ExistentialPhone.hs

{-# LANGUAGE ExistentialQuantification #-}

import Data.Typeable

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

data Selection = Selection Int Int

data ExistPhone = forall t. (IsContact t, Typeable t) => MkPhone t

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

class IsContact a

instance IsContact PhoneContact
instance IsContact PhoneWithSelectionContact

filterSelections :: [ExistPhone] -> [PhoneWithSelectionContact]
filterSelections = concatMap isSelection
    where
        isSelection :: ExistPhone -> [PhoneWithSelectionContact]
        isSelection (MkPhone v)
            | Just x <- cast v = [x]
            | otherwise        = []