HList via GADTs

hlist-gadt.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}

module HList.GADT where

import Data.Kind

data HList (ts :: [Type]) where
  HNil :: HList '[]
  HCons :: forall t ts. t -> HList ts -> HList (t ': ts)

instance (Show t, Show (HList ts)) => Show (HList (t ': ts)) where
  show (HCons t ts) = "HCons " ++ show t ++ " (" ++ show ts ++ ")"

instance Show (HList '[]) where
  show HNil = "HNil"

example :: HList [Int, Bool, String]
example = HCons 2 $ HCons True $ HCons "hi" $ HNil

{-
λ example
HCons 2 (HCons True (HCons "hi" (HNil)))
-}