I am quite rusty in prolog, but I am not sure why things like this fail:
frack(3).
frack(X) :- frack(X-1).
So, if I evaluate frack(4). from the interactive prompt with the above facts defined, I expect that it should not have to endlessly recurse, since 4-1 = 3. But I get this error in SWI-Prolog:
ERROR: Out of global stack
Try it:
?- 4-1 = 3.
false.
Why? Because 4-1 = -(4, 1)
, which clearly is not a number but a compound term.
To reason about integers in Prolog, use clpfd constraints, for example (using GNU Prolog or B-Prolog):
| ?- 4-1 #= X. X = 3
In SWI-Prolog, the graphical tracer may be useful for you to see what happens:
?- gtrace, frack(4).
For more complex debugging, I recommend failure-slice as shown in false's answer.