:- module permutations2.
:- interface.

:- import_module io.

:- pred main(io::di, io::uo) is det.


:- import_module list.
:- import_module set_ordlist.
:- import_module set.
:- import_module solutions.

%% permutationSet(List, Set) is true if List is a permutation of Set:
:- pred permutationSet(list(A)::out,set(A)::in) is nondet.

%% Two ways to compute all permutations of a given list (using backtracking):
:- func all_permutations1(list(int))=set_ordlist.set_ordlist(list(int)).
:- func all_permutations2(list(int))=set_ordlist.set_ordlist(list(int)).

:- implementation.


permutationSet([],set.init).
permutationSet([H|T], S) :- set.member(H,S), permutationSet(T,set.delete(S,H)).

all_permutations1(L) =
    solutions_set(pred(X::out) is nondet:-permutationSet(X,set.from_list(L))).

%%Alternatively, using the imported list.perm predicate:
all_permutations2(L) =
    solutions_set(pred(X::out) is nondet:-perm(L,X)).

main(!IO) :-
    print(all_permutations1([1,2,3,4]),!IO),
    nl(!IO),
    print(all_permutations2([1,2,3,4]),!IO).