guile/doc/ref/sxml.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C) 2013, 2017, 2021 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@c SXPath documentation based on SXPath.scm by Oleg Kiselyov,
@c which is in the public domain according to <http://okmij.org/ftp/>
@c and <http://ssax.sourceforge.net/>.

@node SXML
@section SXML

SXML is a native representation of XML in terms of standard Scheme data
types: lists, symbols, and strings.  For example, the simple XML
fragment:

@example
<parrot type="African Grey"><name>Alfie</name></parrot>
@end example

may be represented with the following SXML:

@example
(parrot (@@ (type "African Grey")) (name "Alfie"))
@end example

SXML is very general, and is capable of representing all of XML.
Formally, this means that SXML is a conforming implementation of the
@uref{http://www.w3.org/TR/xml-infoset/,XML Information Set} standard.

Guile includes several facilities for working with XML and SXML:
parsers, serializers, and transformers.

@menu
* SXML Overview::               XML, as it was meant to be
* Reading and Writing XML::     Convenient XML parsing and serializing
* SSAX::                        Custom functional-style XML parsers
* Transforming SXML::           Munging SXML with @code{pre-post-order}
* SXML Tree Fold::              Fold-based SXML transformations
* SXPath::                      XPath for SXML
* sxml ssax input-parse::       The SSAX tokenizer, optimized for Guile
* sxml apply-templates::        A more XSLT-like approach to SXML transformations
@end menu

@node SXML Overview
@subsection SXML Overview

(This section needs to be written; volunteers welcome.)


@node Reading and Writing XML
@subsection Reading and Writing XML

The @code{(sxml simple)} module presents a basic interface for parsing
XML from a port into the Scheme SXML format, and for serializing it back
to text.

@example
(use-modules (sxml simple))
@end example

@deffn {Scheme Procedure} xml->sxml [string-or-port] [#:namespaces='()] @
       [#:declare-namespaces?=#t] [#:trim-whitespace?=#f] @
       [#:entities='()] [#:default-entity-handler=#f] @
       [#:doctype-handler=#f]
Use SSAX to parse an XML document into SXML. Takes one optional
argument, @var{string-or-port}, which defaults to the current input
port.  Returns the resulting SXML document.  If @var{string-or-port} is
a port, it will be left pointing at the next available character in the
port.
@end deffn

As is normal in SXML, XML elements parse as tagged lists.  Attributes,
if any, are placed after the tag, within an @code{@@} element.  The root
of the resulting XML will be contained in a special tag, @code{*TOP*}.
This tag will contain the root element of the XML, but also any prior
processing instructions.

@example
(xml->sxml "<foo/>")
@result{} (*TOP* (foo))
(xml->sxml "<foo>text</foo>")
@result{} (*TOP* (foo "text"))
(xml->sxml "<foo kind=\"bar\">text</foo>")
@result{} (*TOP* (foo (@@ (kind "bar")) "text"))
(xml->sxml "<?xml version=\"1.0\"?><foo/>")
@result{} (*TOP* (*PI* xml "version=\"1.0\"") (foo))
@end example

All namespaces in the XML document must be declared, via @code{xmlns}
attributes.  SXML elements built from non-default namespaces will have
their tags prefixed with their URI.  Users can specify custom prefixes
for certain namespaces with the @code{#:namespaces} keyword argument to
@code{xml->sxml}.

@example
(xml->sxml "<foo xmlns=\"http://example.org/ns1\">text</foo>")
@result{} (*TOP* (http://example.org/ns1:foo "text"))
(xml->sxml "<foo xmlns=\"http://example.org/ns1\">text</foo>"
           #:namespaces '((ns1 . "http://example.org/ns1")))
@result{} (*TOP* (ns1:foo "text"))
(xml->sxml "<foo xmlns:bar=\"http://example.org/ns2\"><bar:baz/></foo>"
           #:namespaces '((ns2 . "http://example.org/ns2")))
@result{} (*TOP* (foo (ns2:baz)))
@end example

By default, namespaces passed to @code{xml->sxml} are treated as if they
were declared on the root element.  Passing a false
@code{#:declare-namespaces?} argument will disable this behavior,
requiring in-document declarations of namespaces before use..

@example
(xml->sxml "<foo><ns2:baz/></foo>"
           #:namespaces '((ns2 . "http://example.org/ns2")))
@result{} (*TOP* (foo (ns2:baz)))
(xml->sxml "<foo><ns2:baz/></foo>"
           #:namespaces '((ns2 . "http://example.org/ns2"))
           #:declare-namespaces? #f)
@result{} error: undeclared namespace: `bar'
@end example

By default, all whitespace in XML is significant.  Passing the
@code{#:trim-whitespace?} keyword argument to @code{xml->sxml} will trim
whitespace in front, behind and between elements, treating it as
``unsignificant''.  Whitespace in text fragments is left alone.

