Re: Print expected tokens in yyerror function.

[Hans Aberg]

On 29 Mar 2008, at 12:50, Claudio Saavedra wrote:
> > It prints something like that (or used to), but the LALR(1) that
> > Bison uses compacts the statements in a way that when an error token
> > appears in the input, additional reduction can be performed. So to
> > get a fully correct error handling, including tokens, LR without this
> > type of compaction, or some search algorithm would be needed.
>
> The Burke-Fisher technique uses a deferred parsing mechanism to  
> simulate
> a limited degree of unparsing. With this, errors, that are detected
> after some reductions and shifts take place, can still be detected and
> properly diagnosed and fixed.
>
> This do works with Bison's LALR algorithm and table compactation. I  
> have
> a bison skeleton that works this way, and I'll be publishing it pretty
> soon.

The problem is that after an error token has arrived in the input,  
additional reductions can take place before an error is issued by the  
parser. So, although errors are detected, they are issued at a point  
hampering good error recovery. Then also, the set of lookahead tokens  
are not in the current state, but may be in a state after some  
reductions has been done.

  Hans Aberg