Re: How to get a set response interactively

On 9/04/2014, at 1:16 PM, Ross Boylan wrote:

> I tried 
> ?- setof(OS, mount(_, path([OS | _]), _W), Mounts).

Right there I see an obvious problem:  you have a wild-card
in the generator.  More than one, in fact.

If you have a 'don't-care' variable in a generator,
you MUST explicitly existentially quantify it.  And
this requires having at least two occurrences of the
variable: where it's needed and also in the quantifier.

This has absolutely nothing to do with interactive use
of setof/3.  What you need to write is

?- setof(OS, U^V^W^mount(U, path([OS|V]), W), Mounts).

Better still, create an auxiliary predicate:

mount_os(OS) :-
    mount(_, path([OS|_]), _).

and then call

?- setof(OS, mount_os(OS), Mounts).


Jan, SWI Prolog has made so many changes lately to make life
easier for some programmers; is there any chance of making it
complain loudly about single-occurrence variables in setof/3
or bagof/3 generators?