小编Pau*_*l K的帖子

嗨,我是Haskell的新手,主要是从LYAH和Hutton读书.最近我遇到了这个状态monad的Functor实例的片段,表示为:

instance Functor (State st) where
    fmap f m = State $ \st -> let (a, s) = runState m st in (f a, s)

这可以简化为:

instance Functor (State st) where 
    fmap f m = State $ (\(a,s) -> (f a,s)) . runState m

谁能解释这种减少背后的工作流程？

关于如何学习这种减少技术,还有哪些好的资源/建议？

我有一个树的以下定义

 data Tree a = Leaf a
           | Node [Tree a] 
     deriving (Show)

以及可折叠的以下实例:

 instance Foldable (Tree) where
 foldMap f (Leaf t) = (f t)
 foldMap f (Node t) = (foldMap `mappend` (foldMap f) t)     

这段代码引发了我的错误

 Couldn't match type `a' with `Tree a'
  `a' is a rigid type variable bound by
      the type signature for
        foldMap :: Monoid m => (a -> m) -> Tree a -> m
      at trees.hs:8:5
 Expected type: [a]
   Actual type: [Tree a]

如何在Tree a类型的实例声明中而不是？