On Jan 14, 2008 8:27 PM, apfelmus
The type of contPromptM is even more general than that:
casePromptOf' :: (r -> f b) -> (forall a,b. p a -> (a -> f b) -> f b) -> Prompt p r -> f b casePromptOf' done cont (PromptDone r) = done r casePromptOf' done cont (Prompt p c ) = cont p (casePromptOf' done cont . c)
(I guess the forall b inside 'cont' is a typo?) Actually, it can be as general as casePromptOf :: (r -> b) -> (forall a. p a -> (a -> b) -> b) -> Prompt p r -> b casePromptOf done cont (PromptDone r) = done r casePromptOf done cont (Prompt p c ) = cont p (casePromptOf done cont . c) =) And, just for the record, runPromptAgain :: Monad m => (forall a. p a -> m a) -> Prompt p r -> m r runPromptAgain f = casePromptOf return ((>>=) . f)
The link to ContT m a = (forall b . (a -> m b) -> m b) is apparent in the case of casePromptOf' and is no surprise: you can omit p a and Prompt p r entirely and implement them directly as continuations (thereby loosing the ability to use it with different m, which would defeat the whole point here.) See also
Implementing the State Monad. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31486
for the details.
I've read that e-mail when it was sent but didn't understand it fully. I guess now I'm in a better condition, but I still have a lot to learn about these little warm, fuzzy things. Actually, the more I see, the less I understand why some people are afraid of them... it must really be the name 'monad'. Thanks for all the help guys! Next I'll try to reproduce the freezes I was getting with my first forkIO approach. Cheers, -- Felipe.