[Haskell-cafe] How to abort a computation within Continuation Monad?

[Derek Elkins]

On Tue, 2007-11-20 at 00:18 -0500, Dimitry Golubovsky wrote:
> Hi,
> 
> I have been using plain non-monadic CPS for a while in my web-browser
> related stuff. Now I am tempted to switch from plain CPS to
> syntactically sweetened monadic style based on Continuation Monad, but
> I feel stuck with one important issue that I need an advice on.
> 
> In plain CPS, I may write:
> 
> type CPS x y = (y -> x) -> x
> 
> a :: x
> 
> a = f1 x $ \r ->
>   case r of
>     foo1 -> bar  -- of type x
>     foo2 -> f2 r $ \p -> ...  -- something finally evaluating to a
> value of type x
> 
> So, if at any time I return a value of a final type (x) instead of
> doing something with given continuation,
> I abort/suspend the whole computation. This can happen at any function
> call depth. I can also save the continuation reference in some
> persistent place, to resume the remainder of computation later.
> 
> Now, I am trying to do the same with a Continuation Monad. But does
> anything similar exist in this monad? callCC is not exactly what I
> need because it only helps abort the inner computation, and what is
> returned, is a monadic value, not a final value. Besides, callCC
> defines a name of the function corresponding to the current
> continuation, and in order to use it, this name should be visible.
> 
> If I have
> 
> callCC $ \exit -> do
>   foo
> ...
> 
> I cannot jump to `exit' from within foo unless `exit' is given to foo
> as an argument.
> 
> Any suggestions?

The best approach is probably to write a small variant of callCC usually
called control that does abort if it's continuation isn't used and use
that.  I'm not a big fan of call/cc in general; I much prefer control.
Control.Monad.Cont does export the Cont data constructor so you can
easily add it yourself.