Backwards goto and looping via the continuation monad.

Goto.hs

import Control.Monad.Cont 
import Data.Function


getLabel   = ContT fix
goto label = lift label
getLoop x  = ContT (\k -> let f x' = ContT (\_ -> k (f, x')) in k (f, x)) 


gotoTest :: IO ()
gotoTest = (`runContT` return) $ do 
    loop <- getLabel
    word <- lift getLine
    case word of
	"exit" -> return ()
	_      -> do
	    lift $ putStrLn $ "you typed: " ++ word
	    goto loop
    lift $ putStrLn "bye"


loopTest :: IO ()
loopTest = (`runContT` return) $ do
    (loop, i) <- getLoop 10
    word <- lift getLine
    if i == 0
	then return ()
	else do
	    lift $ putStrLn $ "loops remaining: " ++ show i
	    loop (i - 1) 
    lift $ putStrLn "bye"


main = loopTest