February 2010 - Haskell-Cafe

Infix and postfix operators in Parsec
by Xinyu Jiang 12 Feb '10

12 Feb '10

I'm writing a parser for a Haskell-style language, and when I need to use the same symbol for infix, prefix and postfix operators, the combinator "buildExpressionParser" seems not to work as intended. For example, in: (1)x + y (2)x + (3)+ x If I set the priority of the postfix version of "+" to be higher than the infix version, the parser cannot recognize (1), for it stops at the end of "x +". And if the priority of postfix "+" is lower, Parsec complains about (2) by returning an error message which expects something after "+". Is there some way to get over this problem, and let me be able to still benifit from the expression mechanism in Parsec. Or should I write these stuff from scratch? Thanks.

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Haskell RPC / Cluster
by Rick R 12 Feb '10

12 Feb '10

I am preparing to embark on some serious cluster oriented coding (high availability, monitoring, failover, etc). My primary concern is conforming to standards. I would also like to aid any existing project that fall under this scope. HackPar seems currently targeted towards HPC style clustering, but the page seems to hint at future work in the cloud/high-availability area. I was looking around for RPC libs for Haskell and stumbled across this http://github.com/mariusaeriksen/bert It implements BERT, which is based on Erlang's binary serialization protocol. It seems to have quite a bit of support. Does anyone know of any other RPC modules for Haskell? In addition, can anyone recommend other cluster oriented modules for monitoring, process management, etc? If those don't exist, can anyone recommend some standards off of which to base these? SNMP seems obvious (and daunting), any others? Thanks, Rick

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Re: [Haskell-cafe] HDBC convert [SqlValue] without muchos boilerplate
by Iain Barnett 12 Feb '10

12 Feb '10

On 11 Feb 2010, at 10:05, Vasyl Pasternak wrote: > > But fromSql function could convert everything to String, so you never > need to use `show`, just simply write > > convrow2 :: [SqlValue] -> String > convrow2 (x:xs) = foldl (\i j -> i ++ " | " ++ (fromSql j)) (fromSql x) xs > But, IMO, this is more readable version of your function: > > convrow2' :: [SqlValue] -> String > convrow2' = unwords . intersperse "|" . map fromSql > On 11 Feb 2010, at 10:06, Miguel Mitrofanov wrote: > > What if you just omit the "show" function? fromSql seems to be able to convert almost anything to String. Ok, thanks, that's a big help. I'm really glad to get rid off all that extra cruft I had there. I'd forgotten about intersperse, too. The new code works fine. Regards, Iain

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parallel and distributed haskell?
by Scott A. Waterman 12 Feb '10

12 Feb '10

It looks like there was a recent hackathon focusing on implementing distributed haskell. http://hackage.haskell.org/trac/ghc/wiki/HackPar I feel there is quite a bit of latent interest in the subject here, but relatively little active development (compared to erlang, clojure, etc.) Can anyone involved give a quick overview (or pointers to one)? It would be good to hear what directions people are taking, and why, and where it's going. Personally, I'd love to know the current thinking on a variety of subjects: * erlang-like port/actor implementations v. more implicit constructs * serialization * node failure/error handling * "grid" compatibility v. various ad-hoc cluster approaches. Anyone actively looking at this kind of question who would want to talk about it? Thanks, --ts

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Computer Camp for kids 13 - 15 years old in Colorado featuring Functional Reactive Programming
by John Peterson 12 Feb '10

12 Feb '10

Western State College in Colorado has a computer camp for kids aged 13 - 15. Although we don't use Haskell (it's Python on the inside) the underlying engine is Functional Reactive Programming. We use a 3-D game engine to explore more than just programming - we cover a lot of math and physics. We have a very unique camp - every day includes 3 - 4 hours of recreation in the area: rafting, rock climbing, kayaking, mountain biking. Our website is at http://western.edu/academics/computerscience/computer-camp.html The camp is the last week of June. See the website for further details. We're trying to get the software in releasable form - should be ready to go in a few months. This is the fourth year of our camp and FRP has been an ideal way to introduce novices to computing. John (PS - all recreational activities at our camp are approved by Simon PJ - http://haskell.org/haskellwiki/Simon_Has_Fun)

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ANN: hledger 0.8 released
by Simon Michael 12 Feb '10

12 Feb '10

hledger 0.8 is out! http://hledger.org http://hledger.org/MANUAL.html#installing Bug fixes, refactoring and Hi-Res Graphical Charts. (See Roman Cheplyaka's blog: http://www.reddit.com/r/haskell/comments/b0w0q/using_the_hledger_package_to…) Best - Simon Release notes: ...................... * parsing: in date=date2, use first date's year as a default for the second * add: ctrl-d doesn't work on windows, suggest ctrl-c instead * add: --no-new-accounts option disallows new accounts (Roman Cheplyaka) * add: re-use the previous transaction's date as default (Roman Cheplyaka) * add: a command-line argument now filters by account during history matching (Roman Cheplyaka) * chart: new command, generates balances pie chart (requires - fchart flag, gtk2hs) (Roman Cheplyaka, Simon Michael) * register: make reporting intervals honour a display expression (#18) * web: fix help link * web: use today as default when adding with a blank date * web: re-enable account/period fields, they seem to be fixed, along with file re-reading (#16) * web: get static files from the cabal data dir, or the current dir when using make (#13) * web: preserve encoding during add, assuming it's utf-8 (#15) * fix some non-utf8-aware file handling (#15) * filter ledger again for each command, not just once at program start * refactoring, clearer data types Stats: 62 days since last release, 2 contributors, 76 commits, 3464 lines of non-test code, 97 tests, 53% test coverage

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Hackage download statistis
by Maurício CA 12 Feb '10

