用单个常量派生类

Question

我想派生一个具有常量和一些琐碎操作的类：

class  Zeros z where
    zero :: z
    isZero  :: Eq z =>  z -> Bool
    isZero z = zero == z

{-# LANGUAGE DeriveGeneric, DeriveAnyClass
    , GeneralizedNewtypeDeriving #-}
module Lib.DerivingExampleDerive
    where
import Data.Text
import Lib.DerivingExample
import GHC.Generics

newtype Basis1 = Basis1 Text deriving (Show, Read, Eq, Ord, Generic, Zeros)

/home/frank/Workspace8/testSome/Lib/DerivingExampleDerive.hs:26:70: warning:
    • Both DeriveAnyClass and GeneralizedNewtypeDeriving are enabled
      Defaulting to the DeriveAnyClass strategy for instantiating Zeros
    • In the newtype declaration for ‘Basis1’
   |         
26 | newtype Basis1 = Basis1 Text deriving (Show, Read, Eq, Ord, Generic, Zeros)
   |                                                                      ^^^^^

/home/frank/Workspace8/testSome/Lib/DerivingExampleDerive.hs:26:70: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘zero’
    • In the instance declaration for ‘Zeros Basis1’
   |         
26 | newtype Basis1 = Basis1 Text deriving (Show, Read, Eq, Ord, Generic, Zeros)
   |                                                                      ^^^^^

instance Zeros Text where zero = (""::Text)

的派生

instance Zeros Basis1 where zero = Basis1 zero  

应该是deriveanyclass策略的结果，但不是。

Answer 1

GeneralizedNewTypeDeriving将编写您建议的实例，但DeriveAnyClass将编写此实例：

instance Zeros Text => Zeros Basis1

请注意。没有方法定义！要使其工作,您需要为每个方法提供一个默认实现；通常这是通过泛型或类似的泛型编程工具包来完成的。