Re: Singleton variable in branch: right warning message?

Thank you for the explanation, Michael.  It makes sense now.

> Date: Mon, 17 Mar 2014 08:43:07 -0600
> From: [email protected]
> To: [email protected]
> CC: [email protected]
> Subject: Re: [SWIPL] Singleton variable in branch: right warning message?
> 
> On Mon, Mar 17, 2014 at 6:32 AM, Parker Jones <[email protected]> wrote:
> 
> > This code gives the warning "Singleton variable in branch: A", but
> > shouldn't it be a regular singleton warning?
> >
> >
> > p(X) :-
> >     (X > 0 ->
> >         A=1
> >         ;
> >         A=0
> >     ).
> 
> 
> It's not a regular singleton warning because the variable A appears twice
> in this clause.  This code has two branches: X > 0 and X =< 0.  Simplifying
> you have roughly:
> 
> P(X) :- A=1. % when X > 0
> P(X) :- A=0. % when X =< 0
> 
> You can see that A is a singleton (appears only once) in each of those
> clauses (each representing a branch).
> 
> By adding write(A), you add a second occurrence of the variable A to each
> branch, so it's no longer a singleton.
> 
> -- 
> Michael
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