Factorial in Prolog

Example for versions B-Prolog 7.4 #3, gprolog 1.3.0, swipl 5.6.x

Almost identical to Poplog Prolog example, except for the syntax of compiling a file (doesn’t have a - before file name). However, the results of execution depend on the implementation. SWI-Prolog handles large numbers just fine, while in GNU Prolog and B-Prolog 12! overflows the numeric data type, so all values after 11! are incorrect.

| ?- [fact].
compiling /home/nickolas/Desktop/progopedia/prolog/fact.pl for byte code…
/home/nickolas/Desktop/progopedia/prolog/fact.pl compiled, 3 lines read — 1372 bytes written, 5 ms

yes
| ?- fact(16,X).
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
10! = 3628800
11! = 39916800
12! = -57869312
13! = -215430144
14! = 205203456
15! = -143173632
16! = -143294464

X = -143294464 ?` 

% 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).