[Haskell-cafe] Control.Monad.Cont fun

[oleg at pobox.com]

On Thu, Jul 07, 2005 at 07:08:23PM +0200, Tomasz Zielonka wrote:
> Some time ago I wanted to return the escape continuation out of the
> callCC block, like this:
>
>   getCC = callCC (\c -> return c)

It seems using shift/reset is better not only in principle but in
practice as well.

> module Foo where
>
> import Control.Monad.Cont
>
> -- From  http://www.haskell.org/hawiki/MonadCont
> reset :: (Monad m) => ContT a m a -> ContT r m a
> reset e = ContT $ \k -> runContT e return >>= k
>
> shift :: (Monad m) =>
> 	 ((a -> ContT r m b) -> ContT b m b) 
>           -> ContT b m a
> shift e = ContT $ \k -> 
>             runContT (e $ \v -> ContT $ \c -> k v >>= c) return

We can define a more general getCC' form simply as follows:

> getCC' init = shift (\f -> f (init,jump f))
>     where jump f x = f (x,jump f)

The compiler figures out the types.

Here's the usage example from the original Tomasz's message

> test1' :: IO ()
> test1' = (`runContT` return) $ 
>     do
>     reset (do
> 	   (x, jump) <- getCC' 0
> 	   lift (print x)
>          when (x < 10) $ jump (x + 1))
>     lift (putStrLn "finish")

That was using the ContT monad exactly as it comes with GHC 6.4.

With the CC_CPST monad transformer (with multiple polymorphic
prompts) we can do better:

> getCCP' p init = shiftP p (\f -> f (return (init,jump f)))
>     where jump f x = f (return (x,jump f))

The following code (written a couple of weeks) prints 1 through 10,
then again 4 through 10 and then again 5 through 10, and then
"finish". So we can not only note a label to jump to: we can return
the label so to jump to it from different `functions'. In a sense, we
emulate multiple entry points to a block and cross-block jumps.

> test1'' :: IO ()
> test1'' = runCC (
>     do
>     p <- newPrompt
>     p1 <- newPrompt
>     j <- pushPrompt p1 ( pushPrompt p (
> 	   do
> 	   (x, jump) <- getCCP' p 0
> 	   lift (print x)
> 	   when (x < 10) $ jump (x+1)
> 	   shiftP p1 (const (return jump))) >> undefined)
>     lift (putStrLn "again")
>     pushPrompt p1 (j 4 >> undefined)
>     lift (putStrLn "and again")
>     pushPrompt p1 (j 5 >> undefined)
>     lift (putStrLn "finish"))

In this terminating code, undefined plays a great role.