class Left <L>(shared L val) { }
class Right<R>(shared R val) { }

alias Either<L, R> => Left<L>|Right<R>;

T match<L, R, T>(Either<L, R> x, T(L) l, T(R) r)
    => if (is Left<L> x) then l(x.val) else r(x.val);

print(match<String, String, String>(Right("lol"), (_) => "", (x) => x));