List numberer

-- Copies newline-separated text from the clipboard, replaces tab
-- characters with rightwards arrows surrounded by spaces, numbers the
-- lines, replaces the original text in the clipboard with the output.

import System.Clipboard as SC

main :: IO ()
main = do
  input <- SC.getClipboardString
  if input == Nothing
    then SC.setClipboardString "There was nothing in the clipboard!"
    else SC.setClipboardString $ processData input

processData :: Maybe String -> String
processData = numberedListItemsToString . numberListItems . replaceTabsWithArrows . inputToListOfStrings

inputToListOfStrings :: Maybe String -> [String]
inputToListOfStrings Nothing = ["Nothing!"]
inputToListOfStrings (Just string) = lines string

replaceTabsWithArrows :: [String] -> [String]
replaceTabsWithArrows = map aux
  where
    aux [] = []
    aux (x:xs)
      | x == '\t' = ' ' : '→' : ' ' : aux xs
      | otherwise = x : aux xs

numberListItems :: [String] -> [(Int, String)]
numberListItems = zip [1 ..]

numberedListItemsToString :: [(Int, String)] -> String
numberedListItemsToString = unlines . map (\(number, string) -> show number ++ ". " ++ string)