Typeclass hacks to automatically injects elements into a list

inj-list-sugared.hs

{-# LANGUAGE FlexibleInstances #-}

data AllType = TypeA Char
             | TypeB String
             | TypeC Bool
             deriving (Show)

newtype S = S { unS :: String }

data Ii = Ii { runIi :: [AllType] }

list1 = runIi $ iI '5' '1' '4' (S "Hello World") True '8'
list2 = runIi $ iI (S "Just") False '8' (S "Some") (S "Sugar")

class ASTAll a where
  inj :: a -> AllType

instance ASTAll Char where
  inj = TypeA

instance ASTAll S where
  inj = TypeB . unS

instance ASTAll Bool where
  inj = TypeC

class Heterogeneous a where
  iI' :: [AllType] -> a

iI :: Heterogeneous a => a
iI = iI' []

instance Heterogeneous Ii where
  iI' xs = Ii (reverse xs)

instance (ASTAll a, Heterogeneous as) => Heterogeneous (a -> as) where
  iI' xs a = iI' (inj a : xs)