Solving bison conflict over 2nd lookahead -

i'm writing parser project , got stuck on issue. here's self contained example of problem:

%error-verbose  %token id %token var %token end_var %token constant %token @ %token value %%  unit: regular_var_decl     | direct_var_decl;  regular_var_decl: var constant_opt id ':' value ';' end_var;  constant_opt: /* empty */ | constant;  direct_var_decl: var id @ value ':' value ';' end_var;  %% #include <stdlib.h> #include <stdio.h>  yylex() {   static int = 0;    static int tokens[] = {     var,       id, ':', value, ';',     end_var,     0   };    return tokens[i++]; };  yyerror(str) char *str; {   printf("fail: %s\n", str); };  main() {   yyparse();   return 0; }; 

one build bison test.y && cc test.tab.c && ./a.out.

it warns me constant_opt useless due conflicts.

this ambiguity solved using lalr(2), since after id find ':' or at... how solve issue on bison?

a simple solution not abbreviate optional constant:

regular_var_decl:  var id ':' value ';' end_var; constant_var_decl: var constant id ':' value ';' end_var; direct_var_decl:   var id @ value ':' value ';' end_var; 

that allows reduction decision deferred until enough information known. (you factor ':' value ';' end_var non-terminal if useful.)

another possibility leave grammar was, , ask bison produce glr parser (%glr-parser). glr parser retain 2 (or more) parallel parses until ambiguity can resolved, should fix constant_opt problem. (note shift/reduce conflicts still reported bison; cannot tell whether language ambiguous until ambiguous sentence discovered @ runtime.) of time, no additional change grammar needs made, slow parse down little bit.

a final possibility, less useful here, accept superset of language , issue error message in action:

var_decl: var const_opt id at_value_opt ':' value ';' end_var {    if (/* pseudocode */ $2 && $4) { /* flag syntax error */ } } 

that depends on 2 opt terminals returning semantic value can interrogated somehow empty.