Maybe.ds

module Data.Maybe
export  { isNothing; isJust; fromMaybe }
where


-- | A `Maybe` may contain a value, or not.
data Maybe (a: Data) where
        Nothing : Maybe a
        Just    : a -> Maybe a


-- | Check if the given value is a `Nothing`.
isNothing (m: Maybe a): Bool#
 = case m of
        Nothing -> True
        Just x  -> False


-- | Check if the given value is a `Just`.
isJust (m: Maybe a): Bool#
 = case m of
        Nothing -> False
        Just x  -> True


-- | Take the value from a `Just`, or return a default value.
fromMaybe (def: a) (m: Maybe a): a
 = case m of
        Nothing -> def
        Just x  -> x


-- | Apply a function to the value in a `Just`, or return a default value.
maybe (def: b) (f: a -> b) (m: Maybe a): b
 = case m of
        Nothing -> def
        Just x  -> f x