@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts an
-annotated context-free grammar into an @acronym{LALR}(1) or
-@acronym{GLR} parser for that grammar. Once you are proficient with
-Bison, you can use it to develop a wide range of language parsers, from those
-used in simple desk calculators to complex programming languages.
+annotated context-free grammar into a deterministic or @acronym{GLR}
+parser employing @acronym{LALR}(1), @acronym{IELR}(1), or canonical
+@acronym{LR}(1) parser tables.
+Once you are proficient with Bison, you can use it to develop a wide
+range of language parsers, from those used in simple desk calculators to
+complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars
ought to work with Bison with no change. Anyone familiar with Yacc
essentially machine-readable @acronym{BNF}.
@cindex @acronym{LALR}(1) grammars
+@cindex @acronym{IELR}(1) grammars
@cindex @acronym{LR}(1) grammars
-There are various important subclasses of context-free grammar. Although it
-can handle almost all context-free grammars, Bison is optimized for what
-are called @acronym{LALR}(1) grammars.
-In brief, in these grammars, it must be possible to
-tell how to parse any portion of an input string with just a single
-token of lookahead. Strictly speaking, that is a description of an
-@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
-restrictions that are
-hard to explain simply; but it is rare in actual practice to find an
-@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1).
+There are various important subclasses of context-free grammars.
+Although it can handle almost all context-free grammars, Bison is
+optimized for what are called @acronym{LR}(1) grammars.
+In brief, in these grammars, it must be possible to tell how to parse
+any portion of an input string with just a single token of lookahead.
+For historical reasons, Bison by default is limited by the additional
+restrictions of @acronym{LALR}(1), which is hard to explain simply.
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
more information on this.
+To escape these additional restrictions, you can request
+@acronym{IELR}(1) or canonical @acronym{LR}(1) parser tables.
+@xref{Decl Summary,,lr.type}, to learn how.
@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing
-Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
+Parsers for @acronym{LR}(1) grammars are @dfn{deterministic}, meaning
roughly that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
(called a @dfn{lookahead}) of the remaining input. A context-free
@cindex shift/reduce conflicts
@cindex reduce/reduce conflicts
-In some grammars, Bison's standard
-@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
+In some grammars, Bison's deterministic
+@acronym{LR}(1) parsing algorithm cannot decide whether to apply a
certain grammar rule at a given point. That is, it may not be able to
decide (on the basis of the input read so far) which of two possible
reductions (applications of a grammar rule) applies, or whether to apply
(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
(@pxref{Shift/Reduce}).
-To use a grammar that is not easily modified to be @acronym{LALR}(1), a
+To use a grammar that is not easily modified to be @acronym{LR}(1), a
more general parsing algorithm is sometimes necessary. If you include
@code{%glr-parser} among the Bison declarations in your file
(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR}
(@acronym{GLR}) parser. These parsers handle Bison grammars that
contain no unresolved conflicts (i.e., after applying precedence
-declarations) identically to @acronym{LALR}(1) parsers. However, when
+declarations) identically to deterministic parsers. However, when
faced with unresolved shift/reduce and reduce/reduce conflicts,
@acronym{GLR} parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities. Each of
@cindex shift/reduce conflicts
In the simplest cases, you can use the @acronym{GLR} algorithm
-to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
-Such grammars typically require more than one symbol of lookahead,
-or (in rare cases) fall into the category of grammars in which the
-@acronym{LALR}(1) algorithm throws away too much information (they are in
-@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+to parse grammars that are unambiguous but fail to be @acronym{LR}(1).
+Such grammars typically require more than one symbol of lookahead.
Consider a problem that
arises in the declaration of enumerated and subrange types in the
valid, and more-complicated cases can come up in practical programs.)
These two declarations look identical until the @samp{..} token.
-With normal @acronym{LALR}(1) one-token lookahead it is not
+With normal @acronym{LR}(1) one-token lookahead it is not
possible to decide between the two forms when the identifier
@samp{a} is parsed. It is, however, desirable
for a parser to decide this, since in the latter case
The effect of all this is that the parser seems to ``guess'' the
correct branch to take, or in other words, it seems to use more
-lookahead than the underlying @acronym{LALR}(1) algorithm actually allows
-for. In this example, @acronym{LALR}(2) would suffice, but also some cases
-that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+lookahead than the underlying @acronym{LR}(1) algorithm actually allows
+for. In this example, @acronym{LR}(2) would suffice, but also some cases
+that are not @acronym{LR}(@math{k}) for any @math{k} can be handled this way.
