// 표준 라이브러리: Result.

pub fn is_ok[a, err](r: Result[a, err]) -> Bool

pub fn map[a, b, err, e: effects](
    r: Result[a, err],
    f: fn(a) effects e -> b,
) effects e -> Result[b, err]

pub fn map_err[a, err, err2, e: effects](
    r: Result[a, err],
    f: fn(err) effects e -> err2,
) effects e -> Result[a, err2]

pub fn unwrap_or[a, err](r: Result[a, err], fallback: a) -> a