Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!rutgers!psuvax1!psuvm!f0o
From: F0O@psuvm.psu.edu
Newsgroups: comp.lang.prolog
Subject: Finding all solutions in Prolog
Message-ID: <90216.112514F0O@psuvm.psu.edu>
Date: 4 Aug 90 15:25:14 GMT
Organization: Penn State University
Lines: 31


     I have a fairly simple question, but yet have never ran across any
book that explains it.

     If you have a simple PDC prolog program like:

          predicates
               person(symbol)

          clauses
               person(jack).
               person(jill).
               person(mary).

 and from the goal prompt type: person(X), you will get all 3 persons.
But if you have an internal goal like:
         person(X), write(X),nl.

prolog will only find the first person.  You would have to use the fail
predicate to force Prolog to find all solutions.

     Why does Prolog only find the first solution on internal goals?  I'd
think finding all solutions to a query would be more common than only
finding one.
     I can see that only finding one solution and then using the fail
predicate gives you control over the search, however you could also use
the cut if you wanted to limit the search.

     Am I missing something fundamental here?

                                                    [Tim]