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{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DatatypeContexts #-}
module CannotDoRep0_0 where
import GHC.Generics hiding (P, D)
-- We do not support existential quantification
data Dynamic = forall a. Dynamic a deriving Generic
-- Partial instantiation of types is not allowed
data P a = P a
deriving instance Generic (P Int)
-- This gets trickier for data families
data family D a b
data instance D Char b
data instance (Show b) => D Int b
data instance D () ()
-- Bad: second argument is instantiated
deriving instance Generic (D Char Char)
-- Bad: has context
deriving instance Generic (D Int a)
-- Ok
deriving instance Generic (D () ())
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