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- Info file bison.info, produced by Makeinfo, -*- Text -*- from input
file bison.texinfo.
This file documents the Bison parser generator.
Copyright (C) 1988, 1989, 1990 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License" and
"Conditions for Using Bison" are included exactly as in the original,
and provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License", "Conditions for Using Bison" and this permission notice may
be included in translations approved by the Free Software Foundation
instead of in the original English.
File: bison.info, Node: Type Decl, Next: Expect Decl, Prev: Union Decl, Up: Declarations
Nonterminal Symbols
-------------------
When you use `%union' to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used. This is done with a `%type' declaration, like this:
%type <TYPE> NONTERMINAL...
Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the `%union' to the alternative that you want (*note
Union Decl::.). You can give any number of nonterminal symbols in the
same `%type' declaration, if they have the same value type. Use
spaces to separate the symbol names.
File: bison.info, Node: Expect Decl, Next: Start Decl, Prev: Type Decl, Up: Declarations
Suppressing Conflict Warnings
-----------------------------
Bison normally warns if there are any conflicts in the grammar
(*note Shift/Reduce::.), but most real grammars have harmless
shift/reduce conflicts which are resolved in a predictable way and
would be difficult to eliminate. It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes.
You can do this with the `%expect' declaration.
The declaration looks like this:
%expect N
Here N is a decimal integer. The declaration says there should be
no warning if there are N shift/reduce conflicts and no reduce/reduce
conflicts. The usual warning is given if there are either more or
fewer conflicts, or if there are any reduce/reduce conflicts.
In general, using `%expect' involves these steps:
* Compile your grammar without `%expect'. Use the `-v' option to
get a verbose list of where the conflicts occur. Bison will also
print the number of conflicts.
* Check each of the conflicts to make sure that Bison's default
resolution is what you really want. If not, rewrite the grammar
and go back to the beginning.
* Add an `%expect' declaration, copying the number N from the
number which Bison printed.
Now Bison will stop annoying you about the conflicts you have
checked, but it will warn you again if changes in the grammer result
in additional conflicts.
File: bison.info, Node: Start Decl, Next: Pure Decl, Prev: Expect Decl, Up: Declarations
The Start-Symbol
----------------
Bison assumes by default that the start symbol for the grammar is
the first nonterminal specified in the grammar specification section.
The programmer may override this restriction with the `%start'
declaration as follows:
%start SYMBOL
File: bison.info, Node: Pure Decl, Next: Decl Summary, Prev: Start Decl, Up: Declarations
A Pure (Reentrant) Parser
-------------------------
A "reentrant" program is one which does not alter in the course of
execution; in other words, it consists entirely of "pure" (read-only)
code. Reentrancy is important whenever asynchronous execution is
possible; for example, a nonreentrant program may not be safe to call
from a signal handler. In systems with multiple threads of control, a
nonreentrant program must be called only within interlocks.
The Bison parser is not normally a reentrant program, because it
uses statically allocated variables for communication with `yylex'.
These variables include `yylval' and `yylloc'.
The Bison declaration `%pure_parser' says that you want the parser
to be reentrant. It looks like this:
%pure_parser
The effect is that the two communication variables become local
variables in `yyparse', and a different calling convention is used for
the lexical analyzer function `yylex'. *Note Pure Calling::, for the
details of this. The variable `yynerrs' also becomes local in
`yyparse' (*note Error Reporting::.). The convention for calling
`yyparse' itself is unchanged.
File: bison.info, Node: Decl Summary, Prev: Pure Decl, Up: Declarations
Bison Declaration Summary
-------------------------
Here is a summary of all Bison declarations:
`%union'
Declare the collection of data types that semantic values may have
(*note Union Decl::.).
`%token'
Declare a terminal symbol (token type name) with no precedence or
associativity specified (*note Token Decl::.).
`%right'
Declare a terminal symbol (token type name) that is
right-associative (*note Precedence Decl::.).
`%left'
Declare a terminal symbol (token type name) that is
left-associative (*note Precedence Decl::.).
`%nonassoc'
Declare a terminal symbol (token type name) that is nonassociative
(using it in a way that would be associative is a syntax error)
(*note Precedence Decl::.).
`%type'
Declare the type of semantic values for a nonterminal symbol
(*note Type Decl::.).
`%start'
Specify the grammar's start symbol (*note Start Decl::.).
