January 2011 - Haskell-Cafe

Call for Copy: Monad.Reader Issue 18
by Brent Yorgey 27 Jan '11

27 Jan '11

Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 18 will be: **Friday, April 1, 2011** This will be a normal issue -- but don't forget to also submit your Haskell poems and short stories by February 18 for inclusion in the special poetry and fiction edition [1]! The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out. [1] http://themonadreader.wordpress.com/2011/01/19/call-for-copy-special-poetry…

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ANNOUNCE: WAI 0.3.1, Warp 0.3.1, wai-app-static 0.0.0 and warp-static 0.0.0
by Michael Snoyman 27 Jan '11

27 Jan '11

Hi everyone, Firstly, after some feedback, I'm releasing some point updates for WAI and Warp. They fix some documentation issues, add some status aliases, make it possible to handle exceptions more easily, and make things more responsive on Windows. Next, I'm releasing two new packages: wai-app-static is a static file serving application for WAI. It handles all of your basic needs in this department: * Proper 301 requests to fix trailing-slash issues for folder vs directory. * Directory listing support (if turned on), including a default listing function that uses Haskell colors and a Haskell logo in the corner. Thanks to Happstack for the basis of this code. * Directory index support (eg, index.html, index.htm), again configurable. * Mime type lookups. Thanks to Snap for this table. warp-static is simply a standalone executable that runs wai-app-static on the Warp webserver. The executable name is warp. I know I personally have needed such a light-weight web server at work a few times, so if nothing else, putting it on Hackage is a convenience for me :-). I hope people find these useful. Don't expect the performance to beat nginx, but it is still quite good. if there are any other features requests on any of these packages, let me know. Michael

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Re: [Haskell-cafe] Applicative and parsec3
by Henning Thielemann 27 Jan '11

27 Jan '11

On Thu, 27 Jan 2011, Magnus Therning wrote: > 2011/1/27 Henning Thielemann <lemming(a)henning-thielemann.de>: >> >> On Thu, 27 Jan 2011, Kazu Yamamoto (山本和彦) wrote: >> >>> I'm using parsec3 with the applicative style. Since the functions in >>> Control.Applicative and parsec3 conflicts, I need to use the "hiding" >>> keyword as follows: >>> >>> import Control.Applicative hiding (many,optional,(<|>)) >>> import Text.Parsec >> >> There are several reasons to stick to explicit or qualified imports: >> http://www.haskell.org/haskellwiki/Import_modules_properly > > What that article fails to mention is the inconvenience of using > qualified imports in combination with operators. For infix operators you would use unqualified explicit imports.

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Conditionals and case expressions in do blocks
by Edward Amsden 27 Jan '11

27 Jan '11

Inside a do block, I can very conveniently substitute let x = <pure exp> <continued monadic code> for either x <- return <pure exp> <continued monadic code> or let x = <pure exp> in do <continued monadic code> However, I can't do anything similar (that I know of) with if or case expressions. If I use if or case inside a do block, it's likely that I'm still using monadic expressions (at least in my experience). Is there some nasty semantic ambiguity that comes of this? Is it just not in the standard yet? Is there some extension that I'm not aware of? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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Re: [Haskell-cafe] Problem about exception
by oleg＠okmij.org 27 Jan '11

27 Jan '11

It seems that the problem is to follow a protocol, reporting protocol errors (unexpected responses) and permitting recovery if the `supervisor' decided that the error can be fixed by re-reading from the server. This problem -- specifically, communicating with a supervisor `out-of-band' and recovery -- is a good use case for delimited continuations. The following code implements the scenario in Magicloud's message. http://okmij.org/ftp/continuations/CCmonad/ProtocolRecovery.hs Actually, the code is interactive, letting the use play a good or a bad server. One can see for themselves the effect of expected or unexpected responses. The title comments show the examples of good or bad interactions. Here is a sample transcript of several interactions. ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 2 Client Dis-connected ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 3 Exception: Err_bad_resp Res_debug DEBUG Enter response, as a number 1..4 3 Exception: Err_bad_resp Res_debug DEBUG Enter response, as a number 1..4 2 Client Dis-connected *ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 4 Exception: Err_bad_resp Res_disconnect Aborting Client Dis-connected *ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 1 Exception: Err_bad_resp Res_hello Really bad response! Client Dis-connected

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community.haskell.org is down
by Yitzchak Gale 27 Jan '11

27 Jan '11

The community server, on which resides the domains: community.haskell.org projects.haskell.org code.haskell.org trac.haskell.org planet.haskell.org is down, due to a hacker attack. Rather than trying to repair the old machine it was on, this is the time to migrate this server (finally) to the new machine that has been allocated for it. We are now putting together a strike team to get this done. Hopefully, the server will be up again in a few days. We'll try to keep the community updated about progress.

