be in the range
[-128,255].
@end deffn

@anchor{x-bytevector?}
@deffn {Scheme Procedure} bytevector? obj
@deffnx {C Function} scm_bytevector_p (obj)
Return true if @var{obj} is a bytevector.
@end deffn

@deftypefn {C Function} int scm_is_bytevector (SCM obj)
Equivalent to @code{scm_is_true (scm_bytevector_p (obj))}.
@end deftypefn

@anchor{x-bytevector-length}
@deffn {Scheme Procedure} bytevector-length bv
@deffnx {C Function} scm_bytevector_length (bv)
Return the length in bytes of bytevector @var{bv}.
@end deffn

@deftypefn {C Function} size_t scm_c_bytevector_length (SCM bv)
Likewise, return the length in bytes of bytevector @var{bv}.
@end deftypefn

@deffn {Scheme Procedure} bytevector=? bv1 bv2
@deffnx {C Function} scm_bytevector_eq_p (bv1, bv2)
Return @code{#t} if @var{bv1} equals @var{bv2}---i.e., if they have the same
length and contents.
@end deffn

@deffn {Scheme Procedure} bytevector-fill! bv fill [start [end]]
@deffnx {C Function} scm_bytevector_fill_x (bv, fill)
Fill positions [@var{start} ... @var{end}) of bytevector @var{bv} with
byte @var{fill}.  @var{start} defaults to 0 and @var{end} defaults to the
length of @var{bv}.@footnote{R6RS only defines @code{(bytevector-fill! bv
fill)}.  Arguments @var{start} and @var{end} are a Guile extension
(cf. @ref{x-vector-fill!,@code{vector-fill!}},
@ref{x-string-fill!,@code{string-fill!}}).}
@end deffn

@anchor{x-r6:bytevector-copy!}
@deffn {Scheme Procedure} bytevector-copy! source source-start target target-start len
@deffnx {C Function} scm_bytevector_copy_x (source, source_start, target, target_start, len)
Copy @var{len} bytes from @var{source} into @var{target}, starting
reading from @var{source-start} (an index index within @var{source})
and writing at @var{target-start}.

It is permitted for the @var{source} and @var{target} regions to
overlap.  In that case, copying takes place as if the source is first
copied into a temporary bytevector and then into the destination.
@end deffn

@anchor{x-r6:bytevector-copy}
@deffn {Scheme Procedure} bytevector-copy bv
@deffnx {C Function} scm_bytevector_copy (bv)
Return a newly allocated copy of @var{bv}.
@end deffn

@deftypefn {C Function} scm_t_uint8 scm_c_bytevector_ref (SCM bv, size_t index)
Return the byte at @var{index} in bytevector @var{bv}.
@end deftypefn

@deftypefn {C Function} void scm_c_bytevector_set_x (SCM bv, size_t index, scm_t_uint8 value)
Set the byte at @var{index} in @var{bv} to @var{value}.
@end deftypefn

Low-level C macros are available.  They do not perform any
type-checking; as such they should be used with care.

@deftypefn {C Macro} size_t SCM_BYTEVECTOR_LENGTH (bv)
Return the length in bytes of bytevector @var{bv}.
@end deftypefn

@deftypefn {C Macro} {signed char *} SCM_BYTEVECTOR_CONTENTS (bv)
Return a pointer to the contents of bytevector @var{bv}.
@end deftypefn


@node Bytevectors as Integers
@subsubsection Interpreting Bytevector Contents as Integers

The contents of a bytevector can be interpreted as a sequence of
integers of any given size, sign, and endianness.

@lisp
(let ((bv (make-bytevector 4)))
  (bytevector-u8-set! bv 0 #x12)
  (bytevector-u8-set! bv 1 #x34)
  (bytevector-u8-set! bv 2 #x56)
  (bytevector-u8-set! bv 3 #x78)

  (map (lambda (number)
         (number->string number 16))
       (list (bytevector-u8-ref bv 0)
             (bytevector-u16-ref bv 0 (endianness big))
             (bytevector-u32-ref bv 0 (endianness little)))))

@result{} ("12" "1234" "78563412")
@end lisp

The most generic procedures to interpret bytevector contents as integers
are described below.

@deffn {Scheme Procedure} bytevector-uint-ref bv index endianness size
@deffnx {C Function} scm_bytevector_uint_ref (bv, index, endianness, size)
Return the @var{size}-byte long unsigned integer at index @var{index} in
@var{bv}, decoded according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} bytevector-sint-ref bv index endianness size
@deffnx {C Function} scm_bytevector_sint_ref (bv, index, endianness, size)
Return the @var{size}-byte long signed integer at index @var{index} in
@var{bv}, decoded according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} bytevector-uint-set! bv index value endianness size
@deffnx {C Function} scm_bytevector_uint_set_x (bv, index, value, endianness, size)
Set the @var{size}-byte long unsigned integer at @var{index} to
@var{value}, encoded according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} bytevector-sint-set! bv index value endianness size
@deffnx {C Function} scm_bytevector_sint_set_x (bv, index, value, endianness, size)
Set the @var{size}-byte long signed integer at @var{index} to
@var{value}, encoded according to @var{endianness}.
@end deffn

The following procedures are similar to the ones above, but specialized
to a given integer size:

@anchor{x-bytevector-u8-ref}
@deffn {Scheme Procedure} bytevector-u8-ref bv index
@deffnx {Scheme Procedure} bytevector-s8-ref bv index
@deffnx {Scheme Procedure} bytevector-u16-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-s16-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-u32-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-s32-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-u64-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-s64-ref bv index endianness
@deffnx {C Function} scm_bytevector_u8_ref (bv, index)
@deffnx {C Function} scm_bytevector_s8_ref (bv, index)
@deffnx {C Function} scm_bytevector_u16_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_s16_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_u32_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_s32_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_u64_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_s64_ref (bv, index, endianness)
Return the unsigned @var{n}-bit (signed) integer (where @var{n} is 8,
16, 32 or 64) from @var{bv} at @var{index}, decoded according to
@var{endianness}.
@end deffn

@anchor{x-bytevector-u8-set!}
@deffn {Scheme Procedure} bytevector-u8-set! bv index value
@deffnx {Scheme Procedure} bytevector-s8-set! bv index value
@deffnx {Scheme Procedure} bytevector-u16-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-s16-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-u32-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-s32-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-u64-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-s64-set! bv index value endianness
@deffnx {C Function} scm_bytevector_u8_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_s8_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_u16_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_s16_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_u32_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_s32_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_u64_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_s64_set_x (bv, index, value, endianness)
Store @var{value} as an @var{n}-bit (signed) integer (where @var{n} is
8, 16, 32 or 64) in @var{bv} at @var{index}, encoded according to
@var{endianness}.
@end deffn

Finally, a variant specialized for the host's endianness is available
for each of these functions (with the exception of the @code{u8} and
@code{s8} accessors, as endianness is about byte order and there is only
1 byte):

@deffn {Scheme Procedure} bytevector-u16-native-ref bv index
@deffnx {Scheme Procedure} bytevector-s16-native-ref bv index
@deffnx {Scheme Procedure} bytevector-u32-native-ref bv index
@deffnx {Scheme Procedure} bytevector-s32-native-ref bv index
@deffnx {Scheme Procedure} bytevector-u64-native-ref bv index
@deffnx {Scheme Procedure} bytevector-s64-native-ref bv index
@deffnx {C Function} scm_bytevector_u16_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_s16_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_u32_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_s32_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_u64_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_s64_native_ref (bv, index)
Return the unsigned @var{n}-bit (signed) integer (where @var{n} is 8,
16, 32 or 64) from @var{bv} at @var{index}, decoded according to the
host's native endianness.
@end deffn

@deffn {Scheme Procedure} bytevector-u16-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-s16-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-u32-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-s32-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-u64-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-s64-native-set! bv index value
@deffnx {C Function} scm_bytevector_u16_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_s16_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_u32_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_s32_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_u64_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_s64_native_set_x (bv, index, value)
Store @var{value} as an @var{n}-bit (signed) integer (where @var{n} is
8, 16, 32 or 64) in @var{bv} at @var{index}, encoded according to the
host's native endianness.
@end deffn


@node Bytevectors and Integer Lists
@subsubsection Converting Bytevectors to/from Integer Lists

Bytevector contents can readily be converted to/from lists of signed or
unsigned integers:

@lisp
(bytevector->sint-list (u8-list->bytevector (make-list 4 255))
                       (endianness little) 2)
@result{} (-1 -1)
@end lisp

@deffn {Scheme Procedure} bytevector->u8-list bv
@deffnx {C Function} scm_bytevector_to_u8_list (bv)
Return a newly allocated list of unsigned 8-bit integers from the
contents of @var{bv}.
@end deffn

@anchor{x-u8-list->bytevector}
@deffn {Scheme Procedure} u8-list->bytevector lst
@deffnx {C Function} scm_u8_list_to_bytevector (lst)
Return a newly allocated bytevector consisting of the unsigned 8-bit
integers listed in @var{lst}.
@end deffn

@deffn {Scheme Procedure} bytevector->uint-list bv endianness size
@deffnx {C Function} scm_bytevector_to_uint_list (bv, endianness, size)
Return a list of unsigned integers of @var{size} bytes representing the
contents of @var{bv}, decoded according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} bytevector->sint-list bv endianness size
@deffnx {C Function} scm_bytevector_to_sint_list (bv, endianness, size)
Return a list of signed integers of @var{size} bytes representing the
contents of @var{bv}, decoded according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} uint-list->bytevector lst endianness size
@deffnx {C Function} scm_uint_list_to_bytevector (lst, endianness, size)
Return a new bytevector containing the unsigned integers listed in
@var{lst} and encoded on @var{size} bytes according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} sint-list->bytevector lst endianness size
@deffnx {C Function} scm_sint_list_to_bytevector (lst, endianness, size)
Return a new bytevector containing the signed integers listed in
@var{lst} and encoded on @var{size} bytes according to @var{endianness}.
@end deffn

@node Bytevectors as Floats
@subsubsection Interpreting Bytevector Contents as Floating Point Numbers

@cindex IEEE-754 floating point numbers

Bytevector contents can also be accessed as IEEE-754 single- or
double-precision floating point numbers (respectively 32 and 64-bit
long) using the procedures described here.

@deffn {Scheme Procedure} bytevector-ieee-single-ref bv index endianness
@deffnx {Scheme Procedure} bytevector-ieee-double-ref bv index endianness
@deffnx {C Function} scm_bytevector_ieee_single_ref (bv, index, endianness)
@deffnx {C Function} scm_bytevector_ieee_double_ref (bv, index, endianness)
Return the IEEE-754 single-precision floating point number from @var{bv}
at @var{index} according to @var{endianness}.
@end deffn

@deffn {Scheme Procedure} bytevector-ieee-single-set! bv index value endianness
@deffnx {Scheme Procedure} bytevector-ieee-double-set! bv index value endianness
@deffnx {C Function} scm_bytevector_ieee_single_set_x (bv, index, value, endianness)
@deffnx {C Function} scm_bytevector_ieee_double_set_x (bv, index, value, endianness)
Store real number @var{value} in @var{bv} at @var{index} according to
@var{endianness}.
@end deffn

Specialized procedures are also available:

@deffn {Scheme Procedure} bytevector-ieee-single-native-ref bv index
@deffnx {Scheme Procedure} bytevector-ieee-double-native-ref bv index
@deffnx {C Function} scm_bytevector_ieee_single_native_ref (bv, index)
@deffnx {C Function} scm_bytevector_ieee_double_native_ref (bv, index)
Return the IEEE-754 single-precision floating point number from @var{bv}
at @var{index} according to the host's native endianness.
@end deffn

@deffn {Scheme Procedure} bytevector-ieee-single-native-set! bv index value
@deffnx {Scheme Procedure} bytevector-ieee-double-native-set! bv index value
@deffnx {C Function} scm_bytevector_ieee_single_native_set_x (bv, index, value)
@deffnx {C Function} scm_bytevector_ieee_double_native_set_x (bv, index, value)
Store real number @var{value} in @var{bv} at @var{index} according to
the host's native endianness.
@end deffn


@node Bytevectors as Strings
@subsubsection Interpreting Bytevector Contents as Unicode Strings

@cindex Unicode string encoding

Bytevector contents can also be interpreted as Unicode strings encoded
in one of the most commonly available encoding formats.
@xref{Representing Strings as Bytes}, for a more generic interface.

@lisp
(utf8->string (u8-list->bytevector '(99 97 102 101)))
@result{} "cafe"

(string->utf8 "caf@'e") ;; SMALL LATIN LETTER E WITH ACUTE ACCENT
@result{} #vu8(99 97 102 195 169)
@end lisp

@deftypefn {Scheme Procedure} {} string-utf8-length str
@deftypefnx {C function} SCM scm_string_utf8_length (str)
@deftypefnx {C function} size_t scm_c_string_utf8_length (str)
Return the number of bytes in the UTF-8 representation of @var{str}.
@end deftypefn

@deffn {Scheme Procedure} string->utf8 str
@deffnx {Scheme Procedure} string->utf16 str [endianness]
@deffnx {Scheme Procedure} string->utf32 str [endianness]
@deffnx {C Function} scm_string_to_utf8 (str)
@deffnx {C Function} scm_string_to_utf16 (str, endianness)
@deffnx {C Function} scm_string_to_utf32 (str, endianness)
Return a newly allocated bytevector that contains the UTF-8, UTF-16, or
UTF-32 (aka. UCS-4) encoding of @var{str}.  For UTF-16 and UTF-32,
@var{endianness} should be the symbol @code{big} or @code{little}; when omitted,
it defaults to big endian.
@end deffn

@deffn {Scheme Procedure} utf8->string utf
@deffnx {Scheme Procedure} utf16->string utf [endianness]
@deffnx {Scheme Procedure} utf32->string utf [endianness]
@deffnx {C Function} scm_utf8_to_string (utf)
@deffnx {C Function} scm_utf16_to_string (utf, endianness)
@deffnx {C Function} scm_utf32_to_string (utf, endianness)
Return a newly allocated string that contains from the UTF-8-, UTF-16-,
or UTF-32-decoded contents of bytevector @var{utf}.  For UTF-16 and UTF-32,
@var{endianness} should be the symbol @code{big} or @code{little}; when omitted,
it defaults to big endian.
@end deffn

@node Bytevectors as Arrays
@subsubsection Accessing Bytevectors with the Array API

As an extension to the R6RS, Guile allows bytevectors to be manipulated
with the @dfn{array} procedures (@pxref{Arrays}).  When using these
APIs, bytes are accessed one at a time as 8-bit unsigned integers:

@example
(define bv #vu8(0 1 2 3))

(array? bv)
@result{} #t

(array-rank bv)
@result{} 1

(array-ref bv 2)
@result{} 2

;; Note the different argument order on array-set!.
(array-set! bv 77 2)
(array-ref bv 2)
@result{} 77

(array-type bv)
@result{} vu8
@end example


@node Bytevectors as Uniform Vectors
@subsubsection Accessing Bytevectors with the SRFI-4 API

Bytevectors may also be accessed with the SRFI-4 API. @xref{SRFI-4 and
Bytevectors}, for more information.


@node Bytevector Procedures in R7RS
@subsubsection Bytevector Procedures in R7RS

The @ref{R7RS Support,R7RS} (Section 6.9) defines a set of
bytevector manipulation procedures, accessible with

@example
(use-modules (scheme base))
@end example

Of these, @ref{x-make-bytevector,@code{make-bytevector}},
@ref{x-bytevector?,@code{bytevector?}},
@ref{x-bytevector-length,@code{bytevector-length}},
@ref{x-bytevector-u8-ref,@code{bytevector-u8-ref}} and
@ref{x-bytevector-u8-set!,@code{bytevector-u8-set!}} have the same
definition as in R6RS.  The procedures listed below either have a
different definition in R7RS and R6RS, or are not defined in R6RS.

