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Info file bison.info, produced by Makeinfo, -*- Text -*- from input file bison.texinfo. This file documents the Bison parser generator. Copyright (C) 1988, 1989 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled ``GNU General Public License'' and ``Conditions for Using Bison'' are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled ``GNU General Public License'', ``Conditions for Using Bison'' and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. File: bison.info, Node: Infix Calc, Next: Simple Error Recovery, Prev: RPN Calc, Up: Top Infix Notation Calculator: `calc' ================================= We now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for `calc.y', an infix desk-top calculator. /* Infix notation calculator--calc */ %{ #define YYSTYPE double #include <math.h> %} %token NUM %left '-' '+' %left '*' '/' %left NEG /* negation--unary minus */ %right '^' /* exponentiation */ /* Grammar follows */ %% input: /* empty string */ | input line ; line: '\n' | exp '\n' { printf("\t%.10g\n", $1); } ; exp: NUM { $$ = $1; } | exp '+' exp { $$ = $1 + $3; } | exp '-' exp { $$ = $1 - $3; } | exp '*' exp { $$ = $1 * $3; } | exp '/' exp { $$ = $1 / $3; } | '-' exp %prec NEG { $$ = -$2; } | exp '^' exp { $$ = pow ($1, $3); } | '(' exp ')' { $$ = $2; } ; %% The functions `yylex', `yyerror' and `main' can be the same as before. There are two important new features shown in this code. In the second section (Bison declarations), `%left' declares token types and says they are left-associative operators. The declarations `%left' and `%right' (right associativity) take the place of `%token' which is used to declare a token type name without associativity. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify the associativity.) Operator precedence is determined by the line ordering of the declarations; the lower the declaration, the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (`NEG') is next, followed by `*' and `/', and so on. *Note Precedence::. The other important new feature is the `%prec' in the grammar section for the unary minus operator. The `%prec' simply instructs Bison that the rule `| '-' exp' has the same precedence as `NEG'--in this case the next-to-highest. *Note Contextual Precedence::. Here is a sample run of `calc.y': % calc 4 + 4.5 - (34/(8*3+-3)) 6.880952381 -56 + 2 -54 3 ^ 2 9 File: bison.info, Node: Simple Error Recovery, Next: Multi-function Calc, Prev: Infix Calc, Up: Examples Simple Error Recovery ===================== Up to this point, this manual has not addressed the issue of "error recovery"--how to continue parsing after the parser detects a syntax error. All we have handled is error reporting with `yyerror'. Recall that by default `yyparse' returns after calling `yyerror'. This means that an erroneous input line causes the calculator program to exit. Now we show how to rectify this deficiency. The Bison language itself includes the reserved word `error', which may be included in the grammar rules. In the example below it has been added to one of the alternatives for `line': line: '\n' | exp '\n' { printf("\t%.10g\n", $1); } | error '\n' { yyerrok; } ; This addition to the grammar allows for simple error recovery in the event of a parse error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for `line', and parsing will continue. (The `yyerror' function is still called upon to print its message as well.) The action executes the statement `yyerrok', a macro defined automatically by Bison; its meaning is that error recovery is complete (*note Error Recovery::.). Note the difference between `yyerrok' and `yyerror'; neither one is a misprint. This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception signal that is normally fatal. A real calculator program must handle this signal and use `longjmp' to return to `main' and resume parsing input lines; it would also have to discard the rest of the current line of input. We won't discuss this issue further because it is not specific to Bison programs. File: bison.info, Node: Multi-function Calc, Prev: Simple Error Recovery, Up: Examples Multi-Function Calculator: `mfcalc' =================================== Now that the basics of Bison have been discussed, it is time to move on to a more advanced problem. The above calculators provided only five functions, `+', `-', `*', `/' and `^'. It would be nice to have a calculator that provides other mathematical functions such as `sin', `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 `yylex' passes back all non-number 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: FUNCTION_NAME (ARGUMENT) At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator: % acalc pi = 3.141592653589 3.1415926536 sin(pi) 0.0000000000 alpha = beta1 = 2.3 2.3000000000 alpha 2.3000000000 ln(alpha) 0.8329091229 exp(ln(beta1)) 2.3000000000 % Note that multiple assignment and