`%expect'
Declare the expected number of shift-reduce conflicts (*note
Expect Decl::.).
`%pure_parser'
Request a pure (reentrant) parser program (*note Pure Decl::.).
File: bison.info, Node: Multiple Parsers, Prev: Declarations, Up: Grammar File
Multiple Parsers in the Same Program
====================================
Most programs that use Bison parse only one language and therefore
contain only one Bison parser. But what if you want to parse more
than one language with the same program? Then you need to avoid a
name conflict between different definitions of `yyparse', `yylval',
and so on.
The easy way to do this is to use the option `-p PREFIX' (*note
Invocation::.). This renames the interface functions and variables of
the Bison parser to start with PREFIX instead of `yy'. You can use
this to give each parser distinct names that do not conflict.
The precise list of symbols renamed is `yyparse', `yylex',
`yyerror', `yylval', `yychar' and `yydebug'. For example, if you use
`-p c', the names become `cparse', `clex', and so on.
*All the other variables and macros associated with Bison are not
renamed.* These others are not global; there is no conflict if the same
name is used in different parsers. For example, `YYSTYPE' is not
renamed, but defining this in different ways in different parsers
causes no trouble (*note Value Type::.).
The `-p' option works by adding macro definitions to the beginning
of the parser source file, defining `yyparse' as `PREFIXparse', and so
on. This effectively substitutes one name for the other in the entire
parser file.
File: bison.info, Node: Interface, Next: Algorithm, Prev: Grammar File, Up: Top
Parser C-Language Interface
***************************
The Bison parser is actually a C function named `yyparse'. Here we
describe the interface conventions of `yyparse' and the other
functions that it needs to use.
Keep in mind that the parser uses many C identifiers starting with
`yy' and `YY' for internal purposes. If you use such an identifier
(aside from those in this manual) in an action or in additional C code
in the grammar file, you are likely to run into trouble.
* Menu:
* Parser Function:: How to call `yyparse' and what it returns.
* Lexical:: You must supply a function `yylex' which reads tokens.
* Error Reporting:: You must supply a function `yyerror'.
* Action Features:: Special features for use in actions.
File: bison.info, Node: Parser Function, Next: Lexical, Prev: Interface, Up: Interface
The Parser Function `yyparse'
=============================
You call the function `yyparse' to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
write an action which directs `yyparse' to return immediately without
reading further.
The value returned by `yyparse' is 0 if parsing was successful
(return is due to end-of-input).
The value is 1 if parsing failed (return is due to a syntax error).
In an action, you can cause immediate return from `yyparse' by using
these macros:
`YYACCEPT'
Return immediately with value 0 (to report success).
`YYABORT'
Return immediately with value 1 (to report failure).
File: bison.info, Node: Lexical, Next: Error Reporting, Prev: Parser Function, Up: Interface
The Lexical Analyzer Function `yylex'
=====================================
The "lexical analyzer" function, `yylex', recognizes tokens from
the input stream and returns them to the parser. Bison does not create
this function automatically; you must write it so that `yyparse' can
call it. The function is sometimes referred to as a lexical scanner.
In simple programs, `yylex' is often defined at the end of the Bison
grammar file. If `yylex' is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available
there. To do this, use the `-d' option when you run Bison, so that it
will write these macro definitions into a separate header file
`NAME.tab.h' which you can include in the other source files that need
it. *Note Invocation::.
* Menu:
* Calling Convention:: How `yyparse' calls `yylex'.
* Token Values:: How `yylex' must return the semantic value
of the token it has read.
* Token Positions:: How `yylex' must return the text position
(line number, etc.) of the token, if the
actions want that.
* Pure Calling:: How the calling convention differs
in a pure parser (*note Pure Decl::.).
File: bison.info, Node: Calling Convention, Next: Token Values, Prev: Lexical, Up: Lexical
Calling Convention for `yylex'
------------------------------
The value that `yylex' returns must be the numeric code for the type
of token it has just found, or 0 for end-of-input.
When a token is referred to in the grammar rules by a name, that
name in the parser file becomes a C macro whose definition is the
proper numeric code for that token type. So `yylex' can use the name
to indicate that type. *Note Symbols::.
When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type. So `yylex' can simply return that character code. The
null character must not be used this way, because its code is zero and
that is what signifies end-of-input.