@example
(xml->sxml "<foo>\n<bar> Alfie the parrot! </bar>\n</foo>")
@result{} (*TOP* (foo "\n" (bar " Alfie the parrot! ") "\n"))
(xml->sxml "<foo>\n<bar> Alfie the parrot! </bar>\n</foo>"
           #:trim-whitespace? #t)
@result{} (*TOP* (foo (bar " Alfie the parrot! ")))
@end example

Parsed entities may be declared with the @code{#:entities} keyword
argument, or handled with the @code{#:default-entity-handler}.  By
default, only the standard @code{&lt;}, @code{&gt;}, @code{&amp;},
@code{&apos;} and @code{&quot;} entities are defined, as well as the
@code{&#@var{N};} and @code{&#x@var{N};} (decimal and hexadecimal)
numeric character entities.

@example
(xml->sxml "<foo>&amp;</foo>")
@result{} (*TOP* (foo "&"))
(xml->sxml "<foo>&nbsp;</foo>")
@result{} error: undefined entity: nbsp
(xml->sxml "<foo>&#xA0;</foo>")
@result{} (*TOP* (foo "\xa0"))
(xml->sxml "<foo>&nbsp;</foo>"
           #:entities '((nbsp . "\xa0")))
@result{} (*TOP* (foo "\xa0"))
(xml->sxml "<foo>&nbsp; &foo;</foo>"
           #:default-entity-handler
           (lambda (port name)
             (case name
               ((nbsp) "\xa0")
               (else
                (format (current-warning-port)
                        "~a:~a:~a: undefined entitity: ~a\n"
                        (or (port-filename port) "<unknown file>")
                        (port-line port) (port-column port)
                        name)
                (symbol->string name)))))
@print{} <unknown file>:0:17: undefined entitity: foo
@result{} (*TOP* (foo "\xa0 foo"))
@end example

By default, @code{xml->sxml} skips over the @code{<!DOCTYPE>}
declaration, if any.  This behavior can be overridden with the
@code{#:doctype-handler} argument, which should be a procedure of three
arguments: the @dfn{docname} (a symbol), @dfn{systemid} (a string), and
the internal doctype subset (as a string or @code{#f} if not present).

The handler should return keyword arguments as multiple values, as if it
were calling its continuation with keyword arguments.  The continuation
accepts the @code{#:entities} and @code{#:namespaces} keyword arguments,
in the same format that @code{xml->sxml} itself takes.  These entities
and namespaces will be prepended to those given to the @code{xml->sxml}
invocation.

@example
(define (handle-foo docname systemid internal-subset)
  (case docname
    ((foo)
     (values #:entities '((greets . "<i>Hello, world!</i>"))))
    (else
     (values))))

(xml->sxml "<!DOCTYPE foo><p>&greets;</p>"
           #:doctype-handler handle-foo)
@result{} (*TOP* (p (i "Hello, world!")))
@end example

If the document has no doctype declaration, the @var{doctype-handler} is
invoked with @code{#f} for the three arguments.

In the future, the continuation may accept other keyword arguments, for
example to validate the parsed SXML against the doctype.

@deffn {Scheme Procedure} sxml->xml tree [port]
Serialize the SXML tree @var{tree} as XML. The output will be written to
the current output port, unless the optional argument @var{port} is
present.
@end deffn

@deffn {Scheme Procedure} sxml->string sxml
Detag an sxml tree @var{sxml} into a string. Does not perform any
formatting.
@end deffn

@node SSAX
@subsection SSAX: A Functional XML Parsing Toolkit

Guile's XML parser is based on Oleg Kiselyov's powerful XML parsing
toolkit, SSAX.

@subsubsection History

Back in the 1990s, when the world was young again and XML was the
solution to all of its problems, there were basically two kinds of XML
parsers out there: DOM parsers and SAX parsers.

A DOM parser reads through an entire XML document, building up a tree of
``DOM objects'' representing the document structure.  They are very easy
to use, but sometimes you don't actually want all of the information in
a document; building an object tree is not necessary if all you want to
do is to count word frequencies in a document, for example.

SAX parsers were created to give the programmer more control on the
parsing process.  A programmer gives the SAX parser a number of
``callbacks'': functions that will be called on various features of the
XML stream as they are encountered.  SAX parsers are more efficient, but
much harder to use, as users typically have to manually maintain a
stack of open elements.

Kiselyov realized that the SAX programming model could be made much
simpler if the callbacks were formulated not as a linear fold across the
features of the XML stream, but as a @emph{tree fold} over the structure
implicit in the XML.  In this way, the user has a very convenient,
functional-style interface that can still generate optimal parsers.

The @code{xml->sxml} interface from the @code{(sxml simple)} module is a
DOM-style parser built using SSAX, though it returns SXML instead of DOM
objects.

@subsubsection Implementation

@code{(sxml ssax)} is a package of low-to-high level lexing and parsing
procedures that can be combined to yield a SAX, a DOM, a validating
parser, or a parser intended for a particular document type.  The
procedures in the package can be used separately to tokenize or parse
various pieces of XML documents.  The package supports XML Namespaces,
internal and external parsed entities, user-controlled handling of
whitespace, and validation.  This module therefore is intended to be a
framework, a set of ``Lego blocks'' you can use to build a parser
following any discipline and performing validation to any degree.  As an
example of the parser construction, the source file includes a
semi-validating SXML parser.

