Cut

This section is quite advanced. Only go on if you feel really comfortable with the rest of todays exercises.

Given the following knowledge base, which answers will Prolog give when asked the query wizard(X)?

wizard(harry).
wizard(ron).
wizard(hermione).

(Download this knowledge base.)

Now, consider the following variants of this knowledge base.

Variant 1:

wizard(harry) :- !.
wizard(ron).
wizard(hermione).

(Download this knowledge base.)

Variant 2:

wizard(harry).
wizard(ron) :- !.
wizard(hermione).

(Download this knowledge base.)

Variant 3:

wizard(harry).
wizard(ron).
wizard(hermione) :- !.

(Download this knowledge base.)

Type them in and test how Prolog answers to the query wizard(X).

As you have seen the exclamation mark has an effect on the number of answers that Prolog gets. The exclamation mark is called cut. It is a built-in predicate which always succeeds, but which has important side effects: it restricts backtracking. More specifically, when Prolog encounters a cut in a rule such as

q :- p1,...,pn,!,r1,...,rm,

then it commits itself to using this particular clause for predicate q and it commits itself to all choices made when evalauting p1,...,pn. That is, backtracking will not consider alternative ways of proving the goals p1,...,pn nor will it consider alternative ways of proving q, even if it turns out later that the clause cannot be proved. Backtracking is allowed though to check for alternatives of the goals r1,...,rm and to check for alternatives of the goals that were considered before the predicate q was called.

Here are again several variants of the same knowledge base. Pose the query p(X,Y) and try to understand why Prolog answers the way it does. Do a trace.

Variant 1:

q(a).
q(b).

r(1).
r(2).
r(3).

p(X,Y) :- q(X), r(Y).

(Download knowledge base.)

Variant 2:

q(a).
q(b).

r(1).
r(2).
r(3).

p(X,Y) :- !, q(X), r(Y).

(Download knowledge base.)

Variant 3:

q(a).
q(b).

r(1).
r(2).
r(3).

p(X,Y) :- q(X), !, r(Y).

(Download knowledge base.)

Variant 4:

q(a).
q(b).

r(1).
r(2).
r(3).

p(X,Y) :- q(X), r(Y), !.

(Download knowledge base.)

You can find more about cuts in Chapter 10 of Learn Prolog Now!.

Back to the practical session for day 2.