haskell - 无法成为应用的仿函数的具体类型示例？

Question

这个问题已经有答案了:

Good examples of Not a Functor/Functor/Applicative/Monad? (5 个回答)

已关闭 6 年前。

来自functors that are not applicatives :

A type constructor which is a Functor but not an Applicative. A simple example is a pair:

instance Functor ((,) r) where
    fmap f (x,y) = (x, f y)

But there is no way how to define its Applicative instance without imposing additional restrictions on r. In particular, there is no way how to define pure :: a -> (r, a) for an arbitrary r.

在这里，pure无法同时定义所有类型；但是，对于任何具体类型 T ，可以制作((,) T)一个应用程序。

问题:是否有一个具体仿函数(即不涉及类型变量)的示例，它是仿函数但不是应用程序？

Answer 1

我没有 50 声望在这里发表评论，所以我会尝试将其作为答案:

however, for any concrete type T, one can make ((,) T) an applicative.

...

There's a theorem in mathematics that any collection with at least 2 elements can be made into a monoid. So for any concrete type T, it could in principle be made a member of Monoid, and then could in principle be made Applicative. What's wrong with this reasoning?

来自无人居住类型的元组怎么样？ (,) 无效

它是一个仿函数，对吗？

你能为它导出Applicative吗？ pure 如何实现？