@deffn {Scheme Procedure} bytevector arg @dots{}
Return a newly allocated bytevector composed of the given arguments.
Analogous to @code{list}.

@lisp
(bytevector 2 3 4) @result{} #vu8(2 3 4)
@end lisp

See also @ref{x-u8-list->bytevector,@code{u8-list->bytevector}}.
@end deffn

@anchor{x-r7:bytevector-copy}
@deffn {Scheme Procedure} bytevector-copy bv [start [end]]
Returns a newly allocated bytevector containing the elements of @var{bv}
in the range [@var{start} ... @var{end}).  @var{start} defaults to 0 and
@var{end} defaults to the length of @var{bv}.

@lisp
(define bv #vu8(0 1 2 3 4 5))
(bytevector-copy bv) @result{} #vu8(0 1 2 3 4 5)
(bytevector-copy bv 2) @result{} #vu8(2 3 4 5)
(bytevector-copy bv 2 4) @result{} #vu8(2 3)
@end lisp

See also @ref{x-r6:bytevector-copy,the R6RS version}.
@end deffn

@anchor{x-r7:bytevector-copy!}
@deffn {Scheme Procedure} bytevector-copy! dst at src [start [end]]
Copy the block of elements from bytevector @var{src} in the range
[@var{start} ... @var{end}) into bytevector @var{dst}, starting at
position @var{at}.  @var{start} defaults to 0 and @var{end} defaults to
the length of @var{src}.  It is an error for @var{dst}
to have a length less than @var{at} + (@var{end} - @var{start}).

See also @ref{x-r6:bytevector-copy!,the R6RS version}.  With

@lisp
(use-modules ((rnrs bytevectors) #:prefix r6:)
             ((scheme base) #:prefix r7:))
@end lisp

the following calls are equivalent:

@lisp
(r6:bytevector-copy! source source-start target target-start len)
(r7:bytevector-copy! target target-start source source-start (+ source-start len))
@end lisp

@end deffn

@rnindex bytevector-append
@deffn {Scheme Procedure} bytevector-append arg @dots{}
Return a newly allocated bytevector whose characters form the
concatenation of the given bytevectors @var{arg} @enddots{}

@lisp
(bytevector-append #vu8(0 1 2) #vu8(3 4 5))
@result{} #vu8(0 1 2 3 4 5)
@end lisp
@end deffn


@node Bytevector Slices
@subsubsection Bytevector Slices

@cindex subset, of a bytevector
@cindex slice, of a bytevector
@cindex slice, of a uniform vector
As an extension to the R6RS specification, the @code{(rnrs bytevectors
gnu)} module provides the @code{bytevector-slice} procedure, which
returns a bytevector aliasing part of an existing bytevector.

@deffn {Scheme Procedure} bytevector-slice @var{bv} @var{offset} [@var{size}]
@deffnx {C Function} scm_bytevector_slice (@var{bv}, @var{offset}, @var{size})
Return the slice of @var{bv} starting at @var{offset} and counting
@var{size} bytes.  When @var{size} is omitted, the slice covers all
of @var{bv} starting from @var{offset}.  The returned slice shares
storage with @var{bv}: changes to the slice are visible in @var{bv}
and vice-versa.

When @var{bv} is actually a SRFI-4 uniform vector, its element
type is preserved unless @var{offset} and @var{size} are not aligned
on its element type size.
@end deffn

Here is an example showing how to use it:

@lisp
(use-modules (rnrs bytevectors)
             (rnrs bytevectors gnu))

(define bv (u8-list->bytevector (iota 10)))
(define slice (bytevector-slice bv 2 3))

slice
@result{} #vu8(2 3 4)

(bytevector-u8-set! slice 0 77)
slice
@result{} #vu8(77 3 4)

bv
@result{} #vu8(0 1 77 3 4 5 6 7 8 9)
@end lisp


@node Arrays
@subsection Arrays
@tpindex Arrays

@dfn{Arrays} are a collection of cells organized into an arbitrary
number of dimensions.  Each cell can be accessed in constant time by
supplying an index for each dimension.

In the current implementation, an array uses a vector of some kind for
the actual storage of its elements.  Any kind of vector will do, so you
can have arrays of uniform numeric values, arrays of characters, arrays
of bits, and of course, arrays of arbitrary Scheme values.  For example,
arrays with an underlying @code{c64vector} might be nice for digital
signal processing, while arrays made from a @code{u8vector} might be
used to hold gray-scale images.

The number of dimensions of an array is called its @dfn{rank}.  Thus,
a matrix is an array of rank 2, while a vector has rank 1.  When
accessing an array element, you have to specify one exact integer for
each dimension.  These integers are called the @dfn{indices} of the
element.  An array specifies the allowed range of indices for each
dimension via an inclusive lower and upper bound.  These bounds can
well be negative, but the upper bound must be greater than or equal to
the lower bound minus one.  When all lower bounds of an array are
zero, it is called a @dfn{zero-origin} array.

Arrays can be of rank 0, which could be interpreted as a scalar.
Thus, a zero-rank array can store exactly one object and the list of
indices of this element is the empty list.

Arrays contain zero elements when one of their dimensions has a zero
length.  These empty arrays maintain information about their shape: a
matrix with zero columns and 3 rows is different from a matrix with 3
columns and zero rows, which again is different from a vector of
length zero.

The array procedures are all polymorphic, treating strings, uniform
numeric vectors, bytevectors, bit vectors and ordinary vectors as one
dimensional arrays.

@menu
* Array Syntax::                
* Array Procedures::            
* Shared Arrays::               
* Arrays as arrays of arrays::
* Accessing Arrays from C::     
@end menu

@node Array Syntax
@subsubsection Array Syntax

An array is displayed as @code{#} followed by its rank, followed by a
tag that describes the underlying vector, optionally followed by
information about its shape, and finally followed by the cells,
organized into dimensions using parentheses.

In more words, the array tag is of the form

@example
  #<rank><vectag><@@lower><:len><@@lower><:len>...
@end example

where @code{<rank>} is a positive integer in decimal giving the rank of
the array.  It is omitted when the rank is 1 and the array is non-shared
and has zero-origin (see below).  For shared arrays and for a non-zero
origin, the rank is always printed even when it is 1 to distinguish
them from ordinary vectors.

The @code{<vectag>} part is the tag for a uniform numeric vector, like
@code{u8}, @code{s16}, etc, @code{b} for bitvectors, or @code{a} for
strings.  It is empty for ordinary vectors.

The @code{<@@lower>} part is a @samp{@@} character followed by a signed
integer in decimal giving the lower bound of a dimension.  There is one
@code{<@@lower>} for each dimension.  When all lower bounds are zero,
all @code{<@@lower>} parts are omitted.

The @code{<:len>} part is a @samp{:} character followed by an unsigned
integer in decimal giving the length of a dimension.  Like for the lower
bounds, there is one @code{<:len>} for each dimension, and the
@code{<:len>} part always follows the @code{<@@lower>} part for a
dimension.  Lengths are only then printed when they can't be deduced
from the nested lists of elements of the array literal, which can happen
when at least one length is zero.

As a special case, an array of rank 0 is printed as
@code{#0<vectag>(<scalar>)}, where @code{<scalar>} is the result of
printing the single element of the array.

Thus, 

@table @code
@item #(1 2 3)
is an ordinary array of rank 1 with lower bound 0 in dimension 0.
(I.e., a regular vector.)

@item #@@2(1 2 3)
is an ordinary array of rank 1 with lower bound 2 in dimension 0.

@item #2((1 2 3) (4 5 6))
is a non-uniform array of rank 2; a 2@cross{}3 matrix with index ranges 0..1
and 0..2.

@item #u8(0 1 2)
is a uniform u8 array of rank 1.

@item #2u32@@2@@3((1 2) (2 3))
is a uniform u32 array of rank 2 with index ranges 2..3 and 3..4.

@item #2()
is a two-dimensional array with index ranges 0..-1 and 0..-1, i.e.@:
both dimensions have length zero.

@item #2:0:2()
is a two-dimensional array with index ranges 0..-1 and 0..1, i.e.@: the
first dimension has length zero, but the second has length 2.

@item #0(12)
is a rank-zero array with contents 12.

@end table

In addition, bytevectors are also arrays, but use a different syntax
(@pxref{Bytevectors}):

@table @code

@item #vu8(1 2 3)
is a 3-byte long bytevector, with contents 1, 2, 3.

@end table

@node Array Procedures
@subsubsection Array Procedures

When an array is created, the range of each dimension must be
specified, e.g., to create a 2@cross{}3 array with a zero-based index:

@example
(make-array 'ho 2 3) @result{} #2((ho ho ho) (ho ho ho))
@end example

The range of each dimension can also be given explicitly, e.g., another
way to create the same array:

@example
(make-array 'ho '(0 1) '(0 2)) @result{} #2((ho ho ho) (ho ho ho))
@end example

The following procedures can be used with arrays (or vectors).  An
argument shown as @var{idx}@dots{} means one parameter for each
dimension in the array.  A @var{idxlist} argument means a list of such
values, one for each dimension.


@deffn {Scheme Procedure} array? obj
@deffnx {C Function} scm_array_p (obj, unused)
Return @code{#t} if the @var{obj} is an array, and @code{#f} if
not.

The second argument to scm_array_p is there for historical reasons,
but it is not used.  You should always pass @code{SCM_UNDEFINED} as
its value.
@end deffn

@deffn {Scheme Procedure} typed-array? obj type
@deffnx {C Function} scm_typed_array_p (obj, type)
Return @code{#t} if the @var{obj} is an array of type @var{type}, and
@code{#f} if not.
@end deffn

@deftypefn {C Function} int scm_is_array (SCM obj)
Return @code{1} if the @var{obj} is an array and @code{0} if not.
@end deftypefn

@deftypefn {C Function} int scm_is_typed_array (SCM obj, SCM type)
Return @code{0} if the @var{obj} is an array of type @var{type}, and
@code{1} if not.
@end deftypefn

@deffn {Scheme Procedure} make-array fill bound @dots{}
@deffnx {C Function} scm_make_array (fill, bounds)
Equivalent to @code{(make-typed-array #t @var{fill} @var{bound} ...)}.
@end deffn

@deffn {Scheme Procedure} make-typed-array type fill bound @dots{}
@deffnx {C Function} scm_make_typed_array (type, fill, bounds)
Create and return an array that has as many dimensions as there are
@var{bound}s and (maybe) fill it with @var{fill}.

The underlying storage vector is created according to @var{type},
which must be a symbol whose name is the `vectag' of the array as
explained above, or @code{#t} for ordinary, non-specialized arrays.

For example, using the symbol @code{f64} for @var{type} will create an
array that uses a @code{f64vector} for storing its elements, and
@code{a} will use a string.

When @var{fill} is not the special @emph{unspecified} value, the new
array is filled with @var{fill}.  Otherwise, the initial contents of
the array is unspecified.  The special @emph{unspecified} value is
stored in the variable @code{*unspecified*} so that for example
@code{(make-typed-array 'u32 *unspecified* 4)} creates a uninitialized
@code{u32} vector of length 4.

Each @var{bound} may be a positive non-zero integer @var{n}, in which
case the index for that dimension can range from 0 through @var{n}-1; or
an explicit index range specifier in the form @code{(LOWER UPPER)},
where both @var{lower} and @var{upper} are integers, possibly less than
zero, and possibly the same number (however, @var{lower} cannot be
greater than @var{upper}).
@end deffn

@deffn {Scheme Procedure} list->array dimspec list
Equivalent to @code{(list->typed-array #t @var{dimspec}
@var{list})}.
@end deffn

@deffn {Scheme Procedure} list->typed-array type dimspec list
@deffnx {C Function} scm_list_to_typed_array (type, dimspec, list)
Return an array of the type indicated by @var{type} with elements the
same as those of @var{list}.

The argument @var{dimspec} determines the number of dimensions of the
array and their lower bounds.  When @var{dimspec} is an exact integer,
it gives the number of dimensions directly and all lower bounds are
zero.  When it is a list of exact integers, then each element is the
lower index bound of a dimension, and there will be as many dimensions
as elements in the list.
@end deffn

@deffn {Scheme Procedure} array-type array
@deffnx {C Function} scm_array_type (array)
Return the type of @var{array}.  This is the `vectag' used for
printing @var{array} (or @code{#t} for ordinary arrays) and can be
used with @code{make-typed-array} to create an array of the same kind
as @var{array}.
@end deffn

@deffn {Scheme Procedure} array-ref array idx @dots{}
@deffnx {C Function} scm_array_ref (array, idxlist)
Return the element at @code{(idx @dots{})} in @var{array}.

@example
(define a (make-array 999 '(1 2) '(3 4)))
(array-ref a 2 4) @result{} 999
@end example
@end deffn

@deffn {Scheme Procedure} array-in-bounds? array idx @dots{}
@deffnx {C Function} scm_array_in_bounds_p (array, idxlist)
Return @code{#t} if the given indices would be acceptable to
@code{array-ref}.

@example
(define a (make-array #f '(1 2) '(3 4)))
(array-in-bounds? a 2 3) @result{} #t
(array-in-bounds? a 0 0) @result{} #f
@end example
@end deffn

@deffn {Scheme Procedure} array-set! array obj idx @dots{}
@deffnx {C Function} scm_array_set_x (array, obj, idxlist)
Set the element at @code{(idx @dots{})} in @var{array} to @var{obj}.
The return value is unspecified.

@example
(define a (make-array #f '(0 1) '(0 1)))
(array-set! a #t 1 1)
a @result{} #2((#f #f) (#f #t))
@end example
@end deffn

@deffn {Scheme Procedure} array-shape array
@deffnx {Scheme Procedure} array-dimensions array
@deffnx {C Function} scm_array_dimensions (array)
Return a list of the bounds for each dimension of @var{array}.

@code{array-shape} gives @code{(@var{lower} @var{upper})} for each
dimension.  @code{array-dimensions} instead returns just
@math{@var{upper}+1} for dimensions with a 0 lower bound.  Both are
suitable as input to @code{make-array}.

For example,

@example
(define a (make-array 'foo '(-1 3) 5))
(array-shape a)      @result{} ((-1 3) (0 4))
(array-dimensions a) @result{} ((-1 3) 5)
@end example
@end deffn

@deffn {Scheme Procedure} array-length array
@deffnx {C Function} scm_array_length (array)
@deffnx {C Function} size_t scm_c_array_length (array)
Return the length of an array: its first dimension. It is an error to
ask for the length of an array of rank 0.
@end deffn

@deffn {Scheme Procedure} array-rank array
@deffnx {C Function} scm_array_rank (array)
Return the rank of @var{array}.
@end deffn

@deftypefn {C Function} size_t scm_c_array_rank (SCM array)
Return the rank of @var{array} as a @code{size_t}.
@end deftypefn

@deffn {Scheme Procedure} array->list array
@deffnx {C Function} scm_array_to_list (array)
Return a list consisting of all the elements, in order, of
@var{array}.
@end deffn

@c  FIXME: Describe how the order affects the copying (it matters for
@c  shared arrays with the same underlying root vector, presumably).
@c
@deffn {Scheme Procedure} array-copy! src dst
@deffnx {Scheme Procedure} array-copy-in-order! src dst
@deffnx {C Function} scm_array_copy_x (src, dst)
Copy every element from vector or array @var{src} to the corresponding
element of @var{dst}.  @var{dst} must have the same rank as @var{src},
and be at least as large in each dimension.  The return value is
unspecified.
@end deffn

@deffn {Scheme Procedure} array-fill! array fill
@deffnx {C Function} scm_array_fill_x (array, fill)
Store @var{fill} in every element of @var{array}.  The value returned
is unspecified.
@end deffn

@c begin (texi-doc-string "guile" "array-equal?")
@deffn {Scheme Procedure} array-equal? array @dots{}
Return @code{#t} if all arguments are arrays with the same shape, the
same type, and have corresponding elements which are either
@code{equal?} or @code{array-equal?}.  This function differs from
@code{equal?} (@pxref{Equality}) in that all arguments must be arrays.
@end deffn

@c  FIXME: array-for-each doesn't say what happens if the sources have
@c  different index ranges.  The code currently iterates over the
@c  indices of the first and expects the others to cover those.  That
@c  at least vaguely matches array-map!, but is it meant to be a
@c  documented feature?

