将数据定义封装在Haskell中 [英] Encapsulating data definitions in Haskell

问题描述

我试图使用其他数据类型来定义数据类型，如下所示：

I am trying to define a data type using other data types like this:

data A = Something String | SomethingElse Int

data B = Another B | YetAnother A

data C = A | B

x :: [ C ]
x = [ YetAnother (SomethingElse 0), Something "Hello World" ]

但是这给了我一个错误，说我期望B类型时不能有A类型。为什么会这样？

But this is giving me an error saying that I cannot have a type A when expecting a type B. Why is this?

推荐答案

您缺少 C 的数据构造函数。

You're missing the data constructors for C.

data A = Something String
       | SomethingElse Int

data B = Another    B
       | YetAnother A

data C = C0 A
       | C1 B

x :: [ C ]
x = [ C1 (YetAnother (SomethingElse 0))
    , C0 (Something "Hello World")
     ]

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