In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space
@end group
@end example
-When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+When used as a normal @acronym{LR}(1) grammar, Bison correctly complains
about one reduce/reduce conflict. In the conflicting situation the
parser chooses one of the alternatives, arbitrarily the one
declared first. Therefore the following correct input is not
analyze the conflicts reported by Bison to make sure that @acronym{GLR}
splitting is only done where it is intended. A @acronym{GLR} parser
splitting inadvertently may cause problems less obvious than an
-@acronym{LALR} parser statically choosing the wrong alternative in a
+@acronym{LR} parser statically choosing the wrong alternative in a
conflict. Second, consider interactions with the lexer (@pxref{Semantic
Tokens}) with great care. Since a split parser consumes tokens without
performing any actions during the split, the lexer cannot obtain
(@pxref{Action Features}), which you can invoke in a semantic action to
initiate error recovery.
During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
-the same as its effect in an @acronym{LALR}(1) parser.
+the same as its effect in a deterministic parser.
In a deferred semantic action, its effect is undefined.
@c The effect is probably a syntax error at the split point.
@findex %code requires
@findex %code provides
@findex %code top
-(The prologue alternatives described here are experimental.
-More user feedback will help to determine whether they should become permanent
-features.)
The functionality of @var{Prologue} sections can often be subtle and
inflexible.
equivalent groupings. The symbol name is used in writing grammar rules.
By convention, it should be all lower case.
-Symbol names can contain letters, digits (not at the beginning),
-underscores and periods. Periods make sense only in nonterminals.
+Symbol names can contain letters, underscores, periods, dashes, and (not
+at the beginning) digits. Dashes in symbol names are a GNU
+extension, incompatible with @acronym{POSIX} Yacc. Terminal symbols
+that contain periods or dashes make little sense: since they are not
+valid symbols (in most programming languages) they are not exported as
+token names.
There are three ways of writing terminal symbols in the grammar:
Bison reports an error if the number of shift/reduce conflicts differs
from @var{n}, or if there are any reduce/reduce conflicts.
-For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
+For deterministic parsers, reduce/reduce conflicts are more
serious, and should be eliminated entirely. Bison will always report
reduce/reduce conflicts for these parsers. With @acronym{GLR}
parsers, however, both kinds of conflicts are routine; otherwise,
@subsection A Push Parser
@cindex push parser
@cindex push parser
-@findex %define api.push_pull
+@findex %define api.push-pull
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
Normally, Bison generates a pull parser.
The following Bison declaration says that you want the parser to be a push
-parser (@pxref{Decl Summary,,%define api.push_pull}):
+parser (@pxref{Decl Summary,,%define api.push-pull}):
@example
-%define api.push_pull "push"
+%define api.push-pull "push"
@end example
In almost all cases, you want to ensure that your push parser is also
@example
%define api.pure
-%define api.push_pull "push"
+%define api.push-pull "push"
@end example
There is a major notable functional difference between the pure push parser
Bison also supports both the push parser interface along with the pull parser
interface in the same generated parser. In order to get this functionality,
-you should replace the @code{%define api.push_pull "push"} declaration with the
-@code{%define api.push_pull "both"} declaration. Doing this will create all of
+you should replace the @code{%define api.push-pull "push"} declaration with the
+@code{%define api.push-pull "both"} declaration. Doing this will create all of
the symbols mentioned earlier along with the two extra symbols, @code{yyparse}
and @code{yypull_parse}. @code{yyparse} can be used exactly as it normally
would be used. However, the user should note that it is implemented in the
generated parser by calling @code{yypull_parse}.
This makes the @code{yyparse} function that is generated with the
-@code{%define api.push_pull "both"} declaration slower than the normal
+@code{%define api.push-pull "both"} declaration slower than the normal
@code{yyparse} function. If the user
calls the @code{yypull_parse} function it will parse the rest of the input
stream. It is possible to @code{yypush_parse} tokens to select a subgrammar
@end example
Adding the @code{%define api.pure} declaration does exactly the same thing to
-the generated parser with @code{%define api.push_pull "both"} as it did for
-@code{%define api.push_pull "push"}.
+the generated parser with @code{%define api.push-pull "both"} as it did for
+@code{%define api.push-pull "push"}.
@node Decl Summary
@subsection Bison Declaration Summary
For a detailed discussion, see @ref{Prologue Alternatives}.
For Java, the default location is inside the parser class.
-
-(Like all the Yacc prologue alternatives, this directive is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.)
@end deffn
@deffn {Directive} %code @var{qualifier} @{@var{code}@}
@end itemize
@end itemize
-(Like all the Yacc prologue alternatives, this directive is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.)
-
@cindex Prologue
For a detailed discussion of how to use @code{%code} in place of the
traditional Yacc prologue for C/C++, see @ref{Prologue Alternatives}.
