提议本地数据声明/实例 [英] Proposal for local data declarations / instances

问题描述

我很好奇，并且一直无法在Haskell中找到这样的提议。考虑是否已经编写 sort ，但不是 sortBy 。

  sortBy :: forall a。 （a  - > a  - >订购） - > [a]  - > [a] 
 sortBy f = map getX。排序。 map X 
其中
 newtype X = X {getX :: a} 
实例Ord X其中
比较（X a）（X b）= fab 
  

有没有人看过这个建议？


解决方案 http：// web .archiveorange.com / archive / v / eKcS7T2qBpy7czBE2Jei ，更完整地在Oleg的论文Functional Pearl：Implicit Con fi gurations的第6章中。虽然我几乎没有听说过本地数据声明。

I'm curious, and have been unable to find a proposal for something like this in Haskell. Consider if sort had been written but not sortBy.

sortBy :: forall a. (a -> a -> Ordering) -> [a] -> [a]
sortBy f = map getX . sort . map X
    where
    newtype X = X { getX :: a }
    instance Ord X where
        compare (X a) (X b) = f a b

Has anyone seen this proposal?

解决方案

Apparently, local instances have been discussed briefly at Haskell Prime mailing list: http://web.archiveorange.com/archive/v/eKcS7T2qBpy7czBE2Jei, and more fully in the 6^th chapter of Oleg’s paper "Functional Pearl: Implicit Configurations". I have heard practically nothing about local data declarations, though.

这篇关于提议本地数据声明/实例的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！