[Haskell-cafe] [iteratee] empty chunk as special case of input

John Lato jwlato at gmail.com
Thu Jul 14 03:59:06 CEST 2011

Sorry for the followup, but I forgot about one other important reason
(probably the real reason) for the nullC case in bindIteratee.  Note
what happens in the regular case: the iteratee is run, and if it's in
a completed state, the result is passed to the bound function (in the
"m_done" line), which is then also run.  Examine what happens if the
inner iteratee is also complete:

> const . flip onDone stream

which would be more clearly written as

> \b _str -> onDone b stream

so in this case the leftover stream result from the first iteratee
(stream) is used as the result of the second iteratee, and the
leftover stream from the second iteratee (_str) is discarded.

This doesn't seem right; what should happen is that the two streams
should be appended somehow.  It works because at this stage an
iteratee won't have been enumerated over (by the current stream at
least), so it can't have any leftover data, just a null chunk.  But
bindIteratee explicitly checks for the null chunk case also so that's
not a problem.  If the iteratee was enumerated over by another stream
and therefore does have leftover data, then since that data isn't part
of the current stream it's rightfully discarded anyway.

This is why your function produced an unexpected result; it's in a
completed state without having been enumerated over, but also has
leftover data, which bindIteratee ignores.

Now that I've thought about it, I'm not convinced this is always
correct; in particular I suspect it for being responsible for a
slightly convoluted implementation of enumFromCallbackCatch.  I'll
have to expend more brain cells on it, I think.

John L.

On Thu, Jul 14, 2011 at 1:15 AM, John Lato <jwlato at gmail.com> wrote:
> Hi Sergey,
> iteratee (the package) uses a null chunk to signify that no further
> stream data is available within the iteratee, that is, at some point
> the stream has been entirely consumed.  Therefore, if any of the
> composed iteratees haven't run to completion, they need to get more
> data from an enumerator.  Thus 'bindIteratee' has the nullC guard in
> the definition as an optimization; there's no need to send the null
> chunk to bound iteratees because in most cases they won't be able to
> do anything with it.
> I've recently considered removing this, but at present when I take it
> out some unit tests fail and I haven't had time to explore further.
> Since this would have other benefits I would like to do so provided it
> doesn't strongly impact performance.  Rather than simply removing the
> case I could add a null case to the Stream type, but that could cause
> some extra work for users.
> Also, one rule for writing iteratees is that they shouldn't put
> elements into the stream.  Doing so may cause various transformers to
> behave incorrectly.  If you want to modify a stream rather than simply
> consuming elements, the correct approach is to create an enumeratee
> (stream transformer).
> John L.
> On Wed, Jul 13, 2011 at 11:00 PM, Sergey Mironov <ierton at gmail.com> wrote:
>> Hi community, hi John. I find myself reading bindIteratee[1] function
>> for a several days.. there is something that keeps me away from
>> completely understanding of the concept. The most noticeble thing is
>> \nullC\ guard in the definition. To demonstate the consequences of
>> this solution, let me define an iterator like
>> myI = Iteratee $ \onDone _ -> onDone 'a' (Chunk "xyz")
>> It is a bit unusial, since myI substitutes real stream with a fake one
>> (xyz). Now lets define two actions producing different results in
>> unusual manner:
>> printI i = enumPure1Chunk ['a'..'g'] i >>= run >>= print
>> i1 = (return 'b' >> myI >> I.head)  -- myI substitutes the stream,
>> last /I.head/ produces 'x', OK
>> i2 = (I.head >> myI >> I.head) -- produces 'b'!  I expected another
>> 'x' here but myI's stream was ignored by >>=
>> Well, I understand that this is probably an expected behaviour, but
>> what is it for? Why we can't handle null input like non-null? Iterator
>> may just stay in it's current state in that case.
>> Thanks in advance
>> Sergey
>> --
>> [1] - bindIteratee (basically, >>=) code from Data.Iteratee.Base.hs
>> bindIteratee :: (Monad m, Nullable s)
>>    => Iteratee s m a
>>    -> (a -> Iteratee s m b)
>>    -> Iteratee s m b
>> bindIteratee = self
>>    where
>>        self m f = Iteratee $ \onDone onCont ->
>>             let m_done a (Chunk s)
>>                   | nullC s      = runIter (f a) onDone onCont
>>                 m_done a stream = runIter (f a) (const . flip onDone
>> stream) f_cont
>>                   where f_cont k Nothing = runIter (k stream) onDone onCont
>>                         f_cont k e       = onCont k e
>>             in runIter m m_done (onCont . (flip self f .))

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