September 2011 - Haskell-Cafe

HackageDB User Account
by Jonathan Frywater 29 Sep '11

29 Sep '11

How does one go about getting an account? I sent an email to the address provided at http://hackage.haskell.org/packages/accounts.html but haven't received any response yet. Since it's been over 3 weeks, I decided to try my luck here.

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LLVM: function pointer in global struct
by Tom Schouten 29 Sep '11

29 Sep '11

Hi everyone, Using the LLVM bindings, I'm trying to create an initialized global struct variable containing a pointer to a function. > {-# LANGUAGE ScopedTypeVariables #-} > import LLVM.Core > import Data.Word > import LLVM.Util.File(writeCodeGenModule) > sm_module = do > tick :: Function (Word32 -> IO Word32) <- createFunction ExternalLinkage $ \x -> ret x > info <- createNamedGlobal False ExternalLinkage "sm_info" $ constStruct (tick & ()) > return info I run into a complaint that this pointer isn't constant when it's part of a global variable initializer. No instance for (llvm-0.10.0.1:LLVM.Core.CodeGen.IsConstStruct (Function (Word32 -> IO Word32) :& ()) a0) arising from a use of `constStruct' Possible fix: add an instance declaration for (llvm-0.10.0.1:LLVM.Core.CodeGen.IsConstStruct (Function (Word32 -> IO Word32) :& ()) a0) In the second argument of `($)', namely `constStruct (tick & ())' In a stmt of a 'do' expression: info <- createNamedGlobal False ExternalLinkage "sm_info" $ constStruct (tick & ()) In a stmt of a 'do' expression: tick :: Function (Word32 -> IO Word32) <- createFunction ExternalLinkage $ \ x -> ret x I suppose this is because of > -- |A function is simply a pointer to the function. > type Function a = Value (Ptr a) being a Value instead of ConstValue. Is there a way around this? Cheers Tom

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PhD studentship in Nottingham
by Graham Hutton 29 Sep '11

29 Sep '11

Dear all, I am currently advertising a PhD Studentship in Functional Programming at the University of Nottingham in the UK: http://tinyurl.com/phd-nottingham Due to the nature of the funding, the position is only to UK/EU students who have resided in the UK for the last three years. If you are interested in applying yourself, please drop me a note by email. If you know of any good candidates who may be interested in applying, or there is a local mailing list for advertising such things, I'd be much obliged if you could pass on the above link. Many thanks, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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network 2.3.05 on windows 7 with ghc-7.2.1
by Sai Hemanth K 29 Sep '11

29 Sep '11

Hi, Apologies if this is an off-topic for the list, If someone here managed to build network 2.3.05 on windows 7 with ghc-7.2.1 , could you kindly pass me the trick? The config step fails with missing header. I tried running it with mingw (tried with the latest one too - just in case : http://sourceforge.net/projects/mingw/files/Automated%20MinGW%20Installer/) I did not face any issue for the same thing on linux. Many thanks, Hemanth K

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Haskell Weekly News: Issue 201
by Daniel Santa Cruz 28 Sep '11

