Re: [Haskell-cafe] Fwd: Semantics of iteratees, enumerators, enumeratees?
From: "C. McCann"
What sets an iteratee-style design apart from something conventional based on a State monad is that the iteratee conceals its internal state completely (in fact, there's no reason an iteratee even has to be the "same" function step-to-step, or have a single consistent "state" type--almost has an existential flavor, really), but is at another function's mercy when it comes to actually doing anything.
All of which doesn't really shed too much light on the denotation of these things, I suppose, as there's barely anything there to talk about; the iteratee automaton itself is a terribly simple construct, relying on an underlying monad to perform actions, on an external "push" data source to recurse, and being given only bite-size chunks of data at each step. It's little more than foldl with a "pause" button attached.
This is how I think of them. I particularly your description of them as a foldl with a "pause" button. Maybe it would be helpful to consider iteratees along with delimited continuations? John
On 24 August 2010 13:00, John Lato
This is how I think of them. I particularly your description of them as a foldl with a "pause" button. Maybe it would be helpful to consider iteratees along with delimited continuations?
Aren't they closer - in implementation and by supported operations - to resumptions monads? See many papers by William Harrison here: http://www.cs.missouri.edu/~harrisonwl/abstracts.html
Stephen Tetley wrote:
John Lato wrote:
This is how I think of them. I particularly your description of them as a foldl with a "pause" button. Maybe it would be helpful to consider iteratees along with delimited continuations?
Aren't they closer - in implementation and by supported operations - to resumptions monads?
See many papers by William Harrison here: http://www.cs.missouri.edu/~harrisonwl/abstracts.html
A general method to implement resumption monads, or, in fact, any monad, is given in my "Operational Monad Tutorial": http://apfelmus.nfshost.com/articles/operational-monad.html Here a tiny toy implementation of Iteratees: data IterateeI a where Symbol :: IterateeI Char EOF :: IterateeI Bool type Iteratee = ProgramT IterateeI symbol = singleton . Symbol eof = singleton . EOF runString :: Monad m => Iteratee m a -> String -> m a runString m cs = go cs =<< viewT m where go _ (Return x) = return x go [] (Symbol :>>= k) = error "Expecting input" go (c:cs) (Symbol :>>= k) = runString (k c) cs go cs (EOF :>>= k) = runString (k $ null cs) cs -- an iteratee that counts the number of elements in the input count :: Monad m => Iteratee m Int count = go 0 where go n = eof >>= \b -> case b of True -> return n False -> symbol >> go $! (n+1) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com
participants (3)
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Heinrich Apfelmus -
John Lato -
Stephen Tetley