Swap or prepend

-- The app reads two tab-separated columns of text from the clipboard
-- and either swaps the columns or prepends the two columns with the
-- second one. The swap mode is enabled with the "-s" option, the
-- other mode with any non-empty string. No option results in an error
-- message.

import System.Environment (getArgs)
import System.Hclip (getClipboard)
import System.Exit (die)

main :: IO ()
main = do
  args <- getArgs
  if null args
    then die "No option has been given!"
    else do
      input <- getClipboard
      if null input
        then die "The clipboard is emtpy!"
        else mapM_ (putStrLn . process (head args == "-s")) $ lines input


findTab :: String -> Int
findTab = aux 0
  where
    aux index (x:xs)
      | x == '\t' = index
      | otherwise = aux (index + 1) xs

splitAtTab :: String -> (String, String)
splitAtTab [] = ([], [])
splitAtTab list = (take tabIndex list, drop (tabIndex + 1) list)
  where
    tabIndex = findTab list

process :: Bool -> String -> String
process _ [] = []
process bool string
  | bool = snd tuple ++ "\t" ++ fst tuple
  | otherwise = snd tuple ++ "\t" ++ fst tuple ++ "\t" ++ snd tuple
  where
    tuple = splitAtTab string