什么是多态类型的用法？ [英] What are uses of polymorphic kinds?

问题描述

多态类型是Haskell类型系统的扩展，支持 by UHC，允许

Polymorphic kinds are an extension to Haskell's type system, supported by UHC, allowing

data A x y = A (y x)

键入（kinded？）为 a - > （a - > *） - > * 。它们有什么用处？

to be typed (kinded?) as a -> (a -> *) -> *. What are they useful for?

推荐答案

一个可能的用法示例可以使用conal的 TypeCompose 以无点式创作monad变形金刚。

One possible usage example can be using conal's TypeCompose for composing monad transformers in point-free style.

type MyT = StateT Foo :. MaybeT :. ContT Bar

（就像一个例子，我不知道用这些foos来做什么，酒吧..）

(just as an example, I have no idea what one's going to do with those foos and bars..)

而不是：

Instead of:

type MyT m = StateT Foo (MaybeT (ContT Bar m))

（除了 newtype -wrappers）

(this would have the same result apart from newtype-wrappers)

目前您需要复制不同种类的组合代码，并且此扩展消除了重复并允许使用一段代码来统一它们。

Currently you'll need to duplicate the combinators code for different kinds, and this extension abolishes the repetition and allows using one piece of code to rule them all.

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