X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/0817d1eccb613dfff433837219f703c701958f33..1bddda4d85d1d9431a05699ce1aae40e865de797:/doc/bison.texinfo diff --git a/doc/bison.texinfo b/doc/bison.texinfo index 405abefa..022510b8 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -12,25 +12,15 @@ @c the smallbook format. @c @smallbook -@c Set following if you have the new `shorttitlepage' command -@c @clear shorttitlepage-enabled -@c @set shorttitlepage-enabled - @c Set following if you want to document %default-prec and %no-default-prec. @c This feature is experimental and may change in future Bison versions. @c @set defaultprec -@c ISPELL CHECK: done, 14 Jan 1993 --bob - -@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo -@c titlepage; should NOT be changed in the GPL. --mew - -@c FIXME: I don't understand this `iftex'. Obsolete? --akim. -@iftex +@ifnotinfo @syncodeindex fn cp @syncodeindex vr cp @syncodeindex tp cp -@end iftex +@end ifnotinfo @ifinfo @synindex fn cp @synindex vr cp @@ -44,12 +34,12 @@ This manual is for @acronym{GNU} Bison (version @value{VERSION}, @value{UPDATED}), the @acronym{GNU} parser generator. Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, -1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. +1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. @quotation Permission is granted to copy, distribute and/or modify this document under the terms of the @acronym{GNU} Free Documentation License, -Version 1.1 or any later version published by the Free Software +Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled @@ -62,14 +52,11 @@ Copies published by the Free Software Foundation raise funds for @end quotation @end copying -@dircategory GNU programming tools +@dircategory Software development @direntry * bison: (bison). @acronym{GNU} parser generator (Yacc replacement). @end direntry -@ifset shorttitlepage-enabled -@shorttitlepage Bison -@end ifset @titlepage @title Bison @subtitle The Yacc-compatible Parser Generator @@ -82,8 +69,8 @@ Copies published by the Free Software Foundation raise funds for @insertcopying @sp 2 Published by the Free Software Foundation @* -59 Temple Place, Suite 330 @* -Boston, MA 02111-1307 USA @* +51 Franklin Street, Fifth Floor @* +Boston, MA 02110-1301 USA @* Printed copies are available from the Free Software Foundation.@* @acronym{ISBN} 1-882114-44-2 @sp 2 @@ -117,9 +104,10 @@ Reference sections: messy for Bison to handle straightforwardly. * Debugging:: Understanding or debugging Bison parsers. * Invocation:: How to run Bison (to produce the parser source file). +* C++ Language Interface:: Creating C++ parser objects. +* FAQ:: Frequently Asked Questions * Table of Symbols:: All the keywords of the Bison language are explained. * Glossary:: Basic concepts are explained. -* FAQ:: Frequently Asked Questions * Copying This Manual:: License for copying this manual. * Index:: Cross-references to the text. @@ -136,13 +124,19 @@ The Concepts of Bison the name of an identifier, etc.). * Semantic Actions:: Each rule can have an action containing C code. * GLR Parsers:: Writing parsers for general context-free languages. -* Simple GLR Parsers:: Using GLR in its simplest form. * Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? * Stages:: Stages in writing and running Bison grammars. * Grammar Layout:: Overall structure of a Bison grammar file. +Writing @acronym{GLR} Parsers + +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars. +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities. +* GLR Semantic Actions:: Deferred semantic actions have special concerns. +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler. + Examples * RPN Calc:: Reverse polish notation calculator; @@ -219,10 +213,12 @@ Tracking Locations Bison Declarations +* Require Decl:: Requiring a Bison version. * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Initial Action Decl:: Code run before parsing starts. * Destructor Decl:: Declaring how symbols are freed. * Expect Decl:: Suppressing warnings about parsing conflicts. * Start Decl:: Specifying the start symbol. @@ -236,6 +232,8 @@ Parser C-Language Interface which reads tokens. * Error Reporting:: You must supply a function @code{yyerror}. * Action Features:: Special features for use in actions. +* Internationalization:: How to let the parser speak in the user's + native language. The Lexical Analyzer Function @code{yylex} @@ -258,7 +256,7 @@ The Bison Parser Algorithm * Reduce/Reduce:: When two rules are applicable in the same situation. * Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. * Generalized LR Parsing:: Parsing arbitrary context-free grammars. -* Stack Overflow:: What happens when stack gets full. How to avoid it. +* Memory Management:: What happens when memory is exhausted. How to avoid it. Operator Precedence @@ -286,13 +284,40 @@ Invoking Bison * Option Cross Key:: Alphabetical list of long options. * Yacc Library:: Yacc-compatible @code{yylex} and @code{main}. +C++ Language Interface + +* C++ Parsers:: The interface to generate C++ parser classes +* A Complete C++ Example:: Demonstrating their use + +C++ Parsers + +* C++ Bison Interface:: Asking for C++ parser generation +* C++ Semantic Values:: %union vs. C++ +* C++ Location Values:: The position and location classes +* C++ Parser Interface:: Instantiating and running the parser +* C++ Scanner Interface:: Exchanges between yylex and parse + +A Complete C++ Example + +* Calc++ --- C++ Calculator:: The specifications +* Calc++ Parsing Driver:: An active parsing context +* Calc++ Parser:: A parser class +* Calc++ Scanner:: A pure C++ Flex scanner +* Calc++ Top Level:: Conducting the band + Frequently Asked Questions -* Parser Stack Overflow:: Breaking the Stack Limits +* Memory Exhausted:: Breaking the Stack Limits * How Can I Reset the Parser:: @code{yyparse} Keeps some State * Strings are Destroyed:: @code{yylval} Loses Track of Strings -* C++ Parsers:: Compiling Parsers with C++ Compilers -* Implementing Loops:: Control Flow in the Calculator +* Implementing Gotos/Loops:: Control Flow in the Calculator +* Secure? Conform?:: Is Bison @acronym{POSIX} safe? +* I can't build Bison:: Troubleshooting +* Where can I find help?:: Troubleshouting +* Bug Reports:: Troublereporting +* Other Languages:: Parsers in Java and others +* Beta Testing:: Experimenting development versions +* Mailing Lists:: Meeting other Bison users Copying This Manual @@ -305,16 +330,16 @@ Copying This Manual @unnumbered Introduction @cindex introduction -@dfn{Bison} is a general-purpose parser generator that converts a -grammar description for an @acronym{LALR}(1) context-free grammar into a C -program to parse that grammar. Once you are proficient with Bison, -you may use it to develop a wide range of language parsers, from those +@dfn{Bison} is a general-purpose parser generator that converts a grammar +description for an @acronym{LALR}(1) or @acronym{GLR} context-free grammar +into a C or C++ program to parse 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. Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little trouble. You need to be fluent in -C programming in order to use Bison or to understand this manual. +C or C++ programming in order to use Bison or to understand this manual. We begin with tutorial chapters that explain the basic concepts of using Bison and show three explained examples, each building on the last. If you @@ -383,7 +408,6 @@ use Bison or Yacc, we suggest you start by reading this chapter carefully. the name of an identifier, etc.). * Semantic Actions:: Each rule can have an action containing C code. * GLR Parsers:: Writing parsers for general context-free languages. -* Simple GLR Parsers:: Using GLR in its simplest form. * Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? @@ -432,15 +456,15 @@ more information on this. @cindex @acronym{GLR} parsing @cindex generalized @acronym{LR} (@acronym{GLR}) parsing @cindex ambiguous grammars -@cindex non-deterministic parsing +@cindex nondeterministic parsing Parsers for @acronym{LALR}(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{look-ahead}) of the remaining input. A context-free grammar can be @dfn{ambiguous}, meaning that there are multiple ways to -apply the grammar rules to get the some inputs. Even unambiguous -grammars can be @dfn{non-deterministic}, meaning that no fixed +apply the grammar rules to get the same inputs. Even unambiguous +grammars can be @dfn{nondeterministic}, meaning that no fixed look-ahead always suffices to determine the next grammar rule to apply. With the proper declarations, Bison is also able to parse these more general context-free grammars, using a technique known as @acronym{GLR} @@ -475,18 +499,18 @@ Here is a simple C function subdivided into tokens: @ifinfo @example int /* @r{keyword `int'} */ -square (int x) /* @r{identifier, open-paren, identifier,} +square (int x) /* @r{identifier, open-paren, keyword `int',} @r{identifier, close-paren} */ @{ /* @r{open-brace} */ - return x * x; /* @r{keyword `return', identifier, asterisk, - identifier, semicolon} */ + return x * x; /* @r{keyword `return', identifier, asterisk,} + @r{identifier, semicolon} */ @} /* @r{close-brace} */ @end example @end ifinfo @ifnotinfo @example int /* @r{keyword `int'} */ -square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */ +square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */ @{ /* @r{open-brace} */ return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */ @} /* @r{close-brace} */ @@ -661,8 +685,9 @@ from the values of the two subexpressions. @findex %glr-parser @cindex conflicts @cindex shift/reduce conflicts +@cindex reduce/reduce conflicts -In some grammars, there will be cases where Bison's standard +In some grammars, Bison's standard @acronym{LALR}(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 @@ -675,7 +700,7 @@ input. These are known respectively as @dfn{reduce/reduce} conflicts To use a grammar that is not easily modified to be @acronym{LALR}(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 will be a Generalized @acronym{LR} +(@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 @@ -702,6 +727,216 @@ involved, or by performing both actions, and then calling a designated user-defined function on the resulting values to produce an arbitrary merged result. +@menu +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars. +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities. +* GLR Semantic Actions:: Deferred semantic actions have special concerns. +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler. +@end menu + +@node Simple GLR Parsers +@subsection Using @acronym{GLR} on Unambiguous Grammars +@cindex @acronym{GLR} parsing, unambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars +@findex %glr-parser +@findex %expect-rr +@cindex conflicts +@cindex reduce/reduce conflicts +@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 look-ahead, +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}). + +Consider a problem that +arises in the declaration of enumerated and subrange types in the +programming language Pascal. Here are some examples: + +@example +type subrange = lo .. hi; +type enum = (a, b, c); +@end example + +@noindent +The original language standard allows only numeric +literals and constant identifiers for the subrange bounds (@samp{lo} +and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC} +10206) and many other +Pascal implementations allow arbitrary expressions there. This gives +rise to the following situation, containing a superfluous pair of +parentheses: + +@example +type subrange = (a) .. b; +@end example + +@noindent +Compare this to the following declaration of an enumerated +type with only one value: + +@example +type enum = (a); +@end example + +@noindent +(These declarations are contrived, but they are syntactically +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 look-ahead 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 +@samp{a} must become a new identifier to represent the enumeration +value, while in the former case @samp{a} must be evaluated with its +current meaning, which may be a constant or even a function call. + +You could parse @samp{(a)} as an ``unspecified identifier in parentheses'', +to be resolved later, but this typically requires substantial +contortions in both semantic actions and large parts of the +grammar, where the parentheses are nested in the recursive rules for +expressions. + +You might think of using the lexer to distinguish between the two +forms by returning different tokens for currently defined and +undefined identifiers. But if these declarations occur in a local +scope, and @samp{a} is defined in an outer scope, then both forms +are possible---either locally redefining @samp{a}, or using the +value of @samp{a} from the outer scope. So this approach cannot +work. + +A simple solution to this problem is to declare the parser to +use the @acronym{GLR} algorithm. +When the @acronym{GLR} parser reaches the critical state, it +merely splits into two branches and pursues both syntax rules +simultaneously. Sooner or later, one of them runs into a parsing +error. If there is a @samp{..} token before the next +@samp{;}, the rule for enumerated types fails since it cannot +accept @samp{..} anywhere; otherwise, the subrange type rule +fails since it requires a @samp{..} token. So one of the branches +fails silently, and the other one continues normally, performing +all the intermediate actions that were postponed during the split. + +If the input is syntactically incorrect, both branches fail and the parser +reports a syntax error as usual. + +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 +look-ahead 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. + +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 +for some grammars. In practice, this rarely happens, and for many +grammars it is possible to prove that it cannot happen. +The present example contains only one conflict between two +rules, and the type-declaration context containing the conflict +cannot be nested. So the number of +branches that can exist at any time is limited by the constant 2, +and the parsing time is still linear. + +Here is a Bison grammar corresponding to the example above. It +parses a vastly simplified form of Pascal type declarations. + +@example +%token TYPE DOTDOT ID + +@group +%left '+' '-' +%left '*' '/' +@end group + +%% + +@group +type_decl : TYPE ID '=' type ';' + ; +@end group + +@group +type : '(' id_list ')' + | expr DOTDOT expr + ; +@end group + +@group +id_list : ID + | id_list ',' ID + ; +@end group + +@group +expr : '(' expr ')' + | expr '+' expr + | expr '-' expr + | expr '*' expr + | expr '/' expr + | ID + ; +@end group +@end example + +When used as a normal @acronym{LALR}(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 +recognized: + +@example +type t = (a) .. b; +@end example + +The parser can be turned into a @acronym{GLR} parser, while also telling Bison +to be silent about the one known reduce/reduce conflict, by +adding these two declarations to the Bison input file (before the first +@samp{%%}): + +@example +%glr-parser +%expect-rr 1 +@end example + +@noindent +No change in the grammar itself is required. Now the +parser recognizes all valid declarations, according to the +limited syntax above, transparently. In fact, the user does not even +notice when the parser splits. + +So here we have a case where we can use the benefits of @acronym{GLR}, +almost without disadvantages. Even in simple cases like this, however, +there are at least two potential problems to beware. First, always +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 +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 +information via parser actions. Some cases of lexer interactions can be +eliminated by using @acronym{GLR} to shift the complications from the +lexer to the parser. You must check the remaining cases for +correctness. + +In our example, it would be safe for the lexer to return tokens based on +their current meanings in some symbol table, because no new symbols are +defined in the middle of a type declaration. Though it is possible for +a parser to define the enumeration constants as they are parsed, before +the type declaration is completed, it actually makes no difference since +they cannot be used within the same enumerated type declaration. + +@node Merging GLR Parses +@subsection Using @acronym{GLR} to Resolve Ambiguities +@cindex @acronym{GLR} parsing, ambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars +@findex %dprec +@findex %merge +@cindex conflicts +@cindex reduce/reduce conflicts + Let's consider an example, vastly simplified from a C++ grammar. @example @@ -761,8 +996,21 @@ parses as either an @code{expr} or a @code{stmt} @samp{x} as an @code{ID}). Bison detects this as a reduce/reduce conflict between the rules @code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the -time it encounters @code{x} in the example above. The two @code{%dprec} -declarations, however, give precedence to interpreting the example as a +time it encounters @code{x} in the example above. Since this is a +@acronym{GLR} parser, it therefore splits the problem into two parses, one for +each choice of resolving the reduce/reduce conflict. +Unlike the example from the previous section (@pxref{Simple GLR Parsers}), +however, neither of these parses ``dies,'' because the grammar as it stands is +ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and +the other reduces @code{stmt : decl}, after which both parsers are in an +identical state: they've seen @samp{prog stmt} and have the same unprocessed +input remaining. We say that these parses have @dfn{merged.