@end deffn
@deffn {Directive} %debug
-In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
-already defined, so that the debugging facilities are compiled.
+Instrument the output parser for traces. Obsoleted by @samp{%define
+parse.trace}.
@xref{Tracing, ,Tracing Your Parser}.
@end deffn
Some of the accepted @var{variable}s are:
-@itemize @bullet
+@table @code
@item api.pure
@findex %define api.pure
@item Default Value: @code{"false"}
@end itemize
+@c api.pure
-@item api.push_pull
-@findex %define api.push_pull
+@item api.push-pull
+@findex %define api.push-pull
@itemize @bullet
-@item Language(s): C (LALR(1) only)
+@item Language(s): C (deterministic parsers only)
@item Purpose: Requests a pull parser, a push parser, or both.
@xref{Push Decl, ,A Push Parser}.
@item Default Value: @code{"pull"}
@end itemize
+@c api.push-pull
+
+@item error-verbose
+@findex %define error-verbose
+@itemize
+@item Languages(s):
+all.
+@item Purpose:
+Enable the generation of more verbose error messages than a instead of
+just plain @w{@code{"syntax error"}}. @xref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}.
+@item Accepted Values:
+Boolean
+@item Default Value:
+@code{false}
+@end itemize
+@c error-verbose
+
+
+@item lr.default-reductions
+@cindex default reductions
+@findex %define lr.default-reductions
+@cindex delayed syntax errors
+@cindex syntax errors delayed
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Specifies the kind of states that are permitted to
+contain default reductions.
+That is, in such a state, Bison declares the reduction with the largest
+lookahead set to be the default reduction and then removes that
+lookahead set.
+The advantages of default reductions are discussed below.
+The disadvantage is that, when the generated parser encounters a
+syntactically unacceptable token, the parser might then perform
+unnecessary default reductions before it can detect the syntax error.
+
+(This feature is experimental.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values:
+@itemize
+@item @code{"all"}.
+For @acronym{LALR} and @acronym{IELR} parsers (@pxref{Decl
+Summary,,lr.type}) by default, all states are permitted to contain
+default reductions.
+The advantage is that parser table sizes can be significantly reduced.
+The reason Bison does not by default attempt to address the disadvantage
+of delayed syntax error detection is that this disadvantage is already
+inherent in @acronym{LALR} and @acronym{IELR} parser tables.
+That is, unlike in a canonical @acronym{LR} state, the lookahead sets of
+reductions in an @acronym{LALR} or @acronym{IELR} state can contain
+tokens that are syntactically incorrect for some left contexts.
+
+@item @code{"consistent"}.
+@cindex consistent states
+A consistent state is a state that has only one possible action.
+If that action is a reduction, then the parser does not need to request
+a lookahead token from the scanner before performing that action.
+However, the parser only recognizes the ability to ignore the lookahead
+token when such a reduction is encoded as a default reduction.
+Thus, if default reductions are permitted in and only in consistent
+states, then a canonical @acronym{LR} parser reports a syntax error as
+soon as it @emph{needs} the syntactically unacceptable token from the
+scanner.
+
+@item @code{"accepting"}.
+@cindex accepting state
+By default, the only default reduction permitted in a canonical
+@acronym{LR} parser is the accept action in the accepting state, which
+the parser reaches only after reading all tokens from the input.
+Thus, the default canonical @acronym{LR} parser reports a syntax error
+as soon as it @emph{reaches} the syntactically unacceptable token
+without performing any extra reductions.
+@end itemize
+
+@item Default Value:
+@itemize
+@item @code{"accepting"} if @code{lr.type} is @code{"canonical LR"}.
+@item @code{"all"} otherwise.
+@end itemize
+@end itemize
-@item lr.keep_unreachable_states
-@findex %define lr.keep_unreachable_states
+@item lr.keep-unreachable-states
+@findex %define lr.keep-unreachable-states
@itemize @bullet
@item Language(s): all
However, Bison does not compute which goto actions are useless.
@end itemize
@end itemize
+@c lr.keep-unreachable-states
+
+@item lr.type
+@findex %define lr.type
+@cindex @acronym{LALR}
+@cindex @acronym{IELR}
+@cindex @acronym{LR}
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Specifies the type of parser tables within the
+@acronym{LR}(1) family.
+(This feature is experimental.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values:
+@itemize
+@item @code{"LALR"}.
+While Bison generates @acronym{LALR} parser tables by default for
+historical reasons, @acronym{IELR} or canonical @acronym{LR} is almost
+always preferable for deterministic parsers.
+The trouble is that @acronym{LALR} parser tables can suffer from
+mysterious conflicts and thus may not accept the full set of sentences
+that @acronym{IELR} and canonical @acronym{LR} accept.
