读取自定义数据类型失败

Question

data MyNum = One
           | Two
           | Three
           deriving (Show, Eq)

我只是用构造函数 One、Two 和 Three 定义了 MyNum。

*Main> :t One

One :: MyNum

但是当我将 x = read("One")::MyNum 添加到我的程序时 ghci 会产生错误：

No instance for (Read MyNum) arising from a use of ‘read’

In the expression: read ("One") :: MyNum

In an equation for ‘x’: x = read ("One") :: MyNum

为什么我不能 read 它？

Answer 1

您必须在 MyNum 的定义中导出 Read:

data MyNum = One
           | Two
           | Three
           deriving (Show, Eq, Read)

x = read("One")::MyNum

*Main> x

One

读取自定义数据类型失败

Fail to read a self defined data type

haskell

typeclass

deriving