Haskell记录中的名称冲突 [英] Name conflicts in Haskell records

问题描述

Haskell对记录成员没有点符号。对于每个记录成员，编译器使用类型RecType - > FieldType创建一个具有相同名称的函数。这会导致名称冲突。有什么办法可以解决这个问题，即如何使用相同的字段名称来创建多个记录？解析方案

另一种方法避免这个问题是使用镜头包。它提供了一个makeFields模板haskell函数，你可以像这样使用它：

  { - ＃LANGUAGE FlexibleInstances＃ - } $ b $ $ {$＃$} 
 { - ＃语言MultiParamTypeClasses＃ - } 
 { - ＃LANGUAGE TemplateHaskell＃ - } 
 { - ＃LANGUAGE TypeSynonymInstances＃ - } 
 Control.Lens 
 
 data A = A 
 {_aText :: String 
} 
 makeFields''A  - 为每个记录存取器创建一个镜头x，其名称_aX 
 
数据B = B 
 {_bText :: Int 
，_bValue :: Int 
} 
  - 为每个记录访问器创建一个镜头x名称为_bX 
 makeFields''B 
 
 main = do 
 let a = Ahello
 let b = B 42 1 
 
  - （^。）是访问某个值（a）的字段（文本）的镜头的函数
 putStrLn $a：++ a ^的文本。文本
 putStrLn $b的文本：++ show（b ^ .text）
  

如果您不想使用TemplateHaskell和镜头，您还可以手动使用TemplateHaskell自动执行的镜头： ＃LANGUAGE FlexibleInstances＃ - }
{ - ＃LANGUAGE FunctionalDependencies＃ - }
{ - ＃LANGUAGE MultiParamTypeClasses＃ - }
{ - ＃LANGUAGE TypeSynonymInstances＃ - }
data A = A
{aText :: String
}

data B = B
{bText :: Int
，bValue :: Int
}

- 类型为a的类，它具有t
类型的text字段HasText at | a - > t其中

- 文本值的访问者
text :: a - > t

- 使这两个类型的实例成为这些
实例HasText一个字符串其中text = aText
实例HasText B Int其中text = bText

main = do
let a = Ahello
let b = B 42 1
putStrLn $a：++ text a
putStrLn $的文本b ：++ show（text b）

但我真的可以推荐学习镜头，因为它也提供很多其他工具，比如修改或设置一个字段。

Haskell doesn't have dot notation for record members. For each record member a compiler creates a function with the same name with a type RecType -> FieldType. This leads to name conflicts. Are there any ways to work around this, i.e. how can I have several records with the same field names?

解决方案

Another way to avoid this problem is to use the lens package. It provides a makeFields template haskell function, which you can use like this:

{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE TemplateHaskell        #-}
{-# LANGUAGE TypeSynonymInstances   #-}
import           Control.Lens

data A = A
  { _aText :: String
  }
makeFields ''A   -- Creates a lens x for each record accessor with the name _aX

data B = B
  { _bText  :: Int
  , _bValue :: Int
  }
-- Creates a lens x for each record accessor with the name _bX
makeFields ''B  

main = do
  let a = A "hello"
  let b = B 42 1

  -- (^.) is a function of lens which accesses a field (text) of some value (a)
  putStrLn $ "Text of a: " ++ a ^. text 
  putStrLn $ "Text of b: " ++ show (b ^. text)

If you don't want to use TemplateHaskell and lens, you can also do manually what lens automates using TemplateHaskell:

{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE TypeSynonymInstances   #-}
data A = A
  { aText :: String
  }

data B = B
  { bText  :: Int
  , bValue :: Int
  }

-- A class for types a that have a "text" field of type t
class HasText a t | a -> t where

  -- An accessor for the text value
  text :: a -> t

-- Make our two types instances of those
instance HasText A String where text = aText
instance HasText B Int where text = bText

main = do
  let a = A "hello"
  let b = B 42 1
  putStrLn $ "Text of a: " ++ text a
  putStrLn $ "Text of b: " ++ show (text b)

But I can really recommend learning lens, as it also provides lots of other utilities, like modifying or setting a field.

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