Goto

gistfile1.txt

newtype Label r m = Label { runLabel :: ContT r m () }

label :: ContT r m (Label r m)
label = callCC $ \k -> return $ let x = Label (k x) in x

goto :: Label r m -> ContT r m b
goto lbl = runLabel lbl >> goto lbl

usesGoto :: (Monad m) => ContT r m r -> m r
usesGoto = flip runContT return

{-
usesGoto = flip runContT return
label = ContT fix
goto label = lift label
-}

main :: IO ()
main = usesGoto $ do
   abc <- label
   liftIO $ putStrLn "goto"
   goto abc