main1 = do s <- getLine
           if s == "foo" then putStr "You entered foo"

nop :: IO ()
nop = sequence_ []

main2 = do s <- getLine
           if s == "foo" then putStr "You entered foo" else nop

doIf :: Bool -> IO () -> IO ()
doIf b m = if b then m else nop

main3 = do s <- getLine
           doIf (s == "foo") (putStr "You entered foo")

return ()

import Control.Monad
main = do s <- getLine
          when (s == "foo") $ putStr "You entered foo"