Phone list search

-- Task: a function that takes a list of names and phone numbers i.e.
-- a list of tuples (or an empty list) and a string (including an
-- empty one) and returns the values of the matching tuple i.e.
-- containging the argument string. The result should be a string
-- containing the values. The function should warn about empty strings
-- or lists.

phoneList =
  [ ("Betty", "555-2938")
  , ("Bonnie", "452-2928")
  , ("Patsy", "493-2928")
  , ("Lucille", "205-2928")
  , ("Wendy", "939-8282")
  , ("Penny", "853-2492")
  ]

findNumber :: [([Char], [Char])] -> [Char] -> [Char]
findNumber [] _ = "No list given"
findNumber _ [] = "No name given"
findNumber list name = aux list name
  where
    aux (x:xs) n
      | n == fst x = fst x ++ ": " ++ snd x ++ "."
      | null xs = "Not on the list"
      | otherwise = aux xs n

main :: IO ()
main =
  print $
  map
    (findNumber phoneList)
    ["Betty", "Bonnie", "Patsy", "Lucille", "Wendy", "Penny"]

-- ["Betty: 555-2938.","Bonnie: 452-2928.","Patsy: 493-2928.","Lucille: 205-2928.","Wendy: 939-8282.","Penny: 853-2492."]

-- findNumber [] "Betty"
-- "No list given"

-- findNumber phoneList ""
-- "No name given"

-- findNumber phoneList "Sally"
-- "Not on the list"