Here is an example showing these things:
yylex ()
{
...
if (c == EOF) /* Detect end of file. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for `+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the `lex'
utility can be used without change as the definition of `yylex'.
File: bison.info, Node: Token Values, Next: Token Positions, Prev: Calling Convention, Up: Lexical
Semantic Values of Tokens
-------------------------
In an ordinary (nonreentrant) parser, the semantic value of the
token must be stored into the global variable `yylval'. When you are
using just one data type for semantic values, `yylval' has that type.
Thus, if the type is `int' (the default), you might write this in
`yylex':
...
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
When you are using multiple data types, `yylval''s type is a union
made from the `%union' declaration (*note Union Decl::.). So when you
store a token's value, you must use the proper member of the union.
If the `%union' declaration looks like this:
%union {
int intval;
double val;
symrec *tptr;
}
then the code in `yylex' might look like this:
...
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
File: bison.info, Node: Token Positions, Next: Pure Calling, Prev: Token Values, Up: Lexical
Textual Positions of Tokens
---------------------------
If you are using the `@N'-feature (*note Action Features::.) in
actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in `yylex'. The function
`yyparse' expects to find the textual location of a token just parsed
in the global variable `yylloc'. So `yylex' must store the proper
data in that variable. The value of `yylloc' is a structure and you
need only initialize the members that are going to be used by the
actions. The four members are called `first_line', `first_column',
`last_line' and `last_column'. Note that the use of this feature
makes the parser noticeably slower.
The data type of `yylloc' has the name `YYLTYPE'.
File: bison.info, Node: Pure Calling, Prev: Token Positions, Up: Lexical
Calling for Pure Parsers
------------------------
When you use the Bison declaration `%pure_parser' to request a pure,
reentrant parser, the global communication variables `yylval' and
`yylloc' cannot be used. (*Note Pure Decl::.) In such parsers the
two global variables are replaced by pointers passed as arguments to
`yylex'. You must declare them as shown here, and pass the
information back by storing it through those pointers.
yylex (lvalp, llocp)
YYSTYPE *lvalp;
YYLTYPE *llocp;
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
If the grammar file does not use the `@' constructs to refer to
textual positions, then the type `YYLTYPE' will not be defined. In
this case, omit the second argument; `yylex' will be called with only
one argument.
File: bison.info, Node: Error Reporting, Next: Action Features, Prev: Lexical, Up: Interface
The Error Reporting Function `yyerror'
======================================
The Bison parser detects a "parse error" or "syntax error" whenever
it reads a token which cannot satisfy any syntax rule. A action in
the grammar can also explicitly proclaim an error, using the macro
`YYERROR' (*note Action Features::.).
The Bison parser expects to report the error by calling an error
reporting function named `yyerror', which you must supply. It is
called by `yyparse' whenever a syntax error is found, and it receives
one argument. For a parse error, the string is always `"parse error"'.
The parser can detect one other kind of error: stack overflow. This
happens when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
parser extends its stack automatically up to a very large limit. But
if overflow happens, `yyparse' calls `yyerror' in the usual fashion,
except that the argument string is `"parser stack overflow"'.
The following definition suffices in simple programs:
yyerror (s)
char *s;
{
fprintf (stderr, "%s\n", s);
}
After `yyerror' returns to `yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::.). If recovery is impossible, `yyparse' will
immediately return 1.
The variable `yynerrs' contains the number of syntax errors
encountered so far. Normally this variable is global; but if you
request a pure parser (*note Pure Decl::.) then it is a local variable
which only the actions can access.
File: bison.info, Node: Action Features, Prev: Error Reporting, Up: Interface
Special Features for Use in Actions
===================================
Here is a table of Bison constructs, variables and macros that are
useful in actions.
`$$'
Acts like a variable that contains the semantic value for the
grouping made by the current rule. *Note Actions::.
`$N'
Acts like a variable that contains the semantic value for the Nth
component of the current rule. *Note Actions::.
`$<TYPEALT>$'
Like `$$' but specifies alternative TYPEALT in the union
specified by the `%union' declaration. *Note Action Types::.
`$<TYPEALT>N'
Like `$N' but specifies alternative TYPEALT in the union
specified by the `%union' declaration. *Note Action Types::.
`YYABORT;'
Return immediately from `yyparse', indicating failure. *Note
Parser Function::.
