On Mon, 23 Aug 2010 14:38:29 +0200, Heinrich Apfelmus
Luke Palmer wrote:
Heinrich Apfelmus wrote:
Conal Elliott wrote:
For anyone interested in iteratees (etc) and not yet on the iteratees mailing list.
I'm asking about what iteratees *mean* (denote), independent of the various implementations.
In my world view, iteratees are just a monad M with a single operation
symbol :: M Char
that reads the next symbol from an input stream.
So perhaps this could be a reasonable semantics?
Iteratee a = [Char] -> Maybe (a, [Char]) = MaybeT (State [Char]) a
symbol [] = Nothing symbol (c:cs) = Just (c, cs)
I'm not experienced with iteratees. Does this miss something?
From a purely denotational point of view, that's a reasonable semantics.
However, and that's the main point, with this particular semantics, it is impossible to implement
runHandle :: M a -> Handle -> IO a
without using unsafeInterleaveIO . Typical implementations of iteratees do make that possible, by being able to suspend the iteratee after feeding it a character.
There are also enumerators and enumeratees. I think that
purpose of enumerator = run an iteratee on multiple sources (i.e. first part of the input from a Handle , second part from a String )
I would say more simply that an enumerator is a data-producer (or source). Although it is a producer defined as a consummer (or sink) feeder. An iteratee is thus the consummer. It is defined as an action asking either for more food or producing a value and a food left over -- ignoring errors and over-simplifing Stream as Maybe data Step a b = Continue (Maybe a -> IO (Step a b)) | Yield b (Maybe a) type Iteratee a b = IO (Step a b) -- the most important case is when getting Continue as input: -- type Enumerator a b = (Maybe a -> IO (Step a b)) -> IO (Step a b) type Enumerator a b = Step a b -> IO (Step a b) Note that I'm far from an expert on Iteratee but I start to get some intuitions out of it. -- Nicolas Pouillard http://nicolaspouillard.fr