[mercury-users] inst of list in non-empty branch

[Ian MacLarty]

On Thu, Mar 29, 2007 at 02:35:10PM -0400, doug.auclair at logicaltypes.com wrote:
> Dear Ralph (and all), you and I wrote:
> 
> >> print_kids(Element, Kids@[_|_], !IO) :-
> >>          format("The child element%s of ", [s(singular_plural(Kids))], !IO),
> >>          print(Element, !IO),
> >>          is_are(Kids, !IO),
> >>          print(Kids, !IO),
> >>          nl(!IO).
> 
> >The mode for format/4 is pred(in, in, di, uo), so while the compiler
> >knows that Kids has an inst matching non_empty_list before the call to
> >format, it only knows it has inst ground afterwards.
> 
> Okay.  I think, however, that the mode for Kids is being set to just 'in'
> anyway, because when I reduce the clause to:
> 
> print_kids(_, Kids@[_|_], !_IO) :-
>         TheS = singular_plural(Kids).
> 

I think that:

print_kids(_, Kids, !IO) :-
    Kids = [_ | _],
    TheS = singular_plural(Kids).

will do the trick.

Ian.
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