Haskell 中元组的枚举实例 [英] Enum instance for tuples in Haskell

问题描述

我想定义一个元组 (x, y) 作为 Enum 类的实例，知道 x 和 y 是 Enum 的实例.以下尝试:

I'd like to define a tuple (x, y) as an instance of Enum class, knowing that both x and y are instances of Enum. A following try:

instance (Enum x, Enum y) => Enum (x, y) where
    toEnum = y
    enumFrom x = (x, x)

只会导致错误(y 不在范围内).我是 Haskell 的新手，有人能解释一下如何声明这样的实例吗?

only results in error (y not in scope). I'm new to Haskell, could somebody explain how to declare such an instance?

推荐答案

instance (Enum x, Enum y) => Enum (x, y) where

在上一行中，x 和 y 都是类型(类型变量).

In the above line, x and y are both types (type variables).

    toEnum = y
    enumFrom x = (x, x)

在上面两行中，x 和 y 都是值((value) 变量).y-as-a-value 没有在任何地方定义，这就是它不在范围内的意思.

In the above two lines, x and y are both values ((value) variables). y-as-a-value has not been defined anywhere, that's what it not being in scope means.

至于如何声明这样的实例，我不确定您希望 fromEnum 和 toEnum 的行为方式，例如.

As to how to declare such an instance, I'm not sure how you'd want fromEnum and toEnum to behave, for example.

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