hello_world.hs

{-# LANGUAGE ConstraintKinds, DataKinds, DeriveDataTypeable,
             GADTs, TypeFamilies, TypeOperators, UndecidableInstances #-}
module Codewars.Kata.HelloWorld where

import Control.Arrow(first, second)
import Data.Char(toLower)
import Data.Typeable
import GHC.Exts(Constraint)

type family All (c :: * -> Constraint) (ts :: [*]) :: Constraint
type instance All c '[] = ()
type instance All c (t ': ts) = (c t, All c ts)

data HList (ts :: [*]) where
  Nil  :: HList '[]
  (:>) :: t -> HList ts -> HList (t ': ts)
  
infixr 5 :>

data Delta = Greeting deriving Typeable
data Echo = Is deriving Typeable
data Hotel = This deriving Typeable
data Lima = A | Very deriving Typeable
data Oscar = Weird | Of | To deriving Typeable
data Romeo = Say deriving Typeable
data Whiskey = Way deriving Typeable

toTypeList :: All Typeable ts => HList ts -> [TypeRep]
toTypeList Nil = []
toTypeList (t:>ts) = typeOf t : toTypeList ts

greet :: String
greet = (++"!")
      . unwords
      . (\ ~(x,y) -> [x,y])
      . first (map (toLower . head . show) . toTypeList)
      . second (map (toLower . head . show) . toTypeList)
      $ (This :> Is :> A :> Very :> Weird :> Nil,
         Way :> To :> Say :> A :> Greeting :> Nil)

Solution for kata "hello world!" by Aditu_V.