@deffn {Scheme Procedure} array-map! dst proc src @dots{}
@deffnx {Scheme Procedure} array-map-in-order! dst proc src @dots{}
@deffnx {C Function} scm_array_map_x (dst, proc, srclist)
Set each element of the @var{dst} array to values obtained from calls to
@var{proc}.  The list of @var{src} arguments may be empty.  The value
returned is unspecified.

Each call is @code{(@var{proc} @var{elem} @dots{})}, where each
@var{elem} is from the corresponding @var{src} array, at the
@var{dst} index.  @code{array-map-in-order!} makes the calls in
row-major order, @code{array-map!} makes them in an unspecified order.

The @var{src} arrays must have the same number of dimensions as
@var{dst}, and must have a range for each dimension which covers the
range in @var{dst}.  This ensures all @var{dst} indices are valid in
each @var{src}.
@end deffn

@deffn {Scheme Procedure} array-for-each proc src1 src2 @dots{}
@deffnx {C Function} scm_array_for_each (proc, src1, srclist)
Apply @var{proc} to each tuple of elements of @var{src1} @var{src2}
@dots{}, in row-major order.  The value returned is unspecified.
@end deffn

@deffn {Scheme Procedure} array-index-map! dst proc
@deffnx {C Function} scm_array_index_map_x (dst, proc)
Set each element of the @var{dst} array to values returned by calls to
@var{proc}.  The value returned is unspecified.

Each call is @code{(@var{proc} @var{i1} @dots{} @var{iN})}, where
@var{i1}@dots{}@var{iN} is the destination index, one parameter for
each dimension.  The order in which the calls are made is unspecified.

For example, to create a @m{4\times4, 4x4} matrix representing a
cyclic group,

@tex
\advance\leftskip by 2\lispnarrowing {
$\left(\matrix{%
0 & 1 & 2 & 3 \cr
1 & 2 & 3 & 0 \cr
2 & 3 & 0 & 1 \cr
3 & 0 & 1 & 2 \cr
}\right)$} \par
@end tex
@ifnottex
@example
    / 0 1 2 3 \
    | 1 2 3 0 |
    | 2 3 0 1 |
    \ 3 0 1 2 /
@end example
@end ifnottex

@example
(define a (make-array #f 4 4))
(array-index-map! a (lambda (i j)
                      (modulo (+ i j) 4)))
@end example
@end deffn

An additional array function is available in the module
@code{(ice-9 arrays)}. It can be used with:

@example
(use-modules (ice-9 arrays))
@end example

@deffn {Scheme Procedure} array-copy src
Return a new array with the same elements, type and shape as
@var{src}. However, the array increments may not be the same as those of
@var{src}. In the current implementation, the returned array will be in
row-major order, but that might change in the future. Use
@code{array-copy!} on an array of known order if that is a concern.
@end deffn

@node Shared Arrays
@subsubsection Shared Arrays

@deffn {Scheme Procedure} make-shared-array oldarray mapfunc bound @dots{}
@deffnx {C Function} scm_make_shared_array (oldarray, mapfunc, boundlist)
Return a new array which shares the storage of @var{oldarray}.
Changes made through either affect the same underlying storage.  The
@var{bound} @dots{} arguments are the shape of the new array, the same
as @code{make-array} (@pxref{Array Procedures}).

@var{mapfunc} translates coordinates from the new array to the
@var{oldarray}.  It's called as @code{(@var{mapfunc} newidx1 @dots{})}
with one parameter for each dimension of the new array, and should
return a list of indices for @var{oldarray}, one for each dimension of
@var{oldarray}.

@var{mapfunc} must be affine linear, meaning that each @var{oldarray}
index must be formed by adding integer multiples (possibly negative)
of some or all of @var{newidx1} etc, plus a possible integer offset.
The multiples and offset must be the same in each call.

@sp 1
One good use for a shared array is to restrict the range of some
dimensions, so as to apply say @code{array-for-each} or
@code{array-fill!} to only part of an array.  The plain @code{list}
function can be used for @var{mapfunc} in this case, making no changes
to the index values.  For example,

@example
(make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
@result{} #2((a b) (d e) (g h))
@end example

The new array can have fewer dimensions than @var{oldarray}, for
example to take a column from an array.

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i) (list i 2))
                   '(0 2))
@result{} #1(c f i)
@end example

A diagonal can be taken by using the single new array index for both
row and column in the old array.  For example,

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i) (list i i))
                   '(0 2))
@result{} #1(a e i)
@end example

Dimensions can be increased by for instance considering portions of a
one dimensional array as rows in a two dimensional array.
(@code{array-contents} below can do the opposite, flattening an
array.)

@example
(make-shared-array #1(a b c d e f g h i j k l)
                   (lambda (i j) (list (+ (* i 3) j)))
                   4 3)
@result{} #2((a b c) (d e f) (g h i) (j k l))
@end example

By negating an index the order that elements appear can be reversed.
The following just reverses the column order,

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i j) (list i (- 2 j)))
                   3 3)
@result{} #2((c b a) (f e d) (i h g))
@end example

A fixed offset on indexes allows for instance a change from a 0 based
to a 1 based array,

@example
(define x #2((a b c) (d e f) (g h i)))
(define y (make-shared-array x
                             (lambda (i j) (list (1- i) (1- j)))
                             '(1 3) '(1 3)))
(array-ref x 0 0) @result{} a
(array-ref y 1 1) @result{} a
@end example

A multiple on an index allows every Nth element of an array to be
taken.  The following is every third element,

@example
(make-shared-array #1(a b c d e f g h i j k l)
                   (lambda (i) (list (* i 3)))
                   4)
@result{} #1(a d g j)
@end example

The above examples can be combined to make weird and wonderful
selections from an array, but it's important to note that because
@var{mapfunc} must be affine linear, arbitrary permutations are not
possible.

In the current implementation, @var{mapfunc} is not called for every
access to the new array but only on some sample points to establish a
base and stride for new array indices in @var{oldarray} data.  A few
sample points are enough because @var{mapfunc} is linear.
@end deffn

@deffn {Scheme Procedure} shared-array-increments array
@deffnx {C Function} scm_shared_array_increments (array)
For each dimension, return the distance between elements in the root vector.
@end deffn

@deffn {Scheme Procedure} shared-array-offset array
@deffnx {C Function} scm_shared_array_offset (array)
Return the root vector index of the first element in the array.
@end deffn

@deffn {Scheme Procedure} shared-array-root array
@deffnx {C Function} scm_shared_array_root (array)
Return the root vector of a shared array.
@end deffn

@deffn {Scheme Procedure} array-contents array [strict]
@deffnx {C Function} scm_array_contents (array, strict)
If @var{array} may be @dfn{unrolled} into a one dimensional shared array
without changing their order (last subscript changing fastest), then
@code{array-contents} returns that shared array, otherwise it returns
@code{#f}.  All arrays made by @code{make-array} and
@code{make-typed-array} may be unrolled, some arrays made by
@code{make-shared-array} may not be.

If the optional argument @var{strict} is provided, a shared array will
be returned only if its elements are stored internally contiguous in
memory.
@end deffn

@deffn {Scheme Procedure} transpose-array array dim1 dim2 @dots{}
@deffnx {C Function} scm_transpose_array (array, dimlist)
Return an array sharing contents with @var{array}, but with
dimensions arranged in a different order.  There must be one
@var{dim} argument for each dimension of @var{array}.
@var{dim1}, @var{dim2}, @dots{} should be integers between 0
and the rank of the array to be returned.  Each integer in that
range must appear at least once in the argument list.

The values of @var{dim1}, @var{dim2}, @dots{} correspond to
dimensions in the array to be returned, and their positions in the
argument list to dimensions of @var{array}.  Several @var{dim}s
may have the same value, in which case the returned array will
have smaller rank than @var{array}.

@lisp
(transpose-array '#2((a b) (c d)) 1 0) @result{} #2((a c) (b d))
(transpose-array '#2((a b) (c d)) 0 0) @result{} #1(a d)
(transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) @result{}
                #2((a 4) (b 5) (c 6))
@end lisp
@end deffn

@node Arrays as arrays of arrays
@subsubsection Arrays as arrays of arrays

@cindex array cell

One can see an array of rank @math{n} (an
@math{n}-array) as an array of lower rank where the elements are
themselves arrays (`cells').

@cindex array frame
@cindex frame rank

We speak of the first @math{n-k} dimensions of the array as the
@math{n-k}-`frame' of the array, while the last @math{k} dimensions are
the dimensions of the @math{k}-`cells'. For example, a 3-array can be
seen as a 2-array of vectors (1-arrays) or as a 1-array of matrices
(2-arrays). In each case, the vectors or matrices are the 1-cells or
2-cells of the array. This terminology originates in the J language.

@cindex array slice
@cindex prefix slice

The more vague concept of a `slice' refers to a subset of the array
where some indices are fixed and others are left free. As a Guile data
object, a cell is the same as a `prefix slice' (the first @math{n-k}
indices into the original array are fixed), except that a 0-cell is not
a shared array of the original array, but a 0-slice (where all the
indices into the original array are fixed) is.

@cindex enclosed array

Before @w{version 2.0}, Guile had a feature called `enclosed arrays' to
create special `array of arrays' objects. The functions in this section
do not need special types; instead, the frame rank is stated in each
function call, either implicitly or explicitly.

@deffn {Scheme Procedure} array-cell-ref array idx @dots{}
@deffnx {C Function} scm_array_cell_ref (array, idxlist)
If the length of @var{idxlist} equals the rank @math{n} of @var{array},
return the element at @code{(idx @dots{})}, just like @code{(array-ref
array idx @dots{})}. If, however, the length @math{k} of @var{idxlist}
is smaller than @math{n}, then return the @math{(n-k)}-cell of
@var{array} given by @var{idxlist}, as a shared array.

For example:

@lisp
(array-cell-ref #2((a b) (c d)) 0) @result{} #(a b)
(array-cell-ref #2((a b) (c d)) 1) @result{} #(c d)
(array-cell-ref #2((a b) (c d)) 1 1) @result{} d
(array-cell-ref #2((a b) (c d))) @result{} #2((a b) (c d))
@end lisp

@code{(apply array-cell-ref array indices)} is equivalent to

@lisp
(let ((len (length indices)))
  (if (= (array-rank a) len)
    (apply array-ref a indices)
    (apply make-shared-array a
           (lambda t (append indices t))
           (drop (array-dimensions a) len))))
@end lisp

@end deffn

@deffn {Scheme Procedure} array-slice array idx @dots{}
@deffnx {C Function} scm_array_slice (array, idxlist)
Like @code{(array-cell-ref array idx @dots{})}, but return a 0-rank
shared array into @var{ARRAY} if the length of @var{idxlist} matches the
rank of @var{array}. This can be useful when using @var{ARRAY} as a
place to write to.

Compare:

@lisp
(array-cell-ref #2((a b) (c d)) 1 1) @result{} d
(array-slice #2((a b) (c d)) 1 1) @result{} #0(d)
(define a (make-array 'a 2 2))
(array-fill! (array-slice a 1 1) 'b)
a @result{} #2((a a) (a b)).
(array-fill! (array-cell-ref a 1 1) 'b) @result{} error: not an array
@end lisp

@code{(apply array-slice array indices)} is equivalent to

@lisp
(apply make-shared-array a
  (lambda t (append indices t))
  (drop (array-dimensions a) (length indices)))
@end lisp
@end deffn


@deffn {Scheme Procedure} array-cell-set! array x idx @dots{}
@deffnx {C Function} scm_array_cell_set_x (array, x, idxlist)
If the length of @var{idxlist} equals the rank @math{n} of
@var{array}, set the element at @code{(idx @dots{})} of @var{array} to
@var{x}, just like @code{(array-set! array x idx @dots{})}. If,
however, the length @math{k} of @var{idxlist} is smaller than
@math{n}, then copy the @math{(n-k)}-rank array @var{x}
into the @math{(n-k)}-cell of @var{array} given by
@var{idxlist}. In this case, the last @math{(n-k)} dimensions of
@var{array} and the dimensions of @var{x} must match exactly.

This function returns the modified @var{array}.

For example:

@lisp
(array-cell-set! (make-array 'a 2 2) b 1 1)
  @result{} #2((a a) (a b))
(array-cell-set! (make-array 'a 2 2) #(x y) 1)
  @result{} #2((a a) (x y))
@end lisp

Note that @code{array-cell-set!} will expect elements, not arrays, when
the destination has rank 0. Use @code{array-slice} for the opposite
behavior.

@lisp
(array-cell-set! (make-array 'a 2 2) #0(b) 1 1)
  @result{} #2((a a) (a #0(b)))
(let ((a (make-array 'a 2 2)))
  (array-copy! #0(b) (array-slice a 1 1)) a)
  @result{} #2((a a) (a b))
@end lisp

@code{(apply array-cell-set! array x indices)} is equivalent to

@lisp
(let ((len (length indices)))
  (if (= (array-rank array) len)
    (apply array-set! array x indices)
    (array-copy! x (apply array-cell-ref array indices)))
  array)
@end lisp

@end deffn


@deffn {Scheme Procedure} array-slice-for-each frame-rank op x @dots{}
@deffnx {C Function} scm_array_slice_for_each (array, frame_rank, op, xlist)
Each @var{x} must be an array of rank ≥ @var{frame-rank}, and
the first @var{frame-rank} dimensions of each @var{x} must all be the
same. @var{array-slice-for-each} calls @var{op} with each set of
(rank(@var{x}) - @var{frame-rank})-cells from @var{x}, in unspecified order.

@var{array-slice-for-each} allows you to loop over cells of any rank
without having to carry an index list or construct shared arrays
manually. The slices passed to @var{op} are always shared arrays of
@var{X}, even if they are of rank 0, so it is possible to write to them.

This function returns an unspecified value.

For example, to sort the rows of rank-2 array @code{a}:

@lisp
(array-slice-for-each 1 (lambda (x) (sort! x <)) a)
@end lisp

As another example, let @code{a} be a rank-2 array where each row is a
2-element vector @math{(x,y)}.  Let's compute the arguments of these
vectors and store them in rank-1 array @code{b}.
@lisp
(array-slice-for-each 1
  (lambda (a b)
    (array-set! b (atan (array-ref a 1) (array-ref a 0))))
  a b)
@end lisp

@code{(apply array-slice-for-each frame-rank op x)} is equivalent to

@lisp
(let ((frame (take (array-dimensions (car x)) frank)))
  (unless (every (lambda (x)
                   (equal? frame (take (array-dimensions x) frank)))
                 (cdr x))
    (error))
  (array-index-map!
    (apply make-shared-array (make-array #t) (const '()) frame)
    (lambda i (apply op (map (lambda (x) (apply array-slice x i)) x)))))
@end lisp

@end deffn

@deffn {Scheme Procedure} array-slice-for-each-in-order frame-rank op x @dots{}
@deffnx {C Function} scm_array_slice_for_each_in_order (array, frame_rank, op, xlist)
Same as @code{array-slice-for-each}, but the arguments are traversed
sequentially and in row-major order.
@end deffn

@node Accessing Arrays from C
@subsubsection Accessing Arrays from C

For interworking with external C code, Guile provides an API to allow C
code to access the elements of a Scheme array.  In particular, for
uniform numeric arrays, the API exposes the underlying uniform data as a
C array of numbers of the relevant type.

