如何使用GHC.Generics的selName函数？ [英] How to use the selName function of GHC.Generics?

问题描述

考虑以下记录类型：

  { - ＃language DeriveGeneric＃ - } 
 
 data Person = Person {
 firstName :: String 
，lastName :: String 
，age :: Integer 
}派生（通用
  

如何使用selName函数来获取firstName选择器的名称？

解决方案

下面的代码需要以下扩展：

  { - ＃LANGUAGE FlexibleContexts＃ - } 
 { - ＃LANGUAGE DeriveGeneric＃ - } 
 { - ＃LANGUAGE Typefamilies＃ - } 
 { - ＃LANGUAGE TypeOperators＃ - } 
 { - ＃$} > 
 
 
首先，您可以使用GHCi来查找您的人员类型的通用表示：  
 
 
 λ>：kin d！Rep Person（）
 Rep Person（）:: * 
 = M1 
 D 
 Main.D1Person 
（M1 
 C 
 Main.C1_0Person 
（M1 S Main.S1_0_0Person（K1 R [Char]）
：*：（M1 S Main.S1_0_1Person（K1 R [Char]）
：*：M1 S Main .S1_0_2Person（K1 R Integer））））
（）
  
因为 Main.S1_0_1Person 。为了解压，你可以使用一个类型族：
  type family FirstSelector（f :: *  - > *）:: * 
类型实例FirstSelector（M1：xF）= FirstSelector f 
类型实例FirstSelector（M1 C xf）= FirstSelector f 
类型实例FirstSelector（a：*：b）= FirstSelector a  - 选择第一个选择器
类型实例FirstSelector（M1 S sf）= s 
  - 注意：这不支持具有多个构造函数的类型。 
  - 在这种情况下，您会收到类型错误。 
  
我们需要一种方法来传递 Person 键入我们的函数，获取第一个选择器的名称。我们可以使用一个代理类型来实现，它只有一个构造函数，但是用一个类型标记:(你也可以使用一个参数 undefined :: Person 并忽略它，但这种方式是保证你只能忽略它。）
  data Proxy a =代理 - 也由`tagged` hackage包提供
  
现在，  selName 的类型是 selName :: ts（f :: *  - > *）a  - > [Char] ，所以你需要一个匹配模式 t s（f :: *  - > *）a 的类型来使用该函数。我们使用相同的技巧来创建 SelectorProxy ，它只有一个构造函数，但是是必需的形式： 
 
 
  data SelectorProxy s（f :: *  - > *）a = SelectorProxy 
 SelectorProxy's = SelectorProxy s Proxy（）
  
 
 
 
最后，我们准备编写获取选择器名称的函数：
  firstSelectorName :: forall a。 （通用a，选择器（FirstSelector（Rep a）））=>代理a  - > String 
 firstSelectorName Proxy = selName（SelectorProxy :: SelectorProxy'（FirstSelector（Rep a）））
  
 
 
 < 
 λ>>如果您在GHCi中加载它，您可以看到它的工作原理： firstSelectorName（Proxy :: Proxy Person）
firstName
  
 
I am looking for a simple example concerning the use of the selName function of the Haskell GHC.Generics package.

Considering the following record type:
{-# language DeriveGeneric #-}

data Person = Person {
    firstName :: String
  , lastName  :: String
  , age       :: Integer
  } deriving(Generic
How would be used the selName function to get the name of the firstName selector?
解决方案
The code below requires the following extensions:
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
First, you can use GHCi to find out the generic representation of your person type:
λ> :kind! Rep Person ()
Rep Person () :: *
= M1
    D
    Main.D1Person
    (M1
       C
       Main.C1_0Person
       (M1 S Main.S1_0_0Person (K1 R [Char])
        :*: (M1 S Main.S1_0_1Person (K1 R [Char])
             :*: M1 S Main.S1_0_2Person (K1 R Integer))))
    ()
The selector type your looking for is Main.S1_0_1Person. To extract that, you can use a type family:
type family FirstSelector (f :: * -> *) :: *
type instance FirstSelector (M1 D x f) = FirstSelector f
type instance FirstSelector (M1 C x f) = FirstSelector f
type instance FirstSelector (a :*: b) = FirstSelector a -- Choose first selector
type instance FirstSelector (M1 S s f) = s
-- Note: this doesn't support types with multiple constructors. 
-- You'll get a type error in that case.
We need a way to pass the Person type to our function that gets the name of the first selector. We can use a Proxy type to achieve that, which only has one constructor, but is "tagged" with a type: (you could also use an argument undefined :: Person and ignore it, but this way it's guarranted that you can only ignore it).
data Proxy a = Proxy -- also provided by the `tagged` hackage package
Now, the type of selName is selName :: t s (f :: * -> *) a -> [Char], so you need a type that matches the pattern t s (f :: * -> *) a to use the function. We use the same trick to create a SelectorProxy which has only one constructor, but is of the required form:
data SelectorProxy s (f :: * -> *) a = SelectorProxy
type SelectorProxy' s = SelectorProxy s Proxy ()
Finally, we're ready to write the function that gets the selector name:
firstSelectorName :: forall a. (Generic a, Selector (FirstSelector (Rep a))) => Proxy a -> String
firstSelectorName Proxy = selName (SelectorProxy :: SelectorProxy' (FirstSelector (Rep a)))
And if you load that in GHCi, you can see that it works:
λ> firstSelectorName (Proxy :: Proxy Person)
"firstName"


                        
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