Haskell记录访问器由具有相同数据类型的不同构造函数共享

问题描述

关于Haskell记录的一个基本问题。如果我定义了这个数据类型，

  data Pet = Dog {name :: String} | Cat {name :: String}派生（显示）
  

以下工作：

  main = do 
 let d = Dog {name =Spot} 
c = Cat {name =Morris} 
 putStrLn $ name d 
 putStrLn $ name c 
  

但是如果我这样做，

 数据Pet = Dog {name :: String} | Cat {name :: Integer}派生（显示）
  

我会得到这个错误：多次声明'name'。

我直觉地理解为什么会这样，因为第一种情况下 name 的类型是只需宠物 - >字符串，不管使用的构造函数如何。但我不记得在我读过的任何Haskell书籍中看到有关记录访问函数的规则。有人可以对我在上面看到的行为稍作更深入的解释吗？

解决方案

从：

I don认为还有更深入的东西。正如你所说，结果字段访问器的类型是 Pet - >字符串无论如何所以权力决定它很方便，让您在不同的构造函数中重复使用相同的字段名称。

A basic question about Haskell records. If I define this datatype,

data Pet = Dog { name :: String } | Cat { name :: String } deriving (Show)

the following works:

main = do
  let d = Dog { name = "Spot" }
      c = Cat { name = "Morris" }
  putStrLn $ name d
  putStrLn $ name c

But if I do this,

data Pet = Dog { name :: String } | Cat { name :: Integer } deriving (Show)

I'll get this error: Multiple declarations of 'name'.

I think I understand intuitively why this should be the case, since the type of name in the first case is just Pet -> String regardless of the constructor that was used. But I don't recall seeing this rule about record accessor functions in any of the Haskell books I've read. Can someone give a slightly more in-depth explanation about the behavior I'm seeing above?

解决方案

From the Haskell '98 Report:

I don't think there's anything more in-depth to it. As you said, the resulting field accessor has the type Pet -> String anyway so the powers that be decided it convenient to allow you to re-use the same field name in different constructors.

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