Lazy lists in Prolog?

Question

Is it possible to have lazy lists in Prolog? Something like the following:

ones([1 | Y]) :- ones(Y).

Although this obviously doesn\'t work

Answer 1

Yes, it's possible to have lazy lists in Prolog. Here's an infinite, lazy list of ones using freeze/2.

ones(L) :-
  freeze(L, (L=[1|T],ones(T)) ).

Using it at the top-level looks like this:

?- ones(Ones), [A,B,C|_] = Ones.
A = B = C = 1.

You might also enjoy the list_util pack (for SWI-Prolog) which contains several lazy list predicates.