可以部分应用函子吗

Question

我正在尝试为以下类型实施 fmap：

data Tree a = Leaf a | Node a (Tree a) (Tree a) | Empty deriving (Eq,Show)

instance Functor Tree where
        fmap _ Empty=Empty
        fmap f (Leaf x)=Leaf (f x)
        fmap f (Node t left right)=Node (f t) left right

我不断收到类型不匹配错误：

错误

* Couldn't match type `a' with `b'
      `a' is a rigid type variable bound by
        the type signature for:
          fmap :: forall a b. (a -> b) -> Tree a -> Tree b
        at Monad.hs:8:9-12
      `b' is a rigid type variable bound by
        the type signature for:
          fmap :: forall a b. (a -> b) -> Tree a -> Tree b
        at Monad.hs:8:9-12
      Expected type: Tree b
        Actual type: Tree a

为什么我会收到此错误，但是当我也将 fmap 应用于子节点时，它编译没有问题：

fmap f (Node t left right) = Node (f t) (fmap f left) (fmap f right)

这是否意味着 Tree 中的所有 a-s 必须以某种方式变成 b-s ？在第一种情况下我只处理非函子？ ^

Answer 1

Does it mean that all a-s within the Tree must somehow become b-s ? and i am only dealing with the non-functor one in the first case ? ^

是的，没错。您正在尝试实现 fmap :: (a -> b) -> Tree a -> Tree b，但是当您编写：

fmap f (Node t left right) = Node (f t) left right

您正在尝试使用参数 f t :: b、left :: Tree a 和 right :: Tree a 调用 Node :: b -> Tree b -> Tree b -> Tree b。将 Tree a 变成 Tree b 的唯一方法是通过 fmap f :: Tree a -> Tree b，这就是为什么：

fmap f (Node t left right) = Node (f t) (fmap f left) (fmap f right)

按预期工作。

可以部分应用函子吗

Can one partially apply functor

haskell

types

functor