// An optional integer value.
type OptionalInt = union {
    Some: int;
    None: unit
};

fun maybeIncrement(o: OptionalInt): OptionalInt =
    match o with {
        Some{x} -> Some{x + 1};
        None{_} -> None{()}
    };

let x: OptionalInt = maybeIncrement(Some{41});

assert(match x with {
           Some{y} -> y = 42;
           None{_} -> false
       })