instance MapLike ListMap.hs

import Prelude hiding (lookup)
import qualified Data.List as L

class MapLike m where
  empty :: m k v
  lookup :: Ord k => k -> m k v -> Maybe v
  insert :: Ord k => k -> v -> m k v -> m k v
  delete :: Ord k => k -> m k v -> m k v
  fromList :: Ord k => [(k,v)] -> m k v
  fromList [] = empty
  fromList ((k,v):xs) = insert k v (fromList xs)

newtype ListMap k v = ListMap { getListMap :: [(k,v)] }
  deriving (Eq, Show)

instance MapLike ListMap where
  empty = ListMap []

  lookup k (ListMap m) 
    | k `elem` (map fst m) = Just $ snd $ head $ filter (\(k', v') -> k == k') m
    | otherwise = Nothing

  insert k v (ListMap m) 
    | k `elem` (map fst m) = ListMap $ map (\p@(k', v') -> if k' == k then (k', v) else p) m
    | otherwise = ListMap $ (k, v):m

  delete k (ListMap m) = ListMap (filter (\(k', _) -> k /= k') m)