きょうはData.Mapに触れましたよ #CodePiece

CodePiece.hs

import qualified Data.Map as Map
import Data.Char

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

findKey :: (Eq k) => k -> [(k, v)] -> v
findKey key xs =  snd . head . filter (\(k,v) -> key == k) $ xs

findKey' :: (Eq k) => k -> [(k, v)] -> Maybe v
findKey' key [] = Nothing
findKey' key ((k,v):xs)
  | key == k = Just v
  | otherwise = findKey' key xs

findKey'' :: (Eq k) => k -> [(k, v)] -> Maybe v
findKey'' key xs = foldr (\(k,v) acc -> if key == k then Just v else acc) Nothing xs


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

string2digits :: String -> [Int]
string2digits = map digitToInt . filter isDigit

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