不是函子(或不可遍历)的可折叠示例? [英] An example of a Foldable which is not a Functor (or not Traversable)?

问题描述

A 可折叠 实例可能是某种容器，因此也可能是 Functor.事实上，这个说

一个Foldable类型也是一个容器(虽然这个类在技术上不需要Functor，有趣的Foldable都是Functors).

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

那么有没有一个 Foldable 的例子，它不是一个 Functor 或一个 Traversable?(可能是 Haskell wiki 页面遗漏了 :-) )

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:

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

Weird 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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