This says when, why and how to use the exceptional
action in the middle of a rule.
+Actions in Mid-Rule
+
+* Using Mid-Rule Actions:: Putting an action in the middle of a rule.
+* Mid-Rule Action Translation:: How mid-rule actions are actually processed.
+* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts.
+
Tracking Locations
* Location Type:: Specifying a data type for locations.
@node Grammar Outline
@section Outline of a Bison Grammar
+@cindex comment
+@findex // @dots{}
+@findex /* @dots{} */
A Bison grammar file has four main sections, shown here with the
appropriate delimiters:
@end example
Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
-As a GNU extension, @samp{//} introduces a comment that
-continues until end of line.
+As a GNU extension, @samp{//} introduces a comment that continues until end
+of line.
@menu
* Prologue:: Syntax and usage of the prologue.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.
+@menu
+* Using Mid-Rule Actions:: Putting an action in the middle of a rule.
+* Mid-Rule Action Translation:: How mid-rule actions are actually processed.
+* Mid-Rule Conflicts:: Mid-rule actions can cause conflicts.
+@end menu
+
+@node Using Mid-Rule Actions
+@subsubsection Using Mid-Rule Actions
+
A mid-rule action may refer to the components preceding it using
@code{$@var{n}}, but it may not refer to subsequent components because
it is run before they are parsed.
@example
@group
stmt:
- LET '(' var ')'
- @{ $<context>$ = push_context (); declare_variable ($3); @}
+ "let" '(' var ')'
+ @{
+ $<context>$ = push_context ();
+ declare_variable ($3);
+ @}
stmt
- @{ $$ = $6; pop_context ($<context>5); @}
+ @{
+ $$ = $6;
+ pop_context ($<context>5);
+ @}
@end group
@end example
@code{context} in the data-type union. Then it calls
@code{declare_variable} to add the new variable to that list. Once the
first action is finished, the embedded statement @code{stmt} can be
-parsed. Note that the mid-rule action is component number 5, so the
-@samp{stmt} is component number 6.
+parsed.
+
+Note that the mid-rule action is component number 5, so the @samp{stmt} is
+component number 6. Named references can be used to improve the readability
+and maintainability (@pxref{Named References}):
+
+@example
+@group
+stmt:
+ "let" '(' var ')'
+ @{
+ $<context>let = push_context ();
+ declare_variable ($3);
+ @}[let]
+ stmt
+ @{
+ $$ = $6;
+ pop_context ($<context>let);
+ @}
+@end group
+@end example
After the embedded statement is parsed, its semantic value becomes the
value of the entire @code{let}-statement. Then the semantic value from the
let stmt
@{
$$ = $2;
- pop_context ($1);
+ pop_context ($let);
@};
let:
- LET '(' var ')'
+ "let" '(' var ')'
@{
- $$ = push_context ();
+ $let = push_context ();
declare_variable ($3);
@};
Any mid-rule action can be converted to an end-of-rule action in this way, and
this is what Bison actually does to implement mid-rule actions.
+@node Mid-Rule Action Translation
+@subsubsection Mid-Rule Action Translation
+@vindex $@@@var{n}
+@vindex @@@var{n}
+
+As hinted earlier, mid-rule actions are actually transformed into regular
+rules and actions. The various reports generated by Bison (textual,
+graphical, etc., see @ref{Understanding, , Understanding Your Parser})
+reveal this translation, best explained by means of an example. The
+following rule:
+
+@example
+exp: @{ a(); @} "b" @{ c(); @} @{ d(); @} "e" @{ f(); @};
+@end example
+
+@noindent
+is translated into:
+
+@example
+$@@1: /* empty */ @{ a(); @};
+$@@2: /* empty */ @{ c(); @};
+$@@3: /* empty */ @{ d(); @};
+exp: $@@1 "b" $@@2 $@@3 "e" @{ f(); @};
+@end example
+
+@noindent
+with new nonterminal symbols @code{$@@@var{n}}, where @var{n} is a number.