+@xref{Mystery Conflicts}, for details.
+However, there are at least two scenarios where @acronym{LALR} may be
+worthwhile:
+@itemize
+@cindex @acronym{GLR} with @acronym{LALR}
+@item When employing @acronym{GLR} parsers (@pxref{GLR Parsers}), if you
+do not resolve any conflicts statically (for example, with @code{%left}
+or @code{%prec}), then the parser explores all potential parses of any
+given input.
+In this case, the use of @acronym{LALR} parser tables is guaranteed not
+to alter the language accepted by the parser.
+@acronym{LALR} parser tables are the smallest parser tables Bison can
+currently generate, so they may be preferable.
+
+@item Occasionally during development, an especially malformed grammar
+with a major recurring flaw may severely impede the @acronym{IELR} or
+canonical @acronym{LR} parser table generation algorithm.
+@acronym{LALR} can be a quick way to generate parser tables in order to
+investigate such problems while ignoring the more subtle differences
+from @acronym{IELR} and canonical @acronym{LR}.
+@end itemize
+
+@item @code{"IELR"}.
+@acronym{IELR} is a minimal @acronym{LR} algorithm.
+That is, given any grammar (@acronym{LR} or non-@acronym{LR}),
+@acronym{IELR} and canonical @acronym{LR} always accept exactly the same
+set of sentences.
+However, as for @acronym{LALR}, the number of parser states is often an
+order of magnitude less for @acronym{IELR} than for canonical
+@acronym{LR}.
+More importantly, because canonical @acronym{LR}'s extra parser states
+may contain duplicate conflicts in the case of non-@acronym{LR}
+grammars, the number of conflicts for @acronym{IELR} is often an order
+of magnitude less as well.
+This can significantly reduce the complexity of developing of a grammar.
+
+@item @code{"canonical LR"}.
+@cindex delayed syntax errors
+@cindex syntax errors delayed
+The only advantage of canonical @acronym{LR} over @acronym{IELR} is
+that, for every left context of every canonical @acronym{LR} state, the
+set of tokens accepted by that state is the exact set of tokens that is
+syntactically acceptable in that left context.
+Thus, the only difference in parsing behavior is that the canonical
+@acronym{LR} parser can report a syntax error as soon as possible
+without performing any unnecessary reductions.
+@xref{Decl Summary,,lr.default-reductions}, for further details.
+Even when canonical @acronym{LR} behavior is ultimately desired,
+@acronym{IELR}'s elimination of duplicate conflicts should still
+facilitate the development of a grammar.
+@end itemize
+
+@item Default Value: @code{"LALR"}
+@end itemize
@item namespace
@findex %define namespace
The parser namespace is @code{foo} and @code{yylex} is referenced as
@code{bar::lex}.
@end itemize
+@c namespace
+
+@item parse.assert
+@findex %define parse.assert
+
+@itemize
+@item Languages(s): C++
+
+@item Purpose: Issue runtime assertions to catch invalid uses.
+In C++, when variants are used, symbols must be constructed and
+destroyed properly. This option checks these constraints.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{false}
+@end itemize
+@c parse.assert
+
+@item parse.trace
+@findex %define parse.trace
+
+@itemize
+@item Languages(s): C, C++
+
+@item Purpose: Require parser instrumentation for tracing.
+In C/C++, define the macro @code{YYDEBUG} to 1 in the parser file if it
+is not already defined, so that the debugging facilities are compiled.
+@xref{Tracing, ,Tracing Your Parser}.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{false}
+@end itemize
+@c parse.trace
+
+@item token.prefix
+@findex %define token.prefix
+
+@itemize
+@item Languages(s): all
+
+@item Purpose:
+Add a prefix to the token names when generating their definition in the
+target language. For instance
+
+@example
+%token FILE for ERROR
+%define token.prefix "TOK_"
+%%
+start: FILE for ERROR;
+@end example
+
+@noindent
+generates the definition of the symbols @code{TOK_FILE}, @code{TOK_for},
+and @code{TOK_ERROR} in the generated source files. In particular, the
+scanner must use these prefixed token names, while the grammar itself
+may still use the short names (as in the sample rule given above). The
+generated informational files (@file{*.output}, @file{*.xml},
+@file{*.dot}) are not modified by this prefix. See @ref{Calc++ Parser}
+and @ref{Calc++ Scanner}, for a complete example.
+
+@item Accepted Values:
+Any string. Should be a valid identifier prefix in the target language,
+in other words, it should typically be an identifier itself (sequence of
+letters, underscores, and ---not at the beginning--- digits).
