Person.hs

data Person = Person { _name :: String }

data Status a b = New b | Old a b
  deriving (Functor,Foldable,Traversable)

type PersonWithStatus = Status Int Person

sumNewFolks :: [PersonWithStatus] -> Int
sumNewFolks = foldr go 0
  where go (New _) b = b + 1
        go _ b = b

uniqOldFolksIds :: [PersonWithStatus] -> Data.Set.Set Int
uniqOldFolksIds = foldr go Data.Set.empty
  where go (Old oldId _) b = Data.Set.insert oldId b
        go _ b = b