(Show a) => HeteroList a, (Real a) => HeteroList a -- with ConstraintKinds Extension

HList.hs

{-# LANGUAGE GADTs, DataKinds, TypeFamilies, ConstraintKinds, TypeOperators, UndecidableInstances #-}

import GHC.Prim (Constraint)
import Data.Ratio ((%))

type family All (cx :: * -> Constraint) (xs :: [*]) :: Constraint
type instance All cx '[] = ()
type instance All cx (x ': xs) = (cx x, All cx xs)

data HList (as :: [*]) where
  Nil :: HList '[]
  Cons :: a -> HList as -> HList (a ': as)

instance (All Show as) => Show (HList as) where
  show Nil = "[]"
  show (Cons x xs) = show x ++ ":" ++ show xs

sumRational :: (All Real as) => HList as -> Rational
sumRational Nil = 0
sumRational (Cons x xs) = toRational x + sumRational xs

main = do
  print $ Cons 100 (Cons 'q' Nil) -- 100:'q':[]

  print $ sumRational $ Cons (199 :: Int) $ Cons (1.200 :: Float) $ Cons (3 % 4 :: Rational) $ Nil -- 842845389 % 4194304