haskell - monad 转换器 MaybeT 中函数 "return"的两个定义

Question

MaybeT 定义为

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

而且，MaybeT m 也是 Monad 类的实例，函数 return 定义如下

return  = MaybeT . return . Just

但我读到 “也可以(尽管可读性较差)写 return = MaybeT .return .return”，这让我感到困惑。

如何返回 = MaybeT 。返回 。 return 等于 return = MaybeT 。返回 。只是 ?

Answer 1

考虑 MaybeT 的定义:

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

第一个return定义(从下往上读):

return =
  MaybeT .   -- put `m (Maybe a)` into `MaybeT`: MaybeT (m (Maybe a))
    return . -- put `Maybe a` into external monad: m (Maybe a)
      Just   -- put value into `Maybe`: Maybe a

Maybe 也是一个 monad。它的return 是一个Just。这就是为什么第二个 MaybeT 的 return 定义了完全相同的函数:

return =
  MaybeT .   -- put `m (Maybe a)` into `MaybeT`: MaybeT (m (Maybe a))
    return . -- put `Maybe a` into external monad: m (Maybe a)
      return -- it's the same as `Just` for `Maybe`: Maybe a