如何写“显示"?数据的类型类 [英] How to write "Show" Typeclass for data

问题描述

我有一个非常简单的data，并且我想使用Show Typeclass为其提供良好的输出.

I have a very simple data and I want to use Show Typeclass to give it a nice output.

data Fruit = Apple | Orange Int 

instance Show Fruit
         where
            show Apple    = "Hey Apple"
            show Orange a = "X number of Orange"

这会出现以下错误，但我不知道如何解决:

This gives the following error but I don't know how to fix it:

Equations for `show' have different numbers of arguments

推荐答案

您只是忘记了一些括号:)

You just forgot some parens :)

instance Show Fruit where
  show Apple      = "Hey Apple"
  show (Orange a) = show a ++ " number of Orange"

像Orange a这样的模式需要经过parens才能与Apple a之类的东西区分开来，而我们实际上有两个参数.

Patterns like Orange a need parens to disambiguate from something like Apple a where we really have two arguments.

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