nested function calls are permitted. * Menu: * Decl: Mfcalc Decl. Bison declarations for multi-function calculator. * Rules: Mfcalc Rules. Grammar rules for the calculator. * Symtab: Mfcalc Symtab. Symbol table management subroutines. File: bison.info, Node: Mfcalc Decl, Next: Mfcalc Rules, Prev: Multi-function Calc, Up: Multi-function Calc Declarations for `mfcalc' ------------------------- Here are the C and Bison declarations for the multi-function calculator. %{ #include <math.h> /* For math functions, cos(), sin(), etc */ #include "calc.h" /* Contains definition of `symrec' */ %} %union { double val; /* For returning numbers. */ symrec *tptr; /* For returning symbol-table pointers */ } %token <val> NUM /* Simple double precision number */ %token <tptr> VAR FNCT /* Variable and Function */ %type <val> exp %right '=' %left '-' '+' %left '*' '/' %left NEG /* Negation--unary minus */ %right '^' /* Exponentiation */ /* Grammar follows */ %% The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (*note Multiple Types::.). The `%union' declaration specifies the entire list of possible types; this is instead of defining `YYSTYPE'. The allowable types are now double-floats (for `exp' and `NUM') and pointers to entries in the symbol table. *Note Union Decl::. Since values can now have various types, it is necessary to associate a type with each grammar symbol whose semantic value is used. These symbols are `NUM', `VAR', `FNCT', and `exp'. Their declarations are augmented with information about their data type (placed between angle brackets). The Bison construct `%type' is used for declaring nonterminal symbols, just as `%token' is used for declaring token types. We have not used `%type' before because nonterminal symbols are normally declared implicitly by the rules that define them. But `exp' must be declared explicitly so we can specify its value type. *Note Type Decl::. File: bison.info, Node: Mfcalc Rules, Next: Mfcalc Symtab, Prev: Mfcalc Decl, Up: Multi-function Calc Grammar Rules for `mfcalc' -------------------------- Here are the grammar rules for the multi-function calculator. Most of them are copied directly from `calc'; three rules, those which mention `VAR' or `FNCT', are new. input: /* empty */ | input line ; line: '\n' | exp '\n' { printf ("\t%.10g\n", $1); } | error '\n' { yyerrok; } ; exp: NUM { $$ = $1; } | VAR { $$ = $1->value.var; } | VAR '=' exp { $$ = $3; $1->value.var = $3; } | FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($3); } | exp '+' exp { $$ = $1 + $3; } | exp '-' exp { $$ = $1 - $3; } | exp '*' exp { $$ = $1 * $3; } | exp '/' exp { $$ = $1 / $3; } | '-' exp %prec NEG { $$ = -$2; } | exp '^' exp { $$ = pow ($1, $3); } | '(' exp ')' { $$ = $2; } ; /* End of grammar */ %% File: bison.info, Node: Mfcalc Symtab, Prev: Mfcalc Rules, Up: Multi-function Calc Managing the Symbol Table for `mfcalc' -------------------------------------- The multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support. The symbol table itself consists of a linked list of records. Its definition, which is kept in the header `calc.h', is as follows. It provides for either functions or variables to be placed in the table. /* Data type for links in the chain of symbols. */ struct symrec { char *name; /* name of symbol */ int type; /* type of symbol: either VAR or FNCT */ union { double var; /* value of a VAR */ double (*fnctptr)(); /* value of a FNCT */ } value; struct symrec *next; /* link field */ }; typedef struct symrec symrec; /* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; symrec *putsym (); symrec *getsym (); The new version of `main' includes a call to `init_table', a function that initializes the symbol table. Here it is, and `init_table' as well: #include <stdio.h> main() { init_table (); yyparse (); } yyerror (s) /* Called by yyparse on error */ char *s; { printf ("%s\n", s); } struct init { char *fname; double (*fnct)(); }; struct init arith_fncts[] = { "sin", sin, "cos", cos, "atan", atan, "ln", log, "exp", exp, "sqrt", sqrt, 0, 0 }; /* The symbol table: a chain of `struct symrec'. */ symrec *sym_table = (symrec *)0; init_table () /* puts arithmetic functions in table. */ { int i; symrec *ptr; for (i = 0; arith_fncts[i].fname != 0; i++) { ptr = putsym (arith_fncts[i].fname, FNCT); ptr->value.fnctptr = arith_fncts[i].fnct; } } By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator. Two important functions allow look-up and installation of symbols in the symbol table. The function `putsym' is passed a name and the type (`VAR' or `FNCT') of the object to be installed. The object is linked to the front of the list, and a pointer to the object is returned. The function `getsym' is passed the name of the symbol to look up. If found, a pointer to that symbol is returned; otherwise zero is returned. symrec * putsym (sym_name,sym_type) char *sym_name; int sym_type; { symrec *ptr; ptr = (symrec *) malloc (sizeof(symrec)); ptr->name = (char *) malloc (strlen(sym_name)+1); strcpy (ptr->name,sym_name); ptr->type = sym_type; ptr->value.var = 0; /* set value to 0 even if fctn. */ ptr->next = (struct symrec *)sym_table; sym_table = ptr; return ptr; } symrec * getsym (sym_name) char *sym_name; { symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; ptr = (symrec *)ptr->next) if (strcmp (ptr->name,sym_name) == 0) return ptr; return 0; } The function `yylex' must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric characters with a leading nondigit are recognized as either variables or functions depending on what the symbol table says about them. The string is passed to `getsym' for look up in the symbol table. If the name appears in the table, a pointer to its location and its type (`VAR' or `FNCT') is returned to `yyparse'. If it is not already in the table, then it is installed as a `VAR' using `putsym'. Again, a pointer and its type (which must be `VAR') is returned to `yyparse'. No change is needed in the handling of numeric values and arithmetic operators in `yylex'. #include <ctype.h> yylex() { int c; /* Ignore whitespace, get first nonwhite character. */ while ((c = getchar ()) == ' ' || c == '\t'); if (c == EOF) return 0; /* Char starts a number => parse the number. */ if (c == '.' || isdigit (c)) { ungetc (c, stdin); scanf ("%lf", &yylval.val); return NUM; } /* Char starts an identifier => read the name. */ if (isalpha (c)) { symrec *s; static char *symbuf = 0; static int length = 0; int i; /* Initially make the buffer long enough for a 40-character symbol name. */ if (length == 0) length = 40, symbuf = (char *)malloc (length + 1); i = 0; do { /* If buffer is full, make it bigger. */ if (i == length) { length *= 2; symbuf = (char *)realloc (symbuf, length + 1); } /* Add this character to the buffer. */ symbuf[i++] = c; /* Get another character. */ c = getchar (); } while (c != EOF && isalnum (c)); ungetc (c, stdin); symbuf[i] = '\0'; s = getsym (symbuf); if (s == 0) s = putsym (symbuf, VAR); yylval.tptr = s; return s->type; } /* Any other character is a token by itself. */ return c; } This program is both powerful and flexible. You may easily add new functions, and it is a simple job to modify this code to install predefined variables such as `pi' or `e' as well. File: bison.info, Node: Exercises, Prev: Multi-function calc, Up: Examples Exercises ========= 1. Add some new functions from `math.h' to the initialization list. 2. Add another array that contains constants and their values. Then modify `init_table' to add these constants to the symbol table. It will be easiest to give the constants type `VAR'. 3. Make the program report an error if the user refers to an uninitialized variable in any way except to store a value in it. File: bison.info, Node: Grammar File, Next: Interface, Prev: Examples, Up: Top Bison Grammar Files ******************* Bison takes as input a context-free grammar specification and produces a C-language function that recognizes correct instances of the grammar. The Bison grammar input file conventionally has a name ending in `.y'. * Menu: * Grammar Outline:: Overall layout of the grammar file. * Symbols:: Terminal and nonterminal symbols. * Rules:: How to write grammar rules. * Recursion:: Writing recursive rules. * Semantics:: Semantic values and actions. * Declarations:: All kinds of Bison declarations are described here. * Multiple Parsers:: Putting more than one Bison parser in one program. File: bison.info, Node: Grammar Outline, Next: Symbols, Prev: Grammar File, Up: Grammar File Outline of a Bison Grammar ========================== A Bison grammar file has four main sections, shown here with the appropriate delimiters: %{ C DECLARATIONS %} BISON DECLARATIONS %% GRAMMAR RULES %% ADDITIONAL C CODE Comments enclosed in `/* ... */' may appear in any of the sections. * Menu: * C Declarations:: Syntax and usage of the C declarations section. * Bison Declarations:: Syntax and usage of the Bison declarations section. * Grammar Rules:: Syntax and usage of the grammar rules section. * C Code:: Syntax and usage of the additional C code section. File: bison.info, Node: C Declarations, Next: Bison Declarations, Prev: Grammar Outline, Up: Grammar Outline The C Declarations Section -------------------------- The C DECLARATIONS 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 `yylex'. You can use `#include' to get the declarations from a header file. If you don't need any C declarations, you may omit the `%{' and `%}' delimiters that bracket this section. File: bison.info, Node: Bison Declarations, Next: Grammar Rules, Prev: C Declarations, Up: Grammar Outline The Bison Declarations Section ------------------------------ The BISON DECLARATIONS section contains declarations that define terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. *Note Declarations::. File: bison.info, Node: Grammar Rules, Next: C Code, Prev: Bison Declarations, Up: Grammar Outline The Grammar Rules Section ------------------------- The "grammar rules" section contains one or more Bison grammar rules, and nothing else. *Note Rules::. There must always be at least one grammar rule, and the first `%%' (which precedes the grammar rules) may never be omitted even if it is the first thing in the file. File: bison.info, Node: C Code, Prev: Grammar Rules, Up: Grammar Outline The Additional C Code Section ----------------------------- The ADDITIONAL C CODE section is copied verbatim to the end of the parser file, just as the C DECLARATIONS section is copied to the beginning. This is the most convenient place to put anything that you want to have in the parser file but which need not come before the definition of `yylex'. For example, the definitions of `yylex' and `yyerror' often go here. *Note Interface::. If the last section is empty, you may omit the `%%' that separates it from the grammar rules. The Bison parser itself contains many static variables whose names start with `yy' and many macros whose names start with `YY'. It is a good idea to avoid using any such names (except those documented in this manual) in the additional C code section of the grammar file. File: bison.info, Node: Symbols, Next: Rules, Prev: Grammar Outline, Up: Grammar File Symbols, Terminal and Nonterminal ================================= "Symbols" in Bison grammars represent the grammatical classifications of the language. A "terminal symbol" (also known as a "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 `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. A "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. There are two ways of writing terminal symbols in the grammar: * A "named token type" is written with an identifier, like an identifier in C. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as `%token'. *Note Token Decl::. * A "character token type" (or "literal token") is written in the grammar using the same syntax used in C for character constants; for example, `'+'' is a character token type. A character token type doesn't need to be declared unless you need to specify its semantic value data type (*note Value Type::.), associativity, or precedence (*note Precedence::.). By convention, a character token type is used only to represent a token that consists of that particular character. Thus, the token type `'+'' is used to represent the character `+' as a token. Nothing enforces this convention, but if you depart from it, your program will confuse other readers. All the usual escape sequences used in character literals in C can be used in Bison as well, but you must not use the null character as a character literal because its ASCII code, zero, is the code `yylex' returns for end-of-input (*note Calling Convention::.). How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol. The value returned by `yylex' is always one of the terminal symbols (or 0 for end-of-input). Whichever way you write the token type in the grammar rules, you write it the same way in the definition of `yylex'. The numeric code for a character token type is simply the ASCII code for the character, so `yylex' can use the identical character constant to generate the requisite code. Each named token type becomes a C macro in the parser file, so `yylex' can use the name to stand for the code. (This is why periods don't make sense in terminal symbols.) *Note Calling Convention::. If `yylex' is defined in a separate file, you need to arrange for the token-type macro definitions to be available there. Use the `-d' option when you run Bison, so that it will write these macro definitions into a separate header file `NAME.tab.h' which you can include in the other source files that need it. *Note Invocation::. The symbol `error' is a terminal symbol reserved for error recovery (*note Error Recovery::.); you shouldn't use it for any other purpose. In particular, `yylex' should never return this value. File: bison.info, Node: Rules, Next: Recursion, Prev: Symbols, Up: Grammar File Syntax of Grammar Rules ======================= A Bison grammar rule has the following general form: RESULT: COMPONENTS... ; where RESULT is the nonterminal symbol that this rule describes and COMPONENTS are various terminal and nonterminal symbols that are put together by this rule (*note Symbols::.). For example, exp: exp '+' exp ; says that two groupings of type `exp', with a `+' token in between, can be combined into a larger grouping of type `exp'. Whitespace in rules is significant only to separate symbols. You can add extra whitespace as you wish. Scattered among the components can be ACTIONS that determine the semantics of the rule. An action looks like this: {C STATEMENTS} Usually there is only one action and it follows the components. *Note Actions::. Multiple rules for the same RESULT can be written separately or can be joined with the vertical-bar character `|' as follows: RESULT: RULE1-COMPONENTS... | RULE2-COMPONENTS... ... ; They are still considered distinct rules even when joined in this way. If COMPONENTS in a rule is empty, it means that RESULT can match the empty string. For example, here is how to define a comma-separated sequence of zero or more `exp' groupings: expseq: /* empty */ | expseq1 ; expseq1: exp | expseq1 ',' exp ; It is customary to write a comment `/* empty */' in each rule with no components. File: bison.info, Node: Recursion, Next: Semantics, Prev: Rules, Up: Grammar File Recursive Rules =============== A rule is called "recursive" when its 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 somethings. Consider this recursive definition of a comma-separated sequence of one or more expressions: expseq1: exp | expseq1 ',' exp ; Since the recursive use of `expseq1' is the leftmost symbol in the right hand side, we call this "left recursion". By contrast, here the same construct is defined using "right recursion": expseq1: exp | exp ',' expseq1 ; Any kind of sequence can be defined using either left recursion or right recursion, but you should always use left recursion, because it can parse a sequence of any number of elements with bounded stack space. Right recursion uses up space on the Bison stack in proportion to the number of elements in the sequence, because all the elements must be shifted onto the stack before the rule can be applied even once. *Note Algorithm::, for further explanation of this. "Indirect" or "mutual" recursion occurs when the result of the rule does not appear directly on its right hand side, but does appear in rules for other nonterminals which do appear on its right hand side. For example: expr: primary | primary '+' primary ; primary: constant | '(' expr ')' ; defines two mutually-recursive nonterminals, since each refers to the other. File: bison.info, Node: Semantics, Next: Declarations, Prev: Recursion, Up: Grammar File The Semantics of the Language ============================= The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized. For example, the calculator calculates properly because the value associated with each expression is the proper number; it adds properly because the action for the grouping `X + Y' is to add the numbers associated with X and Y. * Menu: * Value Type:: Specifying one data type for all semantic values. * Multiple Types:: Specifying several alternative data types. * Actions:: An action is the semantic definition of a grammar rule. * Action Types:: Specifying data types for actions to operate on. * Mid-Rule Actions:: Most actions go at the end of a rule. This says when, why and how to use the exceptional action in the middle of a rule. File: bison.info, Node: Value Type, Next: Multiple Types, Prev: Semantics, Up: Semantics The Data Types of Semantic Values --------------------------------- In a simple program it may be sufficient to use the same data type for the semantic values of all language constructs. This was true in the RPN and infix calculator examples (*note RPN Calc::.). Bison's default is to use type `int' for all semantic values. To specify some other type, define `YYSTYPE' as a macro, like this: #define YYSTYPE double This macro definition must go in the C declarations section of the grammar file (*note Grammar Outline::.). File: bison.info, Node: Multiple Types, Next: Actions, Prev: Value Type, Up: Semantics More Than One Type for Semantic Values -------------------------------------- In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type `int' or `long', while a string constant needs type `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: * Specify the entire collection of possible data types, with the `%union' Bison declaration (*note Union Decl::.). * Choose one of those types for each symbol (terminal or nonterminal) for which semantic values are used. This is done for tokens with the `%token' Bison declaration (*note Token Decl::.) and for groupings with the `%type' Bison declaration (*note Type Decl::.). File: bison.info, Node: Actions, Next: Action Types, Prev: Multiple Types, Up: Semantics Actions ------- An action accompanies a syntactic rule and contains C code to be executed 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. It 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 (*note Mid-Rule Actions::.). The C code in an action can refer to the semantic values of the components matched by the rule with the construct `$N', which stands for the value of the Nth component. The semantic value for the grouping being constructed is `$$'. (Bison translates both of these constructs into array element references when it copies the actions into the parser file.) Here is a typical example: exp: ... | exp '+' exp { $$ = $1 + $3; } This rule constructs an `exp' from two smaller `exp' groupings connected by a plus-sign token. In the action, `$1' and `$3' refer to the semantic values of the