Derived instance in Haskell

Question

I would like to use derived instance like this:

data Test3D = forall a. (Show a, Eq a, Typeable a, Generic a)
                => Test3D { testDt :: String
                          , testPrm :: a
                          }
   deriving (Show, Eq, Typeable, Generic)

instance Binary (Test3D)
$(deriveJSON defaultOptions ''Test3D)

But I received from GHC:

• Can't make a derived instance of ‘Show Test3D’:
        Constructor ‘Test3D’ has existentials or constraints in its type
        Possible fix: use a standalone deriving declaration instead
• In the data declaration for ‘Test3D’

This way is very convenient for my project. I can not find the solution.

Is any way of using derived instance for such data?

Answer 1

Is any way of using derived instance for such data?

Yes. Do what GHC suggested, make a standalone deriving clause:

{-# LANGUAGE StandaloneDeriving, ExistentialQuantification #-}

data Test3D = forall a. (Show a)
                => Test3D { testDt :: String
                          , testPrm :: a
                          }

deriving instance Show Test3D

What you cannot do is derive an Eq instance, because different values may actually contain different types and it's only possible to compare these with a dynamic-cast hack through Typeable.