使用FlexibleContexts和FlexibleInstances有哪些缺陷？ [英] What are the pitfalls of using FlexibleContexts and FlexibleInstances?

问题描述

由于这些灵活的上下文和实例在Haskell标准中不可用，我假设在使用它们时存在潜在的问题。他们是什么？它们能导致一些含糊不清，重叠的实例等吗？

有一个类似的问题，只询问 FlexibleInstances ，而不是 FlexibleContexts ，但答案只是说这是安全的使用它们。

解决方案

我曾经偶然发现了以下情况。 回答这个问题，我第一次尝试这个代码：

<$ p $
{ - ＃LANGUAGE MultiParamTypeClasses＃ - }
{ - ＃LANGUAGE FunctionalDependencies＃ - }
$ b $ class（Eq a，Show a）=>基因组a其中
crossover ::（小数b）=> b - > a - > a - > IO（a，a）
mutate ::（Fractional b）=> b - > a - > IO a
develop ::（表型b a）=> a - > b

class（Eq a，Show a）=>表型a b | a - > b其中
- 在Coevolution的情况下，每个表型需要与
相比较 - 在​​人口中每隔一个
fitness :: [a] - > a - > Int
genome ::（Genome b）=> a - > b - 在这里，问题

繁殖父母交叉静音= do
children < - mapM（\（爸爸，妈妈） - >交叉交叉（基因组爸爸）（基因组妈妈））
父母
let ch1 = map fst children ++ map snd children
mutated< - mapM（mutate mute）ch1
return $ map开发变异

得到一个编译错误和GHCi建议添加 FlexibleContexts 选项。当我这样做，它编译好。但是这实际上不是一件正确的事情，因为约束声明为类型变量引入了新的范围，并且在基因组 b c>的类型签名变得与类型类中的完全无关;但 FlexibleContexts 为此提供了掩护。

在类型级别指定适当的约束时，

 类（等式a，显示a，基因组b）=>表型a b | a  - > b其中
  - 在Coevolution的情况下，每个表型需要与
相比较 - 在​​人口中每隔一个
 fitness :: [a]  - > a  - > Int 
 genome :: a  - > b 
  

它通过编译而不需要 FlexibleContexts 选项。

Since these flexible contexts and instances aren't available in the Haskell standard, I assume there are potential problems when using them. What are they? Can they lead to some ambiguity, undecidability, overlapping instances, etc.?

There is a similar question that asks only about FlexibleInstances, not FlexibleContexts, but the answer only says "that it's safe to use them".

解决方案

I once stumbled upon the following. Answering this question, I first tried this code:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}

class (Eq a, Show a) => Genome a where
    crossover       :: (Fractional b) => b -> a -> a -> IO (a, a)
    mutate          :: (Fractional b) => b -> a -> IO a
    develop         :: (Phenotype b a)  => a -> b

class (Eq a, Show a) => Phenotype a b | a -> b where
    --  In case of Coevolution where each phenotype needs to be compared to 
    --  every other in the population
    fitness         :: [a] -> a -> Int 
    genome          :: (Genome b) => a -> b    -- here, the problem

breed parents cross mute = do
    children <- mapM (\ (dad, mom) -> crossover cross (genome dad) (genome mom)) 
                     parents
    let ch1 = map fst children ++ map snd children
    mutated <- mapM (mutate mute) ch1
    return $ map develop mutated

And got a compilation error and a suggestion by GHCi to add the FlexibleContexts option. When I did, it compiled OK. But this was actually not a right thing to do, as the constraint declaration introduced new scope for type variables, and b in genome's type signature became completely unrelated to the one in the type class; yet FlexibleContexts provided a cover for this.

With the constraint specified properly at the type class level,

class (Eq a, Show a, Genome b) => Phenotype a b | a -> b where
    --  In case of Coevolution where each phenotype needs to be compared to 
    --  every other in the population
    fitness         :: [a] -> a -> Int 
    genome          :: a -> b

it passed compilation without requiring the FlexibleContexts option.

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