While pointers to the elements of an array are in use, the array itself
must be protected so that the pointer remains valid.  Such a protected
array is said to be @dfn{reserved}.  A reserved array can be read but
modifications to it that would cause the pointer to its elements to
become invalid are prevented.  When you attempt such a modification, an
error is signaled.

(This is similar to locking the array while it is in use, but without
the danger of a deadlock.  In a multi-threaded program, you will need
additional synchronization to avoid modifying reserved arrays.)

You must take care to always unreserve an array after reserving it,
even in the presence of non-local exits.  If a non-local exit can
happen between these two calls, you should install a dynwind context
that releases the array when it is left (@pxref{Dynamic Wind}).

In addition, array reserving and unreserving must be properly
paired.  For instance, when reserving two or more arrays in a certain
order, you need to unreserve them in the opposite order.

Once you have reserved an array and have retrieved the pointer to its
elements, you must figure out the layout of the elements in memory.
Guile allows slices to be taken out of arrays without actually making a
copy, such as making an alias for the diagonal of a matrix that can be
treated as a vector.  Arrays that result from such an operation are not
stored contiguously in memory and when working with their elements
directly, you need to take this into account.

The layout of array elements in memory can be defined via a
@emph{mapping function} that computes a scalar position from a vector of
indices.  The scalar position then is the offset of the element with the
given indices from the start of the storage block of the array.

In Guile, this mapping function is restricted to be @dfn{affine}: all
mapping functions of Guile arrays can be written as @code{p = b +
c[0]*i[0] + c[1]*i[1] + ... + c[n-1]*i[n-1]} where @code{i[k]} is the
@nicode{k}th index and @code{n} is the rank of the array.  For
example, a matrix of size 3x3 would have @code{b == 0}, @code{c[0] ==
3} and @code{c[1] == 1}.  When you transpose this matrix (with
@code{transpose-array}, say), you will get an array whose mapping
function has @code{b == 0}, @code{c[0] == 1} and @code{c[1] == 3}.

The function @code{scm_array_handle_dims} gives you (indirect) access to
the coefficients @code{c[k]}.

@c XXX
Note that there are no functions for accessing the elements of a
character array yet.  Once the string implementation of Guile has been
changed to use Unicode, we will provide them.

@deftp {C Type} scm_t_array_handle
This is a structure type that holds all information necessary to manage
the reservation of arrays as explained above.  Structures of this type
must be allocated on the stack and must only be accessed by the
functions listed below.
@end deftp

@deftypefn {C Function} void scm_array_get_handle (SCM array, scm_t_array_handle *handle)
Reserve @var{array}, which must be an array, and prepare @var{handle} to
be used with the functions below.  You must eventually call
@code{scm_array_handle_release} on @var{handle}, and do this in a
properly nested fashion, as explained above.  The structure pointed to
by @var{handle} does not need to be initialized before calling this
function.
@end deftypefn

@anchor{x-scm_array_handle_release}
@deftypefn {C Function} void scm_array_handle_release (scm_t_array_handle *handle)
End the array reservation represented by @var{handle}.  After a call to
this function, @var{handle} might be used for another reservation.
@end deftypefn

@deftypefn {C Function} size_t scm_array_handle_rank (scm_t_array_handle *handle)
Return the rank of the array represented by @var{handle}.
@end deftypefn

@deftp {C Type} scm_t_array_dim
This structure type holds information about the layout of one dimension
of an array.  It includes the following fields:

@table @code
@item  ssize_t lbnd
@itemx ssize_t ubnd
The lower and upper bounds (both inclusive) of the permissible index
range for the given dimension.  Both values can be negative, but
@var{lbnd} is always less than or equal to @var{ubnd}.

@item ssize_t inc
The distance from one element of this dimension to the next.  Note, too,
that this can be negative.
@end table
@end deftp

@deftypefn {C Function} {const scm_t_array_dim *} scm_array_handle_dims (scm_t_array_handle *handle)
Return a pointer to a C vector of information about the dimensions of
the array represented by @var{handle}.  This pointer is valid as long as
the array remains reserved.  As explained above, the
@code{scm_t_array_dim} structures returned by this function can be used
calculate the position of an element in the storage block of the array
from its indices.

This position can then be used as an index into the C array pointer
returned by the various @code{scm_array_handle_<foo>_elements}
functions, or with @code{scm_array_handle_ref} and
@code{scm_array_handle_set}.

Here is how one can compute the position @var{pos} of an element given
its indices in the vector @var{indices}:

@example
ssize_t indices[RANK];
scm_t_array_dim *dims;
ssize_t pos;
size_t i;

pos = 0;
for (i = 0; i < RANK; i++)
  @{
    if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
      out_of_range ();
    pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
  @}
@end example
@end deftypefn

@deftypefn {C Function} ssize_t scm_array_handle_pos (scm_t_array_handle *handle, SCM indices)
Compute the position corresponding to @var{indices}, a list of
indices.  The position is computed as described above for
@code{scm_array_handle_dims}.  The number of the indices and their
range is checked and an appropriate error is signaled for invalid
indices.
@end deftypefn

@deftypefn {C Function} SCM scm_array_handle_ref (scm_t_array_handle *handle, ssize_t pos)
Return the element at position @var{pos} in the storage block of the
array represented by @var{handle}.  Any kind of array is acceptable.  No
range checking is done on @var{pos}.
@end deftypefn

@deftypefn {C Function} void scm_array_handle_set (scm_t_array_handle *handle, ssize_t pos, SCM val)
Set the element at position @var{pos} in the storage block of the array
represented by @var{handle} to @var{val}.  Any kind of array is
acceptable.  No range checking is done on @var{pos}.  An error is
signaled when the array can not store @var{val}.
@end deftypefn

@deftypefn {C Function} {const SCM *} scm_array_handle_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a ordinary array of general Scheme
values (i.e., a non-uniform array) for reading.  This pointer is valid
as long as the array remains reserved.
@end deftypefn

@deftypefn {C Function} {SCM *} scm_array_handle_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_elements}, but the pointer is good for
reading and writing.
@end deftypefn

@deftypefn {C Function} {const void *} scm_array_handle_uniform_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array for reading.
This pointer is valid as long as the array remains reserved.  The size
of each element is given by @code{scm_array_handle_uniform_element_size}.
@end deftypefn

@deftypefn {C Function} {void *} scm_array_handle_uniform_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_uniform_elements}, but the pointer is good
reading and writing.
@end deftypefn

@deftypefn {C Function} size_t scm_array_handle_uniform_element_size (scm_t_array_handle *handle)
Return the size of one element of the uniform numeric array represented
by @var{handle}.
@end deftypefn

@deftypefn  {C Function} {const scm_t_uint8 *} scm_array_handle_u8_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int8 *} scm_array_handle_s8_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint16 *} scm_array_handle_u16_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int16 *} scm_array_handle_s16_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint32 *} scm_array_handle_u32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int32 *} scm_array_handle_s32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint64 *} scm_array_handle_u64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int64 *} scm_array_handle_s64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const float *} scm_array_handle_f32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const double *} scm_array_handle_f64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const float *} scm_array_handle_c32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const double *} scm_array_handle_c64_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array of the
indicated kind for reading.  This pointer is valid as long as the array
remains reserved.

The pointers for @code{c32} and @code{c64} uniform numeric arrays point
to pairs of floating point numbers.  The even index holds the real part,
the odd index the imaginary part of the complex number.
@end deftypefn

@deftypefn {C Function} {scm_t_uint8 *} scm_array_handle_u8_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int8 *} scm_array_handle_s8_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint16 *} scm_array_handle_u16_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int16 *} scm_array_handle_s16_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint32 *} scm_array_handle_u32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int32 *} scm_array_handle_s32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint64 *} scm_array_handle_u64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int64 *} scm_array_handle_s64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {float *} scm_array_handle_f32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {double *} scm_array_handle_f64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {float *} scm_array_handle_c32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {double *} scm_array_handle_c64_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_<kind>_elements}, but the pointer is good
for reading and writing.
@end deftypefn

@deftypefn {C Function} {const scm_t_uint32 *} scm_array_handle_bit_elements (scm_t_array_handle *handle)
Return a pointer to the words that store the bits of the represented
array, which must be a bit array.

Unlike other arrays, bit arrays have an additional offset that must be
figured into index calculations.  That offset is returned by
@code{scm_array_handle_bit_elements_offset}.

To find a certain bit you first need to calculate its position as
explained above for @code{scm_array_handle_dims} and then add the
offset.  This gives the absolute position of the bit, which is always a
non-negative integer.

Each word of the bit array storage block contains exactly 32 bits, with
the least significant bit in that word having the lowest absolute
position number.  The next word contains the next 32 bits.

Thus, the following code can be used to access a bit whose position
according to @code{scm_array_handle_dims} is given in @var{pos}:

@example
SCM bit_array;
scm_t_array_handle handle;
scm_t_uint32 *bits;
ssize_t pos;
size_t abs_pos;
size_t word_pos, mask;

scm_array_get_handle (&bit_array, &handle);
bits = scm_array_handle_bit_elements (&handle);

pos = ...
abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
word_pos = abs_pos / 32;
mask = 1L << (abs_pos % 32);

if (bits[word_pos] & mask)
  /* bit is set. */

scm_array_handle_release (&handle);
@end example

@end deftypefn

@deftypefn {C Function} {scm_t_uint32 *} scm_array_handle_bit_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_bit_elements} but the pointer is good for
reading and writing.  You must take care not to modify bits outside of
the allowed index range of the array, even for contiguous arrays.
@end deftypefn

@node VLists
@subsection VLists

@cindex vlist

The @code{(ice-9 vlist)} module provides an implementation of the @dfn{VList}
data structure designed by Phil Bagwell in 2002.  VLists are immutable lists,
which can contain any Scheme object.  They improve on standard Scheme linked
lists in several areas:

@itemize
@item
Random access has typically constant-time complexity.

@item
Computing the length of a VList has time complexity logarithmic in the number of
elements.

@item
VLists use less storage space than standard lists.

@item
VList elements are stored in contiguous regions, which improves memory locality
and leads to more efficient use of hardware caches.
@end itemize

The idea behind VLists is to store vlist elements in increasingly large
contiguous blocks (implemented as vectors here).  These blocks are linked to one
another using a pointer to the next block and an offset within that block.  The
size of these blocks form a geometric series with ratio
@code{block-growth-factor} (2 by default).

The VList structure also serves as the basis for the @dfn{VList-based hash
lists} or ``vhashes'', an immutable dictionary type (@pxref{VHashes}).

However, the current implementation in @code{(ice-9 vlist)} has several
noteworthy shortcomings:

@itemize

@item
It is @emph{not} thread-safe.  Although operations on vlists are all
@dfn{referentially transparent} (i.e., purely functional), adding elements to a
vlist with @code{vlist-cons} mutates part of its internal structure, which makes
it non-thread-safe.  This could be fixed, but it would slow down
@code{vlist-cons}.

@item
@code{vlist-cons} always allocates at least as much memory as @code{cons}.
Again, Phil Bagwell describes how to fix it, but that would require tuning the
garbage collector in a way that may not be generally beneficial.

@item
@code{vlist-cons} is a Scheme procedure compiled to bytecode, and it does not
compete with the straightforward C implementation of @code{cons}, and with the
fact that the VM has a special @code{cons} instruction.

@end itemize

We hope to address these in the future.

The programming interface exported by @code{(ice-9 vlist)} is defined below.
Most of it is the same as SRFI-1 with an added @code{vlist-} prefix to function
names.

@deffn {Scheme Procedure} vlist? obj
Return true if @var{obj} is a VList.
@end deffn

@defvr {Scheme Variable} vlist-null
The empty VList.  Note that it's possible to create an empty VList not
@code{eq?} to @code{vlist-null}; thus, callers should always use
@code{vlist-null?} when testing whether a VList is empty.
@end defvr

@deffn {Scheme Procedure} vlist-null? vlist
Return true if @var{vlist} is empty.
@end deffn

@deffn {Scheme Procedure} vlist-cons item vlist
Return a new vlist with @var{item} as its head and @var{vlist} as its tail.
@end deffn

@deffn {Scheme Procedure} vlist-head vlist
Return the head of @var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-tail vlist
Return the tail of @var{vlist}.
@end deffn

@defvr {Scheme Variable} block-growth-factor
A fluid that defines the growth factor of VList blocks, 2 by default.
@end defvr

The functions below provide the usual set of higher-level list operations.

@deffn {Scheme Procedure} vlist-fold proc init vlist
@deffnx {Scheme Procedure} vlist-fold-right proc init vlist
Fold over @var{vlist}, calling @var{proc} for each element, as for SRFI-1
@code{fold} and @code{fold-right} (@pxref{SRFI-1, @code{fold}}).
@end deffn

@deffn {Scheme Procedure} vlist-ref vlist index
Return the element at index @var{index} in @var{vlist}.  This is typically a
constant-time operation.
@end deffn

@deffn {Scheme Procedure} vlist-length vlist
Return the length of @var{vlist}.  This is typically logarithmic in the number
of elements in @var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-reverse vlist
Return a new @var{vlist} whose content are those of @var{vlist} in reverse
order.
@end deffn

@deffn {Scheme Procedure} vlist-map proc vlist
Map @var{proc} over the elements of @var{vlist} and return a new vlist.
@end deffn

@deffn {Scheme Procedure} vlist-for-each proc vlist
Call @var{proc} on each element of @var{vlist}.  The result is unspecified.
@end deffn

@deffn {Scheme Procedure} vlist-drop vlist count
Return a new vlist that does not contain the @var{count} first elements of
@var{vlist}.  This is typically a constant-time operation.
@end deffn

@deffn {Scheme Procedure} vlist-take vlist count
Return a new vlist that contains only the @var{count} first elements of
@var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-filter pred vlist
Return a new vlist containing all the elements from @var{vlist} that satisfy
@var{pred}.
@end deffn

@deffn {Scheme Procedure} vlist-delete x vlist [equal?]
Return a new vlist corresponding to @var{vlist} without the elements
@var{equal?} to @var{x}.
@end deffn

@deffn {Scheme Procedure} vlist-unfold p f g seed [tail-gen]
@deffnx {Scheme Procedure} vlist-unfold-right p f g seed [tail]
Return a new vlist, as for SRFI-1 @code{unfold} and @code{unfold-right}
(@pxref{SRFI-1, @code{unfold}}).
@end deffn

@deffn {Scheme Procedure} vlist-append vlist @dots{}
Append the given vlists and return the resulting vlist.
@end deffn

@deffn {Scheme Procedure} list->vlist lst
Return a new vlist whose contents correspond to @var{lst}.
@end deffn

@deffn {Scheme Procedure} vlist->list vlist
Return a new list whose contents match those of @var{vlist}.
@end deffn

@node Record Overview
@subsection Record Overview

@cindex record
@cindex structure

@dfn{Records}, also called @dfn{structures}, are Scheme's primary
mechanism to define new disjoint types.  A @dfn{record type} defines a
list of @dfn{fields} that instances of the type consist of.  This is like
C's @code{struct}.

