Showable HList

ThisIsWeird.hs

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

module ThisIsWeird where

import           ClassyPrelude
import           Data.Kind (Constraint)

data HList xs where
  HNil :: HList '[]
  HCons :: a -> HList as -> HList (a ': as)

type family   ConstrainAllHList (c :: * -> Constraint) (ls :: [*]) :: Constraint
type instance ConstrainAllHList c '[] = ()
type instance ConstrainAllHList c (x ': xs) = (c x, ConstrainAllHList c xs)

showAllHList :: ConstrainAllHList Show xs => HList xs -> [String]
showAllHList HNil         = []
showAllHList (HCons x xs) = (show x) : (showAllHList xs)