+
+A mid-rule action is expected to generate a value if it uses @code{$$}, or
+the (final) action uses @code{$@var{n}} where @var{n} denote the mid-rule
+action. In that case its nonterminal is rather named @code{@@@var{n}}:
+
+@example
+exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @};
+@end example
+
+@noindent
+is translated into
+
+@example
+@@1: /* empty */ @{ a(); @};
+@@2: /* empty */ @{ $$ = c(); @};
+$@@3: /* empty */ @{ d(); @};
+exp: @@1 "b" @@2 $@@3 "e" @{ f = $1; @}
+@end example
+
+There are probably two errors in the above example: the first mid-rule
+action does not generate a value (it does not use @code{$$} although the
+final action uses it), and the value of the second one is not used (the
+final action does not use @code{$3}). Bison reports these errors when the
+@code{midrule-value} warnings are enabled (@pxref{Invocation, ,Invoking
+Bison}):
+
+@example
+$ bison -fcaret -Wmidrule-value mid.y
+@group
+mid.y:2.6-13: warning: unset value: $$
+ exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @};
+ ^^^^^^^^
+@end group
+@group
+mid.y:2.19-31: warning: unused value: $3
+ exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @};
+ ^^^^^^^^^^^^^
+@end group
+@end example
+
+
+@node Mid-Rule Conflicts
+@subsubsection Conflicts due to Mid-Rule Actions
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute the
action. For example, the following two rules, without mid-rule actions,
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.
+
@node Tracking Locations
@section Tracking Locations
@cindex location
@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
Parsers}, for the details of this. The variable @code{yynerrs}
becomes local in @code{yyparse} in pull mode but it becomes a member
-of yypstate in push mode. (@pxref{Error Reporting, ,The Error
+of @code{yypstate} in push mode. (@pxref{Error Reporting, ,The Error
Reporting Function @code{yyerror}}). The convention for calling
@code{yyparse} itself is unchanged.
@code{yyerror} are:
@example
-void yyerror (char const *msg); /* Yacc parsers. */
-void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
+void yyerror (char const *msg); // Yacc parsers.
+void yyerror (YYLTYPE *locp, char const *msg); // GLR parsers.
@end example
But if @samp{%locations %define api.pure %parse-param @{int *nastiness@}} is
@c api.token.prefix
+@c ================================================== api.value.type
+@item %define api.value.type variant
+@findex %define api.value.type variant
+
+@itemize @bullet
+@item Language(s):
+C++
+
+@item Purpose:
+Request variant-based semantic values.
+@xref{C++ Variants}.
+
+@c FIXME: @item Accepted Values:
+@c FIXME: Boolean.
+
+@c @item Default Value:
+@c @code{false}
+@ @end itemize
+@c api.value.type
+
@c ================================================== lr.default-reduction
@item lr.default-reduction
@item Default Value: @code{false}
@end itemize
@c parse.trace
-
-@c ================================================== variant
-@item variant
-@findex %define variant
-
-@itemize @bullet
-@item Language(s):
-C++
-
-@item Purpose:
-Request variant-based semantic values.
-@xref{C++ Variants}.
-
-@item Accepted Values:
-Boolean.
-
-@item Default Value:
-@code{false}
-@end itemize
-@c variant
@end table
-
@node %code Summary
@subsection %code Summary
@findex %code
@code{YYDEBUG} (not renamed) is used as a default value:
@example
-/* Enabling traces. */
+/* Debug traces. */
#ifndef CDEBUG
# if defined YYDEBUG
# if YYDEBUG
@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl, ,A Push Parser}.
-@deftypefun int yypush_parse (yypstate *yyps)
+@deftypefun int yypush_parse (yypstate *@var{yyps})
The value returned by @code{yypush_parse} is the same as for yyparse with
the following exception: it returns @code{YYPUSH_MORE} if more input is
required to finish parsing the grammar.
declaration is used.
