如何限制关联的数据？ [英] How to put constraints on the associated data?

问题描述

  class（Context（Associated） ab））=>类别a其中
数据关联a :: *  - > * 
 
实例上下文（Associated ab）其中
 func1 =错误func1
  

然而，不在范围内的自由变量 b 阻止了我这样做。其中一个解决方案是从 Context 复制类的功能，但它看起来很难看。

  class Class a where 
 data相关a :: *  - > * 
  - 从类Context中复制所有函数
 contextFunc1 ::关联a b  - >字符串
 
实例类a =>上下文（Associated ab）其中
 func1 = contextFunc1 
  

是否有一种惯用的方式来约束约束在关联的数据类型中有哪些头部没有提到变量？

编辑：我想保持与GHC 7.0.3的兼容性

您可以像这样手动传递字典（使用 Context = Show 作为示例）：

  { - ＃LANGUAGE ScopedTypeVariables，TypeFamilies，ExistentialQuantification＃ - } 
 
 data ShowDict a = Show a => ShowDict 
 
 class Class a where 
 data相关a :: *  - > * 
 
 getShow :: ShowDict（关联a b）
 
  - 便捷函数
 getShowFor :: Class a =>相关联的b  - > ShowDict（Associated a b）
 getShowFor _ = getShow 
 
 showAssociated :: Class a =>相关联的b  - > String 
 showAssociated a = 
 case getShowFor a of 
 ShowDict  - > -  Show（Associated ab）通过这个模式匹配
显示一个
 
实例类Int其中
数据Associated Int b = Foo派生Show 
 
 getShow = ShowDict 
 
 main = print $ showAssociated Foo 
  

这是有点类似于你建议的功能复制，但是它的优点是：
$ b $ ul

避免重复（Context的方法签名）

>

在上下文中显示Baz功能比仅显示Baz功能更强大，因为它允许您调用（库）函数，它需要显示Baz ``，或者使用隐含的实例，比如`Show [Baz]`：

$ b

  showAssociateds :: forall a。 Class a => [Associated a b]  - > String 
 showAssociateds as = 
 case getShow :: ShowDict（关联a b）
 ShowDict  - > 
显示为
  

主要缺点是使用 getShow 总是需要一个明确的类型签名（像 getShowFor 这样的函数可以缓解这个问题）。

I would like to state that the associated data is always an instance of a certain class.

class (Context (Associated a b)) => Class a where
  data Associated a :: * -> *

instance Context (Associated a b) where
  func1 = error "func1"

However, the free variable b that is not in scope prevents me from this. One of the solutions is to copy class functions from Context, but it looks ugly.

class Class a where
  data Associated a :: * -> *
  -- duplicate all functions from class Context
  contextFunc1 :: Associated a b -> String

instance Class a => Context (Associated a b) where
  func1 = contextFunc1

Is there an idiomatic way to put constraints on associated data type which has variables not mentioned in head?

edit: I would like to keep compatibility with GHC 7.0.3

解决方案

I don't have GHC 7.0.3 available, but I think this should work with it.

You could pass the dictionaries around manually like this (using Context = Show as an example):

{-# LANGUAGE ScopedTypeVariables, TypeFamilies, ExistentialQuantification #-}

data ShowDict a = Show a => ShowDict

class Class a where
  data Associated a :: * -> *

  getShow :: ShowDict (Associated a b)

-- Convenience function
getShowFor :: Class a => Associated a b -> ShowDict (Associated a b)
getShowFor _ = getShow

showAssociated :: Class a => Associated a b -> String
showAssociated a = 
  case getShowFor a of
    ShowDict -> -- Show (Associated a b) is made available by this pattern match 
      show a

instance Class Int where
  data Associated Int b = Foo deriving Show

  getShow = ShowDict

main = print $ showAssociated Foo

This is somewhat similar to the function copying you propose, but advantages are:

• Avoids repetition (of `Context`'s method signatures)
• Having `Show Baz` in context is somewhat more powerful than just having a function for showing a `Baz`, since it allows you to call (library) functions which require `Show Baz`, or use implied instances like `Show [Baz]`:

showAssociateds :: forall a b. Class a => [Associated a b] -> String
showAssociateds as = 
  case getShow :: ShowDict (Associated a b) of
    ShowDict ->
      show as

The main disadvantage is that using getShow always requires an explicit type signature (functions like getShowFor can mitigate this).

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