Using defunctionalization to avoid partially applied type synonyms

Defunctionalization.hs

{-# LANGUAGE DataKinds      #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies   #-}
{-# LANGUAGE PolyKinds      #-}

data Fn a = Id | Const a

type family Apply (fn :: Fn k) (a :: k) :: k where
  Apply Id        a = a
  Apply (Const b) a = b

type Schema f = (Apply f Bool, Apply f Int)

type Example1 = Schema Id             -- Example1 ~ (Bool, Int)
type Example2 = Schema (Const Double) -- Example2 ~ (Double, Double)