Lib.hs

{-# LANGUAGE ConstraintKinds      #-}
{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE GADTs                #-}
{-# LANGUAGE ScopedTypeVariables  #-}
{-# LANGUAGE TypeApplications     #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE UndecidableInstances #-}

import Data.Kind (Constraint, Type)


main = undefined

data HList (ts :: [Type]) where
    HNil :: HList '[]
    (:#) :: t -> HList ts -> HList (t ': ts)

infixr 5 :#

instance Eq (HList '[]) where
    HNil == HNil = True

instance (Eq t, Eq (HList ts)) => Eq (HList (t ': ts)) where
    (a :# as) == (b :# bs) = a == b && as == bs


instance Ord (HList '[]) where
    compare HNil HNil = EQ

instance (Ord t, Ord (HList ts)) => Ord (HList (t ': ts)) where
    compare (a :# as) (b :# bs) = case compare a b of
                                    EQ -> compare as bs
                                    x -> x

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

instance (Show t, Show (HList ts)) => Show (HList (t ': ts)) where
    show xs = "[" ++ (showMany xs "") ++ "]"
        where
            showMany (y :# ys) prefix = prefix ++ (show y) ++ rest
                where rest = case ys of
                               HNil -> ""
                               ys -> showMany ys ", "