findKey.hs

findKey :: (Eq k) => k -> [(k, v)] -> Maybe v
-- 該当者なしならNothingでエラーハンドリング
findKey key [] = Nothing
findKey key ((k,v):xs)
        | key == k = Just v
        | otherwise = findKey key xs

-- 該当者なしならエラーになるやつ
findKey' :: (Eq k) => k -> [(k, v)] -> v
findKey' key xs = snd . head . filter (\(k, v) -> key == k) $ xs

-- 畳み込みでどうにかしたい
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 =
    [("betty", "555-2938")
    ,("bonnie", "452-2928")
    ,("patsy", "493-2928")
    ,("lucille", "205-2928")
    ,("wendy", "939-8282")
    ,("penny", "853-2492")
    ]

main = do
    print $ findKey "penny" phoneBook
    print $ findKey "betty" phoneBook
    print $ findKey "wilma" phoneBook -- Nothing
    print $ findKey' "penny" phoneBook
    print $ findKey' "betty" phoneBook
    -- print $ findKey' "wilma" phoneBook -- Nothing -- 該当者なしでエラー
    print $ findKey'' "penny" phoneBook
    print $ findKey'' "betty" phoneBook
    print $ findKey'' "wilma" phoneBook -- Nothing