two component `exp' groupings, which are the first and third symbols on the right hand side of the rule. The sum is stored into `$$' so that it becomes the semantic value of the addition-expression just recognized by the rule. If there were a useful semantic value associated with the `+' token, it could be referred to as `$2'. `$N' with N zero or negative is allowed for reference to tokens and groupings on the stack *before* those that match the current rule. This is a very risky practice, and to use it reliably you must be certain of the context in which the rule is applied. Here is a case in which you can use this reliably: foo: expr bar '+' expr { ... } | expr bar '-' expr { ... } ; bar: /* empty */ { previous_expr = $0; } ; As long as `bar' is used only in the fashion shown here, `$0' always refers to the `expr' which precedes `bar' in the definition of `foo'. File: bison.info, Node: Action Types, Next: Mid-Rule Actions, Prev: Actions, Up: Semantics Data Types of Values in Actions ------------------------------- If you have chosen a single data type for semantic values, the `$$' and `$N' constructs always have that data type. If you have used `%union' to specify a variety of data types, then you must declare a choice among these types for each terminal or nonterminal symbol that can have a semantic value. Then each time you use `$$' or `$N', its data type is determined by which symbol it refers to in the rule. In this example, exp: ... | exp '+' exp { $$ = $1 + $3; } `$3' and `$$' refer to instances of `exp', so they all have the data type declared for the nonterminal symbol `exp'. If `$2' were used, it would have the data type declared for the terminal symbol `'+'', whatever that might be. Alternatively, you can specify the data type when you refer to the value, by inserting `<TYPE>' after the `$' at the beginning of the reference. For example, if you have defined types as shown here: %union { int itype; double dtype; } then you can write `$<itype>1' to refer to the first subunit of the rule as an integer, or `$<dtype>1' to refer to it as a double. File: bison.info, Node: Mid-Rule Actions, Prev: Action Types, Up: Semantics Actions in Mid-Rule ------------------- Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components. A mid-rule action may refer to the components preceding it using `$N', but it may not refer to subsequent components because it is run before they are parsed. The mid-rule action itself counts as one of the components of the rule. This makes a difference when there is another action later in the same rule (and usually there is another at the end): you have to count the actions along with the symbols when working out which number N to use in `$N'. The mid-rule action can also have a semantic value. This can be set within that action by an assignment to `$$', and can referred to by later actions using `$N'. Since there is no symbol to name the action, there is no way to declare a data type for the value in advance, so you must use the `$<...>' construct to specify a data type each time you refer to this value. Here is an example from a hypothetical compiler, handling a `let' statement that looks like `let (VARIABLE) STATEMENT' and serves to create a variable named VARIABLE temporarily for the duration of STATEMENT. To parse this construct, we must put VARIABLE into the symbol table while STATEMENT is parsed, then remove it afterward. Here is how it is done: stmt: LET '(' var ')' { $<context>$ = push_context (); declare_variable ($3); } stmt { $$ = $6; pop_context ($<context>5); } As soon as `let (VARIABLE)' has been recognized, the first action is run. It saves a copy of the current semantic context (the list of accessible variables) as its semantic value, using alternative `context' in the data-type union. Then it calls `declare_variable' to add the new variable to that list. Once the first action is finished, the embedded statement `stmt' can be parsed. Note that the mid-rule action is component number 5, so the `stmt' is component number 6. After the embedded statement is parsed, its semantic value becomes the value of the entire `let'-statement. Then the semantic value from the earlier action is used to restore the prior list of variables. This removes the temporary `let'-variable from the list so that it won't appear to exist while the rest of the program is parsed. 