@xref{Push Decl, ,A Push Parser}.
-@deftypefun int yypull_parse (yypstate *yyps)
+@deftypefun int yypull_parse (yypstate *@var{yyps})
The value returned by @code{yypull_parse} is the same as for @code{yyparse}.
@end deftypefun
@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl, ,A Push Parser}.
-@deftypefun void yypstate_delete (yypstate *yyps)
+@deftypefun void yypstate_delete (yypstate *@var{yyps})
This function will reclaim the memory associated with a parser instance.
After this call, you should no longer attempt to use the parser instance.
@end deftypefun
@end deffn
@deffn {Value} @@$
-@findex @@$
Acts like a structure variable containing information on the textual
location of the grouping made by the current rule. @xref{Tracking
Locations}.
@item
@cindex bison-i18n.m4
Into the directory containing the GNU Autoconf macros used
-by the package---often called @file{m4}---copy the
+by the package ---often called @file{m4}--- copy the
@file{bison-i18n.m4} file installed by Bison under
@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
For example:
Developing a parser can be a challenge, especially if you don't understand
the algorithm (@pxref{Algorithm, ,The Bison Parser Algorithm}). This
-chapter explains how to generate and read the detailed description of the
-automaton, and how to enable and understand the parser run-time traces.
+chapter explains how understand and debug a parser.
+
+The first sections focus on the static part of the parser: its structure.
+They explain how to generate and read the detailed description of the
+automaton. There are several formats available:
+@itemize @minus
+@item
+as text, see @ref{Understanding, , Understanding Your Parser};
+
+@item
+as a graph, see @ref{Graphviz,, Visualizing Your Parser};
+
+@item
+or as a markup report that can be turned, for instance, into HTML, see
+@ref{Xml,, Visualizing your parser in multiple formats}.
+@end itemize
+
+The last section focuses on the dynamic part of the parser: how to enable
+and understand the parser run-time traces (@pxref{Tracing, ,Tracing Your
+Parser}).
@menu
* Understanding:: Understanding the structure of your parser.
As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm})
Bison parsers are @dfn{shift/reduce automata}. In some cases (much more
frequent than one would hope), looking at this automaton is required to
-tune or simply fix a parser. Bison provides two different
-representation of it, either textually or graphically (as a DOT file).
+tune or simply fix a parser.
The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @ref{Invocation, , Invoking
@example
%token NUM STR
+@group
%left '+' '-'
%left '*'
+@end group
%%
+@group
exp:
exp '+' exp
| exp '-' exp
| exp '/' exp
| NUM
;
+@end group
useless: STR;
%%
@end example
@example
calc.y: warning: 1 nonterminal useless in grammar
calc.y: warning: 1 rule useless in grammar
-calc.y:11.1-7: warning: nonterminal useless in grammar: useless
-calc.y:11.10-12: warning: rule useless in grammar: useless: STR
+calc.y:12.1-7: warning: nonterminal useless in grammar: useless
+calc.y:12.10-12: warning: rule useless in grammar: useless: STR
calc.y: conflicts: 7 shift/reduce
@end example
the location of the input cursor.
@example
-state 0
+State 0
0 $accept: . exp $end
@option{--report=itemset} to list the derived items as well:
@example
-state 0
+State 0
0 $accept: . exp $end
1 exp: . exp '+' exp
In the state 1@dots{}
@example
-state 1
+State 1
5 exp: NUM .
@noindent
the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead token
(@samp{$default}), the parser will reduce it. If it was coming from
-state 0, then, after this reduction it will return to state 0, and will
+State 0, then, after this reduction it will return to state 0, and will
jump to state 2 (@samp{exp: go to state 2}).
@example
-state 2
+State 2
0 $accept: exp . $end
1 exp: exp . '+' exp
state}:
@example
-state 3
+State 3
0 $accept: exp $end .
the reader.
