Monad Transformer 'distribute' 函数

Question

给定以下Typeclassopedia 练习：

--Implement join :: M (N (M (N a))) -> M (N a), 
--given distrib :: N (M a) -> M (N a) and 
--assuming M and N are instances of Monad.

distrib :: (Monad m, Monad n) => n (m a) => m (n a)
distrib = undefined

我收到以下编译时错误。

ghci> :l MonadTransformers.hs
[1 of 1] Compiling Main             ( MonadTransformers.hs, interpreted )

MonadTransformers.hs:7:34:
    Expected a constraint, but `n (m a)' has kind `*'
    In the type signature for `distrib':
      distrib :: (Monad m, Monad n) => n (m a) => m (n a)
Failed, modules loaded: none.

我该如何解决？

Answer 1

应该是这样的

distrib :: (Monad m, Monad n) => n (m a) -> m (n a)

=> 介于约束和类型的其余部分之间。

-> 是函数的中缀类型构造函数。

例如

(+) :: Num a => a -> a -> a
       -----     \  /      \
        \         arguments result
         constraint