SSAX has a ``sequential'' feel of SAX yet a ``functional style'' of DOM.
Like a SAX parser, the framework scans the document only once and
permits incremental processing.  An application that handles document
elements in order can run as efficiently as possible.  @emph{Unlike} a
SAX parser, the framework does not require an application register
stateful callbacks and surrender control to the parser.  Rather, it is
the application that can drive the framework -- calling its functions to
get the current lexical or syntax element.  These functions do not
maintain or mutate any state save the input port.  Therefore, the
framework permits parsing of XML in a pure functional style, with the
input port being a monad (or a linear, read-once parameter).

Besides the @var{port}, there is another monad -- @var{seed}.  Most of
the middle- and high-level parsers are single-threaded through the
@var{seed}.  The functions of this framework do not process or affect
the @var{seed} in any way: they simply pass it around as an instance of
an opaque datatype.  User functions, on the other hand, can use the seed
to maintain user's state, to accumulate parsing results, etc.  A user
can freely mix their own functions with those of the framework.  On the
other hand, the user may wish to instantiate a high-level parser:
@code{SSAX:make-elem-parser} or @code{SSAX:make-parser}.  In the latter
case, the user must provide functions of specific signatures, which are
called at predictable moments during the parsing: to handle character
data, element data, or processing instructions (PI).  The functions are
always given the @var{seed}, among other parameters, and must return the
new @var{seed}.

From a functional point of view, XML parsing is a combined
pre-post-order traversal of a ``tree'' that is the XML document itself.
This down-and-up traversal tells the user about an element when its
start tag is encountered.  The user is notified about the element once
more, after all element's children have been handled.  The process of
XML parsing therefore is a fold over the raw XML document.  Unlike a
fold over trees defined in [1], the parser is necessarily
single-threaded -- obviously as elements in a text XML document are laid
down sequentially.  The parser therefore is a tree fold that has been
transformed to accept an accumulating parameter [1,2].

Formally, the denotational semantics of the parser can be expressed as

@smallexample 
 parser:: (Start-tag -> Seed -> Seed) ->
	   (Start-tag -> Seed -> Seed -> Seed) ->
	   (Char-Data -> Seed -> Seed) ->
	   XML-text-fragment -> Seed -> Seed
 parser fdown fup fchar "<elem attrs> content </elem>" seed
  = fup "<elem attrs>" seed
	(parser fdown fup fchar "content" (fdown "<elem attrs>" seed))

 parser fdown fup fchar "char-data content" seed
  = parser fdown fup fchar "content" (fchar "char-data" seed)

 parser fdown fup fchar "elem-content content" seed
  = parser fdown fup fchar "content" (
	parser fdown fup fchar "elem-content" seed)
@end smallexample

Compare the last two equations with the left fold

@smallexample 
 fold-left kons elem:list seed = fold-left kons list (kons elem seed)
@end smallexample

The real parser created by @code{SSAX:make-parser} is slightly more
complicated, to account for processing instructions, entity references,
namespaces, processing of document type declaration, etc.

The XML standard document referred to in this module is
@uref{http://www.w3.org/TR/1998/REC-xml-19980210.html}

The present file also defines a procedure that parses the text of an XML
document or of a separate element into SXML, an S-expression-based model
of an XML Information Set.  SXML is also an Abstract Syntax Tree of an
XML document.  SXML is similar but not identical to DOM; SXML is
particularly suitable for Scheme-based XML/HTML authoring, SXPath
queries, and tree transformations.  See SXML.html for more details.
SXML is a term implementation of evaluation of the XML document [3].
The other implementation is context-passing.

The present frameworks fully supports the XML Namespaces Recommendation:
@uref{http://www.w3.org/TR/REC-xml-names/}.

Other links:

@table @asis
@item [1]
Jeremy Gibbons, Geraint Jones, "The Under-appreciated Unfold," Proc.
ICFP'98, 1998, pp. 273-279.

@item [2]
Richard S. Bird, The promotion and accumulation strategies in
transformational programming, ACM Trans. Progr. Lang. Systems,
6(4):487-504, October 1984.

@item [3]
Ralf Hinze, "Deriving Backtracking Monad Transformers," Functional
Pearl. Proc ICFP'00, pp. 186-197.

@end table

@subsubsection Usage
@deffn {Scheme Procedure} current-ssax-error-port 
@end deffn

@deffn {Scheme Procedure} with-ssax-error-to-port port thunk
@end deffn

@deffn {Scheme Procedure} xml-token? _
@verbatim 
 -- Scheme Procedure: pair? x
     Return `#t' if X is a pair; otherwise return `#f'.