+
+@item Default Value:
+empty
@end itemize
+@c token.prefix
+@end table
@end deffn
+@c ---------------------------------------------------------- %define
@deffn {Directive} %defines
Write a header file containing macro definitions for the token type
More user feedback will help to stabilize it.)
You call the function @code{yypush_parse} to parse a single token. This
-function is available if either the @code{%define api.push_pull "push"} or
-@code{%define api.push_pull "both"} declaration is used.
+function is available if either the @code{%define api.push-pull "push"} or
+@code{%define api.push-pull "both"} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun int yypush_parse (yypstate *yyps)
More user feedback will help to stabilize it.)
You call the function @code{yypull_parse} to parse the rest of the input
-stream. This function is available if the @code{%define api.push_pull "both"}
+stream. This function is available if the @code{%define api.push-pull "both"}
declaration is used.
@xref{Push Decl, ,A Push Parser}.
More user feedback will help to stabilize it.)
You call the function @code{yypstate_new} to create a new parser instance.
-This function is available if either the @code{%define api.push_pull "push"} or
-@code{%define api.push_pull "both"} declaration is used.
+This function is available if either the @code{%define api.push-pull "push"} or
+@code{%define api.push-pull "both"} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun yypstate *yypstate_new (void)
More user feedback will help to stabilize it.)
You call the function @code{yypstate_delete} to delete a parser instance.
-function is available if either the @code{%define api.push_pull "push"} or
-@code{%define api.push_pull "both"} declaration is used.
+function is available if either the @code{%define api.push-pull "push"} or
+@code{%define api.push-pull "both"} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun void yypstate_delete (yypstate *yyps)
receives one argument. For a syntax error, the string is normally
@w{@code{"syntax error"}}.
-@findex %error-verbose
-If you invoke the directive @code{%error-verbose} in the Bison
+@findex %define error-verbose
+If you invoke the directive @code{%define error-verbose} in the Bison
declarations section (@pxref{Bison Declarations, ,The Bison Declarations
Section}), then Bison provides a more verbose and specific error message
string instead of just plain @w{@code{"syntax error"}}.
@cindex @acronym{LR}(1)
@cindex @acronym{LALR}(1)
-However, Bison, like most parser generators, cannot actually handle all
-@acronym{LR}(1) grammars. In this grammar, two contexts, that after
-an @code{ID}
-at the beginning of a @code{param_spec} and likewise at the beginning of
-a @code{return_spec}, are similar enough that Bison assumes they are the
-same. They appear similar because the same set of rules would be
+However, for historical reasons, Bison cannot by default handle all
+@acronym{LR}(1) grammars.
+In this grammar, two contexts, that after an @code{ID} at the beginning
+of a @code{param_spec} and likewise at the beginning of a
+@code{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 @code{name} and that for reducing to
a @code{type}. Bison is unable to determine at that stage of processing
that the rules would require different lookahead tokens in the two
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not @acronym{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 @acronym{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
+For many practical grammars (specifically those that fall into the
+non-@acronym{LR}(1) class), the limitations of @acronym{LALR}(1) result in
+difficulties beyond just mysterious reduce/reduce conflicts.
+The best way to fix all these problems is to select a different parser
+table generation algorithm.
+Either @acronym{IELR}(1) or canonical @acronym{LR}(1) would suffice, but
+the former is more efficient and easier to debug during development.
+@xref{Decl Summary,,lr.type}, for details.
+(Bison's @acronym{IELR}(1) and canonical @acronym{LR}(1) implementations
+are experimental.
+More user feedback will help to stabilize them.)
+
+If you instead wish to work around @acronym{LALR}(1)'s limitations, you
+can often fix a mysterious conflict 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
@code{return_spec} as follows makes the problem go away:
@example
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (@pxref{Mystery Conflicts}),
-there are languages where Bison's particular choice of how to
+there are languages where Bison's default choice of how to
summarize the input seen so far loses necessary information.
When you use the @samp{%glr-parser} declaration in your grammar file,
stream.
Whenever the parser makes a transition from having multiple
-states to having one, it reverts to the normal @acronym{LALR}(1) parsing
+states to having one, it reverts to the normal deterministic parsing
algorithm, after resolving and executing the saved-up actions.
At this transition, some of the states on the stack will have semantic
values that are sets (actually multisets) of possible actions. The
the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the @acronym{GLR} parsing tree that
-permits the processing of any @acronym{LALR}(1) grammar in linear time (in the
+permits the processing of any @acronym{LR}(1) grammar in linear time (in the
size of the input), any unambiguous (not necessarily
-@acronym{LALR}(1)) grammar in
+@acronym{LR}(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time. However, Bison currently
uses a simpler data structure that requires time proportional to the
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local---the parser is ``in
doubt'' only for a few tokens at a time. Therefore, the current data
-structure should generally be adequate. On @acronym{LALR}(1) portions of a
-grammar, in particular, it is only slightly slower than with the default
-Bison parser.
