“分类"在Haskell表演吗? [英] "Subclassing" show in Haskell?

问题描述

让我说以下内容:

data Greek = Alpha | Beta | Gamma | Phi deriving Show

我想使用除Beta以外的所有项目的默认显示，我想说两个".

I want to use the default showing of all items except Beta, which I want to say "two".

我可以这样做吗?

推荐答案

deriving Show使用标准实例化机制(仅返回定义).如果您需要任何特殊的东西，则必须手动实例化它:

deriving Show uses the standard instantiation mechanism (simply returning the definition). If you want any special things, you will have to instantiate it manually:

data Greek = Alpha | Beta | Gamma | Phi

instance Show Greek
    where
        show Alpha = "Alpha"
        show Beta  = "2"
        show Gamma = "Gamma"
        show Phi   = "Phi"

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