January 2010 - Haskell-Cafe

Splitting a Hackage project (was ANN: Streaming Component Combinators 0.4)
by Mario Blazevic 08 Jan '10

08 Jan '10

I couldn't find any Hackage policy guideline on the appropriate balance between the package list size and package size, so I'll just ask the question. I have released the version 0.4 of my SCC package yesterday. In this particular release, there is at least one module that's nicely self-contained, providing useful functionality of its own, and not expected to evolve much: http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concur… My question then is if I should split this module into a separate package or leave it where it is? If I split it out, what should I do about the ParallelizableMonad class in the module? > class Monad m => ParallelizableMonad m where > bindM2 :: (a -> b -> m c) -> m a -> m b -> m c This class and its instances would again be something *usable* both outside the SCC package and outside the Control.Concurrent.Coroutine module. But would they be *useful* and would they be *used*? Should I split the class out into a Control.ParallelizableMonad package? I'm sure I'm not the only one with the same dilemma. It would be nice to have some written rules to follow on what belongs on Hackage as a separate package. > Version 0.4 of Streaming Component Combinators, or SCC for short, has > been released on Hackage. Get it at > > http://hackage.haskell.org/package/scc > > There isn't much new high-level functionality compared to the > previous version, but the implementation has been heavily refactored and > the foundations completely replaced. > > I'm particularly happy to have found a way to drop the ugly reliance > on Data.Dynamic and to encode the required constraints in the type > system instead. The foundation of streaming components in this version > is the new Control.Concurrent.Coroutine module, whose main export is the monad > transformer Coroutine. It can transform any monad into a suspendable, resumable, > trampoline-style-runnable monad. Coroutines can be nested, which was the > requirement for streaming and the main stumbling block for the implementation. > The solution, worth at least 10 milliOlegs according to my estimate, was to > parameterize the Coroutine with a functor that wraps the coroutine suspension, > and to use nested functors for suspension from nested coroutines. The type system > automatically figures out how many wrappings to apply to each suspension > depending on how many intermediate coroutines it suspends. > > In other news is the project's Wiki page at > http://trac.haskell.org/SCC/wiki/. It's still rudimentary, but growing. > > All feedback will be appreciated. > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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ANN: hakyll-0.4
by Jasper Van der Jeugt 08 Jan '10

08 Jan '10

Hello, I am announcing the release of hakyll-0.4. Hakyll is a static site generator library written Haskell. It is written in a very configurable way and uses an xmonad-like DSL for configuration. Notable changes since the last big release (0.1) include: - CSS compression - Dependency handling (aka. not generate everything again every time) - A simple http server for previewing your site - Speed improvements and bug fixes - Several specialized functions for dealing with dates, tags... - Abstraction of context manipulations - Some tutorials are added and documentation is mostly complete - Example sites were added More information can be found at http://jaspervdj.be/hakyll All feedback is welcome. Kind regards, Jasper Van der Jeugt

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Testing for statistical properties
by Lee Pike 08 Jan '10

08 Jan '10

Greg, > Hey everyone! I have some computations that satisfy statistical > properties which I would like to test --- that is, the result of the > computation is non-deterministic, but I want to check that it is > sampling the distribution that it should be sampling. Is anyone > aware of a Haskell library out there that does anything like this? I don't know of a library, but here are a couple of ideas/pointers: I wanted to do something vaguely similar in the realm of real-time hardware testing. I used QuickCheck (QC) <http://hackage.haskell.org/package/QuickCheck > to do some stochastic testing (in my case, generate test cases from the reals satisfying somewhat complex constraints), and I have a small patch to the QC library to make this possible. I have a short '09 Haskell Symposium paper about it (probably Sec. 5 is the most interesting). The paper and patch are here: <http://www.cs.indiana.edu/~lepike/pub_pages/qc-biphase.html >. One caveat though is that I didn't check that the tests generated by QC satisfied any particular distribution---I was assuming it is roughly uniformly distributed. Also, I didn't investigate how to generate other distributions, but I don't think that'd be too hard. FYI, Bryan O'Sullivan has a statistics library <http://hackage.haskell.org/package/statistics >, which I plan to use myself soon! I'm personally interested in how things turn out for you---please keep me posted. Lee

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ANNOUNCE: system-uuid-1.2.0
by Jason Dusek 08 Jan '10

08 Jan '10

Patched by Antoine S. Latter to interoperate with UUID. http://hackage.haskell.org/package/system-uuid-1.2.0 -- Jason Dusek

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ANNOUNCE: New versions of ALL the monadic regions packages
by Bas van Dijk 08 Jan '10

