Re: [eclipse-clp-users] Backtracking over current predicate only

From: Matthew Skala <mskala_at_...203...>
Date: Sat, 15 May 2010 13:50:21 -0400 (EDT)
On Fri, 14 May 2010, Christian Wirth wrote:
> I'm currently using pred(X) :-
> functor(X,F,A),not(F==aPred),anotherCall(X), but i want to replace this
> because speedup is very essential.

It's probably best that you use a cut, as Joachim suggested.  However,
it's also worth pointing out that there's a faster way to do the "X is not
of the form aPred(_)" test:  instead of functor(X,F,A),not(F==aPred) you
could write:

   X \= aPred(_)

That is not exactly the same as functor(X,F,A),not(F==aPred) because it
only matches aPred(_) (with arity one), not (for instance) aPred(_,_)
(with arity two).  But it'll be faster.  If you wanted to really match all
things that don't have the functor aPred, regardless of arity, you could
do this:

   X=..Y, Y \= [aPred|_]

That will be slower than the arity-one version, but probably faster than
your code that uses functor/3.  Joachim's suggestion using cut should be
faster than either of them - and it'll continue to work even  if you add
additional clauses later and want to exclude those too.
-- 
Matthew Skala, postdoctoral researcher, Universities of Toronto and Waterloo
mskala@...203...    mskala@...205...    mskala@...206...
Received on Sat May 15 2010 - 17:50:19 CEST

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