MPTCs + fundepsとtype familiesは型の上の関数

FunWithTypes.hs

{-# LANGUAGE UndecidableInstances, FlexibleInstances #-}
{-# LANGUAGE TypeFamilies, FunctionalDependencies, MultiParamTypeClasses #-}

data Z
data S n

class Plus m n r | m n -> r
instance Plus Z n n
instance Plus m n r => Plus (S m) n (S r)

two = undefined :: Plus (S Z) (S Z) r => r

type family Plus' m n :: *
type instance Plus' Z     n = n
type instance Plus' (S m) n = S (Plus' m n)

two' = undefined :: Plus' (S Z) (S Z)

{-
Prelude> :t two
two :: S (S Z)
Prelude> :t two'
two' :: S (S Z)
-}