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prologbehaviorclauseprolog-cut

Prolog cut operator behaviour

I have these clauses:

a(1).
a(2).
b(a).
c(A,B,C) :- a(A),d(B,C).
c(A,B,C) :- b(A),d(B,C).
d(B,C) :- a(B),!,a(C).
d(B,_) :- b(B).

When I run the query c(X,Y,Z) the answers are:

X = 1, Y = 1, Z = 1 ;
X = 1, Y = 1, Z = 2 ;
X = 2, Y = 1, Z = 1 ;
X = 2, Y = 1, Z = 2 ;
X = a, Y = 1, Z = 1 ;
X = a, Y = 1, Z = 2.

So basically, the cut operator (in here d(B,C) :- a(B),!,a(C).) ignores the most recent choice points, i.e. it does not do a further search for d() and a(). I though that the cut ignores ALL previous choice points and won't do any backtracking.

Can someone explain the exact behavior and why am I wrong?

Solution

  • I did some reading and the cut is working as follows:

    1. Kills off the parent choice-point
2. Commits to all the choices made going through the rule

    Thus : 

    1. d(B,_) :- b(B). is not explored
2. B in d(B,C) :- a(B),!,a(C). is irrevocably bound to 1.