等价于mapM的箭头？ [英] Arrow equivalent of mapM?

问题描述

我正在努力克服&和Arrows一起工作，并且遇到一些困难。我有一个需要 Arrow [a] [b] 的上下文，我想写一个 Arrow ab 并将其映射/排列在箭头内，即 mapM 。具体来说，箭头是一个Hakyll 编译器，但我认为这对于答案来说不重要。

给定一个箭头

  myInnerArrow :: Arrow a => abc 
  

我怎样才能把它放到一个箭头里

  myOuterArrow :: Arrow a => a [b] [c] 
  

？

<我已经搜遍了基础库，特别是在 Data.List 和 Control.Arrow 中，但是我找不到任何东西看起来它会做这项工作。它是否以我不指望的名字存在？它是否由其他图书馆提供？是否因为某些原因而不能编写代码？ 解决方案

你不能没有选择。提升函数将具有以下类型：

  mapA ::（ArrowChoice a）=> a b c  - > a [b] [c] 
  

最简单的实现方法是使用 proc 表示法：

  mapA c = 
 proc xs' - > 
 case xs'of 
 []  - > returnA  - < [] 
（x：xs） - > uncurry（:) ^<< c *** mapA c  - < （x，xs）
  

未经测试的代码，但应该有效。但请注意，泛型将会非常慢的函数。我建议特别为你的箭头编写这个映射函数。

I'm trying to grok & work with Arrows, and am having some difficulty. I have a context where I need an Arrow [a] [b], and I want to write an Arrow a b and map/sequence it inside the arrow, a la mapM. Specifically, the arrow is a Hakyll Compiler, but I don't think that matters much for the answer.

Given an arrow

myInnerArrow :: Arrow a => a b c

How can I lift this into an arrow

myOuterArrow :: Arrow a => a [b] [c]

?

I have scoured the base library, particularly in Data.List and Control.Arrow, but I cannot find anything that looks like it will do the job. Does it exist under a name I do not expect? Is it provided by some other library? Is it impossible to write for some reason?

解决方案

You can't without choice. The lifting function will have this type:

mapA :: (ArrowChoice a) => a b c -> a [b] [c]

The easiest way to implement is by using proc notation:

mapA c =
    proc xs' ->
        case xs' of
            [] -> returnA -< []
            (x:xs) -> uncurry (:) ^<< c *** mapA c -< (x, xs)

Untested code, but should work. Note however that a function that generic is going to be really slow. I recommend writing this mapping function for your arrow specifically.

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