Historically, Guile has offered several different ways to define record
types and to create records, offering different features, and making
different trade-offs.  Over the years, each ``standard'' has also come
with its own new record interface, leading to a maze of record APIs.

At the highest level is SRFI-9, a high-level record interface
implemented by most Scheme implementations (@pxref{SRFI-9 Records}).  It
defines a simple and efficient syntactic abstraction of record types and
their associated type predicate, fields, and field accessors.  SRFI-9 is
suitable for most uses, and this is the recommended way to create record
types in Guile.  Similar high-level record APIs include SRFI-35
(@pxref{SRFI-35}) and R6RS records (@pxref{rnrs records syntactic}).

Then comes Guile's historical ``records'' API (@pxref{Records}).  Record
types defined this way are first-class objects.  Introspection
facilities are available, allowing users to query the list of fields or
the value of a specific field at run-time, without prior knowledge of
the type.

Finally, the common denominator of these interfaces is Guile's
@dfn{structure} API (@pxref{Structures}).  Guile's structures are the
low-level building block for all other record APIs.  Application writers
will normally not need to use it.

Records created with these APIs may all be pattern-matched using Guile's
standard pattern matcher (@pxref{Pattern Matching}).


@node SRFI-9 Records
@subsection SRFI-9 Records

@cindex SRFI-9
@cindex record

SRFI-9 standardizes a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions.  In Guile
this is the recommended option to create new record types (@pxref{Record
Overview}).  It can be used with:

@example
(use-modules (srfi srfi-9))
@end example

@deffn {Scheme Syntax} define-record-type type @* (constructor fieldname @dots{}) @* predicate @* (fieldname accessor [modifier]) @dots{}
@sp 1
Create a new record type, and make various @code{define}s for using
it.  This syntax can only occur at the top-level, not nested within
some other form.

@var{type} is bound to the record type, which is as per the return
from the core @code{make-record-type}.  @var{type} also provides the
name for the record, as per @code{record-type-name}.

@var{constructor} is bound to a function to be called as
@code{(@var{constructor} fieldval @dots{})} to create a new record of
this type.  The arguments are initial values for the fields, one
argument for each field, in the order they appear in the
@code{define-record-type} form.

The @var{fieldname}s provide the names for the record fields, as per
the core @code{record-type-fields} etc, and are referred to in the
subsequent accessor/modifier forms.

@var{predicate} is bound to a function to be called as
@code{(@var{predicate} obj)}.  It returns @code{#t} or @code{#f}
according to whether @var{obj} is a record of this type.

Each @var{accessor} is bound to a function to be called
@code{(@var{accessor} record)} to retrieve the respective field from a
@var{record}.  Similarly each @var{modifier} is bound to a function to
be called @code{(@var{modifier} record val)} to set the respective
field in a @var{record}.
@end deffn

@noindent
An example will illustrate typical usage,

@example
(define-record-type <employee>
  (make-employee name age salary)
  employee?
  (name    employee-name)
  (age     employee-age    set-employee-age!)
  (salary  employee-salary set-employee-salary!))
@end example

This creates a new employee data type, with name, age and salary
fields.  Accessor functions are created for each field, but no
modifier function for the name (the intention in this example being
that it's established only when an employee object is created).  These
can all then be used as for example,

@example
<employee> @result{} #<record-type <employee>>

(define fred (make-employee "Fred" 45 20000.00))

(employee? fred)        @result{} #t
(employee-age fred)     @result{} 45
(set-employee-salary! fred 25000.00)  ;; pay rise
@end example

The functions created by @code{define-record-type} are ordinary
top-level @code{define}s.  They can be redefined or @code{set!} as
desired, exported from a module, etc.

@unnumberedsubsubsec Non-toplevel Record Definitions

The SRFI-9 specification explicitly disallows record definitions in a
non-toplevel context, such as inside @code{lambda} body or inside a
@var{let} block.  However, Guile's implementation does not enforce that
restriction.

@unnumberedsubsubsec Custom Printers

You may use @code{set-record-type-printer!} to customize the default printing
behavior of records.  This is a Guile extension and is not part of SRFI-9.  It
is located in the @nicode{(srfi srfi-9 gnu)} module.

@deffn {Scheme Syntax} set-record-type-printer! type proc
Where @var{type} corresponds to the first argument of @code{define-record-type},
and @var{proc} is a procedure accepting two arguments, the record to print, and
an output port.
@end deffn

@noindent
This example prints the employee's name in brackets, for instance @code{[Fred]}.

@example
(set-record-type-printer! <employee>
  (lambda (record port)
    (write-char #\[ port)
    (display (employee-name record) port)
    (write-char #\] port)))
@end example

@unnumberedsubsubsec Functional ``Setters''

@cindex functional setters

When writing code in a functional style, it is desirable to never alter
the contents of records.  For such code, a simple way to return new
record instances based on existing ones is highly desirable.

The @code{(srfi srfi-9 gnu)} module extends SRFI-9 with facilities to
return new record instances based on existing ones, only with one or
more field values changed---@dfn{functional setters}.  First, the
@code{define-immutable-record-type} works like
@code{define-record-type}, except that fields are immutable and setters
are defined as functional setters.

@deffn {Scheme Syntax} define-immutable-record-type type @* (constructor fieldname @dots{}) @* predicate @* (fieldname accessor [modifier]) @dots{}
Define @var{type} as a new record type, like @code{define-record-type}.
However, the record type is made @emph{immutable} (records may not be
mutated, even with @code{struct-set!}), and any @var{modifier} is
defined to be a functional setter---a procedure that returns a new
record instance with the specified field changed, and leaves the
original unchanged (see example below.)
@end deffn

@noindent
In addition, the generic @code{set-field} and @code{set-fields} macros
may be applied to any SRFI-9 record.

@deffn {Scheme Syntax} set-field record (field sub-fields ...) value
Return a new record of @var{record}'s type whose fields are equal to
the corresponding fields of @var{record} except for the one specified by
@var{field}.

@var{field} must be the name of the getter corresponding to the field of
@var{record} being ``set''.  Subsequent @var{sub-fields} must be record
getters designating sub-fields within that field value to be set (see
example below.)
@end deffn

@deffn {Scheme Syntax} set-fields record ((field sub-fields ...) value) ...
Like @code{set-field}, but can be used to set more than one field at a
time.  This expands to code that is more efficient than a series of
single @code{set-field} calls.
@end deffn

To illustrate the use of functional setters, let's assume these two
record type definitions:

@example
(define-record-type <address>
  (address street city country)
  address?
  (street  address-street)
  (city    address-city)
  (country address-country))

(define-immutable-record-type <person>
  (person age email address)
  person?
  (age     person-age set-person-age)
  (email   person-email set-person-email)
  (address person-address set-person-address))
@end example

@noindent
First, note that the @code{<person>} record type definition introduces
named functional setters.  These may be used like this:

@example
(define fsf-address
  (address "Franklin Street" "Boston" "USA"))

(define rms
  (person 30 "rms@@gnu.org" fsf-address))

(and (equal? (set-person-age rms 60)
             (person 60 "rms@@gnu.org" fsf-address))
     (= (person-age rms) 30))
@result{} #t
@end example

@noindent
Here, the original @code{<person>} record, to which @var{rms} is bound,
is left unchanged.

Now, suppose we want to change both the street and age of @var{rms}.
This can be achieved using @code{set-fields}:

@example
(set-fields rms
  ((person-age) 60)
  ((person-address address-street) "Temple Place"))
@result{} #<<person> age: 60 email: "rms@@gnu.org"
  address: #<<address> street: "Temple Place" city: "Boston" country: "USA">>
@end example

@noindent
Notice how the above changed two fields of @var{rms}, including the
@code{street} field of its @code{address} field, in a concise way.  Also
note that @code{set-fields} works equally well for types defined with
just @code{define-record-type}.

@node Records
@subsection Records

A @dfn{record type} is a first class object representing a user-defined
data type.  A @dfn{record} is an instance of a record type.

Note that in many ways, this interface is too low-level for every-day
use.  Most uses of records are better served by SRFI-9 records.
@xref{SRFI-9 Records}.

@deffn {Scheme Procedure} record? obj
Return @code{#t} if @var{obj} is a record of any type and @code{#f}
otherwise.

Note that @code{record?} may be true of any Scheme value; there is no
promise that records are disjoint with other Scheme types.
@end deffn

@deffn {Scheme Procedure} make-record-type type-name field-names [print] @
       [#:parent=@code{#f}] [#:uid=@code{#f}] @
       [#:extensible?=@code{#f}] [#:opaque?=@code{#f}] @
       [#:allow-duplicate-field-names?=@code{#t}]
Create and return a new @dfn{record-type descriptor}.

@var{type-name} is a string naming the type.  Currently it's only used
in the printed representation of records, and in diagnostics.
@var{field-names} is a list of elements of the form @code{(immutable
@var{name})}, @code{(mutable @var{name})}, or @var{name}, where
@var{name} are symbols naming the fields of a record of the type.
Duplicates are not allowed among these symbols, unless
@var{allow-duplicate-field-names?} is true.

@example
(make-record-type "employee" '(name age salary))
@end example

The optional @var{print} argument is a function used by
@code{display}, @code{write}, etc, for printing a record of the new
type.  It's called as @code{(@var{print} record port)} and should look
at @var{record} and write to @var{port}.

Pass the @code{#:parent} keyword to derive a record type from a
supertype.  A derived record type has the fields from its parent type,
followed by fields declared in the @code{make-record-type} call.  Record
predicates and field accessors for instance of a parent type will also
work on any instance of a subtype.

@cindex extensible record types
@cindex record types, extensible
Allowing record subtyping has a small amount of overhead.  To avoid this
overhead, prevent extensibility by passing @code{#:extensible? #f}.
By default, record types in Guile are not extensible.

@cindex prefab record types
@cindex record types, prefab
@cindex record types, nongenerative
Generally speaking, calling @code{make-record-type} returns a fresh
record type; it @emph{generates} new record types.  However sometimes
you only want to define a record type if one hasn't been defined
already.  For a @emph{nongenerative} record type definition, pass a
symbol as the @code{#:uid} keyword parameter.  If a record with the
given @var{uid} was already defined, it will be returned instead.  The
type name, fields, parent (if any), and so on for the previously-defined
type must be compatible.

@cindex record types, opaque
R6RS defines a notion of ``opaque'' record types.  Given an instance of
an opaque record type, one cannot obtain a run-time representation of
the record type.  @xref{rnrs records procedural}, for full details.  The
@code{#:opaque?} flag is used by Guile's R6RS layer to record this
information.  The default is determined by whether the parent type, if
any, was opaque.

Fields are mutable by default, meaning that @code{record-modifier} will
return a procedure that can update a record in place.  Specifying a
field using the form @code{(immutable @var{name})} instead marks a field
as immutable.
@end deffn

@deffn {Scheme Procedure} record-constructor rtd
Return a procedure for constructing new members of the type represented
by @var{rtd}.  The result will be a procedure accepting exactly as many
arguments as there are fields in the record type.
@end deffn

@deffn {Scheme Procedure} record-predicate rtd
Return a procedure for testing membership in the type represented by
@var{rtd}.  The returned procedure accepts exactly one argument and
returns a true value if the argument is a member of the indicated record
type; it returns a false value otherwise.
@end deffn

@deffn {Scheme Procedure} record-accessor rtd field-name
Return a procedure for reading the value of a particular field of a
member of the type represented by @var{rtd}.  The returned procedure
accepts exactly one argument which must be a record of the appropriate
type; it returns the current value of the field named by the symbol
@var{field-name} in that record.

If @var{field-name} is a symbol, it must be a member of the list of
field-names in the call to @code{make-record-type} that created the type
represented by @var{rtd}.  If multiple fields in @var{rtd} have the same
name, @code{record-accessor} returns the first one.

If @var{field-name} is an integer, it should be an index into
@code{(record-type-fields @var{rtd})}.  This allows accessing fields
with duplicate names.
@end deffn

@deffn {Scheme Procedure} record-modifier rtd field-name
Return a procedure for writing the value of a particular field of a
member of the type represented by @var{rtd}.  The returned procedure
accepts exactly two arguments: first, a record of the appropriate type,
and second, an arbitrary Scheme value; it modifies the field named by
the symbol @var{field-name} in that record to contain the given value.
The returned value of the modifier procedure is unspecified.  The symbol
@var{field-name} is a field name or a field index, as in
@code{record-modifier}.
@end deffn

@deffn {Scheme Procedure} record-type-descriptor record
Return a record-type descriptor representing the type of the given
record.  That is, for example, if the returned descriptor were passed to
@code{record-predicate}, the resulting predicate would return a true
value when passed the given record.  Note that it is not necessarily the
case that the returned descriptor is the one that was passed to
@code{record-constructor} in the call that created the constructor
procedure that created the given record.
@end deffn

@deffn {Scheme Procedure} record-type-name rtd
Return the type-name associated with the type represented by rtd.  The
returned value is @code{eqv?} to the @var{type-name} argument given in
the call to @code{make-record-type} that created the type represented by
@var{rtd}.
@end deffn

@deffn {Scheme Procedure} record-type-fields rtd
Return a list of the symbols naming the fields in members of the type
represented by @var{rtd}.  The returned value is @code{equal?} to the
field-names argument given in the call to @code{make-record-type} that
created the type represented by @var{rtd}.
@end deffn


@node Structures
@subsection Structures
@tpindex Structures

A @dfn{structure} is a first class data type which holds Scheme values
or C words in fields numbered 0 upwards.  A @dfn{vtable} is a structure
that represents a structure type, giving field types and permissions,
and an optional print function for @code{write} etc.

Structures are lower level than records (@pxref{Records}).  Usually,
when you need to represent structured data, you just want to use
records.  But sometimes you need to implement new kinds of structured
data abstractions, and for that purpose structures are useful.  Indeed,
records in Guile are implemented with structures.

@menu
* Vtables::
* Structure Basics::
* Vtable Contents::
* Meta-Vtables::
* Vtable Example::
@end menu

@node Vtables
@subsubsection Vtables

A vtable is a structure type, specifying its layout, and other
information.  A vtable is actually itself a structure, but there's no
need to worry about that initially (@pxref{Vtable Contents}.)

@deffn {Scheme Procedure} make-vtable fields [print]
Create a new vtable.

@var{fields} is a string describing the fields in the structures to be
created.  Each field is represented by two characters, a type letter
and a permissions letter, for example @code{"pw"}.  The types are as
follows.

@itemize @bullet{}
@item
@code{p} -- a Scheme value.  ``p'' stands for ``protected'' meaning
it's protected against garbage collection.

@item
@code{u} -- an arbitrary word of data (an @code{scm_t_bits}).  At the
Scheme level it's read and written as an unsigned integer.  ``u'' stands
for ``unboxed'', as it's stored as a raw value without additional type
annotations.
@end itemize

It used to be that the second letter for each field was a permission
code, such as @code{w} for writable or @code{r} for read-only.  However
over time structs have become more of a raw low-level facility; access
control is better implemented as a layer on top.  After all,
@code{struct-set!} is a cross-cutting operator that can bypass
abstractions made by higher-level record facilities; it's not generally
safe (in the sense of abstraction-preserving) to expose
@code{struct-set!} to ``untrusted'' code, even if the fields happen to
be writable.  Additionally, permission checks added overhead to every
structure access in a way that couldn't be optimized out, hampering the
ability of structs to act as a low-level building block.  For all of
these reasons, all fields in Guile structs are now writable; attempting
to make a read-only field will now issue a deprecation warning, and the
field will be writable regardless.

