{-# LANGUAGE StrictData, GADTs, DataKinds, KindSignatures #-}
{-# LANGUAGE RankNTypes, StandaloneDeriving #-}

-- this gets suggested, but doesn't help
-- {-# LANGUAGE UndecidableInstances #-}

-- this would help but isn't in until 8.6?
-- {-# LANGUAGE QuantifiedConstraints #-}

import Data.Kind (Type)

data State
  = A
  | B

data Thing :: (State -> State -> Type -> Type) -> Type where
  IntThing :: f a b Int -> f b c Int -> Thing f
  DoubleThing :: f a b Double -> f b c Double -> Thing f

-- this doesn't work, but suggests -XUndecidableInstances, which also doesn't work
-- deriving instance (Show (f a b Int), Show (f a b Double)) => Show (Thing f)

-- this also doesn't work, also suggests -XUndecidableInstances, which also doesn't work
{-
instance (Show (f a b Int), Show (f a b Double)) => Show (Thing f) where
  show (IntThing x y) = "IntThing " ++ show x ++ " " ++ show y
  show (DoubleThing x y) = "DoubleThing " ++ show x ++ " " ++ show y
-}

-- doesn't work until 8.6 with -XQuantifiedConstraints?
-- deriving instance (forall i j a . Show a => Show (f i j a)) => Show (Thing f)