通过GeneralizedNewtypeDeriving派生实例时使用自定义实例

Question

假设我们有一个typeclass类（a a,B a）=>C a where。使用NewType将允许我们克隆一个数据类型，然后通过GeneralizedNewTypeDeriving语言扩展自动派生实例（请参见如何编写可派生类？以及使用相同的内部表示和最小的样板处理多个类型？）。

问题：是否可以让ghc自动派生A和C，但在派生C时使用我们自己指定的B实现？

例如，以下代码（其中A=planet，B=live，C=description)不能按预期工作：

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE StandaloneDeriving #-}
module Main (main) where

data Cat = Cat String
newtype Dolphin = Dolphin Cat deriving (Planet)

------------------------------------------------

class Planet a where
  planet :: a -> String

class Lives a where
  lives :: a -> String

class (Planet a, Lives a) => Description a where
  description :: a -> String

------------------------------------------------

instance Planet Cat where
  planet _ = "lives on planet earth,"

instance Lives Cat where
  lives _ = "lives on land"

instance Description Cat where
  description a = (planet a) ++ (lives a)

------------------------------------------------

instance Lives Dolphin where
  lives _ = "lives in the sea"

--want the following derivation to use the instance of 
--"Lives" for "Dolphin" above
deriving instance Description Dolphin

------------------------------------------------

main = do
  print $ description (Cat "test")
  -- > "lives on planet earth,lives on land"
  -- OK
  print $ description (Dolphin (Cat "test"))
  -- > "lives on planet earth,lives on land"
  -- NOT OK. Want "lives on planet earth,lives in the sea"

我期望/想要的是在派生description时调用lives的dolphin实例。

显然，以下程序可以工作，但它需要为dolphin显式实例化description:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE StandaloneDeriving #-}
module Main (main) where

data Cat = Cat String
newtype Dolphin = Dolphin Cat deriving (Planet)

------------------------------------------------

class Planet a where
  planet :: a -> String

class Lives a where
  lives :: a -> String

class (Planet a, Lives a) => Description a where
  description :: a -> String

------------------------------------------------

instance Planet Cat where
  planet _ = "lives on planet earth,"

instance Lives Cat where
  lives _ = "lives on land"

instance Description Cat where
  description a = (planet a) ++ (lives a)

------------------------------------------------

instance Lives Dolphin where
  lives _ = "lives in the sea"

instance Description Dolphin where
  description a = (planet a) ++ (lives a)

------------------------------------------------

main = do
  print $ description (Cat "test")
  -- > "lives on planet earth,lives on land"
  --[OK]
  print $ description (Dolphin (Cat "test"))
  -- > "lives on planet earth,lives in the sea"
  --[OK]

instance Lives Dolphin where
  lives _ = "lives in the sea"

然后ghc抱怨：

Main.hs:36:1:
    No instance for (Lives Dolphin)
      arising from the superclasses of an instance declaration
    In the instance declaration for ‘Description Dolphin’

奇怪的是，如果ghc没有在Dolphin的description的（自动）派生中使用instance Lives Dolphin的话，它会抱怨没有instance Lives Dolphin。

Answer 1

请考虑以下内容：

newtype ProcessID = PID Int deriving Eq

这样做的目的是编写一个实例

instance Eq PID where
  (PID x) == (PID y)    =    x == y

换句话说，当您对pid调用==时，它会将其解包装为普通的int，然后对其执行==。

（或者这是你想要解决的某个更复杂问题的简化？）