+structure should generally be adequate. On @acronym{LR}(1) portions of a
+grammar, in particular, it is only slightly slower than with the
+deterministic @acronym{LR}(1) Bison parser.
For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth
Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
-macro @code{YYINITDEPTH} to a positive integer. For the C
-@acronym{LALR}(1) parser, this value must be a compile-time constant
+macro @code{YYINITDEPTH} to a positive integer. For the deterministic
+parser in C, this value must be a compile-time constant
unless you are assuming C99 or some other target language or compiler
that allows variable-length arrays. The default is 200.
Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
@c FIXME: C++ output.
-Because of semantical differences between C and C++, the
-@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
+Because of semantical differences between C and C++, the deterministic
+parsers in C produced by Bison cannot grow when compiled
by C++ compilers. In this precise case (compiling a C parser as C++) you are
suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix
this deficiency in a future release.
@command{bison} reports:
@example
-calc.y: warning: 1 nonterminal and 1 rule useless in grammar
+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: conflicts: 7 shift/reduce
'+' . exp}. Since there is no default action, any other token than
those listed above will trigger a syntax error.
+@cindex accepting state
The state 3 is named the @dfn{final state}, or the @dfn{accepting
state}:
NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
NUM}, which corresponds to reducing rule 1.
-Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
+Because in deterministic parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
Shift/Reduce Conflicts}. Discarded actions are reported in between
square brackets.
@item the directive @samp{%debug}
@findex %debug
-Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
-Declaration Summary}). This is a Bison extension, which will prove
-useful when Bison will output parsers for languages that don't use a
-preprocessor. Unless @acronym{POSIX} and Yacc portability matter to
-you, this is
-the preferred solution.
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison Declaration
+Summary}). This Bison extension is maintained for backward
+compatibility with previous versions of Bison.
+
+@item the variable @samp{parse.trace}
+@findex %define parse.trace
+Add the @samp{%define parse.trace} directive (@pxref{Decl Summary,
+,Bison Declaration Summary}), or pass the @option{-Dparse.trace} option
+(@pxref{Bison Options}). This is a Bison extension, which is especially
+useful for languages that don't use a preprocessor. Unless
+@acronym{POSIX} and Yacc portability matter to you, this is the
+preferred solution.
@end table
-We suggest that you always enable the debug option so that debugging is
+We suggest that you always enable the trace option so that debugging is
always possible.
The trace facility outputs messages with macro calls of the form
file name conventions, so that the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}.
-Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define}
+Also, if generating a deterministic parser in C, generate @code{#define}
statements in addition to an @code{enum} to associate token numbers with token
names.
Thus, the following shell script can substitute for Yacc, and the Bison
@table @code
@item state
Description of the grammar, conflicts (resolved and unresolved), and
-@acronym{LALR} automaton.
+parser's automaton.
@item lookahead
Implies @code{state} and augments the description of the automaton with
@item -g [@var{file}]
@itemx --graph[=@var{file}]
-Output a graphical representation of the @acronym{LALR}(1) grammar
+Output a graphical representation of the parser's
automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
@uref{http://www.graphviz.org/doc/info/lang.html, @acronym{DOT}} format.
@code{@var{file}} is optional.
@item -x [@var{file}]
@itemx --xml[=@var{file}]
-Output an XML report of the @acronym{LALR}(1) automaton computed by Bison.
+Output an XML report of the parser's automaton computed by Bison.
@code{@var{file}} is optional.
If omitted and the grammar file is @file{foo.y}, the output file will be
@file{foo.xml}.
@c - Always pure
@c - initial action
-The C++ @acronym{LALR}(1) parser is selected using the skeleton directive,
+The C++ deterministic parser is selected using the skeleton directive,
@samp{%skeleton "lalr1.c"}, or the synonymous command-line option
@option{--skeleton=lalr1.c}.
@xref{Decl Summary}.
@subsubsection Calc++ Parser
The parser definition file @file{calc++-parser.yy} starts by asking for
-the C++ LALR(1) skeleton, the creation of the parser header file, and
-specifies the name of the parser class. Because the C++ skeleton
-changed several times, it is safer to require the version you designed
-the grammar for.
+the C++ deterministic parser skeleton, the creation of the parser header
+file, and specifies the name of the parser class.
+Because the C++ skeleton changed several times, it is safer to require
+the version you designed the grammar for.
