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

假设我们有一个类型类 class (A a, B a) => C a where . 使用 newtype 将允许我们克隆数据类型，然后通过 GeneralizedNewtypeDeriving 语言扩展自动派生实例（请参阅how to write a derivable class?和Handling multiple types with the same internal representation and minimal boilerplate?） .

QUESTION ：是否有可能让ghc自动派生 A 和 C ，但是在派生 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 = Lives ， C = Description ）无法按预期工作：

{-# 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"

我期待/想要的是 Description 的 Dolphin 实例在 Description 的派生中被调用 .

显然以下程序有效，但它需要一个为 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]

附：令人费解的是，如果（在第一个程序中）我没有声明：

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’

似乎很奇怪ghc会抱怨缺少 instance Lives Dolphin where 如果 not 在 Description 的 Description （自动）推导中使用 not .