`YYACCEPT;'
Return immediately from `yyparse', indicating success. *Note
Parser Function::.
`YYBACKUP (TOKEN, VALUE);'
Unshift a token. This macro is allowed only for rules that reduce
a single value, and only when there is no look-ahead token. It
installs a look-ahead token with token type TOKEN and semantic
value VALUE; then it discards the value that was going to be
reduced by this rule.
If the macro is used when it is not valid, such as when there is
a look-ahead token already, then it reports a syntax error with a
message `cannot back up' and performs ordinary error recovery.
In either case, the rest of the action is not executed.
`YYEMPTY'
Value stored in `yychar' when there is no look-ahead token.
`YYERROR;'
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error;
however, it does not call `yyerror', and does not print any
message. If you want to print an error message, call `yyerror'
explicitly before the `YYERROR;' statement. *Note Error
Recovery::.
`YYRECOVERING'
This macro stands for an expression that has the value 1 when the
parser is recovering from a syntax error, and 0 the rest of the
time. *Note Error Recovery::.
`yychar'
Variable containing the current look-ahead token. (In a pure
parser, this is actually a local variable within `yyparse'.)
When there is no look-ahead token, the value `YYEMPTY' is stored
in the variable. *Note Look-Ahead::.
`yyclearin;'
Discard the current look-ahead token. This is useful primarily in
error rules. *Note Error Recovery::.
`yyerrok;'
Resume generating error messages immediately for subsequent syntax
errors. This is useful primarily in error rules. *Note Error
Recovery::.
`@N'
Acts like a structure variable containing information on the line
numbers and column numbers of the Nth component of the current
rule. The structure has four members, like this:
struct {
int first_line, last_line;
int first_column, last_column;
};
Thus, to get the starting line number of the third component, use
`@3.first_line'.
In order for the members of this structure to contain valid
information, you must make `yylex' supply this information about
each token. If you need only certain members, then `yylex' need
only fill in those members.
The use of this feature makes the parser noticeably slower.
File: bison.info, Node: Algorithm, Next: Error Recovery, Prev: Interface, Up: Top
The Bison Parser Algorithm
**************************
As Bison reads tokens, it pushes them onto a stack along with their
semantic values. The stack is called the "parser stack". Pushing a
token is traditionally called "shifting".
For example, suppose the infix calculator has read `1 + 5 *', with a
`3' to come. The stack will have four elements, one for each token
that was shifted.
But the stack does not always have an element for each token read.
When the last N tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule. This is
called "reduction". Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand side)
of that rule. Running the rule's action is part of the process of
reduction, because this is what computes the semantic value of the
resulting grouping.
For example, if the infix calculator's parser stack contains this:
1 + 5 * 3
and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:
expr: expr '*' expr;
Then the stack contains just these three elements:
1 + 15
At this point, another reduction can be made, resulting in the single
value 16. Then the newline token can be shifted.
The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar's
start-symbol (*note Language and Grammar::.).
This kind of parser is known in the literature as a bottom-up
parser.
* Menu:
* Look-Ahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Stack Overflow:: What happens when stack gets full. How to avoid it.
File: bison.info, Node: Look-Ahead, Next: Shift/Reduce, Prev: Algorithm, Up: Algorithm
Look-Ahead Tokens
=================
The Bison parser does *not* always reduce immediately as soon as the
last N tokens and groupings match a rule. This is because such a
simple strategy is inadequate to handle most languages. Instead, when
a reduction is possible, the parser sometimes "looks ahead" at the next
token in order to decide what to do.
When a token is read, it is not immediately shifted; first it
becomes the "look-ahead token", which is not on the stack. Now the
parser can perform one or more reductions of tokens and groupings on
the stack, while the look-ahead token remains off to the side. When
no more reductions should take place, the look-ahead token is shifted
onto the stack. This does not mean that all possible reductions have
been done; depending on the token type of the look-ahead token, some
rules may choose to delay their application.
Here is a simple case where look-ahead is needed. These three
rules define expressions which contain binary addition operators and
postfix unary factorial operators (`!'), and allow parentheses for
grouping.
expr: term '+' expr
| term
;
term: '(' expr ')'
| term '!'
| NUMBER
;
Suppose that the tokens `1 + 2' have been read and shifted; what
should be done? If the following token is `)', then the first three
tokens must be reduced to form an `expr'. This is the only valid
course, because shifting the `)' would produce a sequence of symbols
`term ')'', and no rule allows this.
