lookup 関数は Maybe モナドを返す

lookup 関数は Maybe モナドを返す

Prelude> :t lookup

lookup :: (Eq a) => a -> [(a, b)] -> Maybe b

Prelude> let taro = ("taro",[("sex","male"),("address","chiba")])

Prelude> let jiro = ("jiro",[("sex","male"),("address","tokyo")])

Prelude> let person = [taro,jiro]

Prelude> person

[("taro",[("sex","male"),("address","chiba")]),("jiro",[("sex","male"),("address","tokyo")])]

Just 型の値 または Nothing を返す。

Prelude> lookup "taro" person

Just [("sex","male"),("address","chiba")]

Prelude> lookup "saburo" person

Nothing

>>= で連結できる。

Prelude> lookup "taro" person >>= lookup "address"

Just "chiba"

Prelude> lookup "jiro" person >>= lookup "address"

Just "tokyo"

Prelude> lookup "saburo" person >>= lookup "address"

Nothing

Prelude> lookup "jiro" person >>= lookup "age"

Nothing

 

Prelude> return (lookup "jiro" person) >>= print

Just [("sex","male"),("address","tokyo")]

Prelude> return (lookup "saburo" person) >>= print

Nothing