1.5.4 Controlling a Parse with Arbitrary Predicates

In addition to the %dprec and %merge directives, GLR parsers allow you to reject parses on the basis of arbitrary computations executed in user code, without having Bison treat this rejection as an error if there are alternative parses. For example,

widget:
  %?{  new_syntax } "widget" id new_args  { $$ = f($3, $4); }
| %?{ !new_syntax } "widget" id old_args  { $$ = f($3, $4); }
;

is one way to allow the same parser to handle two different syntaxes for widgets. The clause preceded by %? is treated like an ordinary midrule action, except that its text is handled as an expression and is always evaluated immediately (even when in nondeterministic mode). If the expression yields 0 (false), the clause is treated as a syntax error, which, in a nondeterministic parser, causes the stack in which it is reduced to die. In a deterministic parser, it acts like YYERROR.

As the example shows, predicates otherwise look like semantic actions, and therefore you must take them into account when determining the numbers to use for denoting the semantic values of right-hand side symbols. Predicate actions, however, have no defined value, and may not be given labels.

There is a subtle difference between semantic predicates and ordinary actions in nondeterministic mode, since the latter are deferred. For example, we could try to rewrite the previous example as

widget:
  { if (!new_syntax) YYERROR; }
    "widget" id new_args  { $$ = f($3, $4); }
|  { if (new_syntax) YYERROR; }
    "widget" id old_args  { $$ = f($3, $4); }
;

(reversing the sense of the predicate tests to cause an error when they are false). However, this does not have the same effect if new_args and old_args have overlapping syntax. Since the midrule actions testing new_syntax are deferred, a GLR parser first encounters the unresolved ambiguous reduction for cases where new_args and old_args recognize the same string before performing the tests of new_syntax. It therefore reports an error.

Finally, be careful in writing predicates: deferred actions have not been evaluated, so that using them in a predicate will have undefined effects.