Re: mid-rule actions

[Hans Aberg]

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At 21:10 +0300 2005/01/10, hz kto wrote:
>I have a question about mid-rule actions, here is the example from the bison
documentation:
>
>compound: '{' { prepare_for_local_variables (); }
>          declarations statements '}'
>        | '{' statements '}'
>        ;
>
>If we are parsing "declarations statements" from the first part of the rule and
find out that we need to use just "statements" will
prepare_for_local_variables() already be executed or will it run only if the
"declarations statements" match?

I think it may depend on what lookahead the LALR(1) parsing algorithm needs.
There is in no easy way to tell that. Mid-rule actions are implemented as an
empty rule with the action.

>If the former is the case, how do I undo the effects?

There is no way to undo it in Bison. In the example,  workaround may be to
always push the environment stack. I.e.,
compound: { push_stack; } '{' declarations_statements { pop_stack(); } '}'
;
declarations_statements:
    declarations statements
  | statements
;
The environment stack will then always be pushed, even if there are no local
variables declared.

  Hans Aberg