08 Jan '10

Hello, As I explained in my announcement of 'safer-file-handles', I discovered a serious lack of expressive power in my 'regions' package. I have now solved that problem in the way I envisaged by removing the 'resource' parameter from 'RegionT' and using existential quantification to bring the 'resource' type back at the place I need it but hidden from the outside. Now you're able to open multiple types of resources in a single region which all will be closed automatically on termination of the region. These are the new releases: http://hackage.haskell.org/package/regions-0.2 http://hackage.haskell.org/package/regions-monadsfd-0.2 http://hackage.haskell.org/package/regions-monadstf-0.2.0.1 http://hackage.haskell.org/package/usb-safe-0.5 http://hackage.haskell.org/package/safer-file-handles-0.2 (Note that in the process I shuffled a bit with the API's. That's one of the reasons all these are major releases.) Do you want to see this in action? See 'example.hs' in the following package that demonstrates opening multiple different types of resources in a single region. The example extends the example in the 'usb-safe-examples' package by first opening a USB device (my USB mouse) and a temporary file in the same region. Next the device is configured, an interface is claimed and an alternate is set. Next some bytes are read from an endpoint of the device. Finally these bytes are written to the temporary file. It's fun to see this in action so do try it out: darcs get http://code.haskell.org/~basvandijk/code/usb-safe-and-safer-file-handles-ex… regards and see some of you at FP-NL tomorrow! Bas

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Review request for my permutations implementation
by CK Kashyap 07 Jan '10

07 Jan '10

Hi All, I've written this piece of code to do permutations - perms :: String -> [String] perms []= [] perms (x:[])= [[x]] perms (x:xs)= concat (f [x] (perms xs)) spread :: String -> String -> [String] -- interpolate first string at various positions of second string spread str1 str2 = _spread str1 str2 (length str2) where _spread str1 str2 0= [str1 ++ str2] _spread str1 str2 n= [(take n str2) ++ str1 ++ (drop n str2)] ++ (_spread str1 str2 (n-1)) f xs = map (spread xs) The number of outcomes seem to indicate that correctness of the algo .. however, I'd be very obliged if I could get some feedback on the Haskellness etc of this ... also any performance pointers ... Regards, Kashyap

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darcs 2.4 beta 1 release
by Reinier Lamers 07 Jan '10

07 Jan '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 beta 1. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the fast index-based diffing which is now used by all darcs commands, and the interactive hunk-splitting in darcs record. This beta is your chance to test-drive these improvements and make darcs even better. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.3.98.1.tar.gz , and build it by hand as explained in the README file. A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman Plášil) * A 'remove --recursive' option to remove a directory tree from revision control (Roman Plášil) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš Caitt) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) Kind Regards, the darcs release manager, Reinier Lamers

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Explicit garbage collection
by Miguel Mitrofanov 07 Jan '10

07 Jan '10

Is there any kind of "ST" monad that allows to know if some STRef is no longer needed? The problem is, I want to send some data to an external storage over a network and get it back later, but I don't want to send unnecessary data. I've managed to do something like that with weak pointers, System.Mem.performGC and unsafePerformIO, but it seems to me that invoking GC every time is an overkill. Oh, and I'm ready to trade the purity of runST for that, if necessary. Thanks.

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Example of sending headers/cookies with Network.HTTP.simpleHTTP or Network.Browser?
by Matt Brown 07 Jan '10

07 Jan '10

Hi all, Does anyone know of a good/simple example of sending cookies and other headers in an HTTP request via simpleHTTP or Browser? I know what the cookies should be (i.e. I don't want them to be sent by the server). More specifically, I'm looking for an analogue to curl's -b and -H flags. thanks, -matt

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Distinct types in a list
by rodrigo.bonifacio 07 Jan '10

07 Jan '10

Hi all, I have a family of parsers that return either (Success t) or (Fail), using the following data type: > data ParserResult a = Success a | Fail String > deriving (Read, Show, Eq, Ord) > > isSuccess (Success _) = True > isSuccess (Fail _) = False > ... I want to add the results of different parsers to a list. Such as: > m1 = parseFirstModel file1 -- it returns a ParserResult of t1 > m2 = parseSecondModel file2 -- it returns a ParserResult of t2 > ps = [m1, m2] In such a way that I could write something like: > if and (map isSuccess ps) > then process m1 m2 > else ... Actually, in the real program I have to check more than two input models. However, since Lists do only hold elements of a same type, I couldn't proceed in this way. Which improvements to the ParserResult data type should I wrote in order to proceed as I want to. Best regards, Rodrigo.

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