How Can I repeat the list in swi prolog?

Question

How Can I repeat the list in swi prolog?

{ex: repeat(X,Y,N) to be true when Y is the list consisting of each element of X repeated N times (eg repeat([a,b], [a,a,a,b,b,b],3) is true). }

Answer 1

In haskell ...

rep ls n = [i | i<- ls, _ <- [1 .. n]] main = print $ rep ["1","2"] 3

In prolog

:- forall(I,between(1,3),Ls),Ls=[1,2,3].

Answer 2

The trick here is to have nested iterations - iteration over N..1 nested into the iteration over the elements of the original list. This works in SWI-Prolog: repeat_list([],[],_) :- !.

repeat_list(_,[],0) :- !.

repeat_list(X,Y,N) :-
    N > 0,
    repeat_list(N,N,X,Y),
    !.

repeat_list(_,_,[],[]) :- !.

repeat_list(1,N,[H|List1],[H|List2]) :-
    repeat_list(N,N,List1,List2).

repeat_list(M,N,[H|List1],[H|List2]) :-
    L is M - 1,
    repeat_list(L,N,[H|List1],List2).

NB Try not to use standard clause names such as repeat for purposes different from what standard intends to use it for.