} + +At this point, the @acronym{GLR} parser requires a specification in the +grammar of how to choose between the competing parses. +In the example above, the two @code{%dprec} +declarations specify that Bison is to give precedence +to the parse that interprets the example as a @code{decl}, which implies that @code{x} is a declarator. The parser therefore prints @@ -770,18 +1018,21 @@ The parser therefore prints "x" y z + T <init-declare> @end example -Consider a different input string for this parser: +The @code{%dprec} declarations only come into play when more than one +parse survives. Consider a different input string for this parser: @example T (x) + y; @end example @noindent +This is another example of using @acronym{GLR} to parse an unambiguous +construct, as shown in the previous section (@pxref{Simple GLR Parsers}). Here, there is no ambiguity (this cannot be parsed as a declaration). However, at the time the Bison parser encounters @code{x}, it does not have enough information to resolve the reduce/reduce conflict (again, between @code{x} as an @code{expr} or a @code{declarator}). In this -case, no precedence declaration is used. Instead, the parser splits +case, no precedence declaration is used. Again, the parser splits into two, one assuming that @code{x} is an @code{expr}, and the other assuming @code{x} is a @code{declarator}. The second of these parsers then vanishes when it sees @code{+}, and the parser prints @@ -791,7 +1042,7 @@ x T <cast> y + @end example Suppose that instead of resolving the ambiguity, you wanted to see all -the possibilities. For this purpose, we must @dfn{merge} the semantic +the possibilities. For this purpose, you must merge the semantic actions of the two possible parsers, rather than choosing one over the other. To do so, you could change the declaration of @code{stmt} as follows: @@ -803,7 +1054,6 @@ stmt : expr ';' %merge <stmtMerge> @end example @noindent - and define the @code{stmtMerge} function as: @example @@ -827,17 +1077,73 @@ in the C declarations at the beginning of the file: @end example @noindent -With these declarations, the resulting parser will parse the first example -as both an @code{expr} and a @code{decl}, and print +With these declarations, the resulting parser parses the first example +as both an @code{expr} and a @code{decl}, and prints @example "x" y z + T <init-declare> x T <cast> y z + = <OR> @end example -@sp 1 +Bison requires that all of the +productions that participate in any particular merge have identical +@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable, +and the parser will report an error during any parse that results in +the offending merge. + +@node GLR Semantic Actions +@subsection GLR Semantic Actions + +@cindex deferred semantic actions +By definition, a deferred semantic action is not performed at the same time as +the associated reduction. +This raises caveats for several Bison features you might use in a semantic +action in a @acronym{GLR} parser. + +@vindex yychar +@cindex @acronym{GLR} parsers and @code{yychar} +@vindex yylval +@cindex @acronym{GLR} parsers and @code{yylval} +@vindex yylloc +@cindex @acronym{GLR} parsers and @code{yylloc} +In any semantic action, you can examine @code{yychar} to determine the type of +the look-ahead token present at the time of the associated reduction. +After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF}, +you can then examine @code{yylval} and @code{yylloc} to determine the +look-ahead token's semantic value and location, if any. +In a nondeferred semantic action, you can also modify any of these variables to +influence syntax analysis. +@xref{Look-Ahead, ,Look-Ahead Tokens}. + +@findex yyclearin +@cindex @acronym{GLR} parsers and @code{yyclearin} +In a deferred semantic action, it's too late to influence syntax analysis. +In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to +shallow copies of the values they had at the time of the associated reduction. +For this reason alone, modifying them is dangerous. +Moreover, the result of modifying them is undefined and subject to change with +future versions of Bison. +For example, if a semantic action might be deferred, you should never write it +to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free +memory referenced by @code{yylval}. -@cindex @code{incline} +@findex YYERROR +@cindex @acronym{GLR} parsers and @code{YYERROR} +Another Bison feature requiring special consideration is @code{YYERROR} +(@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. +In a deferred semantic action, its effect is undefined. +@c The effect is probably a syntax error at the split point. + +Also, see @ref{Location Default Action, ,Default Action for Locations}, which +describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers. + +@node Compiler Requirements +@subsection Considerations when Compiling @acronym{GLR} Parsers +@cindex @code{inline} @cindex @acronym{GLR} parsers and @code{inline} + The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or later. In addition, they use the @code{inline} keyword, which is not C89, but is C99 and is a common extension in pre-C99 compilers. It is @@ -862,255 +1168,53 @@ will suffice. Otherwise, we suggest %@} @end example -@node Simple GLR Parsers -@section Using @acronym{GLR} in its Simplest Form -@cindex @acronym{GLR} parsing, unambiguous grammars -@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars -@findex %glr-parser -@findex %expect-rr -@cindex conflicts -@cindex reduce/reduce conflicts - -The C++ example for @acronym{GLR} (@pxref{GLR Parsers}) explains how to use -the @acronym{GLR} parsing algorithm with some advanced features such as -@samp{%dprec} and @samp{%merge} to handle syntactically ambiguous -grammars. However, the @acronym{GLR} algorithm can also be used in a simpler -way to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1). -Such grammars typically require more than one symbol of look-ahead, -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}). +@node Locations Overview +@section Locations +@cindex location +@cindex textual location +@cindex location, textual -Here is an example of this situation, using a problem that -arises in the declaration of enumerated and subrange types in the -programming language Pascal. These declarations look like this: +Many applications, like interpreters or compilers, have to produce verbose +and useful error messages. To achieve this, one must be able to keep track of +the @dfn{textual location}, or @dfn{location}, of each syntactic construct. +Bison provides a mechanism for handling these locations. -@example -type subrange = lo .. hi; -type enum = (a, b, c); -@end example +Each token has a semantic value. In a similar fashion, each token has an +associated location, but the type of locations is the same for all tokens and +groupings. Moreover, the output parser is equipped with a default data +structure for storing locations (@pxref{Locations}, for more details). -@noindent -The original language standard allows only numeric -literals and constant identifiers for the subrange bounds (@samp{lo} -and @samp{hi}), but Extended Pascal (ISO/IEC 10206:1990) and many other -Pascal implementations allow arbitrary expressions there. This gives -rise to the following situation, containing a superfluous pair of -parentheses: +Like semantic values, locations can be reached in actions using a dedicated +set of constructs. In the example above, the location of the whole grouping +is @code{@@$}, while the locations of the subexpressions are @code{@@1} and +@code{@@3}. -@example -type subrange = (a) .. b; -@end example +When a rule is matched, a default action is used to compute the semantic value +of its left hand side (@pxref{Actions}). In the same way, another default +action is used for locations. However, the action for locations is general +enough for most cases, meaning there is usually no need to describe for each +rule how @code{@@$} should be formed. When building a new location for a given +grouping, the default behavior of the output parser is to take the beginning +of the first symbol, and the end of the last symbol. -@noindent -Compare this to the following declaration of an enumerated -type with only one value: +@node Bison Parser +@section Bison Output: the Parser File +@cindex Bison parser +@cindex Bison utility +@cindex lexical analyzer, purpose +@cindex parser -@example -type enum = (a); -@end example +When you run Bison, you give it a Bison grammar file as input. The output +is a C source file that parses the language described by the grammar. +This file is called a @dfn{Bison parser}. Keep in mind that the Bison +utility and the Bison parser are two distinct programs: the Bison utility +is a program whose output is the Bison parser that becomes part of your +program. -@noindent -(These declarations are contrived, but they are syntactically -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 look-ahead 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 -@samp{a} must become a new identifier to represent the enumeration -value, while in the former case @samp{a} must be evaluated with its -current meaning, which may be a constant or even a function call. - -You could parse @samp{(a)} as an ``unspecified identifier in parentheses'', -to be resolved later, but this typically requires substantial -contortions in both semantic actions and large parts of the -grammar, where the parentheses are nested in the recursive rules for -expressions. - -You might think of using the lexer to distinguish between the two -forms by returning different tokens for currently defined and -undefined identifiers. But if these declarations occur in a local -scope, and @samp{a} is defined in an outer scope, then both forms -are possible---either locally redefining @samp{a}, or using the -value of @samp{a} from the outer scope. So this approach cannot -work. - -A solution to this problem is to use a @acronym{GLR} parser in its simplest -form, i.e., without using special features such as @samp{%dprec} and -@samp{%merge}. When the @acronym{GLR} parser reaches the critical state, it -simply splits into two branches and pursues both syntax rules -simultaneously. Sooner or later, one of them runs into a parsing -error. If there is a @samp{..} token before the next -@samp{;}, the rule for enumerated types fails since it cannot -accept @samp{..} anywhere; otherwise, the subrange type rule -fails since it requires a @samp{..} token. So one of the branches -fails silently, and the other one continues normally, performing -all the intermediate actions that were postponed during the split. - -If the input is syntactically incorrect, both branches fail and the parser -reports a syntax error as usual. - -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 -look-ahead 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. - -Since there can be only two branches and at least one of them -must fail, you need not worry about merging the branches by -using dynamic precedence or @samp{%merge}. - -Another potential problem of @acronym{GLR} does not arise here, either. 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 -for some grammars. In practice, this rarely happens, and for many -grammars it is possible to prove that it cannot happen. In -in the present example, there is only one conflict between two -rules, and the type-declaration context where the conflict -arises cannot be nested. So the number of -branches that can exist at any time is limited by the constant 2, -and the parsing time is still linear. - -So here we have a case where we can use the benefits of @acronym{GLR}, almost -without disadvantages. There are two things to note, though. -First, one should carefully analyze the conflicts reported by -Bison to make sure that @acronym{GLR} splitting is done only where it is -intended to be. A @acronym{GLR} parser splitting inadvertently may cause -problems less obvious than an @acronym{LALR} parser statically choosing the -wrong alternative in a conflict. - -Second, interactions with the lexer (@pxref{Semantic Tokens}) must -be considered with great care. Since a split parser consumes tokens -without performing any actions during the split, the lexer cannot -obtain information via parser actions. Some cases of -lexer interactions can simply be eliminated by using @acronym{GLR}, i.e., -shifting the complications from the lexer to the parser. Remaining -cases have to be checked for safety. - -In our example, it would be safe for the lexer to return tokens -based on their current meanings in some symbol table, because no new -symbols are defined in the middle of a type declaration. Though it -is possible for a parser to define the enumeration -constants as they are parsed, before the type declaration is -completed, it actually makes no difference since they cannot be used -within the same enumerated type declaration. - -Here is a Bison grammar corresponding to the example above. It -parses a vastly simplified form of Pascal type declarations. - -@example -%token TYPE DOTDOT ID - -@group -%left '+' '-' -%left '*' '/' -@end group - -%% - -@group -type_decl: - TYPE ID '=' type ';' -; -@end group - -@group -type: '(' id_list ')' - | expr DOTDOT expr -; -@end group - -@group -id_list: ID - | id_list ',' ID -; -@end group - -@group -expr: '(' expr ')' - | expr '+' expr - | expr '-' expr - | expr '*' expr - | expr '/' expr - | ID -; -@end group -@end example - -When used as a normal @acronym{LALR}(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 -recognized: - -@example -type t = (a) .. b; -@end example - -The parser can be turned into a @acronym{GLR} parser, while also telling Bison -to be silent about the one known reduce/reduce conflict, simply by -adding these two declarations to the Bison input file: - -@example -%glr-parser -%expect-rr 1 -@end example - -@noindent -No change in the grammar itself is required. Now the -parser recognizes all valid declarations, according to the -limited syntax above, transparently. In fact, the user does not even -notice when the parser splits. - -@node Locations Overview -@section Locations -@cindex location -@cindex textual location -@cindex location, textual - -Many applications, like interpreters or compilers, have to produce verbose -and useful error messages. To achieve this, one must be able to keep track of -the @dfn{textual location}, or @dfn{location}, of each syntactic construct. -Bison provides a mechanism for handling these locations. - -Each token has a semantic value. In a similar fashion, each token has an -associated location, but the type of locations is the same for all tokens and -groupings. Moreover, the output parser is equipped with a default data -structure for storing locations (@pxref{Locations}, for more details). - -Like semantic values, locations can be reached in actions using a dedicated -set of constructs. In the example above, the location of the whole grouping -is @code{@@$}, while the locations of the subexpressions are @code{@@1} and -@code{@@3}. - -When a rule is matched, a default action is used to compute the semantic value -of its left hand side (@pxref{Actions}). In the same way, another default -action is used for locations. However, the action for locations is general -enough for most cases, meaning there is usually no need to describe for each -rule how @code{@@$} should be formed. When building a new location for a given -grouping, the default behavior of the output parser is to take the beginning -of the first symbol, and the end of the last symbol. - -@node Bison Parser -@section Bison Output: the Parser File -@cindex Bison parser -@cindex Bison utility -@cindex lexical analyzer, purpose -@cindex parser - -When you run Bison, you give it a Bison grammar file as input. The output -is a C source file that parses the language described by the grammar. -This file is called a @dfn{Bison parser}. Keep in mind that the Bison -utility and the Bison parser are two distinct programs: the Bison utility -is a program whose output is the Bison parser that becomes part of your -program. - -The job of the Bison parser is to group tokens into groupings according to -the grammar rules---for example, to build identifiers and operators into -expressions. As it does this, it runs the actions for the grammar rules it -uses. +The job of the Bison parser is to group tokens into groupings according to +the grammar rules---for example, to build identifiers and operators into +expressions. As it does this, it runs the actions for the grammar rules it +uses. The tokens come from a function called the @dfn{lexical analyzer} that you must supply in some fashion (such as by writing it in C). The Bison @@ -1137,13 +1241,17 @@ function @code{yyerror} and the parser function @code{yyparse} itself. This also includes numerous identifiers used for internal purposes. Therefore, you should avoid using C identifiers starting with @samp{yy} or @samp{YY} in the Bison grammar file except for the ones defined in -this manual. +this manual. Also, you should avoid using the C identifiers +@samp{malloc} and @samp{free} for anything other than their usual +meanings. In some cases the Bison parser file includes system headers, and in those cases your code should respect the identifiers reserved by those -headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>}, +headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.h>}, @code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to -declare memory allocators and related types. Other system headers may +declare memory allocators and related types. @code{<libintl.h>} is +included if message translation is in use +(@pxref{Internationalization}). Other system headers may be included if you define @code{YYDEBUG} to a nonzero value (@pxref{Tracing, ,Tracing Your Parser}). @@ -1247,11 +1355,8 @@ under BSD Unix 4.3; each produces a usable, though limited, interactive desk-top calculator. These examples are simple, but Bison grammars for real programming -languages are written the same way. -@ifinfo -You can copy these examples out of the Info file and into a source file -to try them. -@end ifinfo +languages are written the same way. You can copy these examples into a +source file to try them. @menu * RPN Calc:: Reverse polish notation calculator; @@ -1370,7 +1475,7 @@ exp: NUM @{ $$ = $1; @} The groupings of the rpcalc ``language'' defined here are the expression (given the name @code{exp}), the line of input (@code{line}), and the complete input transcript (@code{input}). Each of these nonterminal -symbols has several alternate rules, joined by the @samp{|} punctuator +symbols has several alternate rules, joined by the vertical bar @samp{|} which is read as ``or''. The following sections explain what these rules mean. @@ -1654,12 +1759,12 @@ With all the source in a single file, you use the following command to convert it into a parser file: @example -bison @var{file_name}.y +bison @var{file}.y @end example @noindent In this example the file was called @file{rpcalc.y} (for ``Reverse Polish -@sc{calc}ulator''). Bison produces a file named @file{@var{file_name}.tab.c}, +@sc{calc}ulator''). Bison produces a file named @file{@var{file}.tab.c}, removing the @samp{.y} from the original file name. The file output by Bison contains the source code for @code{yyparse}. The additional functions in the input file (@code{yylex}, @code{yyerror} and @code{main}) @@ -2061,7 +2166,7 @@ as @code{sin}, @code{cos}, etc. It is easy to add new operators to the