 
@end verbatim
@end deffn

@deffn {Scheme Syntax} xml-token-kind token
@end deffn

@deffn {Scheme Syntax} xml-token-head token
@end deffn

@deffn {Scheme Procedure} make-empty-attlist 
@end deffn

@deffn {Scheme Procedure} attlist-add attlist name-value
@end deffn

@deffn {Scheme Procedure} attlist-null? x
Return @code{#t} if @var{x} is the empty list, else @code{#f}.
@end deffn

@deffn {Scheme Procedure} attlist-remove-top attlist
@end deffn

@deffn {Scheme Procedure} attlist->alist attlist
@end deffn

@deffn {Scheme Procedure} attlist-fold kons knil lis1
@end deffn

@deffn {Scheme Procedure} define-parsed-entity! entity str
Define a new parsed entity.  @var{entity} should be a symbol.

Instances of &@var{entity}; in XML text will be replaced with the string
@var{str}, which will then be parsed.
@end deffn

@deffn {Scheme Procedure} reset-parsed-entity-definitions! 
Restore the set of parsed entity definitions to its initial state.
@end deffn

@deffn {Scheme Procedure} ssax:uri-string->symbol uri-str
@end deffn

@deffn {Scheme Procedure} ssax:skip-internal-dtd port
@end deffn

@deffn {Scheme Procedure} ssax:read-pi-body-as-string port
@end deffn

@deffn {Scheme Procedure} ssax:reverse-collect-str-drop-ws fragments
@end deffn

@deffn {Scheme Procedure} ssax:read-markup-token port
@end deffn

@deffn {Scheme Procedure} ssax:read-cdata-body port str-handler seed
@end deffn

@deffn {Scheme Procedure} ssax:read-char-ref port
@end deffn

@deffn {Scheme Procedure} ssax:read-attributes port entities
@end deffn

@deffn {Scheme Procedure} ssax:complete-start-tag tag-head port elems entities namespaces
@end deffn

@deffn {Scheme Procedure} ssax:read-external-id port
@end deffn

@deffn {Scheme Procedure} ssax:read-char-data port expect-eof? str-handler seed
@end deffn

@deffn {Scheme Procedure} ssax:xml->sxml port namespace-prefix-assig
@end deffn

@deffn {Scheme Syntax} ssax:make-parser . kw-val-pairs
@end deffn

@deffn {Scheme Syntax} ssax:make-pi-parser orig-handlers
@end deffn

@deffn {Scheme Syntax} ssax:make-elem-parser my-new-level-seed my-finish-element my-char-data-handler my-pi-handlers
@end deffn

@node Transforming SXML
@subsection Transforming SXML
@subsubsection Overview
@heading SXML expression tree transformers
@subheading Pre-Post-order traversal of a tree and creation of a new tree
@smallexample 
pre-post-order:: <tree> x <bindings> -> <new-tree>
@end smallexample

where

@smallexample 
 <bindings> ::= (<binding> ...)
 <binding> ::= (<trigger-symbol> *preorder* . <handler>) |
               (<trigger-symbol> *macro* . <handler>) |
		(<trigger-symbol> <new-bindings> . <handler>) |
		(<trigger-symbol> . <handler>)
 <trigger-symbol> ::= XMLname | *text* | *default*
 <handler> :: <trigger-symbol> x [<tree>] -> <new-tree>
@end smallexample

The @code{pre-post-order} function, in the @code{(sxml transform)}
module, visits the nodes and nodelists pre-post-order (depth-first).
For each @code{<Node>} of the form @code{(@var{name} <Node> ...)}, it
looks up an association with the given @var{name} among its
@var{<bindings>}.  If failed, @code{pre-post-order} tries to locate a
@code{*default*} binding.  It's an error if the latter attempt fails as
well.  Having found a binding, the @code{pre-post-order} function first
checks to see if the binding is of the form

@smallexample 
	(<trigger-symbol> *preorder* . <handler>)
@end smallexample

If it is, the handler is 'applied' to the current node.  Otherwise, the
pre-post-order function first calls itself recursively for each child of
the current node, with @var{<new-bindings>} prepended to the
@var{<bindings>} in effect.  The result of these calls is passed to the
@var{<handler>} (along with the head of the current @var{<Node>}).  To be
more precise, the handler is _applied_ to the head of the current node
and its processed children.  The result of the handler, which should also
be a @code{<tree>}, replaces the current @var{<Node>}.  If the current
@var{<Node>} is a text string or other atom, a special binding with a
symbol @code{*text*} is looked up.

A binding can also be of a form

@smallexample 
	(<trigger-symbol> *macro* . <handler>)
@end smallexample

This is equivalent to @code{*preorder*} described above.  However, the
result is re-processed again, with the current stylesheet.

@subsubsection Usage
@deffn {Scheme Procedure} SRV:send-reply . fragments
Output the @var{fragments} to the current output port.

