Data commit

Task/Permutations/Mercury/permutations.mercury

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+:- 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).