gistfile1.hs

import qualified Data.Map
import qualified Data.Maybe


listInsert :: Ord k => (k, v) -> Data.Map.Map k [v] -> Data.Map.Map k [v]
listInsert (k,v) acc = if Data.Maybe.isNothing kv then Data.Map.insert k [v] acc else Data.Map.insert k (v:Data.Maybe.fromJust kv) acc
    where kv = Data.Map.lookup k acc

fromListToMapList :: Ord k => [(k, a)] -> Data.Map.Map k [a]
fromListToMapList = foldr listInsert Data.Map.empty

{-|
phoneBook =
    [("betty","555-2938")
    ,("betty","342-2492")
    ,("bonnie","452-2928")
    ,("patsy","493-2928")
    ,("patsy","943-2929")
    ,("patsy","827-9162")
    ,("lucille","205-2928")
    ,("wendy","939-8282")
    ,("penny","853-2492")
    ,("penny","555-2111")
    ]

fromListToMapList phoneBook == fromList [("betty",["555-2938","342-2492"])
                                        ,("bonnie",["452-2928"])
                                        ,("lucille",["205-2928"])
                                        ,("patsy",["493-2928","943-2929","827-9162"])
                                        ,("penny",["853-2492","555-2111"])
                                        ,("wendy",["939-8282"])]
-}