@example
(make-vtable "pw")      ;; one scheme field
(make-vtable "pwuwuw")  ;; one scheme and two unboxed fields
@end example

The optional @var{print} argument is a function called by
@code{display} and @code{write} (etc) to give a printed representation
of a structure created from this vtable.  It's called
@code{(@var{print} struct port)} and should look at @var{struct} and
write to @var{port}.  The default print merely gives a form like
@samp{#<struct ADDR:ADDR>} with a pair of machine addresses.

The following print function for example shows the two fields of its
structure.

@example
(make-vtable "pwpw"
             (lambda (struct port)
               (format port "#<~a and ~a>"
                       (struct-ref struct 0)
                       (struct-ref struct 1))))
@end example
@end deffn


@node Structure Basics
@subsubsection Structure Basics

This section describes the basic procedures for working with structures.
@code{make-struct/no-tail} creates a structure, and @code{struct-ref}
and @code{struct-set!} access its fields.

@deffn {Scheme Procedure} make-struct/no-tail vtable init @dots{}
Create a new structure, with layout per the given @var{vtable}
(@pxref{Vtables}).

The optional @var{init}@dots{} arguments are initial values for the
fields of the structure.  This is the only way to
put values in read-only fields.  If there are fewer @var{init}
arguments than fields then the defaults are @code{#f} for a Scheme
field (type @code{p}) or 0 for an unboxed field (type @code{u}).

The name is a bit strange, we admit.  The reason for it is that Guile
used to have a @code{make-struct} that took an additional argument;
while we deprecate that old interface, @code{make-struct/no-tail} is the
new name for this functionality.

For example,

@example
(define v (make-vtable "pwpwpw"))
(define s (make-struct/no-tail v 123 "abc" 456))
(struct-ref s 0) @result{} 123
(struct-ref s 1) @result{} "abc"
@end example
@end deffn

@deftypefn {C Function} SCM scm_make_struct (SCM vtable, SCM tail_size, SCM init_list)
@deftypefnx {C Function} SCM scm_c_make_struct (SCM vtable, SCM tail_size, SCM init, ...)
@deftypefnx {C Function} SCM scm_c_make_structv (SCM vtable, SCM tail_size, size_t n_inits, scm_t_bits init[])
There are a few ways to make structures from C.  @code{scm_make_struct}
takes a list, @code{scm_c_make_struct} takes variable arguments
terminated with SCM_UNDEFINED, and @code{scm_c_make_structv} takes a
packed array.

For all of these, @var{tail_size} should be zero (as a SCM value).
@end deftypefn

@deffn {Scheme Procedure} struct? obj
@deffnx {C Function} scm_struct_p (obj)
Return @code{#t} if @var{obj} is a structure, or @code{#f} if not.
@end deffn

@deffn {Scheme Procedure} struct-ref struct n
@deffnx {C Function} scm_struct_ref (struct, n)
Return the contents of field number @var{n} in @var{struct}.  The
first field is number 0.

An error is thrown if @var{n} is out of range.
@end deffn

@deffn {Scheme Procedure} struct-set! struct n value
@deffnx {C Function} scm_struct_set_x (struct, n, value)
Set field number @var{n} in @var{struct} to @var{value}.  The first
field is number 0.

An error is thrown if @var{n} is out of range, or if the field cannot
be written because it's @code{r} read-only.
@end deffn

Unboxed fields (those with type @code{u}) need to be accessed with
special procedures.

@deffn {Scheme Procedure} struct-ref/unboxed struct n
@deffnx {Scheme Procedure} struct-set!/unboxed struct n value
@deffnx {C Function} scm_struct_ref_unboxed (struct, n)
@deffnx {C Function} scm_struct_set_x_unboxed (struct, n, value)
Like @code{struct-ref} and @code{struct-set!}, except that these may
only be used on unboxed fields.  @code{struct-ref/unboxed} will always
return a positive integer.  Likewise, @code{struct-set!/unboxed} takes
an unsigned integer as the @var{value} argument, and will signal an
error otherwise.
@end deffn

@deffn {Scheme Procedure} struct-vtable struct
@deffnx {C Function} scm_struct_vtable (struct)
Return the vtable that describes @var{struct}.

The vtable is effectively the type of the structure.  See @ref{Vtable
Contents}, for more on vtables.
@end deffn


@node Vtable Contents
@subsubsection Vtable Contents

A vtable is itself a structure.  It has a specific set of fields
describing various aspects of its @dfn{instances}: the structures
created from a vtable.  Some of the fields are internal to Guile, some
of them are part of the public interface, and there may be additional
fields added on by the user.

Every vtable has a field for the layout of their instances, a field for
the procedure used to print its instances, and a field for the name of
the vtable itself.  Access to the layout and printer is exposed directly
via field indexes.  Access to the vtable name is exposed via accessor
procedures.

@defvr {Scheme Variable} vtable-index-layout
@defvrx {C Macro} scm_vtable_index_layout
The field number of the layout specification in a vtable.  The layout
specification is a symbol like @code{pwpw} formed from the fields
string passed to @code{make-vtable}, or created by
@code{make-struct-layout} (@pxref{Meta-Vtables}).

@example
(define v (make-vtable "pwpw" 0))
(struct-ref v vtable-index-layout) @result{} pwpw
@end example

This field is read-only, since the layout of structures using a vtable
cannot be changed.
@end defvr

@defvr {Scheme Variable} vtable-index-printer
@defvrx {C Macro} scm_vtable_index_printer
The field number of the printer function.  This field contains @code{#f}
if the default print function should be used.

@example
(define (my-print-func struct port)
  ...)
(define v (make-vtable "pwpw" my-print-func))
(struct-ref v vtable-index-printer) @result{} my-print-func
@end example

This field is writable, allowing the print function to be changed
dynamically.
@end defvr

@deffn {Scheme Procedure} struct-vtable-name vtable
@deffnx {Scheme Procedure} set-struct-vtable-name! vtable name
@deffnx {C Function} scm_struct_vtable_name (vtable)
@deffnx {C Function} scm_set_struct_vtable_name_x (vtable, name)
Get or set the name of @var{vtable}.  @var{name} is a symbol and is
used in the default print function when printing structures created
from @var{vtable}.

@example
(define v (make-vtable "pw"))
(set-struct-vtable-name! v 'my-name)

(define s (make-struct v 0))
(display s) @print{} #<my-name b7ab3ae0:b7ab3730>
@end example
@end deffn


@node Meta-Vtables
@subsubsection Meta-Vtables

As a structure, a vtable also has a vtable, which is also a structure.
Structures, their vtables, the vtables of the vtables, and so on form a
tree of structures.  Making a new structure adds a leaf to the tree, and
if that structure is a vtable, it may be used to create other leaves.

If you traverse up the tree of vtables, via calling
@code{struct-vtable}, eventually you reach a root which is the vtable of
itself:

@example
scheme@@(guile-user)> (current-module)
$1 = #<directory (guile-user) 221b090>
scheme@@(guile-user)> (struct-vtable $1)
$2 = #<record-type module>
scheme@@(guile-user)> (struct-vtable $2)
$3 = #<<standard-vtable> 12c30a0>
scheme@@(guile-user)> (struct-vtable $3)
$4 = #<<standard-vtable> 12c3fa0>
scheme@@(guile-user)> (struct-vtable $4)
$5 = #<<standard-vtable> 12c3fa0>
scheme@@(guile-user)> <standard-vtable>
$6 = #<<standard-vtable> 12c3fa0>
@end example

In this example, we can say that @code{$1} is an instance of @code{$2},
@code{$2} is an instance of @code{$3}, @code{$3} is an instance of
@code{$4}, and @code{$4}, strangely enough, is an instance of itself.
The value bound to @code{$4} in this console session also bound to
@code{<standard-vtable>} in the default environment.

@defvr {Scheme Variable} <standard-vtable>
A meta-vtable, useful for making new vtables.
@end defvr

All of these values are structures.  All but @code{$1} are vtables.  As
@code{$2} is an instance of @code{$3}, and @code{$3} is a vtable, we can
say that @code{$3} is a @dfn{meta-vtable}: a vtable that can create
vtables.

With this definition, we can specify more precisely what a vtable is: a
vtable is a structure made from a meta-vtable.  Making a structure from
a meta-vtable runs some special checks to ensure that the first field of
the structure is a valid layout.  Additionally, if these checks see that
the layout of the child vtable contains all the required fields of a
vtable, in the correct order, then the child vtable will also be a
meta-table, inheriting a magical bit from the parent.

@deffn {Scheme Procedure} struct-vtable? obj
@deffnx {C Function} scm_struct_vtable_p (obj)
Return @code{#t} if @var{obj} is a vtable structure: an instance of a
meta-vtable.
@end deffn

@code{<standard-vtable>} is a root of the vtable tree.  (Normally there
is only one root in a given Guile process, but due to some legacy
interfaces there may be more than one.)

The set of required fields of a vtable is the set of fields in the
@code{<standard-vtable>}, and is bound to @code{standard-vtable-fields}
in the default environment.  It is possible to create a meta-vtable that
with additional fields in its layout, which can be used to create
vtables with additional data:

@example
scheme@@(guile-user)> (struct-ref $3 vtable-index-layout)
$6 = pwuhuhpwphuhuhpwpwpw
scheme@@(guile-user)> (struct-ref $4 vtable-index-layout)
$7 = pwuhuhpwphuhuh
scheme@@(guile-user)> standard-vtable-fields 
$8 = "pwuhuhpwphuhuh"
scheme@@(guile-user)> (struct-ref $2 vtable-offset-user)
$9 = module
@end example

In this continuation of our earlier example, @code{$2} is a vtable that
has extra fields, because its vtable, @code{$3}, was made from a
meta-vtable with an extended layout.  @code{vtable-offset-user} is a
convenient definition that indicates the number of fields in
@code{standard-vtable-fields}.

@defvr {Scheme Variable} standard-vtable-fields
A string containing the ordered set of fields that a vtable must have.
@end defvr

@defvr {Scheme Variable} vtable-offset-user
The first index in a vtable that is available for a user.
@end defvr

@deffn {Scheme Procedure} make-struct-layout fields
@deffnx {C Function} scm_make_struct_layout (fields)
Return a structure layout symbol, from a @var{fields} string.
@var{fields} is as described under @code{make-vtable}
(@pxref{Vtables}).  An invalid @var{fields} string is an error.
@end deffn

With these definitions, one can define @code{make-vtable} in this way:

@example
(define* (make-vtable fields #:optional printer)
  (make-struct/no-tail <standard-vtable>
    (make-struct-layout fields)
    printer))
@end example


@node Vtable Example
@subsubsection Vtable Example

Let us bring these points together with an example.  Consider a simple
object system with single inheritance.  Objects will be normal
structures, and classes will be vtables with three extra class fields:
the name of the class, the parent class, and the list of fields.

So, first we need a meta-vtable that allocates instances with these
extra class fields.

@example
(define <class>
  (make-vtable
   (string-append standard-vtable-fields "pwpwpw")
   (lambda (x port)
     (format port "<<class> ~a>" (class-name x)))))

(define (class? x)
  (and (struct? x)
       (eq? (struct-vtable x) <class>)))
@end example

To make a structure with a specific meta-vtable, we will use
@code{make-struct/no-tail}, passing it the computed instance layout and
printer, as with @code{make-vtable}, and additionally the extra three
class fields.

@example
(define (make-class name parent fields)
  (let* ((fields (compute-fields parent fields))
         (layout (compute-layout fields)))
    (make-struct/no-tail <class>
      layout 
      (lambda (x port)
        (print-instance x port))
      name
      parent
      fields)))
@end example

Instances will store their associated data in slots in the structure: as
many slots as there are fields.  The @code{compute-layout} procedure
below can compute a layout, and @code{field-index} returns the slot
corresponding to a field.

@example
(define-syntax-rule (define-accessor name n)
  (define (name obj)
    (struct-ref obj n)))

;; Accessors for classes
(define-accessor class-name (+ vtable-offset-user 0))
(define-accessor class-parent (+ vtable-offset-user 1))
(define-accessor class-fields (+ vtable-offset-user 2))

(define (compute-fields parent fields)
  (if parent
      (append (class-fields parent) fields)
      fields))

(define (compute-layout fields)
  (make-struct-layout
   (string-concatenate (make-list (length fields) "pw"))))

(define (field-index class field)
  (list-index (class-fields class) field))

(define (print-instance x port)
  (format port "<~a" (class-name (struct-vtable x)))
  (for-each (lambda (field idx)
              (format port " ~a: ~a" field (struct-ref x idx)))
            (class-fields (struct-vtable x))
            (iota (length (class-fields (struct-vtable x)))))
  (format port ">"))
@end example

So, at this point we can actually make a few classes:

@example
(define-syntax-rule (define-class name parent field ...)
  (define name (make-class 'name parent '(field ...))))

(define-class <surface> #f
  width height)

(define-class <window> <surface>
  x y)
@end example

And finally, make an instance:

@example
(make-struct/no-tail <window> 400 300 10 20)
@result{} <<window> width: 400 height: 300 x: 10 y: 20>
@end example

And that's that.  Note that there are many possible optimizations and
feature enhancements that can be made to this object system, and the
included GOOPS system does make most of them.  For more simple use
cases, the records facility is usually sufficient.  But sometimes you
need to make new kinds of data abstractions, and for that purpose,
structs are here.


@node Dictionary Types
@subsection Dictionary Types

A @dfn{dictionary} object is a data structure used to index
information in a user-defined way.  In standard Scheme, the main
aggregate data types are lists and vectors.  Lists are not really
indexed at all, and vectors are indexed only by number
(e.g.@: @code{(vector-ref foo 5)}).  Often you will find it useful
to index your data on some other type; for example, in a library
catalog you might want to look up a book by the name of its
author.  Dictionaries are used to help you organize information in
such a way.

An @dfn{association list} (or @dfn{alist} for short) is a list of
key-value pairs.  Each pair represents a single quantity or
object; the @code{car} of the pair is a key which is used to
identify the object, and the @code{cdr} is the object's value.

A @dfn{hash table} also permits you to index objects with
arbitrary keys, but in a way that makes looking up any one object
extremely fast.  A well-designed hash system makes hash table
lookups almost as fast as conventional array or vector references.

Alists are popular among Lisp programmers because they use only
the language's primitive operations (lists, @dfn{car}, @dfn{cdr}
and the equality primitives).  No changes to the language core are
necessary.  Therefore, with Scheme's built-in list manipulation
facilities, it is very convenient to handle data stored in an
association list.  Also, alists are highly portable and can be
easily implemented on even the most minimal Lisp systems.

However, alists are inefficient, especially for storing large
quantities of data.  Because we want Guile to be useful for large
software systems as well as small ones, Guile provides a rich set
of tools for using either association lists or hash tables.

@node Association Lists
@subsection Association Lists
@tpindex Association Lists
@tpindex Alist
@cindex association List
@cindex alist
@cindex database

An association list is a conventional data structure that is often used
to implement simple key-value databases.  It consists of a list of
entries in which each entry is a pair.  The @dfn{key} of each entry is
the @code{car} of the pair and the @dfn{value} of each entry is the
@code{cdr}.

@example
ASSOCIATION LIST ::=  '( (KEY1 . VALUE1)
                         (KEY2 . VALUE2)
                         (KEY3 . VALUE3)
                         @dots{}
                       )
@end example

@noindent
Association lists are also known, for short, as @dfn{alists}.

The structure of an association list is just one example of the infinite
number of possible structures that can be built using pairs and lists.
As such, the keys and values in an association list can be manipulated
using the general list structure procedures @code{cons}, @code{car},
@code{cdr}, @code{set-car!}, @code{set-cdr!} and so on.  However,
because association lists are so useful, Guile also provides specific
procedures for manipulating them.

