HeteroListPhone.hs

{-# LANGUAGE TypeOperators, TypeInType, GADTs, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-}

data HList (xs :: [k]) where
    Z    :: HList '[]
    (:.) :: a -> HList xs -> HList (a ': xs)

infixr 5 :.

class Filter a xs where
    typeFilter :: HList xs -> [a]

instance Filter a '[] where
    typeFilter _ = []

instance Filter a xs => Filter a (a ': xs) where
    typeFilter (v :. xs) = v : typeFilter xs

instance Filter a xs => Filter a (b ': xs) where
    typeFilter (v :. xs) = typeFilter xs

newtype Name = Name { getName :: Name }
newtype Phone = Phone { getPhone :: Int }

data Selection = Selection Int Int

data PhoneContact = PhoneContact Name Phone
data PhoneWithSelectionContact = PhoneWithSelectionContact Name Phone Selection

class IsContact a

instance IsContact PhoneContact
instance IsContact PhoneWithSelectionContact

filterSelections :: Filter PhoneWithSelectionContact xs => HList xs -> [PhoneWithSelectionContact]
filterSelections = typeFilter