The Cut

The cut commits to the choices made since the parent goal was called:

13.1 What ! does

The cut commits to the choices made since the parent goal was called:

No other clauses of the current predicate are tried.

No alternative solutions for goals to the left of ! in the same clause.

max(X, Y, X) :- X >= Y, !.
max(_, Y, Y).

?- max(5, 3, M).    % M = 5 (no choice point left)

13.2 Green vs red cuts

Green cut: removes only redundant choice points; meaning is unchanged if removed.

Red cut: changes the meaning; removing it gives wrong results.

Safer version:

sign(X, pos) :- X > 0, !.
sign(X, zero) :- X =:= 0, !.
sign(X, neg) :- X < 0.          % all conditions explicit → green

max(X, Y, X) :- X >= Y, !.
max(_, Y, Y).                   % red: relies on the cut
?- max(5, 3, 3).                % TRUE — wrong! Head unification happens before the cut

max(X, Y, Z) :- ( X >= Y -> Z = X ; Z = Y ).

13.3 Cut-fail combination

(This is how \+ can be defined: \+ G :- call(G), !, fail. \+ _.)

different(X, X) :- !, fail.
different(_, _).

13.4 When to use the cut

To express determinism (once a clause matches, stop).

To improve efficiency by removing useless choice points.

Prefer if-then-else, once/1, memberchk/2, or pure alternatives (dif/2, if_/3, CLP(FD) reification) where possible — they're clearer and keep predicates more general.