MayBe Just x 에서 x 꺼내기

maybe 정의

maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing  = n
maybe _ f (Just x) = f x


maybe (Nothing일 때의 값) (처리할 함수. 인자 하나) (Maybe 값)
-- Type이 똑같이 맞아야 함

보기

-- unwrap from Maybe
let n = Just 3
-- negate n
maybe 0 negate n 
-- 2 * n 
maybe 0 (2 *) n 
-- 꺼내기
maybe 0 id n 

let s = Just "Andrea"
-- show s 
maybe "" show s 
-- "Hello " ++ s
maybe "" ("Hello " ++) s
-- 꺼내기
maybe "" id s