@comment file: calc++-parser.yy
@example
@comment file: calc++-parser.yy
@example
-%debug
-%error-verbose
+%define parse.trace
+%define error-verbose
@end example
@noindent
@noindent
The token numbered as 0 corresponds to end of file; the following line
-allows for nicer error messages referring to ``end of file'' instead
-of ``$end''. Similarly user friendly named are provided for each
-symbol. Note that the tokens names are prefixed by @code{TOKEN_} to
-avoid name clashes.
+allows for nicer error messages referring to ``end of file'' instead of
+``$end''. Similarly user friendly names are provided for each symbol.
+To avoid name clashes in the generated files (@pxref{Calc++ Scanner}),
+prefix tokens with @code{TOK_} (@pxref{Decl Summary,, token.prefix}).
@comment file: calc++-parser.yy
@example
+%define token.prefix "TOK_"
%token END 0 "end of file"
%token ASSIGN ":="
%token <sval> IDENTIFIER "identifier"
%start unit;
unit: assignments exp @{ driver.result = $2; @};
-assignments: assignments assignment @{@}
- | /* Nothing. */ @{@};
+assignments:
+ assignments assignment @{@}
+| /* Nothing. */ @{@};
assignment:
- "identifier" ":=" exp
+ "identifier" ":=" exp
@{ driver.variables[*$1] = $3; delete $1; @};
%left '+' '-';
%left '*' '/';
-exp: exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | '(' exp ')' @{ $$ = $2; @}
- | "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
- | "number" @{ $$ = $1; @};
+exp:
+ exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
+| exp '/' exp @{ $$ = $1 / $3; @}
+| '(' exp ')' @{ $$ = $2; @}
+| "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
+| "number" @{ $$ = $1; @};
%%
@end example
# undef yywrap
# define yywrap() 1
-/* By default yylex returns int, we use token_type.
- Unfortunately yyterminate by default returns 0, which is
+/* By default yylex returns an int; we use token_type.
+ The default yyterminate implementation returns 0, which is
not of token_type. */
-#define yyterminate() return token::END
+#define yyterminate() return TOKEN(END)
%@}
@end example
@end example
@noindent
-The rules are simple, just note the use of the driver to report errors.
-It is convenient to use a typedef to shorten
-@code{yy::calcxx_parser::token::identifier} into
-@code{token::identifier} for instance.
+The rules are simple. The driver is used to report errors. It is
+convenient to use a macro to shorten
+@code{yy::calcxx_parser::token::TOK_@var{Name}} into
+@code{TOKEN(@var{Name})}; note the token prefix, @code{TOK_}.
@comment file: calc++-scanner.ll
@example
%@{
- typedef yy::calcxx_parser::token token;
+# define TOKEN(Name) \
+ yy::calcxx_parser::token::TOK_ ## Name
%@}
/* Convert ints to the actual type of tokens. */
[-+*/()] return yy::calcxx_parser::token_type (yytext[0]);
-":=" return token::ASSIGN;
+":=" return TOKEN(ASSIGN);
@{int@} @{
errno = 0;
long n = strtol (yytext, NULL, 10);
if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
driver.error (*yylloc, "integer is out of range");
yylval->ival = n;
- return token::NUMBER;
+ return TOKEN(NUMBER);
+@}
+@{id@} @{
+ yylval->sval = new std::string (yytext);
+ return TOKEN(IDENTIFIER);
@}
-@{id@} yylval->sval = new std::string (yytext); return token::IDENTIFIER;
. driver.error (*yylloc, "invalid character");
%%
@end example
Java.
Push parsers are currently unsupported in Java and @code{%define
-api.push_pull} have no effect.
+api.push-pull} have no effect.
@acronym{GLR} parsers are currently unsupported in Java. Do not use the
@code{glr-parser} directive.
@code{%defines} directive or the @option{-d}/@option{--defines} options.
@c FIXME: Possible code change.
-Currently, support for debugging is always compiled
-in. Thus the @code{%debug} and @code{%token-table} directives and the
+Currently, support for tracing is always compiled
+in. Thus the @samp{%define parse.trace} and @samp{%token-table}
+directives and the
@option{-t}/@option{--debug} and @option{-k}/@option{--token-table}
options have no effect. This may change in the future to eliminate
-unused code in the generated parser, so use @code{%debug} explicitly
+unused code in the generated parser, so use @samp{%define parse.trace}
+explicitly
if needed. Also, in the future the
@code{%token-table} directive might enable a public interface to
access the token names and codes.
@deftypemethod {YYParser} {boolean} getErrorVerbose ()
@deftypemethodx {YYParser} {void} setErrorVerbose (boolean @var{verbose})
Get or set the option to produce verbose error messages. These are only
-available with the @code{%error-verbose} directive, which also turn on
+available with the @code{%define error-verbose} directive, which also turn on
verbose error messages.