infix calculator as long as they are only single-character literals. The lexical analyzer @code{yylex} passes -back all nonnumber characters as tokens, so new grammar rules suffice for +back all nonnumeric characters as tokens, so new grammar rules suffice for adding a new operator. But we want something more flexible: built-in functions whose syntax has this form: @@ -2235,7 +2340,7 @@ typedef struct symrec symrec; /* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; -symrec *putsym (char const *, func_t); +symrec *putsym (char const *, int); symrec *getsym (char const *); @end group @end smallexample @@ -2346,7 +2451,7 @@ getsym (char const *sym_name) The function @code{yylex} must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric -characters with a leading non-digit are recognized as either variables or +characters with a leading letter are recognized as either variables or functions depending on what the symbol table says about them. The string is passed to @code{getsym} for look up in the symbol table. If @@ -2520,13 +2625,17 @@ continues until end of line. @cindex Prologue @cindex declarations -The @var{Prologue} section contains macro definitions and -declarations of functions and variables that are used in the actions in the -grammar rules. These are copied to the beginning of the parser file so -that they precede the definition of @code{yyparse}. You can use -@samp{#include} to get the declarations from a header file. If you don't -need any C declarations, you may omit the @samp{%@{} and @samp{%@}} -delimiters that bracket this section. +The @var{Prologue} section contains macro definitions and declarations +of functions and variables that are used in the actions in the grammar +rules. These are copied to the beginning of the parser file so that +they precede the definition of @code{yyparse}. You can use +@samp{#include} to get the declarations from a header file. If you +don't need any C declarations, you may omit the @samp{%@{} and +@samp{%@}} delimiters that bracket this section. + +The @var{Prologue} section is terminated by the the first occurrence +of @samp{%@}} that is outside a comment, a string literal, or a +character constant. You may have more than one @var{Prologue} section, intermixed with the @var{Bison declarations}. This allows you to have C and Bison @@ -2590,16 +2699,16 @@ not come before the definition of @code{yyparse}. For example, the definitions of @code{yylex} and @code{yyerror} often go here. Because C requires functions to be declared before being used, you often need to declare functions like @code{yylex} and @code{yyerror} in the Prologue, -even if you define them int he Epilogue. +even if you define them in the Epilogue. @xref{Interface, ,Parser C-Language Interface}. If the last section is empty, you may omit the @samp{%%} that separates it from the grammar rules. -The Bison parser itself contains many macros and identifiers whose -names start with @samp{yy} or @samp{YY}, so it is a -good idea to avoid using any such names (except those documented in this -manual) in the epilogue of the grammar file. +The Bison parser itself contains many macros and identifiers whose names +start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using +any such names (except those documented in this manual) in the epilogue +of the grammar file. @node Symbols @section Symbols, Terminal and Nonterminal @@ -2615,13 +2724,13 @@ A @dfn{terminal symbol} (also known as a @dfn{token type}) represents a class of syntactically equivalent tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The symbol is represented in the Bison parser by a numeric code, and the @code{yylex} -function returns a token type code to indicate what kind of token has been -read. You don't need to know what the code value is; you can use the -symbol to stand for it. +function returns a token type code to indicate what kind of token has +been read. You don't need to know what the code value is; you can use +the symbol to stand for it. -A @dfn{nonterminal symbol} stands for a class of syntactically equivalent -groupings. The symbol name is used in writing grammar rules. By convention, -it should be all lower case. +A @dfn{nonterminal symbol} stands for a class of syntactically +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. @@ -2717,7 +2826,7 @@ into a separate header file @file{@var{name}.tab.h} which you can include in the other source files that need it. @xref{Invocation, ,Invoking Bison}. If you want to write a grammar that is portable to any Standard C -host, you must use only non-null character tokens taken from the basic +host, you must use only nonnull character tokens taken from the basic execution character set of Standard C@. This set consists of the ten digits, the 52 lower- and upper-case English letters, and the characters in the following C-language string: @@ -2726,17 +2835,17 @@ characters in the following C-language string: "\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~" @end example -The @code{yylex} function and Bison must use a consistent character -set and encoding for character tokens. For example, if you run Bison in an -@acronym{ASCII} environment, but then compile and run the resulting program -in an environment that uses an incompatible character set like -@acronym{EBCDIC}, the resulting program may not work because the -tables generated by Bison will assume @acronym{ASCII} numeric values for -character tokens. It is standard -practice for software distributions to contain C source files that -were generated by Bison in an @acronym{ASCII} environment, so installers on -platforms that are incompatible with @acronym{ASCII} must rebuild those -files before compiling them. +The @code{yylex} function and Bison must use a consistent character set +and encoding for character tokens. For example, if you run Bison in an +@acronym{ASCII} environment, but then compile and run the resulting +program in an environment that uses an incompatible character set like +@acronym{EBCDIC}, the resulting program may not work because the tables +generated by Bison will assume @acronym{ASCII} numeric values for +character tokens. It is standard practice for software distributions to +contain C source files that were generated by Bison in an +@acronym{ASCII} environment, so installers on platforms that are +incompatible with @acronym{ASCII} must rebuild those files before +compiling them. The symbol @code{error} is a terminal symbol reserved for error recovery (@pxref{Error Recovery}); you shouldn't use it for any other purpose. @@ -2788,6 +2897,22 @@ the semantics of the rule. An action looks like this: @end example @noindent +@cindex braced code +This is an example of @dfn{braced code}, that is, C code surrounded by +braces, much like a compound statement in C@. Braced code can contain +any sequence of C tokens, so long as its braces are balanced. Bison +does not check the braced code for correctness directly; it merely +copies the code to the output file, where the C compiler can check it. + +Within braced code, the balanced-brace count is not affected by braces +within comments, string literals, or character constants, but it is +affected by the C digraphs @samp{<%} and @samp{%>} that represent +braces. At the top level braced code must be terminated by @samp{@}} +and not by a digraph. Bison does not look for trigraphs, so if braced +code uses trigraphs you should ensure that they do not affect the +nesting of braces or the boundaries of comments, string literals, or +character constants. + Usually there is only one action and it follows the components. @xref{Actions}. @@ -2795,15 +2920,6 @@ Usually there is only one action and it follows the components. Multiple rules for the same @var{result} can be written separately or can be joined with the vertical-bar character @samp{|} as follows: -@ifinfo -@example -@var{result}: @var{rule1-components}@dots{} - | @var{rule2-components}@dots{} - @dots{} - ; -@end example -@end ifinfo -@iftex @example @group @var{result}: @var{rule1-components}@dots{} @@ -2812,7 +2928,6 @@ be joined with the vertical-bar character @samp{|} as follows: ; @end group @end example -@end iftex @noindent They are still considered distinct rules even when joined in this way. @@ -2843,10 +2958,10 @@ with no components. @section Recursive Rules @cindex recursive rule -A rule is called @dfn{recursive} when its @var{result} nonterminal appears -also on its right hand side. Nearly all Bison grammars need to use -recursion, because that is the only way to define a sequence of any number -of a particular thing. Consider this recursive definition of a +A rule is called @dfn{recursive} when its @var{result} nonterminal +appears also on its right hand side. Nearly all Bison grammars need to +use recursion, because that is the only way to define a sequence of any +number of a particular thing. Consider this recursive definition of a comma-separated sequence of one or more expressions: @example @@ -2960,8 +3075,9 @@ This macro definition must go in the prologue of the grammar file In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type -@code{int} or @code{long int}, while a string constant needs type @code{char *}, -and an identifier might need a pointer to an entry in the symbol table. +@code{int} or @code{long int}, while a string constant needs type +@code{char *}, and an identifier might need a pointer to an entry in the +symbol table. To use more than one data type for semantic values in one parser, Bison requires you to do two things: @@ -2991,14 +3107,8 @@ each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings. -An action consists of C statements surrounded by braces, much like a -compound statement in C@. An action can contain any sequence of C -statements. Bison does not look for trigraphs, though, so if your C -code uses trigraphs you should ensure that they do not affect the -nesting of braces or the boundaries of comments, strings, or character -literals. - -An action can be placed at any position in the rule; +An action consists of braced code containing C statements, and can be +placed at any position in the rule; it is executed at that position. Most rules have just one action at the end of the rule, following all the components. Actions in the middle of a rule are tricky and used only for special purposes (@pxref{Mid-Rule @@ -3076,6 +3186,12 @@ As long as @code{bar} is used only in the fashion shown here, @code{$0} always refers to the @code{expr} which precedes @code{bar} in the definition of @code{foo}. +@vindex yylval +It is also possible to access the semantic value of the look-ahead token, if +any, from a semantic action. +This semantic value is stored in @code{yylval}. +@xref{Action Features, ,Special Features for Use in Actions}. + @node Action Types @subsection Data Types of Values in Actions @cindex action data types @@ -3185,6 +3301,46 @@ earlier action is used to restore the prior list of variables. This removes the temporary @code{let}-variable from the list so that it won't appear to exist while the rest of the program is parsed. +@findex %destructor +@cindex discarded symbols, mid-rule actions +@cindex error recovery, mid-rule actions +In the above example, if the parser initiates error recovery (@pxref{Error +Recovery}) while parsing the tokens in the embedded statement @code{stmt}, +it might discard the previous semantic context @code{$<context>5} without +restoring it. +Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing +Discarded Symbols}). +However, Bison currently provides no means to declare a destructor for a +mid-rule action's semantic value. + +One solution is to bury the mid-rule action inside a nonterminal symbol and to +declare a destructor for that symbol: + +@example +@group +%type <context> let +%destructor @{ pop_context ($$); @} let + +%% + +stmt: let stmt + @{ $$ = $2; + pop_context ($1); @} + ; + +let: LET '(' var ')' + @{ $$ = push_context (); + declare_variable ($3); @} + ; + +@end group +@end example + +@noindent +Note that the action is now at the end of its rule. +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. + 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, @@ -3278,10 +3434,7 @@ compound: subroutine @noindent Now Bison can execute the action in the rule for @code{subroutine} without -deciding which rule for @code{compound} it will eventually use. Note that -the action is now at the end of its rule. 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. +deciding which rule for @code{compound} it will eventually use. @node Locations @section Tracking Locations @@ -3393,9 +3546,16 @@ exp: @dots{} @end group @end example +@vindex yylloc +It is also possible to access the location of the look-ahead token, if any, +from a semantic action. +This location is stored in @code{yylloc}. +@xref{Action Features, ,Special Features for Use in Actions}. + @node Location Default Action @subsection Default Action for Locations @vindex YYLLOC_DEFAULT +@cindex @acronym{GLR} parsers and @code{YYLLOC_DEFAULT} Actually, actions are not the best place to compute locations. Since locations are much more general than semantic values, there is room in @@ -3403,44 +3563,51 @@ the output parser to redefine the default action to take for each rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is matched, before the associated action is run. It is also invoked while processing a syntax error, to compute the error's location. +Before reporting an unresolvable syntactic ambiguity, a @acronym{GLR} +parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location +of that ambiguity. Most of the time, this macro is general enough to suppress location dedicated code from semantic actions. The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is the location of the grouping (the result of the computation). When a -rule is matched, the second parameter is an array holding locations of +rule is matched, the second parameter identifies locations of all right hand side elements of the rule being matched, and the third -parameter is the size of the rule's right hand side. When processing -a syntax error, the second parameter is an array holding locations of -the symbols that were discarded during error processing, and the third +parameter is the size of the rule's right hand side. +When a @acronym{GLR} parser reports an ambiguity, which of multiple candidate +right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined. +When processing a syntax error, the second parameter identifies locations +of the symbols that were discarded during error processing, and the third parameter is the number of discarded symbols. -By default, @code{YYLLOC_DEFAULT} is defined this way for simple -@acronym{LALR}(1) parsers: +By default, @code{YYLLOC_DEFAULT} is defined this way: -@example +@smallexample @group -# define YYLLOC_DEFAULT(Current, Rhs, N) \ - ((Current).first_line = (Rhs)[1].first_line, \ - (Current).first_column = (Rhs)[1].first_column, \ - (Current).last_line = (Rhs)[N].last_line, \ - (Current).last_column = (Rhs)[N].last_column) +# define YYLLOC_DEFAULT(Current, Rhs, N) \ + do \ + if (N) \ + @{ \ + (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \ + (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \ + (Current).last_line = YYRHSLOC(Rhs, N).last_line; \ + (Current).last_column = YYRHSLOC(Rhs, N).last_column; \ + @} \ + else \ + @{ \ + (Current).first_line = (Current).last_line = \ + YYRHSLOC(Rhs, 0).last_line; \ + (Current).first_column = (Current).last_column = \ + YYRHSLOC(Rhs, 0).last_column; \ + @} \ + while (0) @end group -@end example - -@noindent -and like this for @acronym{GLR} parsers: +@end smallexample -@example -@group -# define YYLLOC_DEFAULT(yyCurrent, yyRhs, YYN) \ - ((yyCurrent).first_line = YYRHSLOC(yyRhs, 1).first_line, \ - (yyCurrent).first_column = YYRHSLOC(yyRhs, 1).first_column, \ - (yyCurrent).last_line = YYRHSLOC(yyRhs, YYN).last_line, \ - (yyCurrent).last_column = YYRHSLOC(yyRhs, YYN).last_column) -@end group -@end example +where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol +in @var{rhs} when @var{k} is positive, and the location of the symbol +just before the reduction when @var{k} and @var{n} are both zero. When defining @code{YYLLOC_DEFAULT}, you should consider that: @@ -3450,8 +3617,10 @@ All arguments are free of side-effects. However, only the first one (the result) should be modified by @code{YYLLOC_DEFAULT}. @item -For consistency with semantic actions, valid indexes for the location -array range from 1 to @var{n}. +For consistency with semantic actions, valid indexes within the +right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a +valid index, and it refers to the symbol just before the reduction. +During error processing @var{n} is always positive. @item Your macro should parenthesize its arguments, if need be, since the @@ -3480,10 +3649,12 @@ it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}). @menu +* Require Decl:: Requiring a Bison version. * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Initial Action Decl:: Code run before parsing starts. * Destructor Decl:: Declaring how symbols are freed. * Expect Decl:: Suppressing warnings about parsing conflicts. * Start Decl:: Specifying the start symbol. @@ -3491,6 +3662,20 @@ Grammars}). * Decl Summary:: Table of all Bison declarations. @end menu +@node Require Decl +@subsection Require a Version of Bison +@cindex version requirement +@cindex requiring a version of Bison +@findex %require + +You may require the minimum version of Bison to process the grammar. If +the requirement is not met, @command{bison} exits with an error (exit +status 63). + +@example +%require "@var{version}" +@end example + @node Token Decl @subsection Token Type Names @cindex declaring token type names @@ -3624,10 +3809,10 @@ the one declared later has the higher precedence and is grouped first. @cindex value types, declaring @findex %union -The @code{%union} declaration specifies the entire collection of possible -data types for semantic values. The keyword @code{%union} is followed by a -pair of braces containing the same thing that goes inside a @code{union} in -C. +The @code{%union} declaration specifies the entire collection of +possible data types for semantic values. The keyword @code{%union} is +followed by braced code containing the same thing that goes inside a +@code{union} in C@. For example: @@ -3658,10 +3843,15 @@ As an extension to @acronym{POSIX}, a tag is allowed after the @end group @end example +@noindent specifies the union tag @code{value}, so the corresponding C type is @code{union value}. If you do not specify