Re: When %dprec and %merge aren't enough...

[Hans Aberg]

On 14 Nov 2008, at 19:35, Marcel Laverdet wrote:

> This is a pretty good idea, but I'm not sure if it's possible. For  
> instance in the case of an if statement I modified my grammar as such:
>
> if_statement:
>     t_IF t_LPAREN expression t_RPAREN {yyexpect_statement;}  
> statement t_ELSE statement
> <...>
> |   t_IF t_LPAREN expression t_RPAREN {yyexpect_statement;}  
> statement %prec p_IF
> <...>
> ;
>
> Where yyexpect_statement is a macro I defined to set a global flag  
> and then the lexer checks and returns an appropriate t_LCURLY for  
> statements. The problem I'm seeing here is that the glr parser  
> defers my mid-action rule so the lexer can't adjust in time.

Indeed, for that reason, those two methods do not mix well. There has  
been talk about admitting GLR actions that are executed immediately,  
but don't hold your breath.

So you need the parts with the context switches being unambiguous. In  
addition, lookahead is unpredictable, so typically context switches  
should be put close to tokens. But one can go quite far with this  
technique: one has been able to make C++ into a (non-GLR) LALR(1)  
grammar this way.
  http://www.parashift.com/c++-faq-lite/compiler- 
dependencies.html#faq-38.11

  Hans