Re: cex: unifying counterexamples that are not unified

Hey Akim,

I think the problem here is the extra A was derived to be null during the
search. Either the derivation tree is missing information, or there's
something wrong with the printing code. I'm looking into it.

~Vincent

On Fri, Jun 19, 2020 at 2:10 AM Akim Demaille <[email protected]>
wrote:

> Hi Vincent,
>
> There is something not quite right in the current implementation.  On the
> following grammar, taken from the test suite:
>
> $ cat bar.y
> %left b
> %right c
> %%
> S: B C | C B;
> A: B  | C  | %empty;
> B: A b A;
> C: A c A;
>
> bison thinks it found two unifying examples, but the derivations actually
> yield two different sentential forms:
>
> $ bison bar.y -Wcex
> bar.y: avertissement: 4 conflits par réduction/réduction [-Wconflicts-rr]
> Reduce/reduce conflict on tokens b, c:
>   Example              B • b A A c A
>   First derivation     S ::=[ B ::=[ A ::=[ B • ] b A ] C ::=[ A c A ] ]
>   Example              B • b A c A
>   Second derivation    S ::=[ B C ::=[ A ::=[ B ::=[ A ::=[ • ] b A ] ] c
> A ] ]
>
> Reduce/reduce conflict on tokens b, c:
>   Example              C • c A A b A
>   First derivation     S ::=[ C ::=[ A ::=[ C • ] c A ] B ::=[ A b A ] ]
>   Example              C • c A b A
>   Second derivation    S ::=[ C B ::=[ A ::=[ C ::=[ A ::=[ • ] c A ] ] b
> A ] ]
>
> The derivations are difficult to read.  They look as follows (hoping
> nothing will wreck the indentation):
>
> > S
> > |
> > B       C
> > |       |
> > A b A   A c A
> > |
> > B •
>
>
>
> > S
> > |
> > B  C
> >    |
> >    A c A
> >    |
> >    B
> >    |
> >    A b A
> >    |
> >    •
> >
>
>
>
> The code appears to believe the example is unifying because both
> derivations share the same prefix up to the dot, and both comply with
> parse_state_derivation_completed, whose meaning is unclear to me (could you
> please add comments to these routines?).
>
> But here it seems insufficient to dub this as unifying.
>
> Cheers!