@menu
* Alist Key Equality::
* Adding or Setting Alist Entries::
* Retrieving Alist Entries::
* Removing Alist Entries::
* Sloppy Alist Functions::
* Alist Example::
@end menu

@node Alist Key Equality
@subsubsection Alist Key Equality

All of Guile's dedicated association list procedures, apart from
@code{acons}, come in three flavours, depending on the level of equality
that is required to decide whether an existing key in the association
list is the same as the key that the procedure call uses to identify the
required entry.

@itemize @bullet
@item
Procedures with @dfn{assq} in their name use @code{eq?} to determine key
equality.

@item
Procedures with @dfn{assv} in their name use @code{eqv?} to determine
key equality.

@item
Procedures with @dfn{assoc} in their name use @code{equal?} to
determine key equality.
@end itemize

@code{acons} is an exception because it is used to build association
lists which do not require their entries' keys to be unique.

@node Adding or Setting Alist Entries
@subsubsection Adding or Setting Alist Entries

@code{acons} adds a new entry to an association list and returns the
combined association list.  The combined alist is formed by consing the
new entry onto the head of the alist specified in the @code{acons}
procedure call.  So the specified alist is not modified, but its
contents become shared with the tail of the combined alist that
@code{acons} returns.

In the most common usage of @code{acons}, a variable holding the
original association list is updated with the combined alist:

@example
(set! address-list (acons name address address-list))
@end example

In such cases, it doesn't matter that the old and new values of
@code{address-list} share some of their contents, since the old value is
usually no longer independently accessible.

Note that @code{acons} adds the specified new entry regardless of
whether the alist may already contain entries with keys that are, in
some sense, the same as that of the new entry.  Thus @code{acons} is
ideal for building alists where there is no concept of key uniqueness.

@example
(set! task-list (acons 3 "pay gas bill" '()))
task-list
@result{}
((3 . "pay gas bill"))

(set! task-list (acons 3 "tidy bedroom" task-list))
task-list
@result{}
((3 . "tidy bedroom") (3 . "pay gas bill"))
@end example

@code{assq-set!}, @code{assv-set!} and @code{assoc-set!} are used to add
or replace an entry in an association list where there @emph{is} a
concept of key uniqueness.  If the specified association list already
contains an entry whose key is the same as that specified in the
procedure call, the existing entry is replaced by the new one.
Otherwise, the new entry is consed onto the head of the old association
list to create the combined alist.  In all cases, these procedures
return the combined alist.

@code{assq-set!} and friends @emph{may} destructively modify the
structure of the old association list in such a way that an existing
variable is correctly updated without having to @code{set!} it to the
value returned:

@example
address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "16 Bow Street"))

(assoc-set! address-list "james" "1a London Road")
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))

address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
@end example

Or they may not:

@example
(assoc-set! address-list "bob" "11 Newington Avenue")
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
@end example

The only safe way to update an association list variable when adding or
replacing an entry like this is to @code{set!} the variable to the
returned value:

@example
(set! address-list
      (assoc-set! address-list "bob" "11 Newington Avenue"))
address-list
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))
@end example

Because of this slight inconvenience, you may find it more convenient to
use hash tables to store dictionary data.  If your application will not
be modifying the contents of an alist very often, this may not make much
difference to you.

If you need to keep the old value of an association list in a form
independent from the list that results from modification by
@code{acons}, @code{assq-set!}, @code{assv-set!} or @code{assoc-set!},
use @code{list-copy} to copy the old association list before modifying
it.

@deffn {Scheme Procedure} acons key value alist
@deffnx {C Function} scm_acons (key, value, alist)
Add a new key-value pair to @var{alist}.  A new pair is
created whose car is @var{key} and whose cdr is @var{value}, and the
pair is consed onto @var{alist}, and the new list is returned.  This
function is @emph{not} destructive; @var{alist} is not modified.
@end deffn

@deffn {Scheme Procedure} assq-set! alist key val
@deffnx {Scheme Procedure} assv-set! alist key value
@deffnx {Scheme Procedure} assoc-set! alist key value
@deffnx {C Function} scm_assq_set_x (alist, key, val)
@deffnx {C Function} scm_assv_set_x (alist, key, val)
@deffnx {C Function} scm_assoc_set_x (alist, key, val)
Reassociate @var{key} in @var{alist} with @var{value}: find any existing
@var{alist} entry for @var{key} and associate it with the new
@var{value}.  If @var{alist} does not contain an entry for @var{key},
add a new one.  Return the (possibly new) alist.

These functions do not attempt to verify the structure of @var{alist},
and so may cause unusual results if passed an object that is not an
association list.
@end deffn

@node Retrieving Alist Entries
@subsubsection Retrieving Alist Entries
@rnindex assq
@rnindex assv
@rnindex assoc

@code{assq}, @code{assv} and @code{assoc} find the entry in an alist
for a given key, and return the @code{(@var{key} . @var{value})} pair.
@code{assq-ref}, @code{assv-ref} and @code{assoc-ref} do a similar
lookup, but return just the @var{value}.

@deffn {Scheme Procedure} assq key alist
@deffnx {Scheme Procedure} assv key alist
@deffnx {Scheme Procedure} assoc key alist
@deffnx {C Function} scm_assq (key, alist)
@deffnx {C Function} scm_assv (key, alist)
@deffnx {C Function} scm_assoc (key, alist)
Return the first entry in @var{alist} with the given @var{key}.  The
return is the pair @code{(KEY . VALUE)} from @var{alist}.  If there's
no matching entry the return is @code{#f}.

@code{assq} compares keys with @code{eq?}, @code{assv} uses
@code{eqv?} and @code{assoc} uses @code{equal?}.  See also SRFI-1
which has an extended @code{assoc} (@ref{SRFI-1 Association Lists}).
@end deffn

@deffn {Scheme Procedure} assq-ref alist key
@deffnx {Scheme Procedure} assv-ref alist key
@deffnx {Scheme Procedure} assoc-ref alist key
@deffnx {C Function} scm_assq_ref (alist, key)
@deffnx {C Function} scm_assv_ref (alist, key)
@deffnx {C Function} scm_assoc_ref (alist, key)
Return the value from the first entry in @var{alist} with the given
@var{key}, or @code{#f} if there's no such entry.

@code{assq-ref} compares keys with @code{eq?}, @code{assv-ref} uses
@code{eqv?} and @code{assoc-ref} uses @code{equal?}.

Notice these functions have the @var{key} argument last, like other
@code{-ref} functions, but this is opposite to what @code{assq}
etc above use.

When the return is @code{#f} it can be either @var{key} not found, or
an entry which happens to have value @code{#f} in the @code{cdr}.  Use
@code{assq} etc above if you need to differentiate these cases.
@end deffn


@node Removing Alist Entries
@subsubsection Removing Alist Entries

To remove the element from an association list whose key matches a
specified key, use @code{assq-remove!}, @code{assv-remove!} or
@code{assoc-remove!} (depending, as usual, on the level of equality
required between the key that you specify and the keys in the
association list).

As with @code{assq-set!} and friends, the specified alist may or may not
be modified destructively, and the only safe way to update a variable
containing the alist is to @code{set!} it to the value that
@code{assq-remove!} and friends return.

@example
address-list
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
@result{}
(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))
@end example

Note that, when @code{assq/v/oc-remove!} is used to modify an
association list that has been constructed only using the corresponding
@code{assq/v/oc-set!}, there can be at most one matching entry in the
alist, so the question of multiple entries being removed in one go does
not arise.  If @code{assq/v/oc-remove!} is applied to an association
list that has been constructed using @code{acons}, or an
@code{assq/v/oc-set!} with a different level of equality, or any mixture
of these, it removes only the first matching entry from the alist, even
if the alist might contain further matching entries.  For example:

@example
(define address-list '())
(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
address-list
@result{}
(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
@result{}
(("mary" . "11 Elm Street"))
@end example

In this example, the two instances of the string "mary" are not the same
when compared using @code{eq?}, so the two @code{assq-set!} calls add
two distinct entries to @code{address-list}.  When compared using
@code{equal?}, both "mary"s in @code{address-list} are the same as the
"mary" in the @code{assoc-remove!} call, but @code{assoc-remove!} stops
after removing the first matching entry that it finds, and so one of the
"mary" entries is left in place.

@deffn {Scheme Procedure} assq-remove! alist key
@deffnx {Scheme Procedure} assv-remove! alist key
@deffnx {Scheme Procedure} assoc-remove! alist key
@deffnx {C Function} scm_assq_remove_x (alist, key)
@deffnx {C Function} scm_assv_remove_x (alist, key)
@deffnx {C Function} scm_assoc_remove_x (alist, key)
Delete the first entry in @var{alist} associated with @var{key}, and return
the resulting alist.
@end deffn

@node Sloppy Alist Functions
@subsubsection Sloppy Alist Functions

@code{sloppy-assq}, @code{sloppy-assv} and @code{sloppy-assoc} behave
like the corresponding non-@code{sloppy-} procedures, except that they
return @code{#f} when the specified association list is not well-formed,
where the non-@code{sloppy-} versions would signal an error.

Specifically, there are two conditions for which the non-@code{sloppy-}
procedures signal an error, which the @code{sloppy-} procedures handle
instead by returning @code{#f}.  Firstly, if the specified alist as a
whole is not a proper list:

@example
(assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
@result{}
ERROR: In procedure assoc in expression (assoc "mary" (quote #)):
ERROR: Wrong type argument in position 2 (expecting
   association list): ((1 . 2) ("key" . "door") . "open sesame")

(sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
@result{}
#f
@end example

@noindent
Secondly, if one of the entries in the specified alist is not a pair:

@example
(assoc 2 '((1 . 1) 2 (3 . 9)))
@result{}
ERROR: In procedure assoc in expression (assoc 2 (quote #)):
ERROR: Wrong type argument in position 2 (expecting
   association list): ((1 . 1) 2 (3 . 9))

(sloppy-assoc 2 '((1 . 1) 2 (3 . 9)))
@result{}
#f
@end example

Unless you are explicitly working with badly formed association lists,
it is much safer to use the non-@code{sloppy-} procedures, because they
help to highlight coding and data errors that the @code{sloppy-}
versions would silently cover up.

@deffn {Scheme Procedure} sloppy-assq key alist
@deffnx {C Function} scm_sloppy_assq (key, alist)
Behaves like @code{assq} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@deffn {Scheme Procedure} sloppy-assv key alist
@deffnx {C Function} scm_sloppy_assv (key, alist)
Behaves like @code{assv} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@deffn {Scheme Procedure} sloppy-assoc key alist
@deffnx {C Function} scm_sloppy_assoc (key, alist)
Behaves like @code{assoc} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@node Alist Example
@subsubsection Alist Example

Here is a longer example of how alists may be used in practice.

@lisp
(define capitals '(("New York" . "Albany")
                   ("Oregon"   . "Salem")
                   ("Florida"  . "Miami")))

;; What's the capital of Oregon?
(assoc "Oregon" capitals)       @result{} ("Oregon" . "Salem")
(assoc-ref capitals "Oregon")   @result{} "Salem"

;; We left out South Dakota.
(set! capitals
      (assoc-set! capitals "South Dakota" "Pierre"))
capitals
@result{} (("South Dakota" . "Pierre")
    ("New York" . "Albany")
    ("Oregon" . "Salem")
    ("Florida" . "Miami"))

;; And we got Florida wrong.
(set! capitals
      (assoc-set! capitals "Florida" "Tallahassee"))
capitals
@result{} (("South Dakota" . "Pierre")
    ("New York" . "Albany")
    ("Oregon" . "Salem")
    ("Florida" . "Tallahassee"))

;; After Oregon secedes, we can remove it.
(set! capitals
      (assoc-remove! capitals "Oregon"))
capitals
@result{} (("South Dakota" . "Pierre")
    ("New York" . "Albany")
    ("Florida" . "Tallahassee"))
@end lisp

@node VHashes
@subsection VList-Based Hash Lists or ``VHashes''

@cindex VList-based hash lists
@cindex VHash

The @code{(ice-9 vlist)} module provides an implementation of @dfn{VList-based
hash lists} (@pxref{VLists}).  VList-based hash lists, or @dfn{vhashes}, are an
immutable dictionary type similar to association lists that maps @dfn{keys} to
@dfn{values}.  However, unlike association lists, accessing a value given its
key is typically a constant-time operation.

The VHash programming interface of @code{(ice-9 vlist)} is mostly the same as
that of association lists found in SRFI-1, with procedure names prefixed by
@code{vhash-} instead of @code{alist-} (@pxref{SRFI-1 Association Lists}).

In addition, vhashes can be manipulated using VList operations:

@example
(vlist-head (vhash-consq 'a 1 vlist-null))
@result{} (a . 1)

(define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null)))
(define vh2 (vhash-consq 'c 3 (vlist-tail vh1)))

(vhash-assq 'a vh2)
@result{} (a . 1)
(vhash-assq 'b vh2)
@result{} #f
(vhash-assq 'c vh2)
@result{} (c . 3)
(vlist->list vh2)
@result{} ((c . 3) (a . 1))
@end example

However, keep in mind that procedures that construct new VLists
(@code{vlist-map}, @code{vlist-filter}, etc.) return raw VLists, not vhashes:

@example
(define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq))
(vhash-assq 'a vh)
@result{} (a . 1)

(define vl
  ;; This will create a raw vlist.
  (vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh))
(vhash-assq 'a vl)
@result{} ERROR: Wrong type argument in position 2

(vlist->list vl)
@result{} ((a . 1) (c . 3))
@end example

@deffn {Scheme Procedure} vhash? obj
Return true if @var{obj} is a vhash.
@end deffn

@deffn {Scheme Procedure} vhash-cons key value vhash [hash-proc]
@deffnx {Scheme Procedure} vhash-consq key value vhash
@deffnx {Scheme Procedure} vhash-consv key value vhash
Return a new hash list based on @var{vhash} where @var{key} is associated with
@var{value}, using @var{hash-proc} to compute the hash of @var{key}.
@var{vhash} must be either @code{vlist-null} or a vhash returned by a previous
call to @code{vhash-cons}.  @var{hash-proc} defaults to @code{hash} (@pxref{Hash
Table Reference, @code{hash} procedure}).  With @code{vhash-consq}, the
@code{hashq} hash function is used; with @code{vhash-consv} the @code{hashv}
hash function is used.

All @code{vhash-cons} calls made to construct a vhash should use the same
@var{hash-proc}.  Failing to do that, the result is undefined.
@end deffn

@deffn {Scheme Procedure} vhash-assoc key vhash [equal? [hash-proc]]
@deffnx {Scheme Procedure} vhash-assq key vhash
@deffnx {Scheme Procedure} vhash-assv key vhash
Return the first key/value pair from @var{vhash} whose key is equal to @var{key}
according to the @var{equal?} equality predicate (which defaults to
@code{equal?}), and using @var{hash-proc} (which defaults to @code{hash}) to
compute the hash of @var{key}.  The second form uses @code{eq?} as the equality
predicate and @code{hashq} as the hash function; the last form uses @code{eqv?}
and @code{hashv}.

Note that it is important to consistently use the same hash function for
@var{hash-proc} as was passed to @code{vhash-cons}.  Failing to do that, the
result is unpredictable.
@end deffn

@deffn {Scheme Procedure} vhash-delete key vhash [equal? [hash-proc]]
@deffnx {Scheme Procedure} vhash-delq key vhash
@deffnx {Scheme Procedure} vhash-delv key vhash
Remove all associations from @var{vhash} with @var{key}, comparing keys with
@var{equal?} (which defaults to @code{equal?}), and computing the hash of
@var{key} using @var{hash-proc} (which defaults to @code{hash}).  The second
form uses @code{eq?} as the equality predicate and @code{hashq} as the hash
function; the last one uses @code{eqv?} and @code{hashv}.

