{-# LANGUAGE DataKinds, TypeFamilies, AllowAmbiguousTypes, UndecidableInstances #-}

import Data.Kind


main :: IO ()
main = do
  print (check @[Bool, Int])
  -- print (check @[Float, Bool, Int])


check :: forall xs. Extendss Proxy xs => ()
check = ()

type SeqConstraintUnit :: Constraint -> Constraint -> Constraint
type family SeqConstraintUnit a b where
  SeqConstraintUnit () x = x

type Extendss :: ([Type] -> Type) -> [Type] -> Constraint 
type family Extendss p xs where
  Extendss p '[] = ()
  Extendss p (x : xs) = (Extends p x `SeqConstraintUnit` Extendss p xs)
  
  
type Extends :: ([Type] -> Type) -> Type -> Constraint
type family Extends a b

data Proxy a = P

type instance Extends Proxy Bool = ()
type instance Extends Proxy Int = ()