The fragments are a list of strings, characters, numbers, thunks,
@code{#f}, @code{#t} -- and other fragments.  The function traverses the
tree depth-first, writes out strings and characters, executes thunks,
and ignores @code{#f} and @code{'()}.  The function returns @code{#t} if
anything was written at all; otherwise the result is @code{#f} If
@code{#t} occurs among the fragments, it is not written out but causes
the result of @code{SRV:send-reply} to be @code{#t}.
@end deffn

@deffn {Scheme Procedure} foldts fdown fup fhere seed tree
@end deffn

@deffn {Scheme Procedure} post-order tree bindings
@end deffn

@deffn {Scheme Procedure} pre-post-order tree bindings
@end deffn

@deffn {Scheme Procedure} replace-range beg-pred end-pred forest
@end deffn

@node SXML Tree Fold
@subsection SXML Tree Fold
@subsubsection Overview
@code{(sxml fold)} defines a number of variants of the @dfn{fold}
algorithm for use in transforming SXML trees.  Additionally it defines
the layout operator, @code{fold-layout}, which might be described as a
context-passing variant of SSAX's @code{pre-post-order}.

@subsubsection Usage
@deffn {Scheme Procedure} foldt fup fhere tree
The standard multithreaded tree fold.

@var{fup} is of type [a] -> a. @var{fhere} is of type object -> a.
@end deffn

@deffn {Scheme Procedure} foldts fdown fup fhere seed tree
The single-threaded tree fold originally defined in SSAX.  @xref{SSAX},
for more information.
@end deffn

@deffn {Scheme Procedure} foldts* fdown fup fhere seed tree
A variant of @code{foldts} that allows pre-order tree
rewrites.  Originally defined in Andy Wingo's 2007 paper,
@emph{Applications of fold to XML transformation}.
@end deffn

@deffn {Scheme Procedure} fold-values proc list . seeds
A variant of @code{fold} that allows multi-valued seeds.  Note that the
order of the arguments differs from that of @code{fold}.  @xref{SRFI-1
Fold and Map}.
@end deffn

@deffn {Scheme Procedure} foldts*-values fdown fup fhere tree . seeds
A variant of @code{foldts*} that allows multi-valued
seeds.  Originally defined in Andy Wingo's 2007 paper, @emph{Applications
of fold to XML transformation}.
@end deffn

@deffn {Scheme Procedure} fold-layout tree bindings params layout stylesheet
A traversal combinator in the spirit of @code{pre-post-order}.
@xref{Transforming SXML}.

@code{fold-layout} was originally presented in Andy Wingo's 2007 paper,
@emph{Applications of fold to XML transformation}.

@example 
bindings := (<binding>...)
binding  := (<tag> <handler-pair>...)
          | (*default* . <post-handler>)
          | (*text* . <text-handler>)
tag      := <symbol>
handler-pair := (pre-layout . <pre-layout-handler>)
          | (post . <post-handler>)
          | (bindings . <bindings>)
          | (pre . <pre-handler>)
          | (macro . <macro-handler>)
@end example

@table @var
@item pre-layout-handler
A function of three arguments:

@table @var
@item kids
the kids of the current node, before traversal

@item params
the params of the current node

@item layout
the layout coming into this node

@end table

@var{pre-layout-handler} is expected to use this information to return a
layout to pass to the kids.  The default implementation returns the
layout given in the arguments.

@item post-handler
A function of five arguments:

@table @var
@item tag
the current tag being processed

@item params
the params of the current node

@item layout
the layout coming into the current node, before any kids were processed

@item klayout
the layout after processing all of the children

@item kids
the already-processed child nodes

@end table

@var{post-handler} should return two values, the layout to pass to the
next node and the final tree.

@item text-handler
@var{text-handler} is a function of three arguments:

@table @var
@item text
the string

@item params
the current params

@item layout
the current layout

@end table

@var{text-handler} should return two values, the layout to pass to the
next node and the value to which the string should transform.

@end table
@end deffn

@node SXPath
@subsection SXPath
@subsubsection Overview
@heading SXPath: SXML Query Language
SXPath is a query language for SXML, an instance of XML Information set
(Infoset) in the form of s-expressions.  See @code{(sxml ssax)} for the
definition of SXML and more details.  SXPath is also a translation into
Scheme of an XML Path Language, @uref{http://www.w3.org/TR/xpath,XPath}.
XPath and SXPath describe means of selecting a set of Infoset's items or
their properties.

To facilitate queries, XPath maps the XML Infoset into an explicit tree,
and introduces important notions of a location path and a current,
context node.  A location path denotes a selection of a set of nodes
relative to a context node.  Any XPath tree has a distinguished, root
node -- which serves as the context node for absolute location paths.
Location path is recursively defined as a location step joined with a
location path.  A location step is a simple query of the database
relative to a context node.  A step may include expressions that further
filter the selected set.  Each node in the resulting set is used as a
context node for the adjoining location path.  The result of the step is
a union of the sets returned by the latter location paths.