@example
-state 4
+State 4
1 exp: exp '+' . exp
exp go to state 8
-state 5
+State 5
2 exp: exp '-' . exp
exp go to state 9
-state 6
+State 6
3 exp: exp '*' . exp
exp go to state 10
-state 7
+State 7
4 exp: exp '/' . exp
1 shift/reduce}:
@example
-state 8
+State 8
1 exp: exp . '+' exp
1 | exp '+' exp .
@option{--report=lookahead}, Bison specifies these lookahead tokens:
@example
-state 8
+State 8
1 exp: exp . '+' exp
1 | exp '+' exp . [$end, '+', '-', '/']
@example
@group
-state 9
+State 9
1 exp: exp . '+' exp
2 | exp . '-' exp
@end group
@group
-state 10
+State 10
1 exp: exp . '+' exp
2 | exp . '-' exp
@end group
@group
-state 11
+State 11
1 exp: exp . '+' exp
2 | exp . '-' exp
@noindent
Observe that state 11 contains conflicts not only due to the lack of
-precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
-@samp{*}, but also because the
-associativity of @samp{/} is not specified.
+precedence of @samp{/} with respect to @samp{+}, @samp{-}, and @samp{*}, but
+also because the associativity of @samp{/} is not specified.
-Note that Bison may also produce an HTML version of this output, via an XML
-file and XSLT processing (@pxref{Xml}).
+Bison may also produce an HTML version of this output, via an XML file and
+XSLT processing (@pxref{Xml,,Visualizing your parser in multiple formats}).
@c ================================================= Graphical Representation
(@pxref{Invocation, , Invoking Bison}). Its name is made by removing
@samp{.tab.c} or @samp{.c} from the parser implementation file name, and
adding @samp{.dot} instead. If the grammar file is @file{foo.y}, the
-Graphviz output file is called @file{foo.dot}.
+Graphviz output file is called @file{foo.dot}. A DOT file may also be
+produced via an XML file and XSLT processing (@pxref{Xml,,Visualizing your
+parser in multiple formats}).
+
The following grammar file, @file{rr.y}, will be used in the sequel:
@end group
@end example
-The graphical output is very similar to the textual one, and as such it is
-easier understood by making direct comparisons between them. See
-@ref{Debugging, , Debugging Your Parser} for a detailled analysis of the
-textual report.
+The graphical output
+@ifnotinfo
+(see @ref{fig:graph})
+@end ifnotinfo
+is very similar to the textual one, and as such it is easier understood by
+making direct comparisons between them. @xref{Debugging, , Debugging Your
+Parser}, for a detailled analysis of the textual report.
+
+@ifnotinfo
+@float Figure,fig:graph
+@image{figs/example, 430pt}
+@caption{A graphical rendering of the parser.}
+@end float
+@end ifnotinfo
@subheading Graphical Representation of States
@example
@group
-state 3
+State 3
1 exp: a . ";"
This is how reductions are represented in the verbose file @file{rr.output}:
@example
-state 1
+State 1
3 a: "0" . [";"]
4 b: "0" . ["."]
are distinguished by a red filling color on these nodes, just like how they are
reported between square brackets in the verbose file.
-The reduction corresponding to the rule number 0 is the acceptation state. It
-is shown as a blue diamond, labelled "Acc".
+The reduction corresponding to the rule number 0 is the acceptation
+state. It is shown as a blue diamond, labelled ``Acc''.
@subheading Graphical representation of go tos
The @samp{go to} jump transitions are represented as dotted lines bearing
the name of the rule being jumped to.
-Note that a DOT file may also be produced via an XML file and XSLT
-processing (@pxref{Xml}).
