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

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

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