Re: [eclipse-clp-users] testing NOT existence

On Thu, 15 Apr 2010, david wrote:
> For example, assume foo/1 is a dynamic predicate, and that there are
> rules such as:
>
>    foo(X) :- goo(X).
>
> Is there a way to test if there are any assertions of foo/1? One would
> want to avoid any "false positives" due to, say, goo/1 being satisfied.

If you don't care about side effects of goo/1 and just want to know
whether foo(X) succeeds with a specific, not general, value of X, then you
could do this:

specific_foo(X):-
   foo(Y),Y==X.

Note that specific_foo(bar) will succeed with foo(X):-goo(X). goo(bar).

If you want to test for the existence of a clause with exactly foo(bar) as
its head, not any variable, you could do this:

head_foo(X):-
   clause(foo(Y):-_),Y==X.

Note that head_foo(bar) will succeed with foo(bar):-fail.  It *only* looks
at the head.

If you want to test explicitly for a fact with no body, you could do this:

fact_foo(X):-
   clause(foo(Y):-true),Y==X.

That succeeds with foo(bar) but not with foo(bar):-goo(bar).

It should be easy to imagine other variations of using clause/1 and ==/2
depending on just what you want to query for.  Note that all of these
break the logic programming model and you probably should think hard about
whether what you're attempting to do is *really* a good idea.
-- 
Matthew Skala, postdoctoral researcher, Universities of Toronto and Waterloo
mskala_at_cs.toronto.edu    mskala_at_cs.uwaterloo.ca    mskala_at_ansuz.sooke.bc.ca