If the following token is `!', then it must be shifted immediately
so that `2 !' can be reduced to make a `term'. If instead the parser
were to reduce before shifting, `1 + 2' would become an `expr'. It
would then be impossible to shift the `!' because doing so would
produce on the stack the sequence of symbols `expr '!''. No rule
allows that sequence.
The current look-ahead token is stored in the variable `yychar'.
*Note Action Features::.
File: bison.info, Node: Shift/Reduce, Next: Precedence, Prev: Look-Ahead, Up: Algorithm
Shift/Reduce Conflicts
======================
Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
(Here we assume that `IF', `THEN' and `ELSE' are terminal symbols for
specific keyword tokens.)
When the `ELSE' token is read and becomes the look-ahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
`ELSE', because that would lead to eventual reduction by the second
rule.
This situation, where either a shift or a reduction would be valid,
is called a "shift/reduce conflict". Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations. To see the reason for this, let's
contrast it with the other alternative.
Since the parser prefers to shift the `ELSE', the result is to
attach the else-clause to the innermost if-statement, making these two
inputs equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); else lose; end;
But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); end; else lose;
The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate. The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce. (It would
ideally be cleaner to write an unambiguous grammar, but that is very
hard to do in this case.) This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
"dangling `else'" ambiguity.
To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, use the `%expect N' declaration. There will
be no warning as long as the number of shift/reduce conflicts is
exactly N. *Note Expect Decl::.
File: bison.info, Node: Precedence, Next: Contextual Precedence, Prev: Shift/Reduce, Up: Algorithm
Operator Precedence
===================
Another situation where shift/reduce conflicts appear is in
arithmetic expressions. Here shifting is not always the preferred
resolution; the Bison declarations for operator precedence allow you
to specify when to shift and when to reduce.
* Menu:
* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence in Bison grammars.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
File: bison.info, Node: Why Precedence, Next: Using Precedence, Prev: Precedence, Up: Precedence
When Precedence is Needed
-------------------------
Consider the following ambiguous grammar fragment (ambiguous
because the input `1 - 2 * 3' can be parsed in two different ways):
expr: expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
...
;
Suppose the parser has seen the tokens `1', `-' and `2'; should it
reduce them via the rule for the addition operator? It depends on the
next token. Of course, if the next token is `)', we must reduce;
shifting is invalid because no single rule can reduce the token
sequence `- 2 )' or anything starting with that. But if the next
token is `*' or `<', we have a choice: either shifting or reduction
would allow the parse to complete, but with different results.
To decide which one Bison should do, we must consider the results.
If the next operator token OP is shifted, then it must be reduced
first in order to permit another opportunity to reduce the sum. The
result is (in effect) `1 - (2 OP 3)'. On the other hand, if the
subtraction is reduced before shifting OP, the result is
`(1 - 2) OP 3'. Clearly, then, the choice of shift or reduce should
depend on the relative precedence of the operators `-' and OP: `*'
should be shifted first, but not `<'.
What about input such as `1 - 2 - 5'; should this be `(1 - 2) - 5'
or should it be `1 - (2 - 5)'? For most operators we prefer the
former, which is called "left association". The latter alternative,
"right association", is desirable for assignment operators. The
choice of left or right association is a matter of whether the parser
chooses to shift or reduce when the stack contains `1 - 2' and the
look-ahead token is `-': shifting makes right-associativity.
File: bison.info, Node: Using Precedence, Next: Precedence Examples, Prev: Why Precedence, Up: Precedence
Specifying Operator Precedence
------------------------------
Bison allows you to specify these choices with the operator
precedence declarations `%left' and `%right'. Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared. The `%left' declaration makes all
those operators left-associative and the `%right' declaration makes
them right-associative. A third alternative is `%nonassoc', which
declares that it is a syntax error to find the same operator twice "in
a row".
The relative precedence of different operators is controlled by the
order in which they are declared. The first `%left' or `%right'
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.
File: bison.info, Node: Precedence Examples, Next: How Precedence, Prev: Using Precedence, Up: Precedence
Precedence Examples
-------------------
In our example, we would want the following declarations:
%left '<'
%left '-'
%left '*'
In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence. For example,
`'+'' is declared with `'-'':
%left '<' '>' '=' NE LE GE
%left '+' '-'
%left '*' '/'
(Here `NE' and so on stand for the operators for "not equal" and so
on. We assume that these tokens are more than one character long and
therefore are represented by names, not character literals.)
