Another important efficiency feature of SICStus Prolog is last call optimization. This is a space optimization technique, which applies when a predicate is determinate at the point where it is about to call the last goal in the body of a clause. For example,
% for(Int, Lower, Upper)
% Lower and Upper should be integers such that Lower =< Upper.
% Int should be uninstantiated; it will be bound successively on
% backtracking to Lower, Lower+1, ... Upper.
for(Int, Int, _Upper).
for(Int, Lower, Upper) :-
Lower < Upper,
Next is Lower + 1,
for(Int, Next, Upper).
This predicate is determinate at the point where the
recursive call is about to be made, since this is the last clause
and the preceding goals (<)/2 and is/2) are
determinate. Thus last call optimization can be applied;
effectively, the stack space being used for the current predicate
call is reclaimed before the recursive call is made. This means that
this predicate uses only a constant amount of space, no matter how
deep the recursion.