a tag, it defaults to @code{YYSTYPE}. +As another extension to @acronym{POSIX}, you may specify multiple +@code{%union} declarations; their contents are concatenated. However, +only the first @code{%union} declaration can specify a tag. + Note that, unlike making a @code{union} declaration in C, you need not write a semicolon after the closing brace. @@ -3693,35 +3883,61 @@ use the same @code{<@var{type}>} construction in a declaration for the terminal symbol. All kinds of token declarations allow @code{<@var{type}>}. +@node Initial Action Decl +@subsection Performing Actions before Parsing +@findex %initial-action + +Sometimes your parser needs to perform some initializations before +parsing. The @code{%initial-action} directive allows for such arbitrary +code. + +@deffn {Directive} %initial-action @{ @var{code} @} +@findex %initial-action +Declare that the braced @var{code} must be invoked before parsing each time +@code{yyparse} is called. The @var{code} may use @code{$$} and +@code{@@$} --- initial value and location of the look-ahead --- and the +@code{%parse-param}. +@end deffn + +For instance, if your locations use a file name, you may use + +@example +%parse-param @{ char const *file_name @}; +%initial-action +@{ + @@$.initialize (file_name); +@}; +@end example + + @node Destructor Decl @subsection Freeing Discarded Symbols @cindex freeing discarded symbols @findex %destructor -Some symbols can be discarded by the parser, typically during error -recovery (@pxref{Error Recovery}). Basically, during error recovery, -embarrassing symbols already pushed on the stack, and embarrassing -tokens coming from the rest of the file are thrown away until the parser -falls on its feet. If these symbols convey heap based information, this -memory is lost. While this behavior is tolerable for batch parsers, -such as in compilers, it is unacceptable for parsers that can -possibility ``never end'' such as shells, or implementations of -communication protocols. +During error recovery (@pxref{Error Recovery}), symbols already pushed +on the stack and tokens coming from the rest of the file are discarded +until the parser falls on its feet. If the parser runs out of memory, +or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the +symbols on the stack must be discarded. Even if the parser succeeds, it +must discard the start symbol. -The @code{%destructor} directive allows for the definition of code that -is called when a symbol is thrown away. +When discarded symbols convey heap based information, this memory is +lost. While this behavior can be tolerable for batch parsers, such as +in traditional compilers, it is unacceptable for programs like shells or +protocol implementations that may parse and execute indefinitely. + +The @code{%destructor} directive defines code that is called when a +symbol is automatically discarded. @deffn {Directive} %destructor @{ @var{code} @} @var{symbols} @findex %destructor -Declare that the @var{code} must be invoked for each of the -@var{symbols} that will be discarded by the parser. The @var{code} -should use @code{$$} to designate the semantic value associated to the -@var{symbols}. The additional parser parameters are also available -(@pxref{Parser Function, , The Parser Function @code{yyparse}}). - -@strong{Warning:} as of Bison 1.875, this feature is still considered as -experimental, as there was not enough user feedback. In particular, -the syntax might still change. +Invoke the braced @var{code} whenever the parser discards one of the +@var{symbols}. +Within @var{code}, @code{$$} designates the semantic value associated +with the discarded symbol. The additional parser parameters are also +available (@pxref{Parser Function, , The Parser Function +@code{yyparse}}). @end deffn For instance: @@ -3737,26 +3953,34 @@ For instance: @end smallexample @noindent -guarantees that when a @code{STRING} or a @code{string} will be discarded, +guarantees that when a @code{STRING} or a @code{string} is discarded, its associated memory will be freed. -Note that in the future, Bison might also consider that right hand side -members that are not mentioned in the action can be destroyed. For -instance, in: +@sp 1 -@smallexample -comment: "/*" STRING "*/"; -@end smallexample +@cindex discarded symbols +@dfn{Discarded symbols} are the following: -@noindent -the parser is entitled to destroy the semantic value of the -@code{string}. Of course, this will not apply to the default action; -compare: +@itemize +@item +stacked symbols popped during the first phase of error recovery, +@item +incoming terminals during the second phase of error recovery, +@item +the current look-ahead and the entire stack (except the current +right-hand side symbols) when the parser returns immediately, and +@item +the start symbol, when the parser succeeds. +@end itemize -@smallexample -typeless: string; // $$ = $1 does not apply; $1 is destroyed. -typefull: string; // $$ = $1 applies, $1 is not destroyed. -@end smallexample +The parser can @dfn{return immediately} because of an explicit call to +@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory +exhaustion. + +Right-hand size symbols of a rule that explicitly triggers a syntax +error via @code{YYERROR} are not discarded automatically. As a rule +of thumb, destructors are invoked only when user actions cannot manage +the memory. @node Expect Decl @subsection Suppressing Conflict Warnings @@ -3780,19 +4004,18 @@ The declaration looks like this: %expect @var{n} @end example -Here @var{n} is a decimal integer. The declaration says there should be -no warning if there are @var{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. +Here @var{n} is a decimal integer. The declaration says there should +be @var{n} shift/reduce conflicts and no reduce/reduce conflicts. +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 LALR(1) parsers, reduce/reduce conflicts are more serious, -and should be eliminated entirely. Bison will always report -reduce/reduce conflicts for these parsers. With GLR parsers, however, -both shift/reduce and reduce/reduce are routine (otherwise, there -would be no need to use GLR parsing). Therefore, it is also possible -to specify an expected number of reduce/reduce conflicts in GLR -parsers, using the declaration: +For normal @acronym{LALR}(1) 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, +there would be no need to use @acronym{GLR} parsing. Therefore, it is +also possible to specify an expected number of reduce/reduce conflicts +in @acronym{GLR} parsers, using the declaration: @example %expect-rr @var{n} @@ -3813,12 +4036,12 @@ go back to the beginning. @item Add an @code{%expect} declaration, copying the number @var{n} from the -number which Bison printed. +number which Bison printed. With @acronym{GLR} parsers, add an +@code{%expect-rr} declaration as well. @end itemize -Now Bison will stop annoying you if you do not change the number of -conflicts, but it will warn you again if changes in the grammar result -in more or fewer conflicts. +Now Bison will warn you if you introduce an unexpected conflict, but +will keep silent otherwise. @node Start Decl @subsection The Start-Symbol @@ -3844,8 +4067,8 @@ may override this restriction with the @code{%start} declaration as follows: A @dfn{reentrant} program is one which does not alter in the course of execution; in other words, it consists entirely of @dfn{pure} (read-only) code. Reentrancy is important whenever asynchronous execution is possible; -for example, a non-reentrant program may not be safe to call from a signal -handler. In systems with multiple threads of control, a non-reentrant +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. Normally, Bison generates a parser which is not reentrant. This is @@ -3951,13 +4174,12 @@ is named @file{@var{name}.h}. Unless @code{YYSTYPE} is already defined as a macro, the output header declares @code{YYSTYPE}. Therefore, if you are using a @code{%union} -(@pxref{Multiple Types, ,More Than One Value Type}) with components -that require other definitions, or if you have defined a -@code{YYSTYPE} macro (@pxref{Value Type, ,Data Types of Semantic -Values}), you need to arrange for these definitions to be propagated to -all modules, e.g., by putting them in a -prerequisite header that is included both by your parser and by any -other module that needs @code{YYSTYPE}. +(@pxref{Multiple Types, ,More Than One Value Type}) with components that +require other definitions, or if you have defined a @code{YYSTYPE} macro +(@pxref{Value Type, ,Data Types of Semantic Values}), you need to +arrange for these definitions to be propagated to all modules, e.g., by +putting them in a prerequisite header that is included both by your +parser and by any other module that needs @code{YYSTYPE}. Unless your parser is pure, the output header declares @code{yylval} as an external variable. @xref{Pure Decl, ,A Pure (Reentrant) @@ -3968,16 +4190,16 @@ If you have also used locations, the output header declares @code{YYSTYPE} and @code{yylval}. @xref{Locations, ,Tracking Locations}. -This output file is normally essential if you wish to put the -definition of @code{yylex} in a separate source file, because -@code{yylex} typically needs to be able to refer to the -above-mentioned declarations and to the token type codes. -@xref{Token Values, ,Semantic Values of Tokens}. +This output file is normally essential if you wish to put the definition +of @code{yylex} in a separate source file, because @code{yylex} +typically needs to be able to refer to the above-mentioned declarations +and to the token type codes. @xref{Token Values, ,Semantic Values of +Tokens}. @end deffn @deffn {Directive} %destructor -Specifying how the parser should reclaim the memory associated to -discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +Specify how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. @end deffn @deffn {Directive} %file-prefix="@var{prefix}" @@ -3996,12 +4218,14 @@ accurate syntax error messages. @deffn {Directive} %name-prefix="@var{prefix}" Rename the external symbols used in the parser so that they start with @var{prefix} instead of @samp{yy}. The precise list of symbols renamed +in C parsers is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc}, @code{yychar}, @code{yydebug}, and possible @code{yylloc}. For example, if you use @samp{%name-prefix="c_"}, the names become @code{c_parse}, @code{c_lex}, -and so on. @xref{Multiple Parsers, ,Multiple Parsers in the Same -Program}. +and so on. In C++ parsers, it is only the surrounding namespace which is +named @var{prefix} instead of @samp{yy}. +@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}. @end deffn @ifset defaultprec @@ -4018,7 +4242,7 @@ parser file contains just @code{#define} directives and static variable declarations. This option also tells Bison to write the C code for the grammar actions -into a file named @file{@var{filename}.act}, in the form of a +into a file named @file{@var{file}.act}, in the form of a brace-surrounded body fit for a @code{switch} statement. @end deffn @@ -4031,8 +4255,8 @@ associate errors with the parser file, treating it an independent source file in its own right. @end deffn -@deffn {Directive} %output="@var{filename}" -Specify the @var{filename} for the parser file. +@deffn {Directive} %output="@var{file}" +Specify @var{file} for the parser file. @end deffn @deffn {Directive} %pure-parser @@ -4040,6 +4264,11 @@ Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). @end deffn +@deffn {Directive} %require "@var{version}" +Require version @var{version} or higher of Bison. @xref{Require Decl, , +Require a Version of Bison}. +@end deffn + @deffn {Directive} %token-table Generate an array of token names in the parser file. The name of the array is @code{yytname}; @code{yytname[@var{i}]} is the name of the @@ -4049,15 +4278,14 @@ three elements of @code{yytname} correspond to the predefined tokens @code{"error"}, and @code{"$undefined"}; after these come the symbols defined in the grammar file. -For single-character literal tokens and literal string tokens, the name -in the table includes the single-quote or double-quote characters: for -example, @code{"'+'"} is a single-character literal and @code{"\"<=\""} -is a literal string token. All the characters of the literal string -token appear verbatim in the string found in the table; even -double-quote characters are not escaped. For example, if the token -consists of three characters @samp{*"*}, its string in @code{yytname} -contains @samp{"*"*"}. (In C, that would be written as -@code{"\"*\"*\""}). +The name in the table includes all the characters needed to represent +the token in Bison. For single-character literals and literal +strings, this includes the surrounding quoting characters and any +escape sequences. For example, the Bison single-character literal +@code{'+'} corresponds to a three-character name, represented in C as +@code{"'+'"}; and the Bison two-character literal string @code{"\\/"} +corresponds to a five-character name, represented in C as +@code{"\"\\\\/\""}. When you specify @code{%token-table}, Bison also generates macro definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and @@ -4138,6 +4366,8 @@ in the grammar file, you are likely to run into trouble. which reads tokens. * Error Reporting:: You must supply a function @code{yyerror}. * Action Features:: Special features for use in actions. +* Internationalization:: How to let the parser speak in the user's + native language. @end menu @node Parser Function @@ -4155,7 +4385,11 @@ without reading further. The value returned by @code{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). +The value is 1 if parsing failed because of invalid input, i.e., input +that contains a syntax error or that causes @code{YYABORT} to be +invoked. + +The value is 2 if parsing failed due to memory exhaustion. @end deftypefun In an action, you can cause immediate return from @code{yyparse} by using @@ -4177,8 +4411,8 @@ declaration @code{%parse-param}: @deffn {Directive} %parse-param @{@var{argument-declaration}@} @findex %parse-param -Declare that an argument declared by @code{argument-declaration} is an -additional @code{yyparse} argument. +Declare that an argument declared by the braced-code +@var{argument-declaration} is an additional @code{yyparse} argument. The @var{argument-declaration} is used when declaring functions or prototypes. The last identifier in @var{argument-declaration} must be the argument name. @@ -4296,11 +4530,13 @@ the grammar file has no effect on @code{yylex}. table. The index of the token in the table is the token type's code. The name of a multicharacter token is recorded in @code{yytname} with a double-quote, the token's characters, and another double-quote. The -token's characters are not escaped in any way; they appear verbatim in -the contents of the string in the table. +token's characters are escaped as necessary to be suitable as input +to Bison. -Here's code for looking up a token in @code{yytname}, assuming that the -characters of the token are stored in @code{token_buffer}. +Here's code for looking up a multicharacter token in @code{yytname}, +assuming that the characters of the token are stored in +@code{token_buffer}, and assuming that the token does not contain any +characters like @samp{"} that require escaping. @smallexample for (i = 0; i < YYNTOKENS; i++) @@ -4323,7 +4559,7 @@ The @code{yytname} table is generated only if you use the @subsection Semantic Values of Tokens @vindex yylval -In an ordinary (non-reentrant) parser, the semantic value of the token must +In an ordinary (nonreentrant) parser, the semantic value of the token must be stored into the global variable @code{yylval}. When you are using just one data type for semantic values, @code{yylval} has that type. Thus, if the type is @code{int} (the default), you might write this in @@ -4371,12 +4607,11 @@ then the code in @code{yylex} might look like this: @vindex yylloc If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, , -Tracking Locations}) in actions to keep track of the -textual locations of tokens and groupings, then you must provide this -information in @code{yylex}. The function @code{yyparse} expects to -find the textual location of a token just parsed in the global variable -@code{yylloc}. So @code{yylex} must store the proper data in that -variable. +Tracking Locations}) in actions to keep track of the textual locations +of tokens and groupings, then you must provide this information in +@code{yylex}. The function @code{yyparse} expects to find the textual +location of a token just parsed in the global variable @code{yylloc}. +So @code{yylex} must store the proper data in that variable. By default, the value of @code{yylloc} is a structure and you need only initialize the members that are going to be used by the actions. The @@ -4421,8 +4656,8 @@ Function}). @deffn {Directive} lex-param @{@var{argument-declaration}@} @findex %lex-param -Declare that @code{argument-declaration} is an additional @code{yylex} -argument declaration. +Declare that the braced-code @var{argument-declaration} is an +additional @code{yylex} argument declaration. @end deffn For instance: @@ -4481,13 +4716,16 @@ 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"}}. -The parser can detect one other kind of error: stack overflow. This -happens when the input contains constructions that are very deeply +The parser can detect one other kind of error: memory exhaustion. This +can happen 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, @code{yyparse} calls @code{yyerror} in the usual -fashion, except that the argument string is @w{@code{"parser stack -overflow"}}. +parser normally extends its stack automatically up to a very large limit. But +if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual +fashion, except that the argument string is @w{@code{"memory exhausted"}}. + +In some cases diagnostics like @w{@code{"syntax error"}} are +translated automatically from English to some other language before +they are passed to @code{yyerror}. @xref{Internationalization}. The following definition suffices in simple programs: @@ -4509,7 +4747,8 @@ error recovery if you have written suitable error recovery grammar rules immediately return 1. Obviously, in location tracking pure parsers, @code{yyerror} should have -an access to the current location. This is indeed the case for the GLR +an access to the current location. +This is indeed the case for the @acronym{GLR} parsers, but not for the Yacc parser, for historical reasons. I.e., if @samp{%locations %pure-parser} is passed