I am trying to implement "OrdList", which is a list that is guaranteed to be ordered. Here's my implementation so far: module OrdList (empty, fromList, insert, toList, OrdList(OrdList)) where import Data.List as L (span) data OrdList a = OrdList [a] deriving (Show) empty :: OrdList a empty = OrdList [] insert :: (Ord a) => a -> OrdList a -> OrdList a insert x ol = let -- OrdList xs = ol (before, after) = L.span (\el -> el <= x) $ toList xs in OrdList $ before ++ [x] ++ after fromList :: Ord a => [a] -> OrdList a fromList xs = if null xs then empty else insert (head xs) (fromList $ tail xs) toList :: OrdList t -> [t] toList xs = let OrdList xs' = xs in xs' It "works", but no doubt there's plenty to dislike about it. Some specific questions I have in mind: 1. Should OrdList be best implemented as a class, data, or newtype? If so, what changes need to be made? 2. How comes I can't refer to OrdList as a data type? This is a problem because I can't say that I want the type of something to be OrdList Int, for example.