The SXML representation of the XML Infoset (see SSAX.scm) is rather
suitable for querying as it is.  Bowing to the XPath specification, we
will refer to SXML information items as 'Nodes':

@example 
 	<Node> ::= <Element> | <attributes-coll> | <attrib>
 		   | "text string" | <PI>
@end example

This production can also be described as

@example 
	<Node> ::= (name . <Nodeset>) | "text string"
@end example

An (ordered) set of nodes is just a list of the constituent nodes:

@example 
 	<Nodeset> ::= (<Node> ...)
@end example

Nodesets, and Nodes other than text strings are both lists.  A <Nodeset>
however is either an empty list, or a list whose head is not a symbol.  A
symbol at the head of a node is either an XML name (in which case it's a
tag of an XML element), or an administrative name such as '@@'.  This
uniform list representation makes processing rather simple and elegant,
while avoiding confusion.  The multi-branch tree structure formed by the
mutually-recursive datatypes <Node> and <Nodeset> lends itself well to
processing by functional languages.

A location path is in fact a composite query over an XPath tree or its
branch.  A singe step is a combination of a projection, selection or a
transitive closure.  Multiple steps are combined via join and union
operations.  This insight allows us to @emph{elegantly} implement XPath
as a sequence of projection and filtering primitives -- converters --
joined by @dfn{combinators}.  Each converter takes a node and returns a
nodeset which is the result of the corresponding query relative to that
node.  A converter can also be called on a set of nodes.  In that case it
returns a union of the corresponding queries over each node in the set.
The union is easily implemented as a list append operation as all nodes
in a SXML tree are considered distinct, by XPath conventions.  We also
preserve the order of the members in the union.  Query combinators are
high-order functions: they take converter(s) (which is a Node|Nodeset ->
Nodeset function) and compose or otherwise combine them.  We will be
concerned with only relative location paths [XPath]: an absolute
location path is a relative path applied to the root node.

Similarly to XPath, SXPath defines full and abbreviated notations for
location paths.  In both cases, the abbreviated notation can be
mechanically expanded into the full form by simple rewriting rules.  In
the case of SXPath the corresponding rules are given in the
documentation of the @code{sxpath} procedure.
@xref{sxpath-procedure-docs,,SXPath procedure documentation}.

The regression test suite at the end of the file @file{SXPATH-old.scm}
shows a representative sample of SXPaths in both notations, juxtaposed
with the corresponding XPath expressions.  Most of the samples are
borrowed literally from the XPath specification.

Much of the following material is taken from the SXPath sources by Oleg
Kiselyov et al.

@subsubsection Basic Converters and Applicators

A converter is a function mapping a nodeset (or a single node) to another
nodeset.  Its type can be represented like this:

@example
type Converter = Node|Nodeset -> Nodeset
@end example

A converter can also play the role of a predicate: in that case, if a
converter, applied to a node or a nodeset, yields a non-empty nodeset,
the converter-predicate is deemed satisfied.  Likewise, an empty nodeset
is equivalent to @code{#f} in denoting failure.

@deffn {Scheme Procedure} nodeset? x
Return @code{#t} if @var{x} is a nodeset.
@end deffn

@deffn {Scheme Procedure} node-typeof? crit
This function implements a 'Node test' as defined in Sec. 2.3 of the
XPath document.  A node test is one of the components of a location
step.  It is also a converter-predicate in SXPath.

The function @code{node-typeof?} takes a type criterion and returns a
function, which, when applied to a node, will tell if the node satisfies
the test.

The criterion @var{crit} is a symbol, one of the following:

@table @code
@item id
tests if the node has the right name (id)

@item @@
tests if the node is an <attributes-coll>

@item *
tests if the node is an <Element>

@item *text*
tests if the node is a text node

@item *PI*
tests if the node is a PI (processing instruction) node

@item *any*
@code{#t} for any type of node
@end table
@end deffn

@deffn {Scheme Procedure} node-eq? other
A curried equivalence converter predicate that takes a node @var{other}
and returns a function that takes another node.  The two nodes are
compared using @code{eq?}.
@end deffn

@deffn {Scheme Procedure} node-equal? other
A curried equivalence converter predicate that takes a node @var{other}
and returns a function that takes another node.  The two nodes are
compared using @code{equal?}.
@end deffn

@deffn {Scheme Procedure} node-pos n
Select the @var{n}'th element of a nodeset and return as a singular
nodeset.  If the @var{n}'th element does not exist, return an empty
nodeset.  If @var{n} is a negative number the node is picked from the
tail of the list.

@example
((node-pos 1) nodeset)  ; return the the head of the nodeset (if exists)
((node-pos 2) nodeset)  ; return the node after that (if exists)
((node-pos -1) nodeset) ; selects the last node of a non-empty nodeset
((node-pos -2) nodeset) ; selects the last but one node, if exists.
@end example
@end deffn

@deffn {Scheme Procedure} filter pred?
A filter applicator, which introduces a filtering context.  The argument
converter @var{pred?} is considered a predicate, with either @code{#f}
or @code{nil} meaning failure.
@end deffn

@deffn {Scheme Procedure} take-until pred?
@example
take-until:: Converter -> Converter, or
take-until:: Pred -> Node|Nodeset -> Nodeset
@end example

Given a converter-predicate @var{pred?} and a nodeset, apply the
predicate to each element of the nodeset, until the predicate yields
anything but @code{#f} or @code{nil}.  Return the elements of the input
nodeset that have been processed until that moment (that is, which fail
the predicate).

