On Tue, 2007-11-20 at 13:22 +0200, Gleb Alexeyev wrote:
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.
I made no such suggestion. I simply suggested using instead of callCC f = Cont (\k -> runCont (f (\a -> Cont $ \_ -> k a)) k) with control f = Cont (\k -> runCont (f (\a -> Cont $ \_ -> k a)) id) abort is then simply abort = control . const . return callCC is obviously just callCC f = control (\k -> f k >>= k) If you didn't want to "break abstraction" like this, you can implement this in terms of just callCC awkwardly. To do so requires having a top-level continuation and is exactly what is below. The above and below is (part of) why I prefer control.
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.
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