Again the choice of @var{hash-proc} must be consistent with previous calls to
@code{vhash-cons}.
@end deffn

@deffn {Scheme Procedure} vhash-fold proc init vhash
@deffnx {Scheme Procedure} vhash-fold-right proc init vhash
Fold over the key/value elements of @var{vhash} in the given direction,
with each call to @var{proc} having the form @code{(@var{proc} key value
result)}, where @var{result} is the result of the previous call to
@var{proc} and @var{init} the value of @var{result} for the first call
to @var{proc}.
@end deffn

@deffn {Scheme Procedure} vhash-fold* proc init key vhash [equal? [hash]]
@deffnx {Scheme Procedure} vhash-foldq* proc init key vhash
@deffnx {Scheme Procedure} vhash-foldv* proc init key vhash
Fold over all the values associated with @var{key} in @var{vhash}, with each
call to @var{proc} having the form @code{(proc value result)}, where
@var{result} is the result of the previous call to @var{proc} and @var{init} the
value of @var{result} for the first call to @var{proc}.

Keys in @var{vhash} are hashed using @var{hash} are compared using @var{equal?}.
The second form uses @code{eq?} as the equality predicate and @code{hashq} as
the hash function; the third one uses @code{eqv?} and @code{hashv}.

Example:

@example
(define vh
  (alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3))))

(vhash-fold* cons '() 'a vh)
@result{} (3 2 1)

(vhash-fold* cons '() 'z vh)
@result{} (0)
@end example
@end deffn

@deffn {Scheme Procedure} alist->vhash alist [hash-proc]
Return the vhash corresponding to @var{alist}, an association list, using
@var{hash-proc} to compute key hashes.  When omitted, @var{hash-proc} defaults
to @code{hash}.
@end deffn


@node Hash Tables
@subsection Hash Tables
@tpindex Hash Tables

Hash tables are dictionaries which offer similar functionality as
association lists: They provide a mapping from keys to values.  The
difference is that association lists need time linear in the size of
elements when searching for entries, whereas hash tables can normally
search in constant time.  The drawback is that hash tables require a
little bit more memory, and that you can not use the normal list
procedures (@pxref{Lists}) for working with them.

@menu
* Hash Table Examples::         Demonstration of hash table usage.
* Hash Table Reference::        Hash table procedure descriptions.
@end menu


@node Hash Table Examples
@subsubsection Hash Table Examples

For demonstration purposes, this section gives a few usage examples of
some hash table procedures, together with some explanation what they do.

First we start by creating a new hash table with 31 slots, and
populate it with two key/value pairs.

@lisp
(define h (make-hash-table 31))

;; This is an opaque object
h
@result{}
#<hash-table 0/31>

;; Inserting into a hash table can be done with hashq-set!
(hashq-set! h 'foo "bar")
@result{}
"bar"

(hashq-set! h 'braz "zonk")
@result{}
"zonk"

;; Or with hash-create-handle!
(hashq-create-handle! h 'frob #f)
@result{}
(frob . #f)
@end lisp

You can get the value for a given key with the procedure
@code{hashq-ref}, but the problem with this procedure is that you
cannot reliably determine whether a key does exists in the table.  The
reason is that the procedure returns @code{#f} if the key is not in
the table, but it will return the same value if the key is in the
table and just happens to have the value @code{#f}, as you can see in
the following examples.

@lisp
(hashq-ref h 'foo)
@result{}
"bar"

(hashq-ref h 'frob)
@result{}
#f

(hashq-ref h 'not-there)
@result{}
#f
@end lisp

It is often better is to use the procedure @code{hashq-get-handle},
which makes a distinction between the two cases.  Just like @code{assq},
this procedure returns a key/value-pair on success, and @code{#f} if the
key is not found.

@lisp
(hashq-get-handle h 'foo)
@result{}
(foo . "bar")

(hashq-get-handle h 'not-there)
@result{}
#f
@end lisp

Interesting results can be computed by using @code{hash-fold} to work
through each element.  This example will count the total number of
elements:

@lisp
(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
@result{}
3
@end lisp

The same thing can be done with the procedure @code{hash-count}, which
can also count the number of elements matching a particular predicate.
For example, count the number of elements with string values:

@lisp
(hash-count (lambda (key value) (string? value)) h)
@result{}
2
@end lisp

Counting all the elements is a simple task using @code{const}:

@lisp
(hash-count (const #t) h)
@result{}
3
@end lisp

@node Hash Table Reference
@subsubsection Hash Table Reference

@c  FIXME: Describe in broad terms what happens for resizing, and what
@c  the initial size means for this.

Like the association list functions, the hash table functions come in
several varieties, according to the equality test used for the keys.
Plain @code{hash-} functions use @code{equal?}, @code{hashq-}
functions use @code{eq?}, @code{hashv-} functions use @code{eqv?}, and
the @code{hashx-} functions use an application supplied test.

A single @code{make-hash-table} creates a hash table suitable for use
with any set of functions, but it's imperative that just one set is
then used consistently, or results will be unpredictable.

Hash tables are implemented as a vector indexed by a hash value formed
from the key, with an association list of key/value pairs for each
bucket in case distinct keys hash together.  Direct access to the
pairs in those lists is provided by the @code{-handle-} functions.

When the number of entries in a hash table goes above a threshold, the
vector is made larger and the entries are rehashed, to prevent the
bucket lists from becoming too long and slowing down accesses.  When the
number of entries goes below a threshold, the vector is shrunk to save
space.

For the @code{hashx-} ``extended'' routines, an application supplies a
@var{hash} function producing an integer index like @code{hashq} etc
below, and an @var{assoc} alist search function like @code{assq} etc
(@pxref{Retrieving Alist Entries}).  Here's an example of such
functions implementing case-insensitive hashing of string keys,

@example
(use-modules (srfi srfi-1)
             (srfi srfi-13))

(define (my-hash str size)
  (remainder (string-hash-ci str) size))
(define (my-assoc str alist)
  (find (lambda (pair) (string-ci=? str (car pair))) alist))

(define my-table (make-hash-table))
(hashx-set! my-hash my-assoc my-table "foo" 123)

(hashx-ref my-hash my-assoc my-table "FOO")
@result{} 123
@end example

In a @code{hashx-} @var{hash} function the aim is to spread keys
across the vector, so bucket lists don't become long.  But the actual
values are arbitrary as long as they're in the range 0 to
@math{@var{size}-1}.  Helpful functions for forming a hash value, in
addition to @code{hashq} etc below, include @code{symbol-hash}
(@pxref{Symbol Keys}), @code{string-hash} and @code{string-hash-ci}
(@pxref{String Comparison}), and @code{char-set-hash}
(@pxref{Character Set Predicates/Comparison}).

@sp 1
@deffn {Scheme Procedure} make-hash-table [size]
Create a new hash table object, with an optional minimum
vector @var{size}.

When @var{size} is given, the table vector will still grow and shrink
automatically, as described above, but with @var{size} as a minimum.
If an application knows roughly how many entries the table will hold
then it can use @var{size} to avoid rehashing when initial entries are
added.
@end deffn

@deffn {Scheme Procedure} alist->hash-table alist
@deffnx {Scheme Procedure} alist->hashq-table alist
@deffnx {Scheme Procedure} alist->hashv-table alist
@deffnx {Scheme Procedure} alist->hashx-table hash assoc alist
Convert @var{alist} into a hash table. When keys are repeated in
@var{alist}, the leftmost association takes precedence.

@example
(use-modules (ice-9 hash-table))
(alist->hash-table '((foo . 1) (bar . 2)))
@end example

When converting to an extended hash table, custom @var{hash} and
@var{assoc} procedures must be provided.

@example
(alist->hashx-table hash assoc '((foo . 1) (bar . 2)))
@end example

@end deffn

@deffn {Scheme Procedure} hash-table? obj
@deffnx {C Function} scm_hash_table_p (obj)
Return @code{#t} if @var{obj} is a abstract hash table object.
@end deffn

@deffn {Scheme Procedure} hash-clear! table
@deffnx {C Function} scm_hash_clear_x (table)
Remove all items from @var{table} (without triggering a resize).
@end deffn

@deffn {Scheme Procedure} hash-ref table key [dflt]
@deffnx {Scheme Procedure} hashq-ref table key [dflt]
@deffnx {Scheme Procedure} hashv-ref table key [dflt]
@deffnx {Scheme Procedure} hashx-ref hash assoc table key [dflt]
@deffnx {C Function} scm_hash_ref (table, key, dflt)
@deffnx {C Function} scm_hashq_ref (table, key, dflt)
@deffnx {C Function} scm_hashv_ref (table, key, dflt)
@deffnx {C Function} scm_hashx_ref (hash, assoc, table, key, dflt)
Lookup @var{key} in the given hash @var{table}, and return the
associated value.  If @var{key} is not found, return @var{dflt}, or
@code{#f} if @var{dflt} is not given.
@end deffn

@deffn {Scheme Procedure} hash-set! table key val
@deffnx {Scheme Procedure} hashq-set! table key val
@deffnx {Scheme Procedure} hashv-set! table key val
@deffnx {Scheme Procedure} hashx-set! hash assoc table key val
@deffnx {C Function} scm_hash_set_x (table, key, val)
@deffnx {C Function} scm_hashq_set_x (table, key, val)
@deffnx {C Function} scm_hashv_set_x (table, key, val)
@deffnx {C Function} scm_hashx_set_x (hash, assoc, table, key, val)
Associate @var{val} with @var{key} in the given hash @var{table}.  If
@var{key} is already present then it's associated value is changed.
If it's not present then a new entry is created.
@end deffn

@deffn {Scheme Procedure} hash-remove! table key
@deffnx {Scheme Procedure} hashq-remove! table key
@deffnx {Scheme Procedure} hashv-remove! table key
@deffnx {Scheme Procedure} hashx-remove! hash assoc table key
@deffnx {C Function} scm_hash_remove_x (table, key)
@deffnx {C Function} scm_hashq_remove_x (table, key)
@deffnx {C Function} scm_hashv_remove_x (table, key)
@deffnx {C Function} scm_hashx_remove_x (hash, assoc, table, key)
Remove any association for @var{key} in the given hash @var{table}.
If @var{key} is not in @var{table} then nothing is done.
@end deffn

@deffn {Scheme Procedure} hash key size
@deffnx {Scheme Procedure} hashq key size
@deffnx {Scheme Procedure} hashv key size
@deffnx {C Function} scm_hash (key, size)
@deffnx {C Function} scm_hashq (key, size)
@deffnx {C Function} scm_hashv (key, size)
Return a hash value for @var{key}.  This is a number in the range
@math{0} to @math{@var{size}-1}, which is suitable for use in a hash
table of the given @var{size}.

Note that @code{hashq} and @code{hashv} may use internal addresses of
objects, so if an object is garbage collected and re-created it can
have a different hash value, even when the two are notionally
@code{eq?}.  For instance with symbols,

@example
(hashq 'something 123)   @result{} 19
(gc)
(hashq 'something 123)   @result{} 62
@end example

In normal use this is not a problem, since an object entered into a
hash table won't be garbage collected until removed.  It's only if
hashing calculations are somehow separated from normal references that
its lifetime needs to be considered.
@end deffn

@deffn {Scheme Procedure} hash-get-handle table key
@deffnx {Scheme Procedure} hashq-get-handle table key
@deffnx {Scheme Procedure} hashv-get-handle table key
@deffnx {Scheme Procedure} hashx-get-handle hash assoc table key
@deffnx {C Function} scm_hash_get_handle (table, key)
@deffnx {C Function} scm_hashq_get_handle (table, key)
@deffnx {C Function} scm_hashv_get_handle (table, key)
@deffnx {C Function} scm_hashx_get_handle (hash, assoc, table, key)
Return the @code{(@var{key} . @var{value})} pair for @var{key} in the
given hash @var{table}, or @code{#f} if @var{key} is not in
@var{table}.
@end deffn

@deffn {Scheme Procedure} hash-create-handle! table key init
@deffnx {Scheme Procedure} hashq-create-handle! table key init
@deffnx {Scheme Procedure} hashv-create-handle! table key init
@deffnx {Scheme Procedure} hashx-create-handle! hash assoc table key init
@deffnx {C Function} scm_hash_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashq_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashv_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashx_create_handle_x (hash, assoc, table, key, init)
Return the @code{(@var{key} . @var{value})} pair for @var{key} in the
given hash @var{table}.  If @var{key} is not in @var{table} then
create an entry for it with @var{init} as the value, and return that
pair.
@end deffn

@deffn {Scheme Procedure} hash-map->list proc table
@deffnx {Scheme Procedure} hash-for-each proc table
@deffnx {C Function} scm_hash_map_to_list (proc, table)
@deffnx {C Function} scm_hash_for_each (proc, table)
Apply @var{proc} to the entries in the given hash @var{table}.  Each
call is @code{(@var{proc} @var{key} @var{value})}.  @code{hash-map->list}
returns a list of the results from these calls, @code{hash-for-each}
discards the results and returns an unspecified value.

Calls are made over the table entries in an unspecified order, and for
@code{hash-map->list} the order of the values in the returned list is
unspecified.  Results will be unpredictable if @var{table} is modified
while iterating.

For example the following returns a new alist comprising all the
entries from @code{mytable}, in no particular order.

@example
(hash-map->list cons mytable)
@end example
@end deffn

@deffn {Scheme Procedure} hash-for-each-handle proc table
@deffnx {C Function} scm_hash_for_each_handle (proc, table)
Apply @var{proc} to the entries in the given hash @var{table}.  Each
call is @code{(@var{proc} @var{handle})}, where @var{handle} is a
@code{(@var{key} . @var{value})} pair. Return an unspecified value.

@code{hash-for-each-handle} differs from @code{hash-for-each} only in
the argument list of @var{proc}.
@end deffn

@deffn {Scheme Procedure} hash-fold proc init table
@deffnx {C Function} scm_hash_fold (proc, init, table)
Accumulate a result by applying @var{proc} to the elements of the
given hash @var{table}.  Each call is @code{(@var{proc} @var{key}
@var{value} @var{prior-result})}, where @var{key} and @var{value} are
from the @var{table} and @var{prior-result} is the return from the
previous @var{proc} call.  For the first call, @var{prior-result} is
the given @var{init} value.

Calls are made over the table entries in an unspecified order.
Results will be unpredictable if @var{table} is modified while
@code{hash-fold} is running.

For example, the following returns a count of how many keys in
@code{mytable} are strings.

@example
(hash-fold (lambda (key value prior)
             (if (string? key) (1+ prior) prior))
           0 mytable)
@end example
@end deffn

@deffn {Scheme Procedure} hash-count pred table
@deffnx {C Function} scm_hash_count (pred, table)
Return the number of elements in the given hash @var{table} that cause
@code{(@var{pred} @var{key} @var{value})} to return true.  To quickly
determine the total number of elements, use @code{(const #t)} for
@var{pred}.
@end deffn

@node Other Types
@subsection Other Types

Procedures are documented in their own section.  @xref{Procedures}.

Variable objects are documented as part of the description of Guile's
module system: see @ref{Variables}.

@xref{Scheduling}, for discussion of threads, mutexes, and so on.

Ports are described in the section on I/O: see @ref{Input and Output}.

Regular expressions are described in their own section: see @ref{Regular
Expressions}.

There are quite a number of additional data types documented in this
manual; if you feel a link is missing here, please file a bug.

@c Local Variables:
@c TeX-master: "guile.texi"
@c End: