April 2010 - Haskell-Cafe

ANN: shelltestrunner 0.8 released
by Simon Michael 09 Apr '10

09 Apr '10

I'm pleased to announce a new release of shelltestrunner, a tool which aims to make testing command-line programs easy. Thanks to Bernie Pope for contributing features and valuable feedback. Example: $ cabal install shelltestrunner ... $ cat - >a.test # a simple test - run cat, provide stdin, check stdout cat <<< 1 2 >>> 1 2 $ shelltest a.test a.test: [OK] Test Cases Total Passed 1 1 Failed 0 0 Total 1 1 Release notes: 2010/4/9 0.8 * rename executable to shelltest. The package might also be renamed at some point. * better built-in help * shell tests now include a full command line, making them more readable and self-contained. The --with option can be used to replace the first word with something else, unless the test command line begins with a space. * we also accept directory arguments, searching for test files below them, with two new options: --execdir execute tested command in same directory as test file --extension=EXT file extension of test files (default=.test) home: http://hackage.haskell.org/package/shelltestrunner repo: http://joyful.com/repos/shelltestrunner

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GSoC Project: A Haddock + Pandoc documentation tool
by ViaToR (Alvaro V.) 09 Apr '10

09 Apr '10

Hello, I just finished writing my GSoC proposal and I want to have some feedback from the community. I'll try to be brief (this is not the proposal). The project is about creating a new documentation tool for Haskell projects, like Sphinx[1] for Python or Scribble[2] for Scheme. We have Haddock, which is a great tool. It can take the sources of a project and generate a useful API reference. But the reference is just a fragment of the whole project documentation. So, Haddock can only do a part of what Sphinx can do. But we have another tool, Pandoc, that takes files of whatever markup language and transform it to another format. And this is the more general-purposed part of Sphinx that is missing in Haddock. So we have the tools for creating documentation as useful as other systems, we just need the glue and several improvements. To achieve this project, first I'll have to use Haddock as an API. Currently, the Haddock API is rudimentary and highly experimental, so I would have to extend and test it. Then I would have to write a Haddock backend which would generate the reference in an internal independent pandoc format [3]. Finally I would have to write a new command-line program that would manage the projects or I would have to add Pandoc support in Haddock command-line program or Haddock support in Pandoc. IMHO, I think that will be better a new command-line program, everyone with its own purpose. I created an example of what would be a library documented with this system [4]. The file contains a configuration file a la cabal, a reST file (It could be markdown) and the html output. I tried to represent that the html output for the reference have to be almost the same that generates Haddock (css may differ). Note that all the entries of the Haddock reference are first-citizens entities in the documentation, so you can refer to the entries, render one entry, all or by groups (there are examples in the reST file). I am looking forward your impressions and suggestions. Thanks to Simon Marlow, David Waern and Conal Elliott for their suggestions and help. Thanks you all. [1] http://sphinx.pocoo.org/ [2] http://docs.plt-scheme.org/scribble/ [3] http://hackage.haskell.org/packages/archive/pandoc/1.5.1.1/doc/html/Text-Pa… [4] http://bitbucket.org/viator/doc-example/get/tip.zip

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Can Haskell enforce the dimension?
by Haihua 09 Apr '10

09 Apr '10

Hi, In C++, template can be used to enforce the dimension. For example, F=m*a is OK and F=m*t will issue a compile time error. Is there a way to do this in Haskell? Thanks, Haihua

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mapM for vectors
by Khudyakov Alexey 09 Apr '10

09 Apr '10

Hello I found that there is no monadic map for vector. It's possible to define to define such map using conversion to list, but I suppose it's not efficient. I didn't make any measurements. > mapM' :: Monad m => (a -> m b) -> V.Vector a -> m (V.Vector b) > mapM' f = return . V.fromList <=< mapM f . V.toList Any suggestions about implementation of such function? Specifically I want to use Random monad. -- Khudyakov Alexey

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Re: [Haskell-cafe] ANN: darcsum.el 1.2 released
by Jose A. Ortega Ruiz 09 Apr '10

09 Apr '10

Simon Michael <simon(a)joyful.com> writes: > Unfortunately it will still hang if darcs emits something it can't > parse. An emacs process-filter is used to drive an interactive darcs > command, like an expect script. The process-filter can receive darcs > output in random chunks, so it's hard to distinguish a parse failure > from partial output, unless the output has a recognisable prefix, > which some of it does not (eg darcs amend's.) I'm not sure what a more > robust way to drive darcs looks like. Maybe i'm not understanding the problem, but cannot you just accumulate the output in an auxiliary variable and parse the ouput as a whole once the darcs process finishes? Thanks a lot for working on darcsum, btw! jao -- There are two ways to write error-free programs; only the third one works. - Alan Perlis, Epigrams in Programing

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Type constrain in instance?
by Louis Zhuang 09 Apr '10

09 Apr '10

Hi, I got an issue when playing haskell's type system, partically parametic class/instance have no way to specify constrains of their parameters. for example, i want to data struct to chain functions and their derivatives so we can have combined function and derivations, as following import qualified Control.Category as Cat data ChainableFunction a b = CF { cfF :: (a->b), cfDeriv :: (a->b) } class Module a b wher4e (*>) :: a -> b -> b instance Cat.Category CF where id :: (Num a) -- GHC dis-allow this id = CF id (const 1) (.) :: (Num a, Num b, Num c, Module a b, Module b c) => CF a b -> CF b c -> CF a c -- GHC disallow this either (.) (CF f f') (CF g g') = CF (g.f) (\a -> f' a *> g' (f a)) However GHC only has kinds for class/instance like (*->*->*) so we are forced to allow all possible types in instance code. I'm not sure if I'm modelling things correctly or is there another way to do the same thing? Cheers, Louis

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transliteration with Haskell iconv?
by Eric Kow 09 Apr '10

09 Apr '10

Hi all, I'd like transliterate a UTF-8 text to ASCII. With iconv(1), I would do iconv -f UTF-8 -t ASCII//TRANSLIT foo I've tried using Duncan's iconv library, but it seems to give me more question marks than iconv(1) would, which seems odd if it's the same library underneath. In other words, transliterating Aonach Mòr is a mountain in the Highlands of Scotland. On va boire un petit café si ça te dit. gives me with iconv(1) Aonach Mor is a mountain in the Highlands of Scotland. On va boire un petit cafe si ca te dit. but with my attempts at using Codec.Text.IConv and examples/hiconv -f utf-8 -t ascii --transliterate, I get Aonach M?r is a mountain in the Highlands of Scotland. On va boire un petit caf? si ?a te dit. Anybody run into this and know what to do? Thanks! PS. I'm on Ubuntu 9.04 if it makes any difference... -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Call for Contributions - Haskell Communities and Activities Report, May 2010 edition
by Janis Voigtländer 09 Apr '10

09 Apr '10

Dear all, I would like to collect contributions for the 18th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 1 May 2010 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2009 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2010/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2010/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write "wishlist" items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them.

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Re: Asynchronous exception wormholes kill modularity
by Simon Marlow 09 Apr '10

09 Apr '10

On 08/04/2010 21:20, Tyson Whitehead wrote: > On March 26, 2010 15:51:42 Isaac Dupree wrote: >> On 03/25/10 12:36, Simon Marlow wrote: >>> I'd also be amenable to having block/unblock count nesting levels >>> instead, I don't think it would be too hard to implement and it wouldn't >>> require any changes at the library level. >> >> Wasn't there a reason that it didn't nest? >> >> I think it was that operations that block-as-in-takeMVar, for an >> unbounded length of time, are always supposed to C.Exception.unblock and >> in fact be unblocked within that operation. Otherwise the thread might >> never receive its asynchronous exceptions. > > If I'm understanding correctly here, it would be nice if you could just go > > unmask :: IO a -> IO a > > and always have asynchronous exceptions on for the duration of IO a. > > One solution might be if unmask always enabled asynchronous exception, but, in > a masked context, stopped them from propagating beyond the boundary of the IO > a action by re-queueing them be re-raised when next allowed. > > Of course you've then got the problem of umask having to produce a valid value > even when IO a was aborted, so you would have to go with something like > > unmask :: a -> IO a -> IO a > > where the first a gets returned if the IO a computation gets aborted by an > exception. The original problem code would then go from I haven't seen anyone else asking for this kind of design, and it's quite different to both what we have now and the new proposal. What advantages would this have, do you think? Cheers, Simon

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Performance question
by Arnoldo Muller 09 Apr '10

09 Apr '10

Hello! I am trying to implement a binary search function that returns the index of an exact or the (index + 1) where the item should be inserted in an array if the item to be searched is not found (I am not trying to insert data in the array) . Right now, the bottleneck of my program is in binarySearch', the function must be called a few billion times. Do you have any ideas on how to improve the performance of this function? import Data.Array.IArray type IntArray a = Array Int a -- The array must be 0 indexed. binarySearch :: Ord a => a -> IntArray a -> Int binarySearch query array = let (low, high) = bounds array in binarySearch' query array low high binarySearch' :: Ord a => a -> IntArray a -> Int -> Int -> Int binarySearch' query array !low !high | low <= high = let ! mid = low + ((high - low) `div` 2) ! midVal = array ! mid in next mid midVal | otherwise = -(low + 1) where next mid midVal | midVal < query = binarySearch' query array (mid + 1) high | midVal > query = binarySearch' query array low (mid - 1) | otherwise = mid Thank you! Arnoldo Muller

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