@end deftypemethod
@end deffn
@deffn {Directive} %error-verbose
-Bison declaration to request verbose, specific error message strings
-when @code{yyerror} is called.
+An obsolete directive standing for @samp{%define error-verbose}.
@end deffn
@deffn {Directive} %file-prefix "@var{prefix}"
@deffn {Function} yyerror
User-supplied function to be called by @code{yyparse} on error.
-@xref{Error Reporting, ,The Error
-Reporting Function @code{yyerror}}.
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
@end deffn
@deffn {Macro} YYERROR_VERBOSE
-An obsolete macro that you define with @code{#define} in the prologue
-to request verbose, specific error message strings
-when @code{yyerror} is called. It doesn't matter what definition you
-use for @code{YYERROR_VERBOSE}, just whether you define it. Using
-@code{%error-verbose} is preferred.
+An obsolete macro used in the @file{yacc.c} skeleton, that you define
+with @code{#define} in the prologue to request verbose, specific error
+message strings when @code{yyerror} is called. It doesn't matter what
+definition you use for @code{YYERROR_VERBOSE}, just whether you define
+it. Using @code{%define error-verbose} is preferred (@pxref{Error
+Reporting, ,The Error Reporting Function @code{yyerror}}).
@end deffn
@deffn {Macro} YYINITDEPTH
@end deffn
@deffn {Macro} YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca} when the C
-@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0,
+Macro used to control the use of @code{alloca} when the
+deterministic parser in C needs to extend its stacks. If defined to 0,
the parser will use @code{malloc} to extend its stacks. If defined to
1, the parser will use @code{alloca}. Values other than 0 and 1 are
reserved for future Bison extensions. If not defined,
@cindex glossary
@table @asis
+@item Accepting State
+A state whose only action is the accept action.
+The accepting state is thus a consistent state.
+@xref{Understanding,,}.
+
@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
Formal method of specifying context-free grammars originally proposed
by John Backus, and slightly improved by Peter Naur in his 1960-01-02
committee document contributing to what became the Algol 60 report.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+@item Consistent State
+A state containing only one possible action.
+@xref{Decl Summary,,lr.default-reductions}.
+
@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
Thus, if there is a rule which says that an integer can be used as an
permitted. @xref{Language and Grammar, ,Languages and Context-Free
Grammars}.
+@item Default Reduction
+The reduction that a parser should perform if the current parser state
+contains no other action for the lookahead token.
+In permitted parser states, Bison declares the reduction with the
+largest lookahead set to be the default reduction and removes that
+lookahead set.
+@xref{Decl Summary,,lr.default-reductions}.
+
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.
@item Generalized @acronym{LR} (@acronym{GLR})
A parsing algorithm that can handle all context-free grammars, including those
-that are not @acronym{LALR}(1). It resolves situations that Bison's
-usual @acronym{LALR}(1)
+that are not @acronym{LR}(1). It resolves situations that Bison's
+deterministic parsing
algorithm cannot by effectively splitting off multiple parsers, trying all
possible parsers, and discarding those that fail in the light of additional
right context. @xref{Generalized LR Parsing, ,Generalized
for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+@item @acronym{IELR}(1)
+A minimal @acronym{LR}(1) parser table generation algorithm.
+That is, given any context-free grammar, @acronym{IELR}(1) generates
+parser tables with the full language recognition power of canonical
+@acronym{LR}(1) but with nearly the same number of parser states as
+@acronym{LALR}(1).
+This reduction in parser states is often an order of magnitude.
+More importantly, because canonical @acronym{LR}(1)'s extra parser
+states may contain duplicate conflicts in the case of
+non-@acronym{LR}(1) grammars, the number of conflicts for
+@acronym{IELR}(1) is often an order of magnitude less as well.
+This can significantly reduce the complexity of developing of a grammar.
+@xref{Decl Summary,,lr.type}.
+
@item Infix operator
An arithmetic operator that is placed between the operands on which it
performs some operation.
@item @acronym{LALR}(1)
The class of context-free grammars that Bison (like most other parser
-generators) can handle; a subset of @acronym{LR}(1). @xref{Mystery
-Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
+generators) can handle by default; a subset of @acronym{LR}(1).
+@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
@item @acronym{LR}(1)
The class of context-free grammars in which at most one token of
@c LocalWords: infile ypp yxx outfile itemx tex leaderfill
@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
@c LocalWords: nbar yytext fst snd osplit ntwo strdup AST
-@c LocalWords: YYSTACK DVI fdl printindex
+@c LocalWords: YYSTACK DVI fdl printindex IELR
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