@code{take-until} is a variation of the @code{filter} above:
@code{take-until} passes elements of an ordered input set up to (but not
including) the first element that satisfies the predicate.  The nodeset
returned by @code{((take-until (not pred)) nset)} is a subset -- to be
more precise, a prefix -- of the nodeset returned by @code{((filter
pred) nset)}.
@end deffn

@deffn {Scheme Procedure} take-after pred?
@example
take-after:: Converter -> Converter, or
take-after:: Pred -> Node|Nodeset -> Nodeset
@end example

Given a converter-predicate @var{pred?} and a nodeset, apply the
predicate to each element of the nodeset, until the predicate yields
anything but @code{#f} or @code{nil}.  Return the elements of the input
nodeset that have not been processed: that is, return the elements of
the input nodeset that follow the first element that satisfied the
predicate.

@code{take-after} along with @code{take-until} partition an input
nodeset into three parts: the first element that satisfies a predicate,
all preceding elements and all following elements.
@end deffn

@deffn {Scheme Procedure} map-union proc lst
Apply @var{proc} to each element of @var{lst} and return the list of results.
If @var{proc} returns a nodeset, splice it into the result

From another point of view, @code{map-union} is a function
@code{Converter->Converter}, which places an argument-converter in a joining
context.
@end deffn

@deffn {Scheme Procedure} node-reverse node-or-nodeset
@example
node-reverse :: Converter, or
node-reverse:: Node|Nodeset -> Nodeset
@end example

Reverses the order of nodes in the nodeset.  This basic converter is
needed to implement a reverse document order (see the XPath
Recommendation).
@end deffn

@deffn {Scheme Procedure} node-trace title
@example
node-trace:: String -> Converter
@end example

@code{(node-trace title)} is an identity converter.  In addition it
prints out the node or nodeset it is applied to, prefixed with the
@var{title}.  This converter is very useful for debugging.
@end deffn

@subsubsection Converter Combinators

Combinators are higher-order functions that transmogrify a converter or
glue a sequence of converters into a single, non-trivial converter.  The
goal is to arrive at converters that correspond to XPath location paths.

From a different point of view, a combinator is a fixed, named
@dfn{pattern} of applying converters.  Given below is a complete set of
such patterns that together implement XPath location path specification.
As it turns out, all these combinators can be built from a small number
of basic blocks: regular functional composition, @code{map-union} and
@code{filter} applicators, and the nodeset union.

@deffn {Scheme Procedure} select-kids test-pred?
@code{select-kids} takes a converter (or a predicate) as an argument and
returns another converter.  The resulting converter applied to a nodeset
returns an ordered subset of its children that satisfy the predicate
@var{test-pred?}.
@end deffn

@deffn {Scheme Procedure} node-self pred?
Similar to @code{select-kids} except that the predicate @var{pred?} is
applied to the node itself rather than to its children.  The resulting
nodeset will contain either one component, or will be empty if the node
failed the predicate.
@end deffn

@deffn {Scheme Procedure} node-join . selectors
@example
node-join:: [LocPath] -> Node|Nodeset -> Nodeset, or
node-join:: [Converter] -> Converter
@end example

Join the sequence of location steps or paths as described above.
@end deffn

@deffn {Scheme Procedure} node-reduce . converters
@example
node-reduce:: [LocPath] -> Node|Nodeset -> Nodeset, or
node-reduce:: [Converter] -> Converter
@end example

A regular functional composition of converters.  From a different point
of view, @code{((apply node-reduce converters) nodeset)} is equivalent
to @code{(foldl apply nodeset converters)}, i.e., folding, or reducing,
a list of converters with the nodeset as a seed.
@end deffn

@deffn {Scheme Procedure} node-or . converters
@example
node-or:: [Converter] -> Converter
@end example

This combinator applies all converters to a given node and produces the
union of their results.  This combinator corresponds to a union
(@code{|} operation) for XPath location paths.
@end deffn

@deffn {Scheme Procedure} node-closure test-pred?
@example
node-closure:: Converter -> Converter
@end example

Select all @emph{descendants} of a node that satisfy a
converter-predicate @var{test-pred?}.  This combinator is similar to
@code{select-kids} but applies to grand... children as well.  This
combinator implements the @code{descendant::} XPath axis.  Conceptually,
this combinator can be expressed as

@example
(define (node-closure f)
  (node-or
    (select-kids f)
    (node-reduce (select-kids (node-typeof? '*)) (node-closure f))))
@end example

This definition, as written, looks somewhat like a fixpoint, and it will
run forever.  It is obvious however that sooner or later
@code{(select-kids (node-typeof? '*))} will return an empty nodeset.  At
this point further iterations will no longer affect the result and can
be stopped.
@end deffn

@deffn {Scheme Procedure} node-parent rootnode
@example
node-parent:: RootNode -> Converter
@end example

@code{(node-parent rootnode)} yields a converter that returns a parent
of a node it is applied to.  If applied to a nodeset, it returns the
list of parents of nodes in the nodeset.  The @var{rootnode} does not
have to be the root node of the whole SXML tree -- it may be a root node
of a branch of interest.

