Prologue: a bug from the global stack with what looks like ONE recursion level for me

Question

I'm pretty rusty in the prologue, but I'm not sure why such things do not work:

frack(3). frack(X) :- frack(X-1). 

So, if I rate frack (4). from an interactive invitation with the above facts, I expect that it will not have to be infinitely recursive, since 4-1 = 3. But I get this error in SWI-Prolog:

 ERROR: Out of global stack 

Answer 1

Try:

 ?- 4-1 = 3. false. 

Why? Because 4-1 = -(4, 1) , which is clearly not a number, but a composite term.

To learn about integers in Prolog, use clpfd , for example (using GNU Prolog or B-Prolog):

  |  ? - 4-1 # = X.

 X = 3

In SWI-Prolog, a graphical tracer can be useful for you to see what happens:

 ?- gtrace, frack(4). 

For more complex debugging, I recommend failure-slice , as shown in the false answer .

Answer 2

This is the reason for this failure. Your request does not end because there is a failure-slice of your program that does not end:

  ? - frack (4).

 frack (3): - false .
 frack (X): -
    frack (X-1), false .

You can fix this only by changing something in the visible part. Three SO answers suggest using (is)/2 . But this will not remove the default! In fact, using (is)/2 results in essentially the same fragment:

  ? - frack (4).

 frack (3): - false .
 frack (X): -
    Y is X - 1,
    frack (Y), false .

At least frack(4) now doing well, but it will loop on the back track. You must change something in the visible part, for example, an X test to avoid failure. See failure-slice for more details.

Answer 3

 frack(X) :- frack(X-1). 

it should be

 frack(X) :- Y is X - 1, frack(Y). 

As you wrote it, an X-1 expression X-1 first level is combined with a variable X at the next level, never going over to the fact of frack(3) .

Answer 4

Prolog does not do arithmetic unless you use the is statement:

 frack(X) :- X1 is X-1, frack(X1).