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Haskell Weekly News: Issue 166 - January 26, 2011
by Daniel Santa Cruz 26 Jan '11

26 Jan '11

Welcome to issue 166 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 16 to 22, 2011. Announcements Sukit Tretriluxana [2]proudly announced version 1.0 of SiteBridge. He also included a short introductory video. Mark Wright [3]annouced the release of a Haskell binding to ANTLR LL(*) parser generator C runtime library. Jasper Van der Jeugt [4]inveted everyone to the 6th Ghent Functional Group Meeting to be held on Feb 17, 2011 in the Technicum Building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University. I know I'm not the only one who wishing I could attend!. Quotes of the Week * Guy Steele: Although Fortress is originally designed as an object-oriented framework in which to build an array-style scientific programming language, [...] as we've experimented with it and tried to get the parallelism going we found ourselves pushed more and more in the direction of using immutable data structures and a functional style of programming. [...] If I'd known seven years ago what I know now, I would have started with Haskell and pushed it a tenth of the way toward Fortran instead of starting with Fortran and pushing it nine tenths of the way toward Haskell. * geheimdienst:ah, yes. now we have to make a lambdacat captioned "kittehs no bery gud at speling. we says leksah fur lexer and monad fur moniod in catagory of ennobunktors" * mreh:some jerk said to me over my shoulder once, if you're doing functional programming, why do you have all those assignments? * revenantphx:You don't need category theory to understand Haskell, though you may need it to understand #haskell. * tolkad:haskell isn't even turing complete without infinite memory * monochrom:C-c C-p was the secret command for sending a report to the kgb * Draconx|Laptop:Rice's theorem: Let P be an interesting problem that you want to solve with a computer. Haha, you're screwed! * monochrom:one ring to rule them all! and in your monad >>= them! Top Reddit Stories * Haskell's Own DLL Hell Domain: cdsmith.wordpress.com, Score: 51, Comments: 27 On Reddit: http://www.reddit.com/r/haskell/comments/f3lh5/haskells_own_dll_hell/ Original: http://cdsmith.wordpress.com/2011/01/16/haskells-own-dll-hell/ * [CCC] On how Haskell’s Arrows are NOT just function compositions with a fancier name Domain: ro-che.info, Score: 40, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/f3e06/ccc_on_how_haskells_arrows_a… Original: http://ro-che.info/ccc/12.html * Announcing the Warp web server (benchmarks inside) Domain: docs.yesodweb.com, Score: 36, Comments: 12 On Reddit: http://www.reddit.com/r/haskell/comments/f4bn8/announcing_the_warp_web_serv… Original: http://docs.yesodweb.com/blog/announcing-warp/ * The ABCs of semantics Domain: haskell.org, Score: 33, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/f4v8f/the_abcs_of_semantics/ Original: http://www.haskell.org/pipermail/haskell-cafe/2011-January/088315.html * Thoughts on node.js and Haskell Domain: moreindirection.blogspot.com, Score: 33, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/f6o5i/thoughts_on_nodejs_and_haske… Original: http://moreindirection.blogspot.com/2011/01/thoughts-on-nodejs.html * A low level performance comparison of simple operations on unboxed integer arrays between Haskell/GHC (6.12.1) and OCaml (3.12.0) (and also C) [originally on Proggit] Domain: lix.polytechnique.fr, Score: 32, Comments: 16 On Reddit: http://www.reddit.com/r/haskell/comments/f3yf8/a_low_level_performance_comp… Original: http://www.lix.polytechnique.fr/~kaustuv/expo/incr_uarray/ * The Butterfly Effect in Cabal Domain: cdsmith.wordpress.com, Score: 29, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/f4179/the_butterfly_effect_in_caba… Original: http://cdsmith.wordpress.com/2011/01/17/the-butterfly-effect-in-cabal/ * Painless NP-complete problems: an embedded DSL for SMT solving Domain: donsbot.wordpress.com, Score: 27, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f3fp6/painless_npcomplete_problems… Original: http://donsbot.wordpress.com/2011/01/16/painless-np-complete-problems-an-em… * Merging SMT solvers and programming languages Domain: corp.galois.com, Score: 26, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f4r5r/merging_smt_solvers_and_prog… Original: http://corp.galois.com/blog/2011/1/18/merging-smt-solvers-and-programming-l… * Tell Haskellers: GHC's Trac now has source code browsing (courtesy of Git) Domain: hackage.haskell.org, Score: 26, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f6391/tell_haskellers_ghcs_trac_no… Original: http://hackage.haskell.org/trac/ghc/browser Top StackOverflow Questions * Criticize simple monad votes: 11, answers: 1 Read on SO: http://stackoverflow.com/questions/4765260/criticize-simple-monad * Parsing function in haskell votes: 8, answers: 1 Read on SO: http://stackoverflow.com/questions/4711893/parsing-function-in-haskell * Possible optimizations in Haskell that are not yet implemented in GHC? votes: 7, answers: 1 Read on SO: http://stackoverflow.com/questions/4720499/possible-optimizations-in-haskel… * Is there a Haskell/ML-like compiler to C? votes: 6, answers: 3 Read on SO: http://stackoverflow.com/questions/4722439/is-there-a-haskell-ml-like-compi… * How to find the longest word in list? votes: 5, answers: 4 Read on SO: http://stackoverflow.com/questions/4708028/how-to-find-the-longest-word-in-… About the Haskell Weekly News You can find an html version of this newsletter at: http://contemplatecode.blogspot.com/2011/01/haskell-weekly-news-issue-166.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85348 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18437 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85519