Given the notation of Philip Wadler's paper on semantics of XSLT,

@verbatim
  parent(x) = { y | y=subnode*(root), x=subnode(y) }
@end verbatim

Therefore, @code{node-parent} is not the fundamental converter: it can
be expressed through the existing ones.  Yet @code{node-parent} is a
rather convenient converter.  It corresponds to a @code{parent::} axis
of SXPath.  Note that the @code{parent::} axis can be used with an
attribute node as well.
@end deffn

@anchor{sxpath-procedure-docs}
@deffn {Scheme Procedure} sxpath path
Evaluate an abbreviated SXPath.

@example
sxpath:: AbbrPath -> Converter, or
sxpath:: AbbrPath -> Node|Nodeset -> Nodeset
@end example

@var{path} is a list.  It is translated to the full SXPath according to
the following rewriting rules:

@example
(sxpath '())
@result{} (node-join)

(sxpath '(path-component ...))
@result{} (node-join (sxpath1 path-component) (sxpath '(...)))

(sxpath1 '//)
@result{} (node-or
   (node-self (node-typeof? '*any*))
   (node-closure (node-typeof? '*any*)))

(sxpath1 '(equal? x))
@result{} (select-kids (node-equal? x))

(sxpath1 '(eq? x))
@result{} (select-kids (node-eq? x))

(sxpath1 ?symbol)
@result{} (select-kids (node-typeof? ?symbol)

(sxpath1 procedure)
@result{} procedure

(sxpath1 '(?symbol ...))
@result{} (sxpath1 '((?symbol) ...))

(sxpath1 '(path reducer ...))
@result{} (node-reduce (sxpath path) (sxpathr reducer) ...)

(sxpathr number)
@result{} (node-pos number)

(sxpathr path-filter)
@result{} (filter (sxpath path-filter))
@end example
@end deffn

@node sxml ssax input-parse
@subsection (sxml ssax input-parse)
@subsubsection Overview
A simple lexer.

The procedures in this module surprisingly often suffice to parse an
input stream.  They either skip, or build and return tokens, according to
inclusion or delimiting semantics.  The list of characters to expect,
include, or to break at may vary from one invocation of a function to
another.  This allows the functions to easily parse even
context-sensitive languages.

EOF is generally frowned on, and thrown up upon if encountered.
Exceptions are mentioned specifically.  The list of expected characters
(characters to skip until, or break-characters) may include an EOF
"character", which is to be coded as the symbol, @code{*eof*}.

The input stream to parse is specified as a @dfn{port}, which is usually
the last (and optional) argument.  It defaults to the current input port
if omitted.

If the parser encounters an error, it will throw an exception to the key
@code{parser-error}.  The arguments will be of the form @code{(@var{port}
@var{message} @var{specialising-msg}*)}.

The first argument is a port, which typically points to the offending
character or its neighborhood.  You can then use @code{port-column} and
@code{port-line} to query the current position.  @var{message} is the
description of the error.  Other arguments supply more details about the
problem.

@subsubsection Usage
@deffn {Scheme Procedure} peek-next-char [port]
@end deffn

@deffn {Scheme Procedure} assert-curr-char expected-chars comment [port]
@end deffn

@deffn {Scheme Procedure} skip-until arg [port]
@end deffn

@deffn {Scheme Procedure} skip-while skip-chars [port]
@end deffn

@deffn {Scheme Procedure} next-token prefix-skipped-chars break-chars [comment] [port]
@end deffn

@deffn {Scheme Procedure} next-token-of incl-list/pred [port]
@end deffn

@deffn {Scheme Procedure} read-text-line [port]
@end deffn

@deffn {Scheme Procedure} read-string n [port]
@end deffn

@deffn {Scheme Procedure} find-string-from-port? _ _ . _
Looks for @var{str} in @var{<input-port>}, optionally within the first
@var{max-no-char} characters.
@end deffn

@node sxml apply-templates
@subsection (sxml apply-templates)
@subsubsection Overview
Pre-order traversal of a tree and creation of a new tree:

@smallexample 
	apply-templates:: tree x <templates> -> <new-tree>
@end smallexample

where

@smallexample 
 <templates> ::= (<template> ...)
 <template>  ::= (<node-test> <node-test> ... <node-test> . <handler>)
 <node-test> ::= an argument to node-typeof? above
 <handler>   ::= <tree> -> <new-tree>
@end smallexample

This procedure does a @emph{normal}, pre-order traversal of an SXML
tree.  It walks the tree, checking at each node against the list of
matching templates.

If the match is found (which must be unique, i.e., unambiguous), the
corresponding handler is invoked and given the current node as an
argument.  The result from the handler, which must be a @code{<tree>},
takes place of the current node in the resulting tree.  The name of the
function is not accidental: it resembles rather closely an
@code{apply-templates} function of XSLT.

@subsubsection Usage
@deffn {Scheme Procedure} apply-templates tree templates
@end deffn