then the prototypes for @code{yyerror} are: @@ -4526,7 +4765,7 @@ void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */ void yyerror (int *nastiness, char const *msg); /* GLR parsers. */ @end example -Finally, GLR and Yacc parsers share the same @code{yyerror} calling +Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling convention for absolutely pure parsers, i.e., when the calling convention of @code{yylex} @emph{and} the calling convention of @code{%pure-parser} are pure. I.e.: @@ -4567,7 +4806,7 @@ preferable since it more accurately describes the return type for @vindex yynerrs The variable @code{yynerrs} contains the number of syntax errors -encountered so far. Normally this variable is global; but if you +reported so far. Normally this variable is global; but if you request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable which only the actions can access. @@ -4633,6 +4872,12 @@ In either case, the rest of the action is not executed. Value stored in @code{yychar} when there is no look-ahead token. @end deffn +@deffn {Macro} YYEOF +@vindex YYEOF +Value stored in @code{yychar} when the look-ahead is the end of the input +stream. +@end deffn + @deffn {Macro} YYERROR; @findex YYERROR Cause an immediate syntax error. This statement initiates error @@ -4649,15 +4894,20 @@ is recovering from a syntax error, and 0 the rest of the time. @end deffn @deffn {Variable} yychar -Variable containing the current look-ahead token. (In a pure parser, -this is actually a local variable within @code{yyparse}.) When there is -no look-ahead token, the value @code{YYEMPTY} is stored in the variable. +Variable containing either the look-ahead token, or @code{YYEOF} when the +look-ahead is the end of the input stream, or @code{YYEMPTY} when no look-ahead +has been performed so the next token is not yet known. +Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic +Actions}). @xref{Look-Ahead, ,Look-Ahead Tokens}. @end deffn @deffn {Macro} yyclearin; Discard the current look-ahead token. This is useful primarily in -error rules. @xref{Error Recovery}. +error rules. +Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR +Semantic Actions}). +@xref{Error Recovery}. @end deffn @deffn {Macro} yyerrok; @@ -4666,6 +4916,22 @@ errors. This is useful primarily in error rules. @xref{Error Recovery}. @end deffn +@deffn {Variable} yylloc +Variable containing the look-ahead token location when @code{yychar} is not set +to @code{YYEMPTY} or @code{YYEOF}. +Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic +Actions}). +@xref{Actions and Locations, ,Actions and Locations}. +@end deffn + +@deffn {Variable} yylval +Variable containing the look-ahead token semantic value when @code{yychar} is +not set to @code{YYEMPTY} or @code{YYEOF}. +Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic +Actions}). +@xref{Actions, ,Actions}. +@end deffn + @deffn {Value} @@$ @findex @@$ Acts like a structure variable containing information on the textual location @@ -4699,21 +4965,104 @@ of the @var{n}th component of the current rule. @xref{Locations, , Tracking Locations}. @end deffn +@node Internationalization +@section Parser Internationalization +@cindex internationalization +@cindex i18n +@cindex NLS +@cindex gettext +@cindex bison-po + +A Bison-generated parser can print diagnostics, including error and +tracing messages. By default, they appear in English. However, Bison +also supports outputting diagnostics in the user's native language. To +make this work, the user should set the usual environment variables. +@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}. +For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might +set the user's locale to French Canadian using the @acronym{UTF}-8 +encoding. The exact set of available locales depends on the user's +installation. + +The maintainer of a package that uses a Bison-generated parser enables +the internationalization of the parser's output through the following +steps. Here we assume a package that uses @acronym{GNU} Autoconf and +@acronym{GNU} Automake. -@node Algorithm -@chapter The Bison Parser Algorithm -@cindex Bison parser algorithm -@cindex algorithm of parser -@cindex shifting -@cindex reduction -@cindex parser stack -@cindex stack, parser +@enumerate +@item +@cindex bison-i18n.m4 +Into the directory containing the @acronym{GNU} Autoconf macros used +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: -As Bison reads tokens, it pushes them onto a stack along with their -semantic values. The stack is called the @dfn{parser stack}. Pushing a -token is traditionally called @dfn{shifting}. +@example +cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4 +@end example -For example, suppose the infix calculator has read @samp{1 + 5 *}, with a +@item +@findex BISON_I18N +@vindex BISON_LOCALEDIR +@vindex YYENABLE_NLS +In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT} +invocation, add an invocation of @code{BISON_I18N}. This macro is +defined in the file @file{bison-i18n.m4} that you copied earlier. It +causes @samp{configure} to find the value of the +@code{BISON_LOCALEDIR} variable, and it defines the source-language +symbol @code{YYENABLE_NLS} to enable translations in the +Bison-generated parser. + +@item +In the @code{main} function of your program, designate the directory +containing Bison's runtime message catalog, through a call to +@samp{bindtextdomain} with domain name @samp{bison-runtime}. +For example: + +@example +bindtextdomain ("bison-runtime", BISON_LOCALEDIR); +@end example + +Typically this appears after any other call @code{bindtextdomain +(PACKAGE, LOCALEDIR)} that your package already has. Here we rely on +@samp{BISON_LOCALEDIR} to be defined as a string through the +@file{Makefile}. + +@item +In the @file{Makefile.am} that controls the compilation of the @code{main} +function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro, +either in @samp{DEFS} or in @samp{AM_CPPFLAGS}. For example: + +@example +DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"' +@end example + +or: + +@example +AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"' +@end example + +@item +Finally, invoke the command @command{autoreconf} to generate the build +infrastructure. +@end enumerate + + +@node Algorithm +@chapter The Bison Parser Algorithm +@cindex Bison parser algorithm +@cindex algorithm of parser +@cindex shifting +@cindex reduction +@cindex parser stack +@cindex stack, parser + +As Bison reads tokens, it pushes them onto a stack along with their +semantic values. The stack is called the @dfn{parser stack}. Pushing a +token is traditionally called @dfn{shifting}. + +For example, suppose the infix calculator has read @samp{1 + 5 *}, with a @samp{3} to come. The stack will have four elements, one for each token that was shifted. @@ -4765,7 +5114,7 @@ This kind of parser is known in the literature as a bottom-up parser. * Reduce/Reduce:: When two rules are applicable in the same situation. * Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. * Generalized LR Parsing:: Parsing arbitrary context-free grammars. -* Stack Overflow:: What happens when stack gets full. How to avoid it. +* Memory Management:: What happens when memory is exhausted. How to avoid it. @end menu @node Look-Ahead @@ -4820,7 +5169,11 @@ doing so would produce on the stack the sequence of symbols @code{expr '!'}. No rule allows that sequence. @vindex yychar -The current look-ahead token is stored in the variable @code{yychar}. +@vindex yylval +@vindex yylloc +The look-ahead token is stored in the variable @code{yychar}. +Its semantic value and location, if any, are stored in the variables +@code{yylval} and @code{yylloc}. @xref{Action Features, ,Special Features for Use in Actions}. @node Shift/Reduce @@ -5383,12 +5736,19 @@ return_spec: ; @end example +For a more detailed exposition of @acronym{LALR}(1) parsers and parser +generators, please see: +Frank DeRemer and Thomas Pennello, Efficient Computation of +@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on +Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982), +pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}. + @node Generalized LR Parsing @section Generalized @acronym{LR} (@acronym{GLR}) Parsing @cindex @acronym{GLR} parsing @cindex generalized @acronym{LR} (@acronym{GLR}) parsing @cindex ambiguous grammars -@cindex non-deterministic parsing +@cindex nondeterministic parsing Bison produces @emph{deterministic} parsers that choose uniquely when to reduce and which reduction to apply @@ -5453,32 +5813,33 @@ 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 length of the input times the maximum number of stacks required for any -prefix of the input. Thus, really ambiguous or non-deterministic +prefix of the input. Thus, really ambiguous or nondeterministic grammars can require exponential time and space to process. Such badly behaving examples, however, are not generally of practical interest. -Usually, non-determinism in a grammar is local---the parser is ``in +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. -For a more detailed exposition of GLR parsers, please see: Elizabeth +For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style Generalised @acronym{LR} Parsers, Royal Holloway, University of London, Department of Computer Science, TR-00-12, @uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}, (2000-12-24). -@node Stack Overflow -@section Stack Overflow, and How to Avoid It +@node Memory Management +@section Memory Management, and How to Avoid Memory Exhaustion +@cindex memory exhaustion +@cindex memory management @cindex stack overflow @cindex parser stack overflow @cindex overflow of parser stack -The Bison parser stack can overflow if too many tokens are shifted and +The Bison parser stack can run out of memory if too many tokens are shifted and not reduced. When this happens, the parser function @code{yyparse} -returns a nonzero value, pausing only to call @code{yyerror} to report -the overflow. +calls @code{yyerror} and then returns 2. Because Bison parsers have growing stacks, hitting the upper limit usually results from using a right recursion instead of a left @@ -5486,34 +5847,41 @@ recursion, @xref{Recursion, ,Recursive Rules}. @vindex YYMAXDEPTH By defining the macro @code{YYMAXDEPTH}, you can control how deep the -parser stack can become before a stack overflow occurs. Define the +parser stack can become before memory is exhausted. 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 @code{YYMAXDEPTH}, the parser actually allocates a small +large value for @code{YYMAXDEPTH}, the parser normally 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 @code{YYMAXDEPTH} painfully small merely to save space for ordinary inputs that do not need much stack. +However, do not allow @code{YYMAXDEPTH} to be a value so large that +arithmetic overflow could occur when calculating the size of the stack +space. Also, do not allow @code{YYMAXDEPTH} to be less than +@code{YYINITDEPTH}. + @cindex default stack limit The default value of @code{YYMAXDEPTH}, if you do not define it, is 10000. @vindex YYINITDEPTH You can control how much stack is allocated initially by defining the -macro @code{YYINITDEPTH}. This value too must be a compile-time -constant integer. The default is 200. +macro @code{YYINITDEPTH} to a positive integer. For the C +@acronym{LALR}(1) parser, 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 by compiled as C++ cannot grow. In this precise -case (compiling a C parser as C++) you are suggested to grow -@code{YYINITDEPTH}. In the near future, a C++ output output will be -provided which addresses this issue. +@acronym{LALR}(1) 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. @node Error Recovery @chapter Error Recovery @@ -5622,6 +5990,7 @@ The previous look-ahead token is reanalyzed immediately after an error. If this is unacceptable, then the macro @code{yyclearin} may be used to clear this token. Write the statement @samp{yyclearin;} in the error rule's action. +@xref{Action Features, ,Special Features for Use in Actions}. For example, suppose that on a syntax error, an error handling routine is called that advances the input stream to some point where parsing should @@ -5689,9 +6058,13 @@ redeclare a typedef name provided an explicit type has been specified earlier: @example -typedef int foo, bar, lose; -static foo (bar); /* @r{redeclare @code{bar} as static variable} */ -static int foo (lose); /* @r{redeclare @code{foo} as function} */ +typedef int foo, bar; +int baz (void) +@{ + static bar (bar); /* @r{redeclare @code{bar} as static variable} */ + extern foo foo (foo); /* @r{redeclare @code{foo} as function} */ + return foo (bar); +@} @end example Unfortunately, the name being declared is separated from the declaration @@ -6247,8 +6620,9 @@ state 11 @end example @noindent -Observe that state 11 contains conflicts due to the lack of precedence -of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the +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. @@ -6379,14 +6753,15 @@ bison @var{infile} Here @var{infile} is the grammar file name, which usually ends in @samp{.y}. The parser file's name is made by replacing the @samp{.y} -with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields -@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields -@file{hack/foo.tab.c}. It's also possible, in case you are writing +with @samp{.tab.c} and removing any leading directory. Thus, the +@samp{bison foo.y} file name yields +@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields +@file{foo.tab.c}. It's also possible, in case you are writing C++ code instead of C in your grammar file, to name it @file{foo.ypp} or @file{foo.y++}. Then, the output files will take an extension like the given one as input (respectively @file{foo.tab.cpp} and @file{foo.tab.c++}). -This feature takes effect with all options that manipulate filenames like +This feature takes effect with all options that manipulate file names like @samp{-o} or @samp{-d}. For example : @@ -6440,13 +6815,17 @@ Print a summary of the command-line options to Bison and exit. @itemx --version Print the version number of Bison and exit. -@need 1750 +@item --print-localedir +Print the name of the directory containing locale-dependent data. + @item -y @itemx --yacc -Equivalent to @samp{-o y.tab.c}; the parser output file is called +Act more like the traditional Yacc command. This can cause +different diagnostics to be generated, and may change behavior in +other minor ways. Most importantly, imitate Yacc's output +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}. The purpose of this option is to imitate Yacc's output -file name conventions. Thus, the following shell script can substitute +@file{y.tab.h}. Thus, the following shell script can substitute for Yacc, and the Bison distribution contains such a script for compatibility with @acronym{POSIX}: @@ -6454,6 +6833,12 @@ compatibility with @acronym{POSIX}: #! /bin/sh bison -y "$@@" @end example + +The @option{-y}/@option{--yacc} option is intended for use with +traditional Yacc grammars. If your grammar uses a Bison extension +like @samp{%glr-parser}, Bison might not be Yacc-compatible even if +this option is specified. + @end table @noindent @@ -6511,7 +6896,7 @@ Same as above, but save in the file @var{defines-file}. @item -b @var{file-prefix} @itemx --file-prefix=@var{prefix} -Pretend that @code{%verbose} was specified, i.e, specify prefix to use +Pretend that @code{%file-prefix} was specified, i.e, specify prefix to use for all Bison output file names. @xref{Decl Summary}. @item -r @var{things} @@ -6533,19 +6918,17 @@ Implies @code{state} and augments the description of the automaton with the full set of items for each state, instead of its core only. @end table -For instance, on the following grammar - @item -v @itemx --verbose Pretend that @code{%verbose} was specified, i.e, write an extra output file containing verbose descriptions of the grammar and parser. @xref{Decl Summary}. -@item -o @var{filename} -@itemx --output=@var{filename} -Specify the @var{filename} for the parser file. +@item -o @var{file} +@itemx --output=@var{file} +Specify the @var{file} for the parser file. -The other output files' names are constructed from @var{filename} as +The other output files' names are constructed from @var{file} as described under the @samp{-v} and @samp{-d} options. @item -g @@ -6557,52 +6940,33 @@ be @file{foo.vcg}. @item --graph=@var{graph-file} The behavior of @var{--graph} is the same than @samp{-g}. The only difference is that it has an optional argument which is the name of -the output graph filename. +the output graph file. @end table @node Option Cross Key @section Option Cross Key +@c FIXME: How about putting the directives too? Here is a list of options, alphabetized by long option, to help you find the corresponding short option. -@tex -\def\leaderfill{\leaders\hbox to 1em{\hss.