为什么我们需要Control.Lens.Reified？ [英] Why do we need Control.Lens.Reified?

问题描述

为什么我们需要 Control.Lens.Reified ？有什么理由不能将 Lens 直接放入容器中？什么是 reify 意味着什么？

Why do we need Control.Lens.Reified? Is there some reason I can't place a Lens directly into a container? What does reify mean anyway?

推荐答案

因为Haskell's类型系统是预测性的。我不知道技术细节究竟是什么意思，但它禁止类型像

We need reified lenses because Haskell's type system is predicative. I don't know the technical details of exactly what that means, but it prohibits types like

[Lens s t a b]

出于某些目的，可以使用

For some purposes, it's acceptable to use

Functor f => [(a -> f b) -> s -> f t]

来代替，但是当您进入该范围时，您不会收到镜头;你得到一个专门用于某个仿函数或另一个函数的 LensLike 。 ReifiedBlah newtypes可以让你保持完整的多态性。

instead, but when you reach into that, you don't get a Lens; you get a LensLike specialized to some functor or another. The ReifiedBlah newtypes let you hang on to the full polymorphism.

在操作上， [ReifiedLens stab] 是一个函数列表，每个函数都需要一个 Functor f 字典，而 forall f。 Functor f => [LensLike fstab] 是一个函数，它接收 Functor f 字典并返回一个列表。

Operationally, [ReifiedLens s t a b] is a list of functions each of which takes a Functor f dictionary, while forall f . Functor f => [LensLike f s t a b] is a function that takes a Functor f dictionary and returns a list.

至于reify是什么意思，那么字典会说些什么，这似乎转化为Haskell中相当惊人的各种具体含义。所以没有评论。

As for what "reify" means, well, the dictionary will say something, and that seems to translate into a rather stunning variety of specific meanings in Haskell. So no comment on that.

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