{-# language TypeFamilies, DataKinds, UndecidableSuperClasses, UndecidableInstances, TypeFamilyDependencies, QuantifiedConstraints #-}
module M where
import GHC.TypeLits

data FArray (n :: Nat)
instance Num (FArray n)

class Tensor r where
  type TensorOf (n :: Nat) r = t | t -> n r
  type Primal r

instance Tensor Float where
  type TensorOf n Float = FArray n
  type Primal Float = Float

class c (TensorOf n r) => CTensorOf c n r where
instance c (TensorOf n r) => CTensorOf c n r where

foo :: (forall n. CTensorOf Num n r) => TensorOf n r -> TensorOf n r
foo x = x

foo2 :: (p ~ Primal r, forall n. CTensorOf Num n p) => TensorOf n r -> TensorOf n (Primal r) -> TensorOf n (Primal r)
foo2 _ x = x

bar :: FArray n -> FArray n
bar x = foo x