不是Functor(或不可穿越)的Foldable的示例? [英] An example of a Foldable which is not a Functor (or not Traversable)?

问题描述

A Foldable instance可能是某种容器，因此也可能是Functor.实际上，这说

Foldable类型也是一个容器(尽管从技术上讲该类不需要Functor，但有趣的Foldable都是Functor s).

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

那么有一个Foldable的例子，它自然不是Functor或Traversable吗? (也许Haskell维基页面可能没有找到:-))

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

下面是一个完全参数化的示例:

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出现在负位置.

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

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