File: bison.info, Node: How Precedence, Prev: Precedence Examples, Up: Precedence
How Precedence Works
--------------------
The first effect of the precedence declarations is to assign
precedence levels to the terminal symbols declared. The second effect
is to assign precedence levels to certain rules: each rule gets its
precedence from the last terminal symbol mentioned in the components.
(You can also specify explicitly the precedence of a rule. *Note
Contextual Precedence::.)
Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the look-ahead
token. If the token's precedence is higher, the choice is to shift.
If the rule's precedence is higher, the choice is to reduce. If they
have equal precedence, the choice is made based on the associativity
of that precedence level. The verbose output file made by `-v' (*note
Invocation::.) says how each conflict was resolved.
Not all rules and not all tokens have precedence. If either the
rule or the look-ahead token has no precedence, then the default is to
shift.
File: bison.info, Node: Contextual Precedence, Next: Parser States, Prev: Precedence, Up: Algorithm
Context-Dependent Precedence
============================
Often the precedence of an operator depends on the context. This
sounds outlandish at first, but it is really very common. For
example, a minus sign typically has a very high precedence as a unary
operator, and a somewhat lower precedence (lower than multiplication)
as a binary operator.
The Bison precedence declarations, `%left', `%right' and
`%nonassoc', can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the `%prec'
modifier for rules.
The `%prec' modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that
rule. It's not necessary for that symbol to appear otherwise in the
rule. The modifier's syntax is:
%prec TERMINAL-SYMBOL
and it is written after the components of the rule. Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way. The
altered rule precedence then affects how conflicts involving that rule
are resolved (*note Precedence::.).
Here is how `%prec' solves the problem of unary minus. First,
declare a precedence for a fictitious terminal symbol named `UMINUS'.
There are no tokens of this type, but the symbol serves to stand for
its precedence:
...
%left '+' '-'
%left '*'
%left UMINUS
Now the precedence of `UMINUS' can be used in specific rules:
exp: ...
| exp '-' exp
...
| '-' exp %prec UMINUS
File: bison.info, Node: Parser States, Next: Reduce/Reduce, Prev: Contextual Precedence, Up: Algorithm
Parser States
=============
The function `yyparse' is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes;
they represent the entire sequence of terminal and nonterminal symbols
at or near the top of the stack. The current state collects all the
information about previous input which is relevant to deciding what to
do next.
Each time a look-ahead token is read, the current parser state
together with the type of look-ahead token are looked up in a table.
This table entry can say, "Shift the look-ahead token." In this case,
it also specifies the new parser state, which is pushed onto the top
of the parser stack. Or it can say, "Reduce using rule number N."
This means that a certain of tokens or groupings are taken off the top
of the stack, and replaced by one grouping. In other words, that
number of states are popped from the stack, and one new state is
pushed.
There is one other alternative: the table can say that the
look-ahead token is erroneous in the current state. This causes error
processing to begin (*note Error Recovery::.).
File: bison.info, Node: Reduce/Reduce, Next: Mystery Conflicts, Prev: Parser States, Up: Algorithm
Reduce/Reduce Conflicts
=======================
A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input. This usually indicates a serious
error in the grammar.
For example, here is an erroneous attempt to define a sequence of
zero or more `word' groupings.
sequence: /* empty */
{ printf ("empty sequence\n"); }
| word
{ printf ("single word %s\n", $1); }
| sequence word
{ printf ("added word %s\n", $2); }
;
The error is an ambiguity: there is more than one way to parse a single
`word' into a `sequence'. It could be reduced directly via the second
rule. Alternatively, nothing-at-all could be reduced into a
`sequence' via the first rule, and this could be combined with the
`word' using the third rule.
You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or
not. But it does affect which actions are run. One parsing order
runs the second rule's action; the other runs the first rule's action
and the third rule's action. In this example, the output of the
program changes.
Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on
this. Every reduce/reduce conflict must be studied and usually
eliminated. Here is the proper way to define `sequence':
sequence: /* empty */
{ printf ("empty sequence\n"); }
| sequence word
{ printf ("added word %s\n", $2); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
The intention here is to define a sequence which can contain either
`word' or `redirect' groupings. The individual definitions of
`sequence', `words' and `redirects' are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!
