:- module bug271.

:- interface.

:- import_module string.

:- typeclass stringable(X) where [
    func to_string(X) = string
].

:- type a_or_b
    --->    a
    ;       b.

:- some [Y] pred p(a_or_b::in, Y::out) is det => stringable(Y).

:- implementation.

:- import_module int.
:- import_module list.
:- import_module io.
:- import_module string.

p(a, 13).
p(b, 14).

:- instance stringable(int) where [
    (to_string(Int) = format("int %d", [i(Int)]))
].