Re: Singleton variable in branch

Thanks!


2013/12/16 Paulo Moura <[email protected]>

>
> On 16/12/2013, at 15:58, FERNANDO SAENZ PEREZ <[email protected]> wrote:
>
> > Hello,
> > Given the following system session in SWI-Prolog 6.6.1 (Windows, stable
> > version):
> >
> > %%%%%%%%%%%%%%%%%%%%%%
> >
> > Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 6.6.1)
> > Copyright (c) 1990-2013 University of Amsterdam, VU Amsterdam
> > SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
> > and you are welcome to redistribute it under certain conditions.
> > Please visit http://www.swi-prolog.org for details.
> >
> > For help, use ?- help(Topic). or ?- apropos(Word).
> >
> > 1 ?- [p].
> > Warning: c:/fernan/research/bddeduc/des/des3.3.3/p.dl:1:
> >        Singleton variable in branch: B
> > % p.dl compiled 0.00 sec, 2 clauses
> > true.
> >
> > 2 ?- listing.
> >
> > :- thread_local thread_message_hook/3.
> > :- dynamic thread_message_hook/3.
> > :- volatile thread_message_hook/3.
> >
> >
> > p(A) :-
> >        (   functor(A, _, B)
> >        ->  (   foo(B)
> >            ->  foo(B)
> >            ;   foo(B)
> >            )
> >        ;   foo(B)
> >        ).
> > true.
> >
> > %%%%%%%%%%%%%%%%%%%%%%
> >
> > I do not understand the warning. I think that B is used in all branches,
> > isn't it?
>
> That's not the issue. If functor/3 fails, B is a singleton variable. If
> you take into account a failure of functor/3, your code becomes equivalent
> to:
>
> p(A) :-
>        (   fail
>        ;   foo(B)
>        ).
>
> > In addition, it is said that 2 clauses have been compiled. Is it so? The
> > source file includes the very same clause as listed.
>
> My only explanation involves parallel universes :-) But maybe Jan can shed
> some light here.
>
> Cheers,
>
> Paulo
>
> -----------------------------------------------------------------
> Paulo Moura
> Logtalk developer
>
> Email: <mailto:[email protected]>
> Web:   <http://logtalk.org/>
> -----------------------------------------------------------------
>
>
>
>
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