Consider: nothing-at-all could be a `words'. Or it could be two
`words' in a row, or three, or any number. It could equally well be a
`redirects', or two, or any number. Or it could be a `words' followed
by three `redirects' and another `words'. And so on.
Here are two ways to correct these rules. First, to make it a
single level of sequence:
sequence: /* empty */
| sequence word
| sequence redirect
;
Second, to prevent either a `words' or a `redirects' from being
empty:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: word
| words word
;
redirects:redirect
| redirects redirect
;
File: bison.info, Node: Mystery Conflicts, Next: Stack Overflow, Prev: Reduce/Reduce, Up: Algorithm
Mysterious Reduce/Reduce Conflicts
==================================
Sometimes reduce/reduce conflicts can occur that don't look
warranted. Here is an example:
%token ID
%%
def: param_spec return_spec ','
;
param_spec:
type
| name_list ':' type
;
return_spec:
type
| name ':' type
;
type: ID
;
name: ID
;
name_list:
name
| name ',' name_list
;
It would seem that this grammar can be parsed with only a single
token of look-ahead: when a `param_spec' is being read, an `ID' is a
`name' if a comma or colon follows, or a `type' if another `ID'
follows. In other words, this grammar is LR(1).
However, Bison, like most parser generators, cannot actually handle
all LR(1) grammars. In this grammar, two contexts, that after an `ID'
at the beginning of a `param_spec' and likewise at the beginning of a
`return_spec', are similar enough that Bison assumes they are the
same. They appear similar because the same set of rules would be
active--the rule for reducing to a `name' and that for reducing to a
`type'. Bison is unable to determine at that stage of processing that
the rules would require different look-ahead tokens in the two
contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not LALR(1).
In general, it is better to fix deficiencies than to document them.
But this particular deficiency is intrinsically hard to fix; parser
generators that can handle LR(1) grammars are hard to write and tend to
produce parsers that are very large. In practice, Bison is more useful
as it is now.
When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make
them look distinct. In the above example, adding one rule to
`return_spec' as follows makes the problem go away:
%token BOGUS
...
%%
...
return_spec:
type
| name ':' type
/* This rule is never used. */
| ID BOGUS
;
This corrects the problem because it introduces the possibility of
an additional active rule in the context after the `ID' at the
beginning of `return_spec'. This rule is not active in the
corresponding context in a `param_spec', so the two contexts receive
distinct parser states. As long as the token `BOGUS' is never
generated by `yylex', the added rule cannot alter the way actual input
is parsed.
In this particular example, there is another way to solve the
problem: rewrite the rule for `return_spec' to use `ID' directly
instead of via `name'. This also causes the two confusing contexts to
have different sets of active rules, because the one for `return_spec'
activates the altered rule for `return_spec' rather than the one for
`name'.
param_spec:
type
| name_list ':' type
;
return_spec:
type
| ID ':' type
;
File: bison.info, Node: Stack Overflow, Prev: Mystery Conflicts, Up: Algorithm
Stack Overflow, and How to Avoid It
===================================
The Bison parser stack can overflow if too many tokens are shifted
and not reduced. When this happens, the parser function `yyparse'
returns a nonzero value, pausing only to call `yyerror' to report the
overflow.
By defining the macro `YYMAXDEPTH', you can control how deep the
parser stack can become before a stack overflow occurs. Define the
macro with a value that is an integer. This value is the maximum
number of tokens that can be shifted (and not reduced) before overflow.
It must be a constant expression whose value is known at compile time.
The stack space allowed is not necessarily allocated. If you
specify a large value for `YYMAXDEPTH', the parser actually allocates
a small stack at first, and then makes it bigger by stages as needed.
This increasing allocation happens automatically and silently.
Therefore, you do not need to make `YYMAXDEPTH' painfully small merely
to save space for ordinary inputs that do not need much stack.
The default value of `YYMAXDEPTH', if you do not define it, is
10000.
You can control how much stack is allocated initially by defining
the macro `YYINITDEPTH'. This value too must be a compile-time
constant integer. The default is 200.
