:- module evenodd.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module list, string.

:- type coin
    --->    heads
    ;       tails.

:- inst heads for coin/0
    --->    heads.
:- inst tails for coin/0
    --->    tails.

main(!IO) :-
    (if heads(heads, N) then
        print_heads(N, !IO)
    else true).

:- pred print_heads(coin::in(heads), io::di, io::uo) is det.
print_heads(_, !IO) :-
    io.write_string("heads\n", !IO).

:- pred print_tails(coin::in(tails), io::di, io::uo) is det.
print_tails(_, !IO) :-
    io.write_string("tails\n", !IO).

:- pragma foreign_export_enum("C", coin/0, [prefix("COIN_"), uppercase]).

:- pred heads(coin::in, coin::out(heads)) is semidet.
:- pragma foreign_proc("C",
    heads(N::in, M::out(heads)),
    [promise_pure, will_not_call_mercury],
"
    M = N;
    SUCCESS_INDICATOR = N == COIN_HEADS;
").