Haskell：一个不是Functor（或不可穿越）的Foldable的例子？ [英] Haskell : An example of a Foldable which is not a Functor (or not Traversable)?

问题描述

可折叠 实例很可能是某种容器，所以很可能也是 Functor 。的确，这个说：

A Foldable instance is likely to be some sort of container, and so is likely to be a Functor as well. Indeed, this says

A Foldable type也是一个容器（尽管这个类在技术上并不需要 Functor ，有趣的 Foldable s都是 Functor s）。

A Foldable type is also a container (although the class does not technically require Functor, interesting Foldables are all Functors).

那么是否存在一个 Foldable 的例子，它本质上不是一个 Functor 或一个 Traversable的？ （这可能是哈斯克尔的wiki页面错过了:-)）

So is there an example of a Foldable which is not naturally a Functor or a Traversable? (which perhaps the Haskell wiki page missed :-) )

推荐答案

这是一个完全参数化的例子： b

Here's a fully parametric example:

data Weird a = Weird a (a -> a)

instance Foldable Weird where
  foldMap f (Weird a b) = f $ b a

奇怪的是不是 Functor ，因为 a 出现在负数位置。

Weird is not a Functor because a occurs in a negative position.

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