File: bison.info, Node: Error Recovery, Next: Context Dependency, Prev: Algorithm, Up: Top
Error Recovery
**************
It is not usually acceptable to have a program terminate on a parse
error. For example, a compiler should recover sufficiently to parse
the rest of the input file and check it for errors; a calculator
should accept another expression.
In a simple interactive command parser where each input is one
line, it may be sufficient to allow `yyparse' to return 1 on error and
have the caller ignore the rest of the input line when that happens
(and then call `yyparse' again). But this is inadequate for a
compiler, because it forgets all the syntactic context leading up to
the error. A syntax error deep within a function in the compiler
input should not cause the compiler to treat the following line like
the beginning of a source file.
You can define how to recover from a syntax error by writing rules
to recognize the special token `error'. This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an `error' token whenever a
syntax error happens; if you have provided a rule to recognize this
token in the current context, the parse can continue.
For example:
stmnts: /* empty string */
| stmnts '\n'
| stmnts exp '\n'
| stmnts error '\n'
The fourth rule in this example says that an error followed by a
newline makes a valid addition to any `stmnts'.
What happens if a syntax error occurs in the middle of an `exp'?
The error recovery rule, interpreted strictly, applies to the precise
sequence of a `stmnts', an `error' and a newline. If an error occurs
in the middle of an `exp', there will probably be some additional
tokens and subexpressions on the stack after the last `stmnts', and
there will be tokens to read before the next newline. So the rule is
not applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding
part of the semantic context and part of the input. First it discards
states and objects from the stack until it gets back to a state in
which the `error' token is acceptable. (This means that the
subexpressions already parsed are discarded, back to the last complete
`stmnts'.) At this point the `error' token can be shifted. Then, if
the old look-ahead token is not acceptable to be shifted next, the
parser reads tokens and discards them until it finds a token which is
acceptable. In this example, Bison reads and discards input until the
next newline so that the fourth rule can apply.
The choice of error rules in the grammar is a choice of strategies
for error recovery. A simple and useful strategy is simply to skip
the rest of the current input line or current statement if an error is
detected:
stmnt: error ';' /* on error, skip until ';' is read */
It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed. Otherwise the
close-delimiter will probably appear to be unmatched, and generate
another, spurious error message:
primary: '(' expr ')'
| '(' error ')'
...
;
Error recovery strategies are necessarily guesses. When they guess
wrong, one syntax error often leads to another. In the above example,
the error recovery rule guesses that an error is due to bad input
within one `stmnt'. Suppose that instead a spurious semicolon is
inserted in the middle of a valid `stmnt'. After the error recovery
rule recovers from the first error, another syntax error will be found
straightaway, since the text following the spurious semicolon is also
an invalid `stmnt'.
To prevent an outpouring of error messages, the parser will output
no error message for another syntax error that happens shortly after
the first; only after three consecutive input tokens have been
successfully shifted will error messages resume.
Note that rules which accept the `error' token may have actions,
just as any other rules can.
You can make error messages resume immediately by using the macro
`yyerrok' in an action. If you do this in the error rule's action, no
error messages will be suppressed. This macro requires no arguments;
`yyerrok;' is a valid C statement.
The previous look-ahead token is reanalyzed immediately after an
error. If this is unacceptable, then the macro `yyclearin' may be
used to clear this token. Write the statement `yyclearin;' in the
error rule's action.
For example, suppose that on a parse error, an error handling
routine is called that advances the input stream to some point where
parsing should once again commence. The next symbol returned by the
lexical scanner is probably correct. The previous look-ahead token
ought to be discarded with `yyclearin;'.
The macro `YYRECOVERING' stands for an expression that has the
value 1 when the parser is recovering from a syntax error, and 0 the
rest of the time. A value of 1 indicates that error messages are
currently suppressed for new syntax errors.
File: bison.info, Node: Context Dependency, Next: Debugging, Prev: Error Recovery, Up: Top
Handling Context Dependencies
*****************************
The Bison paradigm is to parse tokens first, then group them into
larger syntactic units. In many languages, the meaning of a token is
affected by its context. Although this violates the Bison paradigm,
certain techniques (known as "kludges") may enable you to write Bison
parsers for such languages.
* Menu:
* Semantic Tokens:: Token parsing can depend on the semantic context.
* Lexical Tie-ins:: Token parsing can depend on the syntactic context.
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.
(Actually, "kludge" means any technique that gets its job done but
is neither clean nor robust.)
|