\hss}\hfill} - -{\tt -\line{ --debug \leaderfill -t} -\line{ --defines \leaderfill -d} -\line{ --file-prefix \leaderfill -b} -\line{ --graph \leaderfill -g} -\line{ --help \leaderfill -h} -\line{ --name-prefix \leaderfill -p} -\line{ --no-lines \leaderfill -l} -\line{ --no-parser \leaderfill -n} -\line{ --output \leaderfill -o} -\line{ --token-table \leaderfill -k} -\line{ --verbose \leaderfill -v} -\line{ --version \leaderfill -V} -\line{ --yacc \leaderfill -y} -} -@end tex - -@ifinfo -@example ---debug -t ---defines=@var{defines-file} -d ---file-prefix=@var{prefix} -b @var{file-prefix} ---graph=@var{graph-file} -d ---help -h ---name-prefix=@var{prefix} -p @var{name-prefix} ---no-lines -l ---no-parser -n ---output=@var{outfile} -o @var{outfile} ---token-table -k ---verbose -v ---version -V ---yacc -y -@end example -@end ifinfo +@multitable {@option{--defines=@var{defines-file}}} {@option{-b @var{file-prefix}XXX}} +@headitem Long Option @tab Short Option +@item @option{--debug} @tab @option{-t} +@item @option{--defines=@var{defines-file}} @tab @option{-d} +@item @option{--file-prefix=@var{prefix}} @tab @option{-b @var{file-prefix}} +@item @option{--graph=@var{graph-file}} @tab @option{-d} +@item @option{--help} @tab @option{-h} +@item @option{--name-prefix=@var{prefix}} @tab @option{-p @var{name-prefix}} +@item @option{--no-lines} @tab @option{-l} +@item @option{--no-parser} @tab @option{-n} +@item @option{--output=@var{outfile}} @tab @option{-o @var{outfile}} +@item @option{--print-localedir} @tab +@item @option{--token-table} @tab @option{-k} +@item @option{--verbose} @tab @option{-v} +@item @option{--version} @tab @option{-V} +@item @option{--yacc} @tab @option{-y} +@end multitable @node Yacc Library @section Yacc Library @@ -6630,437 +6994,1120 @@ If you use the Yacc library's @code{main} function, your int yyparse (void); @end example -@c ================================================= Invoking Bison - -@node FAQ -@chapter Frequently Asked Questions -@cindex frequently asked questions -@cindex questions +@c ================================================= C++ Bison -Several questions about Bison come up occasionally. Here some of them -are addressed. +@node C++ Language Interface +@chapter C++ Language Interface @menu -* Parser Stack Overflow:: Breaking the Stack Limits -* How Can I Reset the Parser:: @code{yyparse} Keeps some State -* Strings are Destroyed:: @code{yylval} Loses Track of Strings -* C++ Parsers:: Compiling Parsers with C++ Compilers -* Implementing Loops:: Control Flow in the Calculator +* C++ Parsers:: The interface to generate C++ parser classes +* A Complete C++ Example:: Demonstrating their use @end menu -@node Parser Stack Overflow -@section Parser Stack Overflow +@node C++ Parsers +@section C++ Parsers -@display -My parser returns with error with a @samp{parser stack overflow} -message. What can I do? -@end display +@menu +* C++ Bison Interface:: Asking for C++ parser generation +* C++ Semantic Values:: %union vs. C++ +* C++ Location Values:: The position and location classes +* C++ Parser Interface:: Instantiating and running the parser +* C++ Scanner Interface:: Exchanges between yylex and parse +@end menu -This question is already addressed elsewhere, @xref{Recursion, -,Recursive Rules}. +@node C++ Bison Interface +@subsection C++ Bison Interface +@c - %skeleton "lalr1.cc" +@c - Always pure +@c - initial action + +The C++ parser @acronym{LALR}(1) skeleton is named @file{lalr1.cc}. To +select it, you may either pass the option @option{--skeleton=lalr1.cc} +to Bison, or include the directive @samp{%skeleton "lalr1.cc"} in the +grammar preamble. When run, @command{bison} will create several +entities in the @samp{yy} namespace. Use the @samp{%name-prefix} +directive to change the namespace name, see @ref{Decl Summary}. The +various classes are generated in the following files: + +@table @file +@item position.hh +@itemx location.hh +The definition of the classes @code{position} and @code{location}, +used for location tracking. @xref{C++ Location Values}. + +@item stack.hh +An auxiliary class @code{stack} used by the parser. + +@item @var{file}.hh +@itemx @var{file}.cc +The declaration and implementation of the C++ parser class. +@var{file} is the name of the output file. It follows the same +rules as with regular C parsers. + +Note that @file{@var{file}.hh} is @emph{mandatory}, the C++ cannot +work without the parser class declaration. Therefore, you must either +pass @option{-d}/@option{--defines} to @command{bison}, or use the +@samp{%defines} directive. +@end table -@node How Can I Reset the Parser -@section How Can I Reset the Parser +All these files are documented using Doxygen; run @command{doxygen} +for a complete and accurate documentation. + +@node C++ Semantic Values +@subsection C++ Semantic Values +@c - No objects in unions +@c - YSTYPE +@c - Printer and destructor + +The @code{%union} directive works as for C, see @ref{Union Decl, ,The +Collection of Value Types}. In particular it produces a genuine +@code{union}@footnote{In the future techniques to allow complex types +within pseudo-unions (similar to Boost variants) might be implemented to +alleviate these issues.}, which have a few specific features in C++. +@itemize @minus +@item +The type @code{YYSTYPE} is defined but its use is discouraged: rather +you should refer to the parser's encapsulated type +@code{yy::parser::semantic_type}. +@item +Non POD (Plain Old Data) types cannot be used. C++ forbids any +instance of classes with constructors in unions: only @emph{pointers} +to such objects are allowed. +@end itemize -The following phenomenon has several symptoms, resulting in the -following typical questions: +Because objects have to be stored via pointers, memory is not +reclaimed automatically: using the @code{%destructor} directive is the +only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded +Symbols}. + + +@node C++ Location Values +@subsection C++ Location Values +@c - %locations +@c - class Position +@c - class Location +@c - %define "filename_type" "const symbol::Symbol" + +When the directive @code{%locations} is used, the C++ parser supports +location tracking, see @ref{Locations, , Locations Overview}. Two +auxiliary classes define a @code{position}, a single point in a file, +and a @code{location}, a range composed of a pair of +@code{position}s (possibly spanning several files). + +@deftypemethod {position} {std::string*} file +The name of the file. It will always be handled as a pointer, the +parser will never duplicate nor deallocate it. As an experimental +feature you may change it to @samp{@var{type}*} using @samp{%define +"filename_type" "@var{type}"}. +@end deftypemethod + +@deftypemethod {position} {unsigned int} line +The line, starting at 1. +@end deftypemethod + +@deftypemethod {position} {unsigned int} lines (int @var{height} = 1) +Advance by @var{height} lines, resetting the column number. +@end deftypemethod + +@deftypemethod {position} {unsigned int} column +The column, starting at 0. +@end deftypemethod + +@deftypemethod {position} {unsigned int} columns (int @var{width} = 1) +Advance by @var{width} columns, without changing the line number. +@end deftypemethod + +@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width}) +Various forms of syntactic sugar for @code{columns}. +@end deftypemethod + +@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p}) +Report @var{p} on @var{o} like this: +@samp{@var{file}:@var{line}.@var{column}}, or +@samp{@var{line}.@var{column}} if @var{file} is null. +@end deftypemethod + +@deftypemethod {location} {position} begin +@deftypemethodx {location} {position} end +The first, inclusive, position of the range, and the first beyond. +@end deftypemethod + +@deftypemethod {location} {unsigned int} columns (int @var{width} = 1) +@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1) +Advance the @code{end} position. +@end deftypemethod + +@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end}) +@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width}) +@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width}) +Various forms of syntactic sugar. +@end deftypemethod + +@deftypemethod {location} {void} step () +Move @code{begin} onto @code{end}. +@end deftypemethod + + +@node C++ Parser Interface +@subsection C++ Parser Interface +@c - define parser_class_name +@c - Ctor +@c - parse, error, set_debug_level, debug_level, set_debug_stream, +@c debug_stream. +@c - Reporting errors + +The output files @file{@var{output}.hh} and @file{@var{output}.cc} +declare and define the parser class in the namespace @code{yy}. The +class name defaults to @code{parser}, but may be changed using +@samp{%define "parser_class_name" "@var{name}"}. The interface of +this class is detailed below. It can be extended using the +@code{%parse-param} feature: its semantics is slightly changed since +it describes an additional member of the parser class, and an +additional argument for its constructor. + +@defcv {Type} {parser} {semantic_value_type} +@defcvx {Type} {parser} {location_value_type} +The types for semantics value and locations. +@end defcv + +@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...) +Build a new parser object. There are no arguments by default, unless +@samp{%parse-param @{@var{type1} @var{arg1}@}} was used. +@end deftypemethod + +@deftypemethod {parser} {int} parse () +Run the syntactic analysis, and return 0 on success, 1 otherwise. +@end deftypemethod + +@deftypemethod {parser} {std::ostream&} debug_stream () +@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o}) +Get or set the stream used for tracing the parsing. It defaults to +@code{std::cerr}. +@end deftypemethod + +@deftypemethod {parser} {debug_level_type} debug_level () +@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l}) +Get or set the tracing level. Currently its value is either 0, no trace, +or nonzero, full tracing. +@end deftypemethod + +@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m}) +The definition for this member function must be supplied by the user: +the parser uses it to report a parser error occurring at @var{l}, +described by @var{m}. +@end deftypemethod + + +@node C++ Scanner Interface +@subsection C++ Scanner Interface +@c - prefix for yylex. +@c - Pure interface to yylex +@c - %lex-param + +The parser invokes the scanner by calling @code{yylex}. Contrary to C +parsers, C++ parsers are always pure: there is no point in using the +@code{%pure-parser} directive. Therefore the interface is as follows. + +@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...) +Return the next token. Its type is the return value, its semantic +value and location being @var{yylval} and @var{yylloc}. Invocations of +@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments. +@end deftypemethod + + +@node A Complete C++ Example +@section A Complete C++ Example + +This section demonstrates the use of a C++ parser with a simple but +complete example. This example should be available on your system, +ready to compile, in the directory @dfn{../bison/examples/calc++}. It +focuses on the use of Bison, therefore the design of the various C++ +classes is very naive: no accessors, no encapsulation of members etc. +We will use a Lex scanner, and more precisely, a Flex scanner, to +demonstrate the various interaction. A hand written scanner is +actually easier to interface with. -@display -I invoke @code{yyparse} several times, and on correct input it works -properly; but when a parse error is found, all the other calls fail -too. How can I reset the error flag of @code{yyparse}? -@end display +@menu +* Calc++ --- C++ Calculator:: The specifications +* Calc++ Parsing Driver:: An active parsing context +* Calc++ Parser:: A parser class +* Calc++ Scanner:: A pure C++ Flex scanner +* Calc++ Top Level:: Conducting the band +@end menu -@noindent -or +@node Calc++ --- C++ Calculator +@subsection Calc++ --- C++ Calculator -@display -My parser includes support for an @samp{#include}-like feature, in -which case I run @code{yyparse} from @code{yyparse}. This fails -although I did specify I needed a @code{%pure-parser}. -@end display +Of course the grammar is dedicated to arithmetics, a single +expression, possibly preceded by variable assignments. An +environment containing possibly predefined variables such as +@code{one} and @code{two}, is exchanged with the parser. An example +of valid input follows. -These problems typically come not from Bison itself, but from -Lex-generated scanners. Because these scanners use large buffers for -speed, they might not notice a change of input file. As a -demonstration, consider the following source file, -@file{first-line.l}: +@example +three := 3 +seven := one + two * three +seven * seven +@end example -@verbatim -%{ -#include <stdio.h> -#include <stdlib.h> -%} -%% -.*\n ECHO; return 1; -%% -int -yyparse (char const *file) -{ - yyin = fopen (file, "r"); - if (!yyin) - exit (2); - /* One token only. */ - yylex (); - if (fclose (yyin) != 0) - exit (3); - return 0; -} +@node Calc++ Parsing Driver +@subsection Calc++ Parsing Driver +@c - An env +@c - A place to store error messages +@c - A place for the result -int -main (void) -{ - yyparse ("input"); - yyparse ("input"); - return 0; -} -@end verbatim +To support a pure interface with the parser (and the scanner) the +technique of the ``parsing context'' is convenient: a structure +containing all the data to exchange. Since, in addition to simply +launch the parsing, there are several auxiliary tasks to execute (open +the file for parsing, instantiate the parser etc.), we recommend +transforming the simple parsing context structure into a fully blown +@dfn{parsing driver} class. -@noindent -If the file @file{input} contains +The declaration of this driver class, @file{calc++-driver.hh}, is as +follows. The first part includes the CPP guard and imports the +required standard library components, and the declaration of the parser +class. + +@comment file: calc++-driver.hh +@example +#ifndef CALCXX_DRIVER_HH +# define CALCXX_DRIVER_HH +# include <string> +# include <map> +# include "calc++-parser.hh" +@end example -@verbatim -input:1: Hello, -input:2: World! -@end verbatim @noindent -then instead of getting the first line twice, you get: +Then comes the declaration of the scanning function. Flex expects +the signature of @code{yylex} to be defined in the macro +@code{YY_DECL}, and the C++ parser expects it to be declared. We can +factor both as follows. +@comment file: calc++-driver.hh @example -$ @kbd{flex -ofirst-line.c first-line.l} -$ @kbd{gcc -ofirst-line first-line.c -ll} -$ @kbd{./first-line} -input:1: Hello, -input:2: World! +// Announce to Flex the prototype we want for lexing function, ... +# define YY_DECL \ + yy::calcxx_parser::token_type \ + yylex (yy::calcxx_parser::semantic_type* yylval, \ + yy::calcxx_parser::location_type* yylloc, \ + calcxx_driver& driver) +// ... and declare it for the parser's sake. +YY_DECL; @end example -Therefore, whenever you change @code{yyin}, you must tell the -Lex-generated scanner to discard its current buffer and switch to the -new one. This depends upon your implementation of Lex; see its -documentation for more. For Flex, it suffices to call -@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your -Flex-generated scanner needs to read from several input streams to -handle features like include files, you might consider using Flex -functions like @samp{yy_switch_to_buffer} that manipulate multiple -input buffers. +@noindent +The @code{calcxx_driver} class is then declared with its most obvious +members. -If your Flex-generated scanner uses start conditions (@pxref{Start -conditions, , Start conditions, flex, The Flex Manual}), you might -also want to reset the scanner's state, i.e., go back to the initial -start condition, through a call to @samp{BEGIN (0)}. +@comment file: calc++-driver.hh +@example +// Conducting the whole scanning and parsing of Calc++. +class calcxx_driver +@{ +public: + calcxx_driver (); + virtual ~calcxx_driver (); -@node Strings are Destroyed -@section Strings are Destroyed + std::map<std::string, int> variables; -@display -My parser seems to destroy old strings, or maybe it loses track of -them. Instead of reporting @samp{"foo", "bar"}, it reports -@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}. -@end display + int result; +@end example -This error is probably the single most frequent ``bug report'' sent to -Bison lists, but is only concerned with a misunderstanding of the role -of scanner. Consider the following Lex code: +@noindent +To encapsulate the coordination with the Flex scanner, it is useful to +have two members function to open and close the scanning phase. +members. -@verbatim -%{ -#include <stdio.h> -char *yylval = NULL; -%} -%% -.* yylval = yytext; return 1; -\n /* IGNORE */ -%% -int -main () -{ - /* Similar to using $1, $2 in a Bison action. */ - char *fst = (yylex (), yylval); - char *snd = (yylex (), yylval); - printf ("\"%s\", \"%s\"\n", fst, snd); - return 0; -} -@end verbatim +@comment file: calc++-driver.hh +@example + // Handling the scanner. + void scan_begin (); + void scan_end (); + bool trace_scanning; +@end example -If you compile and run this code, you get: +@noindent +Similarly for the parser itself. +@comment file: calc++-driver.hh @example -$ @kbd{flex -osplit-lines.c split-lines.l} -$ @kbd{gcc -osplit-lines split-lines.c -ll} -$ @kbd{printf 'one\ntwo\n' | ./split-lines} -"one -two", "two" + // Handling the parser. + void parse (const std::string& f); + std::string file; + bool trace_parsing; @end example @noindent -this is because @code{yytext} is a buffer provided for @emph{reading} -in the action, but if you want to keep it, you have to duplicate it -(e.g., using @code{strdup}). Note that the output may depend on how -your implementation of Lex handles @code{yytext}. For instance, when -given the Lex compatibility option @option{-l} (which triggers the -option @samp{%array}) Flex generates a different behavior: +To demonstrate pure handling of parse errors, instead of simply +dumping them on the standard error output, we will pass them to the +compiler driver using the following two member functions. Finally, we +close the class declaration and CPP guard. +@comment file: calc++-driver.hh @example -$ @kbd{flex -l -osplit-lines.c split-lines.l} -$ @kbd{gcc -osplit-lines split-lines.c -ll} -$ @kbd{printf 'one\ntwo\n' | ./split-lines} -"two", "two" + // Error handling. + void error (const yy::location& l, const std::string& m); + void error (const std::string& m); +@}; +#endif // ! CALCXX_DRIVER_HH @end example +The implementation of the driver is straightforward. The @code{parse} +member function deserves some attention. The @code{error} functions +are simple stubs, they should actually register the located error +messages and set error state. -@node C++ Parsers -@section C++ Parsers +@comment file: calc++-driver.cc +@example +#include "calc++-driver.hh" +#include "calc++-parser.hh" -@display -How can I generate parsers in C++? -@end display +calcxx_driver::calcxx_driver () + : trace_scanning (false), trace_parsing (false) +@{ + variables["one"] = 1; + variables["two"] = 2; +@} -We are working on a C++ output for Bison, but unfortunately, for lack -of time, the skeleton is not finished. It is functional, but in -numerous respects, it will require additional work which @emph{might} -break backward compatibility. Since the skeleton for C++ is not -documented, we do not consider ourselves bound to this interface, -nevertheless, as much as possible we will try to keep compatibility. +calcxx_driver::~calcxx_driver () +@{ +@} -Another possibility is to use the regular C parsers, and to compile -them with a C++ compiler. This works properly, provided that you bear -some simple C++ rules in mind, such as not including ``real classes'' -(i.e., structure with constructors) in unions. Therefore, in the -@code{%union}, use pointers to classes, or better yet, a single -pointer type to the root of your lexical/syntactic hierarchy. +void +calcxx_driver::parse (const std::string &f) +@{ + file = f; + scan_begin (); + yy::calcxx_parser parser (*this); + parser.set_debug_level (trace_parsing); + parser.parse (); + scan_end (); +@} +void +calcxx_driver::error (const yy::location& l, const std::string& m) +@{ + std::cerr << l << ": " << m << std::endl; +@} -@node Implementing Loops -@section Implementing Loops +void +calcxx_driver::error (const std::string& m) +@{ + std::cerr << m << std::endl; +@} +@end example -@display -My simple calculator supports variables, assignments, and functions, -but how can I implement loops? -@end display +@node Calc++ Parser +@subsection Calc++ Parser -Although very pedagogical, the examples included in the document blur -the distinction to make between the parser---whose job is to recover -the structure of a text and to transmit it to subsequent modules of -the program---and the processing (such as the execution) of this -structure. This works well with so called straight line programs, -i.e., precisely those that have a straightforward execution model: -execute simple instructions one after the others. +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. -@cindex abstract syntax tree -@cindex @acronym{AST} -If you want a richer model, you will probably need to use the parser -to construct a tree that does represent the structure it has -recovered; this tree is usually called the @dfn{abstract syntax tree}, -or @dfn{@acronym{AST}} for short. Then, walking through this tree, -traversing it in various ways, will enable treatments such as its -execution or its translation, which will result in an interpreter or a -compiler. +@comment file: calc++-parser.yy +@example +%skeleton "lalr1.cc" /* -*- C++ -*- */ +%require "2.1a" +%defines +%define "parser_class_name" "calcxx_parser" +@end example -This topic is way beyond the scope of this manual, and the reader is -invited to consult the dedicated literature. +@noindent +Then come the declarations/inclusions needed to define the +@code{%union}. Because the parser uses the parsing driver and +reciprocally, both cannot include the header of the other. Because the +driver's header needs detailed knowledge about the parser class (in +particular its inner types), it is the parser's header which will simply +use a forward declaration of the driver. +@comment file: calc++-parser.yy +@example +%@{ +# include <string> +class calcxx_driver; +%@} +@end example +@noindent +The driver is passed by reference to the parser and to the scanner. +This provides a simple but effective pure interface, not relying on +global variables. -@c ================================================= Table of Symbols +@comment file: calc++-parser.yy +@example +// The parsing context. +%parse-param @{ calcxx_driver& driver @} +%lex-param @{ calcxx_driver& driver @} +@end example -@node Table of Symbols -@appendix Bison Symbols -@cindex Bison symbols, table of -@cindex symbols in Bison, table of +@noindent +Then we request the location tracking feature, and initialize the +first location's file name. Afterwards new locations are computed +relatively to the previous locations: the file name will be +automatically propagated. -@deffn {Variable} @@$ -In an action, the location of the left-hand side of the rule. -@xref{Locations, , Locations Overview}. -@end deffn +@comment file: calc++-parser.yy +@example +%locations +%initial-action +@{ + // Initialize the initial location. + @@$.begin.filename = @@$.end.filename = &driver.file; +@}; +@end example -@deffn {Variable} @@@var{n} -In an action, the location of the @var{n}-th symbol of the right-hand -side of the rule. @xref{Locations, , Locations Overview}. -@end deffn +@noindent +Use the two following directives to enable parser tracing and verbose +error messages. -@deffn {Variable} $$ -In an action, the semantic value of the left-hand side of the rule. -@xref{Actions}. -@end deffn +@comment file: calc++-parser.yy +@example +%debug +%error-verbose +@end example -@deffn {Variable} $@var{n} -In an action, the semantic value of the @var{n}-th symbol of the -right-hand side of the rule. @xref{Actions}. -@end deffn +@noindent +Semantic values cannot use ``real'' objects, but only pointers to +them. -@deffn {Symbol} $accept -The predefined nonterminal whose only rule is @samp{$accept: @var{start} -$end}, where @var{start} is the start symbol. @xref{Start Decl, , The -Start-Symbol}. It cannot be used in the grammar. -@end deffn +@comment file: calc++-parser.yy +@example +// Symbols. +%union +@{ + int ival; + std::string *sval; +@}; +@end example -@deffn {Symbol} $end -The predefined token marking the end of the token stream. It cannot be -used in the grammar. -@end deffn +@noindent +The code between @samp{%@{} and @samp{%@}} after the introduction of the +@samp{%union} is output in the @file{*.cc} file; it needs detailed +knowledge about the driver. -@deffn {Symbol} $undefined -The predefined token onto which all undefined values returned by -@code{yylex} are mapped. It cannot be used in the grammar, rather, use -@code{error}. -@end deffn +@comment file: calc++-parser.yy +@example +%@{ +# include "calc++-driver.hh" +%@} +@end example -@deffn {Symbol} error -A token name reserved for error recovery. This token may be used in -grammar rules so as to allow the Bison parser to recognize an error in -the grammar without halting the process. In effect, a sentence -containing an error may be recognized as valid. On a syntax error, the -token @code{error} becomes the current look-ahead token. Actions -corresponding to @code{error} are then executed, and the look-ahead -token is reset to the token that originally caused the violation. -@xref{Error Recovery}. -@end deffn -@deffn {Macro} YYABORT -Macro to pretend that an unrecoverable syntax error has occurred, by -making @code{yyparse} return 1 immediately. The error reporting -function @code{yyerror} is not called. @xref{Parser Function, ,The -Parser Function @code{yyparse}}. -@end deffn +@noindent +The token numbered as 0 corresponds to end of file; the following line +allows for nicer error messages referring to ``end of file'' instead +of ``$end''. Similarly user friendly named are provided for each +symbol. Note that the tokens names are prefixed by @code{TOKEN_} to +avoid name clashes. -@deffn {Macro} YYACCEPT -Macro to pretend that a complete utterance of the language has been -read, by making @code{yyparse} return 0 immediately. -@xref{Parser Function, ,The Parser Function @code{yyparse}}. -@end deffn +@comment file: calc++-parser.yy +@example +%token END 0 "end of file" +%token ASSIGN ":=" +%token <sval> IDENTIFIER "identifier" +%token <ival> NUMBER "number" +%type <ival> exp "expression" +@end example -@deffn {Macro} YYBACKUP -Macro to discard a value from the parser stack and fake a look-ahead -token. @xref{Action Features, ,Special Features for Use in Actions}. -@end deffn +@noindent +To enable memory deallocation during error recovery, use +@code{%destructor}. -@deffn {Macro} YYDEBUG -Macro to define to equip the parser with tracing code. @xref{Tracing, -,Tracing Your Parser}. -@end deffn +@c FIXME: Document %printer, and mention that it takes a braced-code operand. +@comment file: calc++-parser.yy +@example +%printer @{ debug_stream () << *$$; @} "identifier" +%destructor @{ delete $$; @} "identifier" -@deffn {Macro} YYERROR -Macro to pretend that a syntax error has just been detected: call -@code{yyerror} and then perform normal error recovery if possible -(@pxref{Error Recovery}), or (if recovery is impossible) make -@code{yyparse} return 1. @xref{Error Recovery}. -@end deffn +%printer @{ debug_stream () << $$; @} "number" "expression" +@end example -@deffn {Macro} YYERROR_VERBOSE -An obsolete macro that you define with @code{#define} in the prologue -to request verbose, specific error message strings -when @code{yyerror} is called. It doesn't matter what definition you -use for @code{YYERROR_VERBOSE}, just whether you define it. Using -@code{%error-verbose} is preferred. -@end deffn +@noindent +The grammar itself is straightforward. -@deffn {Macro} YYINITDEPTH -Macro for specifying the initial size of the parser stack. -@xref{Stack Overflow}. -@end deffn +@comment file: calc++-parser.yy +@example +%% +%start unit; +unit: assignments exp @{ driver.result = $2; @}; + +assignments: assignments assignment @{@} + | /* Nothing. */ @{@}; + +assignment: "identifier" ":=" exp @{ driver.variables[*$1] = $3; @}; + +%left '+' '-'; +%left '*' '/'; +exp: exp '+' exp @{ $$ = $1 + $3; @} + | exp '-' exp @{ $$ = $1 - $3; @} + | exp '*' exp @{ $$ = $1 * $3; @} + | exp '/' exp @{ $$ = $1 / $3; @} + | "identifier" @{ $$ = driver.variables[*$1]; @} + | "number" @{ $$ = $1; @}; +%% +@end example -@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}. he use of this -macro is deprecated, and is supported only for Yacc like parsers. -@xref{Pure Calling,, Calling Conventions for Pure Parsers}. -@end deffn +@noindent +Finally the @code{error} member function registers the errors to the +driver. -@deffn {Type} YYLTYPE -Data type of @code{yylloc}; by default, a structure with four -members. @xref{Location Type, , Data Types of Locations}. -@end deffn +@comment file: calc++-parser.yy +@example +void +yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l, + const std::string& m) +@{ + driver.error (l, m); +@} +@end example -@deffn {Macro} YYMAXDEPTH -Macro for specifying the maximum size of the parser stack. @xref{Stack -Overflow}. -@end deffn +@node Calc++ Scanner +@subsection Calc++ Scanner + +The Flex scanner first includes the driver declaration, then the +parser's to get the set of defined tokens. + +@comment file: calc++-scanner.ll +@example +%@{ /* -*- C++ -*- */ +# include <cstdlib> +# include <errno.h> +# include <limits.h> +# include <string> +# include "calc++-driver.hh" +# include "calc++-parser.hh" +/* Work around a bug in flex 2.5.31. See Debian bug 333231 + <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */ +# undef yywrap +# define yywrap() 1 +/* By default yylex returns int, we use token_type. + Unfortunately yyterminate by default returns 0, which is + not of token_type. */ +#define yyterminate() return token::END +%@} +@end example -@deffn {Macro} YYPARSE_PARAM -An obsolete macro for specifying the name of a parameter that -@code{yyparse} should accept. 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 +@noindent +Because there is no @code{#include}-like feature we don't need +@code{yywrap}, we don't need @code{unput} either, and we parse an +actual file, this is not an interactive session with the user. +Finally we enable the scanner tracing features. + +@comment file: calc++-scanner.ll +@example +%option noyywrap nounput batch debug +@end example + +@noindent +Abbreviations allow for more readable rules. + +@comment file: calc++-scanner.ll +@example +id [a-zA-Z][a-zA-Z_0-9]* +int [0-9]+ +blank [ \t] +@end example + +@noindent +The following paragraph suffices to track locations accurately. Each +time @code{yylex} is invoked, the begin position is moved onto the end +position. Then when a pattern is matched, the end position is +advanced of its width. In case it matched ends of lines, the end +cursor is adjusted, and each time blanks are matched, the begin cursor +is moved onto the end cursor to effectively ignore the blanks +preceding tokens. Comments would be treated equally. + +@comment file: calc++-scanner.ll +@example +%@{ +# define YY_USER_ACTION yylloc->columns (yyleng); +%@} +%% +%@{ + yylloc->step (); +%@} +@{blank@}+ yylloc->step (); +[\n]+ yylloc->lines (yyleng); yylloc->step (); +@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. + +@comment file: calc++-scanner.ll +@example +%@{ + typedef yy::calcxx_parser::token token; +%@} + /* Convert ints to the actual type of tokens. */ +[-+*/] return yy::calcxx_parser::token_type (yytext[0]); +":=" 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; +@} +@{id@} yylval->sval = new std::string (yytext); return token::IDENTIFIER; +. driver.error (*yylloc, "invalid character"); +%% +@end example + +@noindent +Finally, because the scanner related driver's member function depend +on the scanner's data, it is simpler to implement them in this file. + +@comment file: calc++-scanner.ll +@example +void +calcxx_driver::scan_begin () +@{ + yy_flex_debug = trace_scanning; + if (!(yyin = fopen (file.c_str (), "r"))) + error (std::string ("cannot open ") + file); +@} + +void +calcxx_driver::scan_end () +@{ + fclose (yyin); +@} +@end example + +@node Calc++ Top Level +@subsection Calc++ Top Level + +The top level file, @file{calc++.cc}, poses no problem. + +@comment file: calc++.cc +@example +#include <iostream> +#include "calc++-driver.hh" + +int +main (int argc, char *argv[]) +@{ + calcxx_driver driver; + for (++argv; argv[0]; ++argv) + if (*argv == std::string ("-p")) + driver.trace_parsing = true; + else if (*argv == std::string ("-s")) + driver.trace_scanning = true; + else + @{ + driver.parse (*argv); + std::cout << driver.result << std::endl; + @} +@} +@end example + +@c ================================================= FAQ + +@node FAQ +@chapter Frequently Asked Questions +@cindex frequently asked questions +@cindex questions + +Several questions about Bison come up occasionally. Here some of them +are addressed. + +@menu +* Memory Exhausted:: Breaking the Stack Limits +* How Can I Reset the Parser:: @code{yyparse} Keeps some State +* Strings are Destroyed:: @code{yylval} Loses Track of Strings +* Implementing Gotos/Loops:: Control Flow in the Calculator +* Secure? Conform?:: Is Bison @acronym{POSIX} safe? +* I can't build Bison:: Troubleshooting +* Where can I find help?:: Troubleshouting +* Bug Reports:: Troublereporting +* Other Languages:: Parsers in Java and others +* Beta Testing:: Experimenting development versions +* Mailing Lists:: Meeting other Bison users +@end menu + +@node Memory Exhausted +@section Memory Exhausted + +@display +My parser returns with error with a @samp{memory exhausted} +message. What can I do? +@end display + +This question is already addressed elsewhere, @xref{Recursion, +,Recursive Rules}. + +@node How Can I Reset the Parser +@section How Can I Reset the Parser + +The following phenomenon has several symptoms, resulting in the +following typical questions: + +@display +I invoke @code{yyparse} several times, and on correct input it works +properly; but when a parse error is found, all the other calls fail +too. How can I reset the error flag of @code{yyparse}? +@end display + +@noindent +or + +@display +My parser includes support for an @samp{#include}-like feature, in +which case I run @code{yyparse} from @code{yyparse}. This fails +although I did specify I needed a @code{%pure-parser}. +@end display + +These problems typically come not from Bison itself, but from +Lex-generated scanners. Because these scanners use large buffers for +speed, they might not notice a change of input file. As a +demonstration, consider the following source file, +@file{first-line.l}: + +@verbatim +%{ +#include <stdio.h> +#include <stdlib.h> +%} +%% +.*\n ECHO; return 1; +%% +int +yyparse (char const *file) +{ + yyin = fopen (file, "r"); + if (!yyin) + exit (2); + /* One token only. */ + yylex (); + if (fclose (yyin) != 0) + exit (3); + return 0; +} + +int +main (void) +{ + yyparse ("input"); + yyparse ("input"); + return 0; +} +@end verbatim + +@noindent +If the file @file{input} contains + +@verbatim +input:1: Hello, +input:2: World! +@end verbatim + +@noindent +then instead of getting the first line twice, you get: + +@example +$ @kbd{flex -ofirst-line.c first-line.l} +$ @kbd{gcc -ofirst-line first-line.c -ll} +$ @kbd{./first-line} +input:1: Hello, +input:2: World! +@end example + +Therefore, whenever you change @code{yyin}, you must tell the +Lex-generated scanner to discard its current buffer and switch to the +new one. This depends upon your implementation of Lex; see its +documentation for more. For Flex, it suffices to call +@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your +Flex-generated scanner needs to read from several input streams to +handle features like include files, you might consider using Flex +functions like @samp{yy_switch_to_buffer} that manipulate multiple +input buffers. + +If your Flex-generated scanner uses start conditions (@pxref{Start +conditions, , Start conditions, flex, The Flex Manual}), you might +also want to reset the scanner's state, i.e., go back to the initial +start condition, through a call to @samp{BEGIN (0)}. + +@node Strings are Destroyed +@section Strings are Destroyed + +@display +My parser seems to destroy old strings, or maybe it loses track of +them. Instead of reporting @samp{"foo", "bar"}, it reports +@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}. +@end display + +This error is probably the single most frequent ``bug report'' sent to +Bison lists, but is only concerned with a misunderstanding of the role +of the scanner. Consider the following Lex code: + +@verbatim +%{ +#include <stdio.h> +char *yylval = NULL; +%} +%% +.