Re: [Yap-users] Forall/2 Problem

[Eyal D. <eya...@gm...>]

HI Ed, there are all sorts of reasons this could print 'why?'. Its hard to
say without knowing more about what your code does.  If the code is pure,
I'm not sure why this would happen, but otherwise it's not hard to come up
with examples. For example:

 ?- [user].
 % consulting user_input...
c(X) :- nonvar(X), !, fail.
|     c(1).
|     c(2).
|      % consulted user_input in module user, 1 msec 0 bytes
true.
 ?- forall(c(X), (c(X); writeln(why))).
why
why
true.

All any use of mutable variables, set_arg, asserta, or similar things can
cause this. Not sure about tabling.

Eyal

On Mon, Feb 22, 2016 at 3:28 PM, Edward Schwartz <esc...@ce...> wrote:

> Hi all,
>
> I am encountering another strange problem.  It's difficult to reproduce
> in a small example, but should the following rule ever print "WHY?":
>
> testcase :-
>     forall(foo(I, F, P, O, T, E, M),
>             (foo(I, F, P, O, T, E, M);
>             writeln('why?'))).
>
> Is it possible this is a bug relating to tabling (foo is tabled)?  How
> can I debug this?
>
> Thanks,
>
> Ed
>
>
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