Haskell-如何制作"div"或"/"只是在实例中? [英] Haskell - How to make "div" or "/" simply in Instance?

问题描述

示例

    data SomeData = Number Integer | String String | Bool Bool
                  deriving Eq

而且，我想对SomeData使用'div'.

And,I want to use 'div' with SomeData.

    instance Num SomeData where
            (Number x) + (Number y) = Number $ x + y -- It's Ok.
            (Number x) - (Number y) = Number $ x - y -- It's Ok.
            (Number x) * (Number y) = Number $ x * y -- It's ok.
    instance Integral SomeData Where
            (Number x) `div` (Number y) = Number $ x `div` y

但是我收到了错误消息.

But I got Error Message.

main.hs:105:10:
    No instance for (Real SomeData)
      arising from the superclasses of an instance declaration
    Possible fix: add an instance declaration for (Real SomeData)
    In the instance declaration for `Integral SomeData'

main.hs:105:10:
    No instance for (Enum SomeData)
      arising from the superclasses of an instance declaration
    Possible fix: add an instance declaration for (Enum SomeData)
    In the instance declaration for `Integral SomeData'

好，如果我要对SomeData数据使用div，我应该为SomeData逐步编写Real Instance和Enum Instance吗?还是还有另一种好方法?

Ok,If I want to use div with SomeData data,should I write Real Instance and Enum Instance for SomeData step-by-step? Or Is there another good way ?

推荐答案

解决此类问题的通用方法是在ghci中使用:info命令:

A general method to solve problems like this is to use :info command in ghci:

Prelude> :i Integral
class (Real a, Enum a) => Integral a where
  quot :: a -> a -> a
  rem :: a -> a -> a
  div :: a -> a -> a
  mod :: a -> a -> a
  quotRem :: a -> a -> (a, a)
  divMod :: a -> a -> (a, a)
  toInteger :: a -> Integer

因此，不能，如果不实现Real，就不能实现Integral.您可能直接从div开始:

So no, you cannot implement Integral without implementing Real. You could have started straight from div:

Prelude> :i div
class (Real a, Enum a) => Integral a where
  ...
  div :: a -> a -> a
  ...
        -- Defined in GHC.Real
infixl 7 div

因此您可以看到div是整数类型类的方法，并且还需要定义Real和Enum.如果选中Real，则需要Num和Ord等.

So you can see that div is a method of Integral type class and you need Real and Enum to be defined as well. If you check Real, it requires Num and Ord etc.

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