Haskell 数据族和可类型化

以下是一段完全可编译的代码：

data family DF a
data instance DF Int = DFInt deriving (Show)
data instance DF Char = DFChar deriving (Show)

但是，尝试按以下方式导出可键入的

会出现“重复实例声明”错误：

那么，如何为数据族派生
Typeable
？有可能吗？如果不是，那么原因是什么？
添加独立的派生
Typeable1
实例（如以下代码）解决了问题，但我没有解释为什么这样做，因此问题仍然悬而未决

data family DF a
deriving instance Typeable1 DF
data instance DF Int = DFInt deriving (Show)
data instance DF Char = DFChar deriving (Show)

如果使用
-ddump deriv
查看派生的输出，您会发现每个数据族实例声明都生成了相同的
实例类型1 DF

data family DF a
data instance DF Int = DFInt deriving (Typeable)
data instance DF Char = DFChar deriving (Typeable)

给予

因此，这种类型派生实际上并不是您希望它做的事情——它是将族派生为
Typeable1
而不是实例。但是，很可能您需要的是内置的，因为
实例（typeable1s，typeablea）=>Typeable（sa）

data family DF a
deriving instance Typeable1 DF
data instance DF Int = DFInt deriving (Show)
data instance DF Char = DFChar deriving (Show)

data DFAll = forall a . Typeable a => DFAll (DF a)

toDFA :: Typeable a => DF a -> DFAll
toDFA = DFAll

fromDFA :: Typeable a => DFAll -> Maybe (DF a)
fromDFA (DFAll x) = cast x

及

因此，实例派生问题看起来像一个bug。关于你的答案，你引入存在主义到底是为了什么？只是为了证明你从中得到了一个功能性的
cast
。
instance Data.Typeable.Internal.Typeable1 Main.DF where
  Data.Typeable.Internal.typeOf1 _
    = Data.Typeable.Internal.mkTyConApp
        (Data.Typeable.Internal.mkTyCon
           17188516606402618172## 4748486123618388125## "main" "Main" "DF")
        []

instance Data.Typeable.Internal.Typeable1 Main.DF where
  Data.Typeable.Internal.typeOf1 _
    = Data.Typeable.Internal.mkTyConApp
        (Data.Typeable.Internal.mkTyCon
           17188516606402618172## 4748486123618388125## "main" "Main" "DF")
        []

data family DF a
deriving instance Typeable1 DF
data instance DF Int = DFInt deriving (Show)
data instance DF Char = DFChar deriving (Show)

data DFAll = forall a . Typeable a => DFAll (DF a)

toDFA :: Typeable a => DF a -> DFAll
toDFA = DFAll

fromDFA :: Typeable a => DFAll -> Maybe (DF a)
fromDFA (DFAll x) = cast x

*Main> fromDFA (toDFA DFInt) :: Maybe (DF Int)
Just DFInt