Abjader

-- The app reads text from stdin, picks words that are four or more characters
-- long and removes non-initial vowels from them thus making words shorter but
-- still in may cases comprehensible. All characters apart from letters and
-- spaces are removed, spaces are replaced with underscores, uppercase letters
-- are made lowercase. This sentence, processed by the app, looks like this:
-- ths_sntnc_prcssd_by_the_app_lks_lk_ths

import System.Exit (die)
import Data.Char (toLower)
import Data.List (intercalate)

main :: IO ()
main = do
  input <- getContents
  if null input
    then die "No input!"
    else putStr $
         intercalate "_" $
         map removeVowelsInLongWords $
         words $ removeInvalidChars $ map toLower input

removeInvalidChars :: String -> String
removeInvalidChars = filter (`elem` ['a' .. 'z'] ++ " ")

removeVowelsInLongWords :: String -> String
removeVowelsInLongWords word
  | length word < 4 = word
  | otherwise = revomeVowels word
  where
    revomeVowels [] = []
    revomeVowels (x:xs) = x : filter (not . (`elem` "aeiouy")) xs