12 Feb '10

Hi, all, Some time ago a download statistic of hackage was made available, and analysed in a few ways. Googling for it still find that at a Galois web page. I though that, since the tools to get that were there, this would be output once in a while, but it seems it hasn't been done since then. Does anyone know if there are plans about that? Thanks, Maurício

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Seen on reddit: or, foldl and foldr considered slightly harmful
by Johann Höchtl 12 Feb '10

12 Feb '10

In a presentation of Guy Steele for ICFP 2009 in Edinburgh: http://www.vimeo.com/6624203 he "considers foldl and foldr" harmful as they hinder parallelism because of "Process first element, then the rest" Instead he proposes a divide and merge aproach, especially in the light of going parallel. The slides at http://docs.google.com/viewer?url=http%3A%2F%2Fresearch.sun.com%2Fprojects%… [Bware: Google docs] are somewhat geared towards Fortress, but I wonder what Haskellers have to say about his position. Greetings, Johann

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ANN: Happstack 0.4.1
by Jeremy Shaw 11 Feb '10

11 Feb '10

Happstack 0.4.1 STABLE is now available. We recommend that all users of Happstack update to 0.4.1 immediately. Migration from Happstack 0.3 should be trivial. There was one race condition in happstack-state which could cause a state event to fail under heavy congestion. Happstack is the Haskell Application Server Stack. It is a scalable framework for developing web applications. It is similar in purpose to Django, RoR, Turbogears, Grails, Seaside, etc. Using less buzzwords, it is a collection of libraries which can be used to develop a self-contained server application which does not require SQL or Apache (though it can integrate with both). It includes: * an HTTP server * a rich DSL for deconstructing HTTP Requests and generating Responses * HTML/XML/etc templating via HStringTemplate, HSP, or Text.XHtml * a persistent data store based on user defined Haskell data types with ACID properties and state migration * and more!! Happstack's modular nature means it is easy to integrate unrelated 3rd party modules such as Pandoc, or to add support for an alternative templating system. Additionally, it does not exclude you from using SQL, or supporting other protocols besides HTTP. HOMEPAGE: http://www.happstack.com/ SUPPORTED PLATFORMS: Happstack supports GHC 6.8, 6.10, and 6.12, under GNU/Linux, FreeBSD, OS X, and Windows. GHC 6.12 currently requires a minor patch to HJScript. Details here: http://groups.google.com/group/haskell-server-pages/browse_thread/thread/56… INSTALL: 1. ensure that ~/.cabal/bin is in your $PATH 2. cabal update 3. cabal install happstack * see SUPPORTED PLATFORMS for information about GHC 6.12 NOTABLE CHANGES: For a complete list of changes see: http://www.patch-tag.com/r/mae/happstack/snapshots/all/history Removed: * happstack-contrib has been completely removed. If you were using something in it, please complain on the mailing list. * removed Happstack.Server.S3 -- newer, better maintained S3 libraries are on hackage now * removed unsupported spread-based multimaster support fileServe: * now with sendfile() support! On supported platforms sendfile allows files to be served from the disk to the network in a zero-copy fashion. This means faster transfers, less memory and CPU usage, and no file handle leaks. (hopefully). sendfile uses native sendfile() support on Linux, Windows, OS X, and FreeBSD. Other platforms will use a sendfile emulation mode. We recommend that you use sendFile instead of sendFileStrict or sendFileLazy, unless you have specific needs and know some reason why you must use sendFileStrict or sendFileLazy. * FileServe internals majorly refactored. FileServe now provides a modular way of building your own fileServe-like functions. Also includes functions for serving a single file instead of the contents of a directory. * added more complete list of mime-types * automatically redirect from foo to foo/ if foo is a directory * filter . and .. from request path, but allow filenames and directories which start with . happstack-server: * ServerMonad instance for ErrorT * Added ServerMonad, FilterMonad, and WebMonad for XMLGenT * Added secure flag to cookies. Defaults to False. * fix supportsIPv6 to work with newer versions of network library * simpleHTTPWithSocket + bindPort for H.S.SimpleHTTP (you can use this to start the server as root, bind to the socket, and then drop privileges.) * Fixed Accept-Encoding parser to accepts encodings with a - in them. Added additional tests to acceptEncodingParserTest * fix so basicAuth sets correct response code and headers. * SimpleHTTP: added trailingSlash. Guard which checks that Request URI ends with a / * decode body of PUT requests not just POST * added ifModifiedSince guard * added host and withHost guards to SimpleHTTP (similar to dir and path, but for the requested hostname) other: * happstack-state: Correct an embarrassing race condition bug. * happstack-data: Exported getSafeGet and getSafePut * happstack: add state transition code to guestbook (add email to form, handler, and state), and tar of example _local state before transition experimental: * happstack-state: now has experimental replication support using Amazon Web Services. This support is disabled by default. This feature is highly experimental, may not work, and may even be completely removed in future versions (though, hopefully not). THANKS: Alex Jacobson David Himmelstrup / Lemmih Matthew Elder / mae Antoine Latter Erik Hesselink Pasqualino 'Titto' Assini Nils / McManiaC Thomas Hartman Creighton Hogg / wchogg John MacFarlane Jochem Berndsen / koeien MightByte And everyone who filed bug reports, tested RC1 and RC2, and anyone else I missed.

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Examples/docs for simulated annealing bindings
by Dougal Stanton 11 Feb '10

11 Feb '10

I found the HsASA library [1] on Hackage, but there's no documentation and it's not particularly intuitive. I can't see any obvious way of choosing initial config or generating new configurations. Google reveals no one using it. Does anyone have ideas? [1]: http://hackage.haskell.org/package/HsASA-0.1 Cheers, D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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