Question

I’m trying to generate multiple permutations using this code:

:- use_module(library(clpfd)).

p(N, Indexes) :-
    M in 1..N,
    M #=< N,
    length(Indexes, M),
    Indexes ins 1..N.

It returns me all the results, but in the end crashes with ERROR: Out of global stack

Answer 1

What happens in your code is that the length of the list is implicitly set by calling length/2. This will start with a list of length 0, which binds M to 0, which in turn wakes up the constraint M in 1..N, which fails. Next it returns a list of length 1, which succeeds, then on backtracking a list of length 2, which succeeds again. After that, any further backtracking into length/2 will return longer and longer lists, but waking M in 1..N will always fail until the list becomes so large that you run out of memory.

What you need to do is place the choicepoint before the length/2 call instead of inside of it by, e.g., replacing

M #=< N,

(which is a redundant constraint anyway) with

indomain(M),

This gives you:

[debug] [1]  ?- p(2,I).
I = [_G3025],
_G3025 in 1..2 ;
I = [_G3102, _G3105],
_G3102 in 1..2,
_G3105 in 1..2.

[debug] [1]  ?-