phoneBook.hs

import qualified Data.Map as Map
phoneBook :: Map.Map String String
phoneBook = Map.fromList $
            [("betty", "555-2938")
            ,("bonnie", "452-2928")
            ,("patsy", "493-2928")
            ,("lucille", "205-2928")
            ,("wendy", "939-8282")
            ,("penny", "853-2492")
            ]

main = do
    -- lookup :: Eq a => a -> [(a, b)] -> Maybe b
    -- Map(連想リスト)からキーを検索して、ValueとしてMaybeを返している
    print $ Map.lookup "betty" phoneBook -- Just "555-2938"
    print $ Map.lookup "wendy" phoneBook -- Just "939-8282"
    print $ Map.lookup "grace" phoneBook -- 該当しない場合はNothing