Re: [Haskell-cafe] Re: Fwd: Semantics of iteratees, enumerators, enumeratees?

At first I regarded this as simply a bug in the Iteratee.map definition, but like Ben, it's started to bother me a lot. I think this is precisely the sort of issue a proper denotational semantics would fix. Unfortunately the only general solution I see is to abandon chunking and work strictly element-wise. I say unfortunately because my current best implementation is about 5 times slower than the main tree. I'm open to ideas. John

From: Ben

Sorry to be late coming into this conversation.....

Something that has bothered me (which I have mentioned to John Lato privately) is that it is very easy to write non-compositional code due to the chunking. For example, there is a standard function

map :: (a -> b) -> Enumeratee a b c

whose meaning I hope is clear : use the function to transform the type of a stream and pass it to an iteratee. However last I checked the versions provided in both the iteratee and enumerator packages fail to satisfy the equation

map f (it1 >> it2) == (map f it1) >> (map f it 2)

because of chunking, essentially. You can check this with f == id and it1 and it2 are head:

let r = runIdentity . runIteratee

runIdentity $ run $ enumList 10 [1..100] $ r $ joinI $ map id $ r (head >> head) --> Right (Just 2)

runIdentity $ run $ enumList 10 [1..100] $ r $ joinI $ (map id $ r head) >> (map id $ r head) --> Right (Just 11)

It is possible to fix this behavior, but it complicates the "obvious" definitions a lot.

B

On Wed, Sep 1, 2010 at 5:10 AM, Heinrich Apfelmus wrote:

...

Tilo Wiklund wrote:

...

Daniel Fischer wrote:

...

[...] Well, I just gave an example where one would want chunking for reasons other than performance. That iteratees don't provide the desired functionality is a different matter. [...]

In the case of hashing, wouldn't it be more reasonable to consider iterators over streams of fixed (or at least predictable) sized chunks (where a set of chunks can themselves be chunked), with the chunking behaviour being given by another iteratee over the original stream?

It seems to me that one of the major points of iteratees is to provide an abstraction from the kind of chunking irrelevant to the parsing logic, otherwise I fail to see any difference (at least relevant to chunking) to plain strict IO.

I thought so, too, but I was informed[1] that iteratees are just a small step up the abstraction ladder. The difference compared to an ordinary file Handle is that you can now reuse one and the same iteratee for reading from a String , for instance, without changing the source code of the iteratee.

Furthermore, iteratees can be suspended, which facilities resource management like closing files handles after they've been read.

[1]:

http://www.reddit.com/r/haskell/comments/ar4wb/understanding_iteratees/c0j0f...

...

Regards, Heinrich Apfelmus

-- http://apfelmus.nfshost.com

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe