phoneBookMap.hs

import Data.Char
import qualified Data.Map as Map

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")
    ]

phoneBookToMap :: (Ord k) => [(k, String)] -> Map.Map k String
phoneBookToMap xs = Map.fromListWith add xs
    where add number1 number2 = number1 ++ ", " ++ number2

phoneBookToMap' :: (Ord k) => [(k, a)] -> Map.Map k [a]
phoneBookToMap' xs = Map.fromListWith (++) $ map (\(k, v) -> (k, [v])) xs
main = do
    print $ Map.lookup "patsy" $ phoneBookToMap phoneBook
    print $ Map.lookup "wendy" $ phoneBookToMap phoneBook
    print $ Map.lookup "patsy" $ phoneBookToMap' phoneBook