[Haskell-cafe] A home-brew iteration-alike library: some extension quiestions

Michael Snoyman michael at snoyman.com
Thu Dec 9 21:43:35 CET 2010

On Thu, Dec 9, 2010 at 10:32 PM, Permjacov Evgeniy <permeakra at gmail.com> wrote:
> On 12/09/2010 11:17 PM, Antoine Latter wrote:
>> Also, one thing that tripped me up is that your "Stream" type is
>> fundamentally different from the "Stream" types in the
>> iteratee/enumerator libraries - yours is more of a monadic list in the
>> inner monad, with explicit errors.
>> How does this change the operation of the Iterator type?
> The problem was that I wished Zippee. It means that external enumerator
> must be "suspended" at some points so Zippee can process elements from
> both left and right streams in desired order. It makes any other
> approach I considered impossible to use.
>> Also, in what way are the other libraries not Haskell-2010 compliant?
>> I haven't experimented too much with this sort of thing, since Cabal
>> defaults to the Haskell '98 language, and that's how I install most
>> things.
> Haskell-2010 does not include functional dependencies (wich are
> considered evil by many) and, as I recall, type families. This makes mtl
> haskell-2010 and haskell-98 uncompilant -(. Functional dependencies and
> type familes are tricky things, so it is better to avoid them.

For the record, enumerator (and I believe iteratee as well) uses
transformers, not mtl. transformers itself is Haskell98; all FunDep
code is separated out to monads-fd.


>> Thanks for your response,
>> Antoine
>> On Thu, Dec 9, 2010 at 2:07 PM, Permjacov Evgeniy <permeakra at gmail.com> wrote:
>>> On 12/09/2010 10:54 PM, Antoine Latter wrote:
>>>> I only have some surface level questions/comments -
>>>> What existing packages is this similar to? How is it different from
>>>> any previous work in the area?
>>> Main idea was taken from Iteratees invented by Oleg Kiselev (there are
>>> two packages on hackage implementing this ideas: data-iteraties and
>>> enumerator packages)
>>> The difference is, that I wished haskell-2010 compilant package for
>>> left-foldable streams, including support for easy builing, transcoding,
>>> merging and folding of streams relying on do-notation (see
>>> Data.Iteration.Unicode.* for examples of transcoding streams: it is
>>> quite clean and easily understandable) and ability to specify easily
>>> monadic actions in stream processors.
>>>> Also, likes looks like you don't need the 'Monad m' constraint on your
>>>> various Monad and Functor instances in Data.Iteration.Types, which I
>>>> think is one of the nicest properties of the continuation-based
>>>> approach to something like this.
>>> Errgh. That may be true, but I did not consider non-monadic context at
>>> all, so I enforced this constrain mindlessly
>>>> It's a mater of taste which way to go, but I prefer importing modules
>>>> qualified rather than have type-suffixes on functions - so I would
>>>> rather use 'I.next' and 'A.next' instead of 'nextI' and 'nextA'. But
>>>> reasonable people can disagree on this.
>>>> Take care,
>>>> Antoine
>>> Thanks!
>>>> On Thu, Dec 9, 2010 at 1:42 PM, Permjacov Evgeniy <permeakra at gmail.com> wrote:
>>>>> Hi. I Wrote a simple iteration library. It was not intensively tested,
>>>>> so it MAY contatin bugs, but it is very unlikely. The library is
>>>>> currently on github: https://github.com/permeakra/iteration
>>>>> I'm not ready to upload it to hackage, as some testing and extension is
>>>>> really needed. However, I'd like to know about possible flaws.
>>>>> Current goal is addition of byte-stream (de)compression and IO functions
>>>>> extenstion. After this package will be cabalized and uploaded to
>>>>> hackage. So, while design is not frozen yet, I'm interested in criticism -)/
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