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.
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