Factorial in Prolog

Example for versions Poplog 15.5 (Prolog)

This example consists of two parts — the first part of code should be stored in a file fact.pl placed in working folder of Poplog, and the second one has to be entered manually. [-fact]. downloads facts and rules from this file to current Prolog session (and outputs fact reconsulted to note that download succeeded). Query fact(16,X). tries to find value of X for which this predicate will evaluate to true. The output required in the example is side effect of query evaluation, and the actual result will be X = 20922789888000 ?. This means that if you’re not satisfied with this binding, you can reject it (by entering ;), and the search for better binding will continue. 

% fact.pl
fact(X, F) :- 
    ( X=0, F=1; 
      Y is X-1, fact(Y, Z), F is X*Z), 
    write(X), write('! = '), write(F), nl.

% interactive
[-fact].
fact(16,X).