-
@c ================================================= XML
@node Xml
@cindex xml
Bison supports two major report formats: textual output
-(@pxref{Understanding}) when invoked with option @option{--verbose}, and DOT
-(@pxref{Graphviz}) when invoked with option @option{--graph}. However,
+(@pxref{Understanding, ,Understanding Your Parser}) when invoked
+with option @option{--verbose}, and DOT
+(@pxref{Graphviz,, Visualizing Your Parser}) when invoked with
+option @option{--graph}. However,
another alternative is to output an XML file that may then be, with
@command{xsltproc}, rendered as either a raw text format equivalent to the
verbose file, or as an HTML version of the same file, with clickable
XSLT have no difference whatsoever with those obtained by invoking
@command{bison} with options @option{--verbose} or @option{--graph}.
-The textual file is generated when the options @option{-x} or
+The XML file is generated when the options @option{-x} or
@option{--xml[=FILE]} are specified, see @ref{Invocation,,Invoking Bison}.
If not specified, its name is made by removing @samp{.tab.c} or @samp{.c}
from the parser implementation file name, and adding @samp{.xml} instead.
@item xml2dot.xsl
Used to output a copy of the DOT visualization of the automaton.
@item xml2text.xsl
-Used to output a copy of the .output file.
+Used to output a copy of the @samp{.output} file.
@item xml2xhtml.xsl
-Used to output an xhtml enhancement of the .output file.
+Used to output an xhtml enhancement of the @samp{.output} file.
@end table
-Sample usage (requires @code{xsltproc}):
+Sample usage (requires @command{xsltproc}):
@example
-$ bison -x input.y
+$ bison -x gr.y
@group
$ bison --print-datadir
/usr/local/share/bison
@end group
-$ xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl input.xml > input.html
+$ xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl gr.xml >gr.html
@end example
@c ================================================= Tracing
@noindent
The previous reduction demonstrates the @code{%printer} directive for
-@code{<val>}: both the token @code{NUM} and the resulting non-terminal
+@code{<val>}: both the token @code{NUM} and the resulting nonterminal
@code{exp} have @samp{1} as value.
@example
$ bison -Werror=yacc,conflicts-sr input.y
$ bison -Werror=yacc,error=conflicts-sr input.y
@end example
+
+@item -f [@var{feature}]
+@itemx --feature[=@var{feature}]
+Activate miscellaneous @var{feature}. @var{feature} can be one of:
+@table @code
+@item caret
+@itemx diagnostics-show-caret
+Show caret errors, in a manner similar to GCC's
+@option{-fdiagnostics-show-caret}, or Clang's @option{-fcaret-diagnotics}. The
+location provided with the message is used to quote the corresponding line of
+the source file, underlining the important part of it with carets (^). Here is
+an example, using the following file @file{in.y}:
+
+@example
+%type <ival> exp
+%%
+exp: exp '+' exp @{ $exp = $1 + $2; @};
+@end example
+
+When invoked with @option{-fcaret}, Bison will report:
+
+@example
+@group
+in.y:3.20-23: error: ambiguous reference: '$exp'
+ exp: exp '+' exp @{ $exp = $1 + $2; @};
+ ^^^^
+@end group
+@group
+in.y:3.1-3: refers to: $exp at $$
+ exp: exp '+' exp @{ $exp = $1 + $2; @};
+ ^^^
+@end group
+@group
+in.y:3.6-8: refers to: $exp at $1
+ exp: exp '+' exp @{ $exp = $1 + $2; @};
+ ^^^
+@end group
+@group
+in.y:3.14-16: refers to: $exp at $3
+ exp: exp '+' exp @{ $exp = $1 + $2; @};
+ ^^^
+@end group
+@group
+in.y:3.32-33: error: $2 of 'exp' has no declared type
+ exp: exp '+' exp @{ $exp = $1 + $2; @};
+ ^^
+@end group
+@end example
+
+@end table
@end table
@noindent
@node C++ Variants
@subsubsection C++ Variants
-Starting with version 2.6, Bison provides a @emph{variant} based
-implementation of semantic values for C++. This alleviates all the
-limitations reported in the previous section, and in particular, object
-types can be used without pointers.
