Question

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

Answer 1

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.