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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
