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From: Pereira%SRI-AI@sri-unix.UUCP
Newsgroups: net.lang.prolog
Subject: More on findall Vs. bagof
Message-ID: <12709@sri-arpa.UUCP>
Date: Fri, 14-Oct-83 11:26:56 EDT
Article-I.D.: sri-arpa.12709
Posted: Fri Oct 14 11:26:56 1983
Date-Received: Sun, 23-Oct-83 04:06:07 EDT
Lines: 23

Consider the following code:

        p(S) :- q(a,X), r(X,S).
        u(S) :- r(X,S), q(a,X).

        r(X,S) :- findall(Y,a(X,Y),S).

        q(X,X).

        a(a,1).
        a(b,2).
        a(a,3).

?- p(S). will return S=[2], whereas ?- u(S) will return S=[1,2,3].
This just because the two goals q and r where exchanged !  Is this
the kind of behavior one should expect from a logic programming
system ?  Of course not! The problem is similar to the one of using
\+P when P has unbound variables.

In contrast, if findall is replaced by bagof in the example,
both queries will return S=[2] as they should.

-- Fernando Pereira