+Bison provides a @emph{variant} based implementation of semantic values for
+C++. This alleviates all the limitations reported in the previous section,
+and in particular, object types can be used without pointers.
To enable variant-based semantic values, set @code{%define} variable
@code{variant} (@pxref{%define Summary,, variant}). Once this defined,
@node Complete Symbols
@subsubsection Complete Symbols
-If you specified both @code{%define variant} and
+If you specified both @code{%define api.value.type variant} and
@code{%define api.token.constructor},
the @code{parser} class also defines the class @code{parser::symbol_type}
which defines a @emph{complete} symbol, aggregating its type (i.e., the
@noindent
@findex %define api.token.constructor
-@findex %define variant
+@findex %define api.value.type variant
This example will use genuine C++ objects as semantic values, therefore, we
require the variant-based interface. To make sure we properly use it, we
enable assertions. To fully benefit from type-safety and more natural
@comment file: calc++-parser.yy
@example
%define api.token.constructor
+%define api.value.type variant
%define parse.assert
-%define variant
@end example
@noindent
@end deffn
@deffn {Variable} @@@var{n}
+@deffnx {Symbol} @@@var{n}
In an action, the location of the @var{n}-th symbol of the right-hand side
of the rule. @xref{Tracking Locations}.
+
+In a grammar, the Bison-generated nonterminal symbol for a mid-rule action
+with a semantical value. @xref{Mid-Rule Action Translation}.
@end deffn
@deffn {Variable} @@@var{name}
-In an action, the location of a symbol addressed by name. @xref{Tracking
-Locations}.
+@deffnx {Variable} @@[@var{name}]
+In an action, the location of a symbol addressed by @var{name}.
+@xref{Tracking Locations}.
@end deffn
-@deffn {Variable} @@[@var{name}]
-In an action, the location of a symbol addressed by name. @xref{Tracking
-Locations}.
+@deffn {Symbol} $@@@var{n}
+In a grammar, the Bison-generated nonterminal symbol for a mid-rule action
+with no semantical value. @xref{Mid-Rule Action Translation}.
@end deffn
@deffn {Variable} $$
@end deffn
@deffn {Variable} $@var{name}
-In an action, the semantic value of a symbol addressed by name.
-@xref{Actions}.
-@end deffn
-
-@deffn {Variable} $[@var{name}]
-In an action, the semantic value of a symbol addressed by name.
+@deffnx {Variable} $[@var{name}]
+In an action, the semantic value of a symbol addressed by @var{name}.
@xref{Actions}.
@end deffn
feature.
@end deffn
-@deffn {Construct} /*@dots{}*/
-Comment delimiters, as in C.
+@deffn {Construct} /* @dots{} */
+@deffnx {Construct} // @dots{}
+Comments, as in C/C++.
@end deffn
@deffn {Delimiter} :
@code{yylex}}.
@end deffn
-@deffn {Macro} YYLEX_PARAM
-An obsolete macro for specifying an extra argument (or list of extra
-arguments) for @code{yyparse} to pass to @code{yylex}. The use of this
-macro is deprecated, and is supported only for Yacc like parsers.
-@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
-@end deffn
-
@deffn {Variable} yylloc
External variable in which @code{yylex} should place the line and column
numbers associated with a token. (In a pure parser, it is a local
@deffn {Variable} yynerrs
Global variable which Bison increments each time it reports a syntax error.
(In a pure parser, it is a local variable within @code{yyparse}. In a
-pure push parser, it is a member of yypstate.)
+pure push parser, it is a member of @code{yypstate}.)
@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
@end deffn
@item Accepting state
A state whose only action is the accept action.
The accepting state is thus a consistent state.
-@xref{Understanding,,}.
+@xref{Understanding, ,Understanding Your Parser}.
@item Backus-Naur Form (BNF; also called ``Backus Normal Form'')
Formal method of specifying context-free grammars originally proposed
projects / bison.git / blobdiff