haskell求和型多重声明错误 [英] haskell sum type multiple declaration error

问题描述

 数据A = A 
数据B = B 
数据AB = A | B 
  

从A和B得到AB类型的和。

但最后一行会导致编译错误multiple declarations的B

我也试过这样的：

  data A = Int | Bool 
  

编译。但为什么ghc不允许我为用户定义的类型创建和类型？

解决方案

你被骗了。你认为当你写数据A = Int | Bool 时，你认为类型 A 的值可以是一个类型 Int 的值或类型 Bool 的值;但实际上你所说的是有两个新的值级构造函数，分别是 Int 和 Bool ，每个都不包含任何信息，类型 A 。同样，您认为 data AB = A | B 表示您可以是 A 类型或 B ，但实际上你说你可以拥有值 A 或值 B 。

要记住的关键是有两个命名空间，type-level和术语级别，并且它们是截然不同的。

以下是一个简单的示例：

  data A = A 
 data B = B 
 data AB = LA | RB 
  

最后一行声明两个新的术语级构造函数， L 和 R 。 L 构造函数的值为 type A ，而 R 构造函数的值为 type B 。

<您可能还喜欢或者类型，定义如下：

 数据ab =左a |右b 
  

你可以用它来实现你的 AB 如果你想要的话：

  type AB = AB 
  

类似地，您可以使用 Either Bool 作为 Int 和 Bool 。

data A=A
data B=B
data AB=A|B

Which makes a sum type AB from A and B.

but the last line induces a compile error "multiple declarations of B"

I also tried sth like this:

data A=Int|Bool

It compiles. but why ghc disallows me from making sum types for user-defined types?

解决方案

You're getting fooled. You think when you write data A=Int|Bool that you are saying that a value of type A can be a value of type Int or a value of type Bool; but what you are actually saying is that there are two new value-level constructors named Int and Bool, each containing no information at all, of type A. Similarly, you think that data AB=A|B says you can either be of type A or type B, but in fact you are saying you can either have value A or value B.

The key thing to keep in mind is that there are two namespaces, type-level and term-level, and that they are distinct.

Here is a simple example of how to do it right:

data A=A
data B=B
data AB=L A|R B

The last line declares two new term-level constructors, L and R. The L constructor carries a value of type A, while the R constructor carries a value of type B.

You might also like the Either type, defined as follows:

data Either a b = Left a | Right b

You could use this to implement your AB if you wanted:

type AB = Either A B

Similarly, you could use Either Int Bool for your tagged union of Int and Bool.

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