1.5.3 GLR 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 GLR parser.

In any semantic action, you can examine yychar to determine the type of the lookahead token present at the time of the associated reduction. After checking that yychar is not set to YYEMPTY or YYEOF, you can then examine yylval and yylloc to determine the lookahead 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. See section Lookahead Tokens.

In a deferred semantic action, it's too late to influence syntax analysis. In this case, yychar, yylval, and 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 yyclearin (see section Special Features for Use in Actions) or to attempt to free memory referenced by yylval.

Another Bison feature requiring special consideration is YYERROR (see section Special Features for Use in Actions), which you can invoke in a semantic action to initiate error recovery. During deterministic GLR operation, the effect of YYERROR is the same as its effect in an LALR(1) parser. In a deferred semantic action, its effect is undefined.

Also, see Default Action for Locations, which describes a special usage of YYLLOC_DEFAULT in GLR parsers.