haskell - “加入”申请？

Question

我正在寻找以下功能:
Applicative f => f (f a) -> f a
Hoogle给我看join :

>:t join
join :: Monad m => m (m a) -> m a

是否有与我想要的签名相匹配的功能？

Answer 1

稍微扩展一下 Carl's answer , 如果有 join 这样的东西，但对于应用程序:

class Applicative f => ApplicativeWithJoin f where
    join' :: f (f a) -> f a

然后你会自动拥有一个 monad:

instance ApplicativeWithJoin m => Monad m where
    return = pure
    x >>= f = join' (f <$> x)