在所有类型族实例上输入约束 [英] Type constraints on all type family instances

问题描述

我想如果没有Template Haskell，我想要的是不可能的，但我仍然会问。

我有一个类似 Data的接口。设置和 Data.IntSet ：

  type family Elem s :: * 
 class SetLike s where 
 insert :: Elem s  - > s  - > s 
会员:: Elem s  - > s  - > Bool 
 ... 
 
类型实例Elem（Set a）= a 
实例Ord a => SetLike（Set a）其中
 ... 
  

我有一个类型族选择最佳设置实现：

 类型系列EfficientSet elem :: * 
类型实例EfficientSet Int = IntSet 
类型实例EfficientSet String = Set String  - 或另一个实现
  

有没有办法保证 EfficientSet 实例将总是 SetLike ，并且 Elem（EfficientSet a）是如果没有这个保证，所有的函数签名都是这样的：
$

  type LocationSet = EfficientSet Location 
f ::（SetLike LocationSet，Elem LocationSet〜Location）=> ... 
  

每次写入 SetLike LocationSet 在某种程度上是可以忍受的，但是 Elem LocationSet〜Location 使得代码理解变得更加困难，就像我一样。

解决方案

使用GHC 7.4的约束类型，您可以使用类似于

 类型EfficientSetLike a =（SetLike （EfficientSet a），Elem（EfficientSet a）〜a）
  

GHC早期版本中的约束

  class（SetLike（EfficientSet a），Elem（EfficientSet a）〜a）=> ; EfficientSet类似于
实例（SetLike（EfficientSet a），Elem（EfficientSet a）〜a）=> EfficientSetLike a 
  

但是，新样式类型声明更好。

我不确定你在找什么，但听起来你只是想更容易编写/理解约束签名，在这种情况下这将工作。

I suppose what I want is impossible without Template Haskell but I'll ask anyway.

I have an interface for types like Data.Set and Data.IntSet:

type family Elem s :: *
class SetLike s where
  insert :: Elem s -> s -> s
  member :: Elem s -> s -> Bool
  ...

type instance Elem (Set a) = a
instance Ord a => SetLike (Set a) where
  ...

And I have a type family which chooses optimal set implementation:

type family EfficientSet elem :: *
type instance EfficientSet Int = IntSet
type instance EfficientSet String = Set String -- or another implementation

Is there a way to guarantee that EfficientSet instances will be always SetLike and that Elem (EfficientSet a) is a ?

Without this guarantee all function signatures will be like this:

type LocationSet = EfficientSet Location
f :: (SetLike LocationSet, Elem LocationSet ~ Location) => ...

To write each time SetLike LocationSet is somewhat tolerable, but Elem LocationSet ~ Location makes code understanding only harder, as for me.

解决方案

Using GHC 7.4's constraint kinds you could have something like

type EfficientSetLike a = (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a)

You can (with appropriate extensions) get constraints like this in earlier versions of GHC

class (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 
instance (SetLike (EfficientSet a),Elem (EfficientSet a) ~ a) => EfficientSetLike a 

But, the new style type declaration is much nicer.

I'm not exactly sure what you are looking for, but it sounds like you just want easier to write/understand constraint signatures, in which case this will work.

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