gistfile1.txt

{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DataKinds #-}
import GHC.Exts

type family AllAllowed k xs where
	AllAllowed k '[] = ()
	AllAllowed k (x : xs) = (Allowed k x, AllAllowed k xs)

class Category k where
	type Allowed k a :: Constraint
	type Allowed k a = ()

	id :: Allowed k a => k a a
	(.) :: AllAllowed k [a,b,c] => k b c -> k a b -> k a c

class (Category k, forall a b. (Allowed k a, Allowed k b) => Allowed k (a, b)) => Monoidal k where
	x :: AllAllowed k [a,b,c,d] => k a c -> k b d -> k (a,b) (c,d)

instance Category (->) where
	id = \x -> x
	f . g = \x -> f (g x)

instance Monoidal (->) where
	x f g (a,b) = (f a, g b)