* yylval = yytext; return 1; +\n /* IGNORE */ +%% +int +main () +{ + /* Similar to using $1, $2 in a Bison action. */ + char *fst = (yylex (), yylval); + char *snd = (yylex (), yylval); + printf ("\"%s\", \"%s\"\n", fst, snd); + return 0; +} +@end verbatim + +If you compile and run this code, you get: + +@example +$ @kbd{flex -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"one +two", "two" +@end example + +@noindent +this is because @code{yytext} is a buffer provided for @emph{reading} +in the action, but if you want to keep it, you have to duplicate it +(e.g., using @code{strdup}). Note that the output may depend on how +your implementation of Lex handles @code{yytext}. For instance, when +given the Lex compatibility option @option{-l} (which triggers the +option @samp{%array}) Flex generates a different behavior: + +@example +$ @kbd{flex -l -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"two", "two" +@end example + + +@node Implementing Gotos/Loops +@section Implementing Gotos/Loops + +@display +My simple calculator supports variables, assignments, and functions, +but how can I implement gotos, or loops? +@end display + +Although very pedagogical, the examples included in the document blur +the distinction to make between the parser---whose job is to recover +the structure of a text and to transmit it to subsequent modules of +the program---and the processing (such as the execution) of this +structure. This works well with so called straight line programs, +i.e., precisely those that have a straightforward execution model: +execute simple instructions one after the others. + +@cindex abstract syntax tree +@cindex @acronym{AST} +If you want a richer model, you will probably need to use the parser +to construct a tree that does represent the structure it has +recovered; this tree is usually called the @dfn{abstract syntax tree}, +or @dfn{@acronym{AST}} for short. Then, walking through this tree, +traversing it in various ways, will enable treatments such as its +execution or its translation, which will result in an interpreter or a +compiler. + +This topic is way beyond the scope of this manual, and the reader is +invited to consult the dedicated literature. + + +@node Secure? Conform? +@section Secure? Conform? + +@display +Is Bison secure? Does it conform to POSIX? +@end display + +If you're looking for a guarantee or certification, we don't provide it. +However, Bison is intended to be a reliable program that conforms to the +@acronym{POSIX} specification for Yacc. If you run into problems, +please send us a bug report. + +@node I can't build Bison +@section I can't build Bison + +@display +I can't build Bison because @command{make} complains that +@code{msgfmt} is not found. +What should I do? +@end display + +Like most GNU packages with internationalization support, that feature +is turned on by default. If you have problems building in the @file{po} +subdirectory, it indicates that your system's internationalization +support is lacking. You can re-configure Bison with +@option{--disable-nls} to turn off this support, or you can install GNU +gettext from @url{ftp://ftp.gnu.org/gnu/gettext/} and re-configure +Bison. See the file @file{ABOUT-NLS} for more information. + + +@node Where can I find help? +@section Where can I find help? + +@display +I'm having trouble using Bison. Where can I find help? +@end display + +First, read this fine manual. Beyond that, you can send mail to +@email{help-bison@@gnu.org}. This mailing list is intended to be +populated with people who are willing to answer questions about using +and installing Bison. Please keep in mind that (most of) the people on +the list have aspects of their lives which are not related to Bison (!), +so you may not receive an answer to your question right away. This can +be frustrating, but please try not to honk them off; remember that any +help they provide is purely voluntary and out of the kindness of their +hearts. + +@node Bug Reports +@section Bug Reports + +@display +I found a bug. What should I include in the bug report? +@end display + +Before you send a bug report, make sure you are using the latest +version. Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its +mirrors. Be sure to include the version number in your bug report. If +the bug is present in the latest version but not in a previous version, +try to determine the most recent version which did not contain the bug. + +If the bug is parser-related, you should include the smallest grammar +you can which demonstrates the bug. The grammar file should also be +complete (i.e., I should be able to run it through Bison without having +to edit or add anything). The smaller and simpler the grammar, the +easier it will be to fix the bug. + +Include information about your compilation environment, including your +operating system's name and version and your compiler's name and +version. If you have trouble compiling, you should also include a +transcript of the build session, starting with the invocation of +`configure'. Depending on the nature of the bug, you may be asked to +send additional files as well (such as `config.h' or `config.cache'). + +Patches are most welcome, but not required. That is, do not hesitate to +send a bug report just because you can not provide a fix. + +Send bug reports to @email{bug-bison@@gnu.org}. + +@node Other Languages +@section Other Languages + +@display +Will Bison ever have C++ support? How about Java or @var{insert your +favorite language here}? +@end display + +C++ support is there now, and is documented. We'd love to add other +languages; contributions are welcome. + +@node Beta Testing +@section Beta Testing + +@display +What is involved in being a beta tester? +@end display + +It's not terribly involved. Basically, you would download a test +release, compile it, and use it to build and run a parser or two. After +that, you would submit either a bug report or a message saying that +everything is okay. It is important to report successes as well as +failures because test releases eventually become mainstream releases, +but only if they are adequately tested. If no one tests, development is +essentially halted. + +Beta testers are particularly needed for operating systems to which the +developers do not have easy access. They currently have easy access to +recent GNU/Linux and Solaris versions. Reports about other operating +systems are especially welcome. + +@node Mailing Lists +@section Mailing Lists -@deffn {Macro} YYRECOVERING -Macro whose value indicates whether the parser is recovering from a -syntax error. @xref{Action Features, ,Special Features for Use in Actions}. -@end deffn +@display +How do I join the help-bison and bug-bison mailing lists? +@end display -@deffn {Macro} YYSTACK_USE_ALLOCA -Macro used to control the use of @code{alloca}. If defined to @samp{0}, -the parser will not use @code{alloca} but @code{malloc} when trying to -grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA} -to anything else. -@end deffn +See @url{http://lists.gnu.org/}. -@deffn {Type} YYSTYPE -Data type of semantic values; @code{int} by default. -@xref{Value Type, ,Data Types of Semantic Values}. +@c ================================================= Table of Symbols + +@node Table of Symbols +@appendix Bison Symbols +@cindex Bison symbols, table of +@cindex symbols in Bison, table of + +@deffn {Variable} @@$ +In an action, the location of the left-hand side of the rule. +@xref{Locations, , Locations Overview}. @end deffn -@deffn {Variable} yychar -External integer variable that contains the integer value of the current -look-ahead token. (In a pure parser, it is a local variable within -@code{yyparse}.) Error-recovery rule actions may examine this variable. -@xref{Action Features, ,Special Features for Use in Actions}. +@deffn {Variable} @@@var{n} +In an action, the location of the @var{n}-th symbol of the right-hand +side of the rule. @xref{Locations, , Locations Overview}. @end deffn -@deffn {Variable} yyclearin -Macro used in error-recovery rule actions. It clears the previous -look-ahead token. @xref{Error Recovery}. +@deffn {Variable} $$ +In an action, the semantic value of the left-hand side of the rule. +@xref{Actions}. @end deffn -@deffn {Variable} yydebug -External integer variable set to zero by default. If @code{yydebug} -is given a nonzero value, the parser will output information on input -symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. +@deffn {Variable} $@var{n} +In an action, the semantic value of the @var{n}-th symbol of the +right-hand side of the rule. @xref{Actions}. @end deffn -@deffn {Macro} yyerrok -Macro to cause parser to recover immediately to its normal mode -after a syntax error. @xref{Error Recovery}. +@deffn {Delimiter} %% +Delimiter used to separate the grammar rule section from the +Bison declarations section or the epilogue. +@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. @end deffn -@deffn {Function} yyerror -User-supplied function to be called by @code{yyparse} on error. -@xref{Error Reporting, ,The Error -Reporting Function @code{yyerror}}. +@c Don't insert spaces, or check the DVI output. +@deffn {Delimiter} %@{@var{code}%@} +All code listed between @samp{%@{} and @samp{%@}} is copied directly to +the output file uninterpreted. Such code forms the prologue of the input +file. @xref{Grammar Outline, ,Outline of a Bison +Grammar}. @end deffn -@deffn {Function} yylex -User-supplied lexical analyzer function, called with no arguments to get -the next token. @xref{Lexical, ,The Lexical Analyzer Function -@code{yylex}}. +@deffn {Construct} /*@dots{}*/ +Comment delimiters, as in C. @end deffn -@deffn {Variable} yylval -External variable in which @code{yylex} should place the semantic -value associated with a token. (In a pure parser, it is a local -variable within @code{yyparse}, and its address is passed to -@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}. +@deffn {Delimiter} : +Separates a rule's result from its components. @xref{Rules, ,Syntax of +Grammar Rules}. @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 -variable within @code{yyparse}, and its address is passed to -@code{yylex}.) You can ignore this variable if you don't use the -@samp{@@} feature in the grammar actions. @xref{Token Locations, -,Textual Locations of Tokens}. +@deffn {Delimiter} ; +Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. @end deffn -@deffn {Variable} yynerrs -Global variable which Bison increments each time there is a syntax error. -(In a pure parser, it is a local variable within @code{yyparse}.) -@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@deffn {Delimiter} | +Separates alternate rules for the same result nonterminal. +@xref{Rules, ,Syntax of Grammar Rules}. @end deffn -@deffn {Function} yyparse -The parser function produced by Bison; call this function to start -parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@deffn {Symbol} $accept +The predefined nonterminal whose only rule is @samp{$accept: @var{start} +$end}, where @var{start} is the start symbol. @xref{Start Decl, , The +Start-Symbol}. It cannot be used in the grammar. @end deffn @deffn {Directive} %debug @@ -7081,8 +8128,8 @@ Bison declaration to create a header file meant for the scanner. @end deffn @deffn {Directive} %destructor -Specifying how the parser should reclaim the memory associated to -discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +Specify how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. @end deffn @deffn {Directive} %dprec @@ -7091,6 +8138,22 @@ time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. @end deffn +@deffn {Symbol} $end +The predefined token marking the end of the token stream. It cannot be +used in the grammar. +@end deffn + +@deffn {Symbol} error +A token name reserved for error recovery. This token may be used in +grammar rules so as to allow the Bison parser to recognize an error in +the grammar without halting the process. In effect, a sentence +containing an error may be recognized as valid. On a syntax error, the +token @code{error} becomes the current look-ahead token. Actions +corresponding to @code{error} are then executed, and the look-ahead +token is reset to the token that originally caused the violation. +@xref{Error Recovery}. +@end deffn + @deffn {Directive} %error-verbose Bison declaration to request verbose, specific error message strings when @code{yyerror} is called. @@ -7106,6 +8169,10 @@ Bison declaration to produce a @acronym{GLR} parser. @xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. @end deffn +@deffn {Directive} %initial-action +Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}. +@end deffn + @deffn {Directive} %left Bison declaration to assign left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @@ -7142,11 +8209,11 @@ parser file. @xref{Decl Summary}. @end deffn @deffn {Directive} %nonassoc -Bison declaration to assign non-associativity to token(s). +Bison declaration to assign nonassociativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @end deffn -@deffn {Directive} %output="@var{filename}" +@deffn {Directive} %output="@var{file}" Bison declaration to set the name of the parser file. @xref{Decl Summary}. @end deffn @@ -7167,6 +8234,11 @@ Bison declaration to request a pure (reentrant) parser. @xref{Pure Decl, ,A Pure (Reentrant) Parser}. @end deffn +@deffn {Directive} %require "@var{version}" +Require version @var{version} or higher of Bison. @xref{Require Decl, , +Require a Version of Bison}. +@end deffn + @deffn {Directive} %right Bison declaration to assign right associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @@ -7192,45 +8264,177 @@ Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}. @end deffn +@deffn {Symbol} $undefined +The predefined token onto which all undefined values returned by +@code{yylex} are mapped. It cannot be used in the grammar, rather, use +@code{error}. +@end deffn + @deffn {Directive} %union Bison declaration to specify several possible data types for semantic values. @xref{Union Decl, ,The Collection of Value Types}. @end deffn -@sp 1 +@deffn {Macro} YYABORT +Macro to pretend that an unrecoverable syntax error has occurred, by +making @code{yyparse} return 1 immediately. The error reporting +function @code{yyerror} is not called. @xref{Parser Function, ,The +Parser Function @code{yyparse}}. +@end deffn + +@deffn {Macro} YYACCEPT +Macro to pretend that a complete utterance of the language has been +read, by making @code{yyparse} return 0 immediately. +@xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -These are the punctuation and delimiters used in Bison input: +@deffn {Macro} YYBACKUP +Macro to discard a value from the parser stack and fake a look-ahead +token. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@deffn {Delimiter} %% -Delimiter used to separate the grammar rule section from the -Bison declarations section or the epilogue. -@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. +@deffn {Variable} yychar +External integer variable that contains the integer value of the +look-ahead token. (In a pure parser, it is a local variable within +@code{yyparse}.) Error-recovery rule actions may examine this variable. +@xref{Action Features, ,Special Features for Use in Actions}. @end deffn -@c Don't insert spaces, or check the DVI output. -@deffn {Delimiter} %@{@var{code}%@} -All code listed between @samp{%@{} and @samp{%@}} is copied directly to -the output file uninterpreted. Such code forms the prologue of the input -file. @xref{Grammar Outline, ,Outline of a Bison -Grammar}. +@deffn {Variable} yyclearin +Macro used in error-recovery rule actions. It clears the previous +look-ahead token. @xref{Error Recovery}. @end deffn -@deffn {Construct} /*@dots{}*/ -Comment delimiters, as in C. +@deffn {Macro} YYDEBUG +Macro to define to equip the parser with tracing code. @xref{Tracing, +,Tracing Your Parser}. @end deffn -@deffn {Delimiter} : -Separates a rule's result from its components. @xref{Rules, ,Syntax of -Grammar Rules}. +@deffn {Variable} yydebug +External integer variable set to zero by default. If @code{yydebug} +is given a nonzero value, the parser will output information on input +symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. @end deffn -@deffn {Delimiter} ; -Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Macro} yyerrok +Macro to cause parser to recover immediately to its normal mode +after a syntax error. @xref{Error Recovery}. @end deffn -@deffn {Delimiter} | -Separates alternate rules for the same result nonterminal. -@xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Macro} YYERROR +Macro to pretend that a syntax error has just been detected: call +@code{yyerror} and then perform normal error recovery if possible +(@pxref{Error Recovery}), or (if recovery is impossible) make +@code{yyparse} return 1. @xref{Error Recovery}. +@end deffn + +@deffn {Function} yyerror +User-supplied function to be called by @code{yyparse} on error. +@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. +@end deffn + +@deffn {Macro} YYINITDEPTH +Macro for specifying the initial size of the parser stack. +@xref{Memory Management}. +@end deffn + +@deffn {Function} yylex +User-supplied lexical analyzer function, called with no arguments to get +the next token. @xref{Lexical, ,The Lexical Analyzer Function +@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 +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) +You can ignore this variable if you don't use the @samp{@@} feature in the +grammar actions. +@xref{Token Locations, ,Textual Locations of Tokens}. +In semantic actions, it stores the location of the look-ahead token. +@xref{Actions and Locations, ,Actions and Locations}. +@end deffn + +@deffn {Type} YYLTYPE +Data type of @code{yylloc}; by default, a structure with four +members. @xref{Location Type, , Data Types of Locations}. +@end deffn + +@deffn {Variable} yylval +External variable in which @code{yylex} should place the semantic +value associated with a token. (In a pure parser, it is a local +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) +@xref{Token Values, ,Semantic Values of Tokens}. +In semantic actions, it stores the semantic value of the look-ahead token. +@xref{Actions, ,Actions}. +@end deffn + +@deffn {Macro} YYMAXDEPTH +Macro for specifying the maximum size of the parser stack. @xref{Memory +Management}. +@end deffn + +@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}.) +@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@end deffn + +@deffn {Function} yyparse +The parser function produced by Bison; call this function to start +parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn + +@deffn {Macro} YYPARSE_PARAM +An obsolete macro for specifying the name of a parameter that +@code{yyparse} should accept. 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 {Macro} YYRECOVERING +Macro whose value indicates whether the parser is recovering from a +syntax error. @xref{Action Features, ,Special Features for Use in Actions}. +@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, +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, +@code{YYSTACK_USE_ALLOCA} defaults to 0. + +In the all-too-common case where your code may run on a host with a +limited stack and with unreliable stack-overflow checking, you should +set @code{YYMAXDEPTH} to a value that cannot possibly result in +unchecked stack overflow on any of your target hosts when +@code{alloca} is called. You can inspect the code that Bison +generates in order to determine the proper numeric values. This will +require some expertise in low-level implementation details. +@end deffn + +@deffn {Type} YYSTYPE +Data type of semantic values; @code{int} by default. +@xref{Value Type, ,Data Types of Semantic Values}. @end deffn @node Glossary @@ -7441,7 +8645,7 @@ grammatically indivisible. The piece of text it represents is a token. @c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES @c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param @c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP -@c LocalWords: YYEMPTY YYRECOVERING yyclearin GE def UMINUS maybeword +@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword @c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH @c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype @c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args