Dimitry Golubovsky wrote:
If I have
callCC $ \exit -> do foo ...
I cannot jump to `exit' from within foo unless `exit' is given to foo as an argument.
As Derek Elkins has written, one of the options is to use delimited continuations, see http://research.microsoft.com/~simonpj/papers/control/ for Haskell implementation. But in this case Cont may be enough. If you don't like passing `exit' explicitly, you can put in into Reader monad. This is the idea: -------------------------------------------------------------------- import Control.Monad.Cont import Control.Monad.Reader type Abortable r a = ReaderT (r -> Cont r r) (Cont r) a runAbortable :: Abortable a a -> a runAbortable m = runCont (callCC $ \exit -> runReaderT m exit) id abort :: r -> Abortable r a abort x = do exit <- ask lift (exit x) undefined -- this hack is needed to make abort polymorphic test a b c = do x <- if a then abort "a" else return 1 y <- if b then abort "b" else return False z <- foo c -- calling foo without explicit abort continuation return $ show (x, y, z) where foo True = abort "c" foo False = return 5.39 run m = putStrLn (runAbortable m) main = do run (test False False False) run (test False False True) run (test False True False) run (test True False False) ------------------------------------------------------------------ This implementation is a bit hackish, since it uses undefined to make abort polymorphic in return type. You can use rank-2 types to avoid it, see http://www.vex.net/~trebla/tmp/ContMonad.lhs by Albert C. Lai.