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5.8.2 Default Reductions

After parser table construction, Bison identifies the reduction with the largest lookahead set in each parser state. To reduce the size of the parser state, traditional Bison behavior is to remove that lookahead set and to assign that reduction to be the default parser action. Such a reduction is known as a default reduction.

Default reductions affect more than the size of the parser tables. They also affect the behavior of the parser:

For canonical LR, the only default reduction that Bison enables by default is the accept action, which appears only in the accepting state, which has no other action and is thus a defaulted state. However, the default accept action does not delay any yylex invocation or syntax error detection because the accept action ends the parse.

For LALR and IELR, Bison enables default reductions in nearly all states by default. There are only two exceptions. First, states that have a shift action on the error token do not have default reductions because delayed syntax error detection could then prevent the error token from ever being shifted in that state. However, parser state merging can cause the same effect anyway, and LAC fixes it in both cases, so future versions of Bison might drop this exception when LAC is activated. Second, GLR parsers do not record the default reduction as the action on a lookahead token for which there is a conflict. The correct action in this case is to split the parse instead.

To adjust which states have default reductions enabled, use the %define lr.default-reduction directive.

Directive: %define lr.default-reduction where

Specify the kind of states that are permitted to contain default reductions. The accepted values of where are:

  • most (default for LALR and IELR)
  • consistent
  • accepting (default for canonical LR)

(The ability to specify where default reductions are permitted is experimental. More user feedback will help to stabilize it.)

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