:- module implied_instance.

:- interface.

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

:- import_module io.

:- implementation.

:- import_module int, list.

:- typeclass sumable(A) where [
	pred p(A::in, int::out) is det
].

:- instance sumable(int) where [
	pred(p/2) is copy_int
].

:- instance sumable(list(T)) <= sumable(T) where [
	pred(p/2) is sum_int_list
].

main -->
	{ p(2, SumA) },
	{ p([42, 24, 1, 2, 3], SumB) },
	io__write_int(SumA),
	io__write_string("\n"),
	io__write_int(SumB),
	io__write_string("\n").

:- pred copy_int(int, int).
:- mode copy_int(in, out) is det.

copy_int(N, N).

:- pred sum_int_list(list(T), int) <= sumable(T).
:- mode sum_int_list(in, out) is det.

sum_int_list([], 0).
sum_int_list([X|Xs], Sum) :-
	p(X, SumA),
	sum_int_list(Xs, SumB),
	Sum = SumA + SumB.