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, can coexist in a working parser because the parser can shift the open-brace token and look at what follows before deciding whether there is a declaration or not: compound: '{' declarations statements '}' | '{' statements '}' ; But when we add a mid-rule action as follows, the rules become nonfunctional: compound: { prepare_for_local_variables (); } '{' declarations statements '}' | '{' statements '}' ; Now the parser is forced to decide whether to run the mid-rule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace token is what is called the "look-ahead" token at this time, since the parser is still deciding what to do about it. *Note Look-Ahead::.) You might think that you could correct the problem by putting identical actions into the two rules, like this: compound: { prepare_for_local_variables (); } '{' declarations statements '}' | { prepare_for_local_variables (); } '{' statements '}' ; But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.) If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this: compound: '{' { prepare_for_local_variables (); } declarations statements '}' | '{' statements '}' ; Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so. Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine: subroutine: /* empty */ { prepare_for_local_variables (); } ; compound: subroutine '{' declarations statements '}' | subroutine '{' statements '}' ; Now Bison can execute the action in the rule for `subroutine' without deciding which rule for `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. File: bison.info, Node: Declarations, Next: Multiple Parsers, Prev: Semantics, Up: Grammar File Bison Declarations ================== The "Bison declarations" section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. *Note Symbols::. All token type names (but not single-character literal tokens such as `'+'' and `'*'') must be declared. Nonterminal symbols must be declared if you need to specify which data type to use for the semantic value (*note Multiple Types::.). The first rule in the file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (*note Language and Grammar::.). * Menu: * 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. * Expect Decl:: Suppressing warnings about shift/reduce conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. * Decl Summary:: Table of all Bison declarations. File: bison.info, Node: Token Decl, Next: Precedence Decl, Prev: Declarations, Up: Declarations Declaring Token Type Names -------------------------- The basic way to declare a token type name (terminal symbol) is as follows: %token NAME Bison will convert this into a `#define' directive in the parser, so that the function `yylex' (if it is in this file) can use the name NAME to stand for this token type's code. Alternatively you can use `%left', `%right', or `%nonassoc' instead of `%token', if you wish to specify precedence. *Note Precedence Decl::. You can explicitly specify the numeric code for a token type by appending an integer value in the field immediately following the token name: %token NUM 300 It is generally best, however, to let Bison choose the numeric codes for all token types. Bison will automatically select codes that don't conflict with each other or with ASCII characters. In the event that the stack type is a union, you must augment the `%token' or other token declaration to include the data type alternative delimited by angle-brackets (*note Multiple Types::.). For example: %union { /* define stack type */ double val; symrec *tptr; } %token <val> NUM /* define token NUM and its type */ File: bison.info, Node: Precedence Decl, Next: Union Decl, Prev: Token Decl, Up: Declarations Declaring Operator Precedence ----------------------------- Use the `%left', `%right' or `%nonassoc' declaration to declare a token and specify its precedence and associativity, all at once. These are called "precedence declarations". *Note Precedence::, for general information on operator precedence. The syntax of a precedence declaration is the same as that of `%token': either %left SYMBOLS... or %left <TYPE> SYMBOLS... And indeed any of these declarations serves the purposes of `%token'. But in addition, they specify the associativity and relative precedence for all the SYMBOLS: * The associativity of an operator OP determines how repeated uses of the operator nest: whether `X OP Y OP Z' is parsed by grouping X with Y first or by grouping Y with Z first. `%left' specifies left-associativity (grouping X with Y first) and `%right' specifies right-associativity (grouping Y with Z first). `%nonassoc' specifies no associativity, which means that `X OP Y OP Z' is considered a syntax error. * The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity. When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first. File: bison.info, Node: Union Decl, Next: Type Decl, Prev: Precedence Decl, Up: Declarations Declaring the Collection of Value Types --------------------------------------- The `%union' declaration specifies the entire collection of possible data types for semantic values. The keyword `%union' is followed by a pair of braces containing the same thing that goes inside a `union' in C. For example: %union { double val; symrec *tptr; } This says that the two alternative types are `double' and `symrec *'. They are given names `val' and `tptr'; these names are used in the `%token' and `%type' declarations to pick one of the types for a terminal or nonterminal symbol (*note Type Decl::.). Note that, unlike making a `union' declaration in C, you do not write a semicolon after the closing brace. File: bison.info, Node: Type Decl, Next: Expect Decl, Prev: Union Decl, Up: Declarations Declaring Value Types of Nonterminal Symbols -------------------------------------------- When you use `%union' to specify multiple value types, you must declare the value type of each nonterminal symbol for which values are used. This is done with a `%type' declaration, like this: %type <TYPE> NONTERMINAL... Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the name given in the `%union' to the alternative that you want (*note Union Decl::.). You can give any number of nonterminal symbols in the same `%type' declaration, if they have the same value type. Use spaces to separate the symbol names. File: bison.info, Node: Expect Decl, Next: Start Decl, Prev: Type Decl, Up: Declarations Preventing Warnings about Conflicts ----------------------------------- Bison normally warns if there are any conflicts in the grammar (*note Shift/Reduce::.), but most real grammars have harmless shift/reduce conflicts which are resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress the warning about these conflicts unless the number of conflicts changes. You can do this with the `%expect' declaration. The declaration looks like this: %expect N Here N is a decimal integer. The declaration says there should be no warning if there are N shift/reduce conflicts and no reduce/reduce conflicts. The usual warning is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts. In general, using `%expect' involves these steps: * Compile your grammar without `%expect'. Use the `-v' option to get a verbose list of where the conflicts occur. Bison will also print the number of conflicts. * Check each of the conflicts to make sure that Bison's default resolution is what you really want. If not, rewrite the grammar and go back to the beginning. * Add an `%expect' declaration, copying the number N from the number which Bison printed. Now Bison will stop annoying you about the conflicts you have checked, but it will warn you again if changes in the grammer result in additional conflicts. File: bison.info, Node: Start Decl, Next: Pure Decl, Prev: Expect Decl, Up: Declarations Declaring the Start-Symbol -------------------------- Bison assumes by default that the start symbol for the grammar is the first nonterminal specified in the grammar specification section. The programmer may override this restriction with the `%start' declaration as follows: %start SYMBOL File: bison.info, Node: Pure Decl, Next: Decl Summary, Prev: Start Decl, Up: Declarations Requesting a Pure (Reentrant) Parser ------------------------------------ A "reentrant" program is one which does not alter in the course of execution; in other words, it consists entirely of "pure" (read-only) code. Reentrancy is important whenever asynchronous execution is possible; for example, a nonreentrant program may not be safe to call from a signal handler. In systems with multiple threads of control, a nonreentrant program must be called only within interlocks. The Bison parser is not normally a reentrant program, because it uses statically allocated variables for communication with `yylex'. These variables include `yylval' and `yylloc'. The Bison declaration `%pure_parser' says that you want the parser to be reentrant. It looks like this: %pure_parser The effect is that the the two communication variables become local variables in `yyparse', and a different calling convention is used for the lexical analyzer function `yylex'. *Note Pure Calling::, for the details of this. The variable `yynerrs' also becomes local in `yyparse' (*note Error Reporting::.). The convention for calling `yyparse' itself is unchanged. File: bison.info, Node: Decl Summary, Prev: Pure Decl, Up: Declarations Bison Declaration Summary ------------------------- Here is a summary of all Bison declarations: `%union' Declare the collection of data types that semantic values may have (*note Union Decl::.). `%token' Declare a terminal symbol (token type name) with no precedence or associativity specified (*note Token Decl::.). `%right' Declare a terminal symbol (token type name) that is right-associative (*note Precedence Decl::.). `%left' Declare a terminal symbol (token type name) that is left-associative (*note Precedence Decl::.). `%nonassoc' Declare a terminal symbol (token type name) that is nonassociative (using it in a way that would be associative is a syntax error) (*note Precedence Decl::.). `%type' Declare the type of semantic values for a nonterminal symbol (*note Type Decl::.). `%start' Specify the grammar's start symbol (*note Start Decl::.). `%expect' Declare the expected number of shift-reduce conflicts (*note Expect Decl::.). `%pure_parser' Request a pure (reentrant) parser program (*note Pure Decl::.).