Prolog Termination of Generated List

Question

N #>= 0, N #< 3, length(Ls, N), false.

The expression above does not terminate when posted on the swi prolog terminal.

I have tried exchanging the order of goals.

length(Ls, N), N #>= 0, N #< 3,  false.

and

length(Ls, N), N >= 0, N < 3, false.

I am using SWI-Prolog version 8.4.3 for x86_64-linux

See [this](https://stackoverflow.com/a/32478496/772868) for a definition that does terminate in that case. — false, Sep 30 '22 at 17:37

score 1 · Answer 1 · answered Sep 30 '22 at 09:23

1

N is still var at that point. Use instead (if you really want to use clpfd - I'm not seeing a reason):

?- N #>= 0, N #< 3, label([N]), length(Ls, N), false.
false.

answered Sep 30 '22 at 09:23

brebs

Thanks. The version without clpfd still does not terminate. `?- length(Ls, N), N<10, false.` Sorry for the noob question, I am still trying to understand the conditions for termination in prolog. – Edgar Sep 30 '22 at 09:41
1

That's because `length` would be incrementing N into infinity. Use e.g. `between(1, 9, N)` *before* `length`. – brebs Sep 30 '22 at 09:51
Great. `between` will work for me – Edgar Sep 30 '22 at 10:00

score 0 · Answer 2 · answered Sep 30 '22 at 14:02

0

length(Ls, N), N >= 0, N < 3, false.

This will count up N trying different values until all the conditions are true, but false is never true.

So this is "count until false is true", which is "count forever", an infinite loop.

answered Sep 30 '22 at 14:02

1

Don't blame it on `false`! `length(Ls,N)` alone does not terminate after all. – false Sep 30 '22 at 17:35

2 Answers2