-- From http://learnyouahaskell.com/input-and-output, somewhat modified

main = interact (unlines . map reverseWords . lines)

-- These are equivalent:
--   interact (\ x -> unlines $ map reverseWords (lines x))
--   interact (\ x -> unlines (map reverseWords (lines x)))
--   (unlines . map reverseWords . lines)

reverseWords :: String -> String
reverseWords = unwords . map reverse . words

-- Note what happens for 
--   main = interact reverseWords

-- This program *also* works, but prints its output only when the end
--  of file is reached. You will not notice any difference if
--  ./reverse-interact gets its input from a file (i.e., via input
--  redirection from the shell) or from a Unix pipe. However,
--  "interactive" line-by-line operation of the above main will not
--  work. See loop3.hs (and loop2.hs, too)

-- This happens because there's nothing to clue the IO actions in
--  interact that newlines are somehow special. In the above main,
--  on the other hand, the (pure) function lines (defined in Data.List,
--  :i tells us--look up its code) causes a sequence of reads in
--  interact.