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Problem about exception.
by Magicloud Magiclouds 26 Jan '11

26 Jan '11

Hi, Consider such a case: I'm making a client program. There is a process, client and server exchange some information in a strict order. So I do (persudo code): exchange = do sendHello readMsg >>= expect hello processHelloReply sendWhatsyourname readMsg >>= expect name processNameReply And expect is something like: expect c f (cmd, msg) = if c == cmd then f msg else fail "unexpected" This is OK until this situation: The server may send some special command, for example DEBUG, DISCONNECT. This breaks the process above. So I think I could throw the unexpected command to outer function to handle. Something like: main = do connect catch exchange $ \e -> do case e of UnexpectedCMD DEBUG -> -- ignore process UnexpectedCMD DISCONNECT -> -- disconnect process _ -> -- something really wrong Well, with Control.Exception, I do not know how to make this done. And with something like Data.Either, I got more problem like: 36| Couldn't match expected type `Control.Monad.Trans.State.Lazy.StateT 37| InternalState IO b' 38| with actual type `Either (Int8, ByteString) b1' 39| Expected type: (Int8, ByteString) 40| -> Control.Monad.Trans.State.Lazy.StateT InternalState IO b 41| Actual type: (Int8, ByteString) -> Either (Int8, ByteString) b1 -- 竹密岂妨流水过山高哪阻野云飞

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web-routes and forms
by Corentin Dupont 26 Jan '11

26 Jan '11

Hello, I have difficulties mixing web-routes and forms: I have put routes in all my site, except for forms which remains with the type ServerPartT IO Response. How to make them work together? I have: type NomicServer = ServerPartT IO type RoutedNomicServer = RouteT PlayerCommand NomicServer newRule :: ServerHandle -> NomicServer Response newRule sh = do methodM POST -- only accept a post method mbEntry <- getData -- get the data case mbEntry of Nothing -> error $ "error: newRule" Just (NewRule name text code pn) -> do *html <- nomicPageComm pn sh (submitRule name text code pn))* ok $ toResponse html nomicPageComm :: PlayerNumber -> ServerHandle -> Comm () -> RoutedNomicServer Html nomicPageComm pn sh comm = (..) launchWebServer :: ServerHandle -> IO () launchWebServer sh = do putStrLn "Starting web server...\nTo connect, drive your browser to \" http://localhost:8000/Login\"" d <- liftIO getDataDir simpleHTTP nullConf $ mconcat [dir "postLogin" $ postLogin, fileServe [] d, dir "Login" $ ok $ toResponse $ loginPage, dir "NewRule" $ newRule sh, dir "NewGame" $ newGameWeb sh, dir "Nomic" $ do html <- implSite " http://localhost:8000/Nomic/" "" (nomicSite sh) ok $ toResponse html ] The red line doesn't compile. I don't know how to transform a RoutedNomicServer into a NomicServer. For the future I intend to use formlets: is these some examples of programs using happstack + web-routes + formlets? Thanks, Corentin On Fri, Jan 7, 2011 at 5:10 PM, Jeremy Shaw <jeremy(a)n-heptane.com> wrote: > Hello, > > The [(String, String)] argument is for adding query parameters. > > > encodePathInfo ["foo", "bar", "baz"] [("key","value")] > > "foo/bar/baz?key=value" > > Instead of showURL you would use showURLParams. > > hope this helps!d > - jeremy > > On Fri, Jan 7, 2011 at 8:12 AM, Corentin Dupont > <corentin.dupont(a)gmail.com> wrote: > > Hello Jeremy, > > I'm using Web routes with happstack. > > I'm following this tutorial: > > http://tutorialpedia.org/tutorials/Happstack+type+safe+URLs.html > > > > But It seems out of synch with the latest version of web-routes: 0.23.2. > > The haddock documentation seems out of date also: > > > > encodePathInfo :: [String] -> [(String, String)] -> String > > > > For example: > > > > encodePathInfo [\"foo\", \"bar\", \"baz\"] > > > > "foo/bar/baz" > > > > And I can't figure out what this [(String, String)] is for ;) > > > > Thanks, > > > > Corentin > > >

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stat64 missing when loading download in GHCi
by Magnus Therning 26 Jan '11

26 Jan '11

Don, I'm having issues with loading download in GHCi: Loading package download-0.3.1.1 ... linking ... <interactive>: /usr/lib/ghc-6.12.3/site-local/download-0.3.1.1/HSdownload-0.3.1.1.o: unknown symbol `stat64' ghc: unable to load package `download-0.3.1.1' I found an old email you sent in 2008[1] on the same issue, but you don't seem to have received any replies back then. Did you ever found out why GHCi complains, but it it possible to compile and link into an executable? I'm CC-ing to Haskell Cafe as well, in case someone else has any information on this. /M [1] http://www.haskell.org/pipermail/glasgow-haskell-bugs/2008-July/014071.html -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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