On Jan 13, 2008 6:49 PM, apfelmus
K. Claessen. Poor man's concurrency monad. http://www.cs.chalmers.se/~koen/pubs/jfp99-monad.ps
P. Li, S. Zdancewic. Combining events and threads for scalable network services. http://www.seas.upenn.edu/~lipeng/homepage/papers/lz07pldi.pdf
Two great papers! Thanks for pointing them out!
Eventually this feature rang some bells: you can save not only when you want to undo, but also when you want to ask something to the user. Unfortunately, I still haven't come up with a nice higher order function that generalizes this work without reinventing Prompt on an isomorphic type.
Oh, what kind of generalization do you have in mind?
Leaking Prompt(..) in the export list to the GUI code seems wrong to me, I like 'runPromptM' because it hides the Prompt(..) data type from the user [module]. But after some rest I think I found a nice corresponding function:
contPromptM :: Monad m => (r -> m ()) -> (forall a. p a -> (a -> m ()) -> m ()) -> Prompt p r -> m () contPromptM done _ (PromptDone r) = done r contPromptM done cont (Prompt p c) = cont p (contPromptM done cont . c)
This way all the Prompts get hidden so that 'lastAttempt' may be coded as
lastAttempt' :: AttemptCode lastAttempt' showInfo entry button = guessGameNew >>= contPromptM done cont where cont :: forall a. GuessP a -> (a -> IO ()) -> IO () -- signature needed cont (Print s) c = showInfo s >> c () cont Guess c = do mfix $ \cid -> onClicked button $ do {signalDisconnect cid; guess <- entryGetText entry; c (read guess)} return () done attempts = showInfo $ "You took " ++ show attempts ++ " attempts."
Nice and clean, and much better to read as well. Now the only question unanswered for me is if there are any relations between (forall a. p a -> (a -> m ()) -> m ()) -- from contPromptM and (ContT r m a -> (a -> m r) -> m r) -- from runContT besides the fact that both carry a continuation. I have a feeling that I am missing something clever here. Cheers, -- Felipe.