class Applicative f => Alternative f where
    -- | The identity of '<|>'
    empty :: f a
    -- | An associative binary operation
    (<|>) :: f a -> f a -> f a

    -- | One or more.
    some :: f a -> f [a]
    some v = some_v
      where
        many_v = some_v <|> pure []
        some_v = (fmap (:) v) <*> many_v

    -- | Zero or more.
    many :: f a -> f [a]
    many v = many_v
      where
        many_v = some_v <|> pure []
        some_v = (fmap (:) v) <*> many_v

と同じ要領で、左がNothingのときだけ右を評価する

import Control.Applicative
main = do
    print $ Just 1 <|> Nothing -- 左のみ
    print $ Nothing <|> Just 2 -- 左→右

Alternative型クラスを実装している型