[Haskell-cafe] Re: Fwd: Semantics of iteratees, enumerators,
apfelmus at quantentunnel.de
Mon Aug 23 08:38:29 EDT 2010
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 )
purpose of enumeratee =
iteratee as a stream transformer, i.e. as a map [x] -> [y]
I am not sure whether this elaborate reinvention of the standard lists
functions is worth the trouble.
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