28 Sep '11

Welcome to issue 201 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of September 18 to 24, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/09/haskell-weekly-news-issue-201.h… Announcements Jasper Van der Jeugt sent more details about the 9th GhentFPG meeting, which will take place on October the 4th, at 19:30 in the Technicum building of Ghent University. "You don't have to be a Functional Programming guru to attend, everyone eager to learn is welcome." [1] http://goo.gl/hAa5Zn New and Updated Projects * usb, bindings-libusb, usb-iteratee (Bas van Dijk; 1.0, 1.4.4, 0.4) Wrapers around the portable C library libusb-1.0 [2] http://goo.gl/lTMNJn * ivy-web (James Deng; 0.1) A lightweight web framework, with type-safe routes, based on invertible syntax, and i18n support, influenced by Django, Snap and Yesod. [3] http://goo.gl/dTcPEn * lushtags (Bit Connor) ctags compatible tags files for Haskell programs. [4] http://goo.gl/30TYen * lightweight monadic regions family (Bas van Dijk; multiple versions) Updates to regios, regions-mtl, regions-monadstf, regional-pointers, safer-file-handles, safer-file-handles-bytestring, safer-file-handles-text, and usb-safe. [5] http://goo.gl/g8P6Kn * skein (Felipe Almeida Lessa; 0.1) A family of fast secure cryptographic hash functions. [6] http://goo.gl/xwafmn Quotes of the Week * dafis: I thought a C++ friend class was one you allowed to touch your private parts * edwardk: any time i can help, just ask. i may not actually help, but i'll be sure to note your suffering. * edwardk: i have a lot fewer [haddock problems] now since i stopped bothering trying to document ;) * edwardk: Had i thought of it earlier i could have named trifecta kessel run. at this point it looks like it'll contain at least 12 parsecs. * ddarius: The history of iteratees is making a simple idea complex. * shachaf: You can never escape having learned monads. If you learn two monads, though, you can go back to only knowing one. Top Reddit Stories * GHCi now supports all top-level declarations Domain: github.com, Score: 104, Comments: 35 On Reddit: [7] http://goo.gl/GfTc9n Original: [8] http://goo.gl/Md5B3n * The best ghc bug ever Domain: twitter.com, Score: 93, Comments: 27 On Reddit: [9] http://goo.gl/3sCHUn Original: [10] http://goo.gl/N7IGdn * Running a startup on Haskell Domain: bos.github.com, Score: 73, Comments: 33 On Reddit: [11] http://goo.gl/HevD3n Original: [12] http://goo.gl/IsXedn * No chrome, Haskell is not Danish. Domain: i.imgur.com, Score: 73, Comments: 26 On Reddit: [13] http://goo.gl/AYyq4n Original: [14] http://goo.gl/whKijn * Did Haskell Platform miss the July release? It's already September. Domain: trac.haskell.org, Score: 46, Comments: 12 On Reddit: [15] http://goo.gl/6m8Yln Original: [16] http://goo.gl/r9k6tn * Iteratees at Tsuru Capital: various contributions to the hackage iteratee library used in a production trading system Domain: blog.kfish.org, Score: 36, Comments: 12 On Reddit: [17] http://goo.gl/rCTO6n Original: [18] http://goo.gl/PikaAn * Apprendre Haskell vous fera le plus grand bien ! -- LYAH in French Domain: lyah.haskell.fr, Score: 33, Comments: 15 On Reddit: [19] http://goo.gl/pV3jvn Original: [20] http://goo.gl/ixiXWn * Would You Bet $100,000,000 on Haskell? Domain: digitalmars.com, Score: 28, Comments: 44 On Reddit: [21] http://goo.gl/oPI5Kn Original: [22] http://goo.gl/YYeOln * Galois offers Haskell course: The Tao of Functional Programming Domain: corp.galois.com, Score: 26, Comments: 9 On Reddit: [23] http://goo.gl/6HX8Kn Original: [24] http://goo.gl/n7urdn Top StackOverflow Questions * What are Haskell's strictness points? votes: 59, answers: 7 Read on SO: [25] http://goo.gl/5PiDHn * Can someone explain the traverse function in Haskell votes: 14, answers: 3 Read on SO: [26] http://goo.gl/P5WtFn * Why do we have map, fmap and liftM? votes: 13, answers: 1 Read on SO: [27] http://goo.gl/TCRKwn * Does a function like this already exist? (Or, what's a better name for this function?) votes: 12, answers: 4 Read on SO: [28] http://goo.gl/QY6i8n * Question about the ~ and @ operators in Haskell votes: 11, answers: 2 Read on SO: [29] http://goo.gl/pXnV6n * Folding across Maybes in Haskell votes: 8, answers: 5 Read on SO: [30] http://goo.gl/y1ALtn * How is foldl lazy? votes: 7, answers: 4 Read on SO: [31] http://goo.gl/LQAxNn * avoid explicit passing of lookup table votes: 7, answers: 4 Read on SO: [32] http://goo.gl/tKh2wn * Why is [Char]-based input so much slower than the [Char]-based output in Haskell? votes: 7, answers: 1 Read on SO: [33] http://goo.gl/2nuqNn Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92680 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18935 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18936 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92480 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18940 6. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92528 7. https://github.com/ghc/ghc/commit/3db757241ce7fb99c096c30481aefa86bb9855a1 8. http://www.reddit.com/r/haskell/comments/kmxf2/ghci_now_supports_all_toplev… 9. http://twitter.com/#!/bos31337/status/116372971509121025 10. http://www.reddit.com/r/haskell/comments/kmk6q/the_best_ghc_bug_ever/ 11. http://bos.github.com/strange-loop-2011/talk/talk.html#(1) 12. http://www.reddit.com/r/haskell/comments/km2b8/running_a_startup_on_haskell/ 13. http://i.imgur.com/D4QEO.png 14. http://www.reddit.com/r/haskell/comments/koom1/no_chrome_haskell_is_not_dan… 15. http://trac.haskell.org/haskell-platform/wiki/ReleaseTimetable 16. http://www.reddit.com/r/haskell/comments/kk9hl/did_haskell_platform_miss_th… 17. http://blog.kfish.org/2011/09/iteratees-at-tsuru.html 18. http://www.reddit.com/r/haskell/comments/kjem1/iteratees_at_tsuru_capital_v… 19. http://lyah.haskell.fr/ 20. http://www.reddit.com/r/haskell/comments/km1l6/apprendre_haskell_vous_fera_… 21. http://www.digitalmars.com/d/archives/digitalmars/D/Would_You_Bet_100_000_0… 22. http://www.reddit.com/r/haskell/comments/klhln/would_you_bet_100000000_on_h… 23. http://corp.galois.com/haskell-course 24. http://www.reddit.com/r/haskell/comments/kp6as/galois_offers_haskell_course… 25. http://stackoverflow.com/questions/7490768/what-are-haskells-strictness-poi… 26. http://stackoverflow.com/questions/7460809/can-someone-explain-the-traverse… 27. http://stackoverflow.com/questions/7463500/why-do-we-have-map-fmap-and-liftm 28. http://stackoverflow.com/questions/7518589/does-a-function-like-this-alread… 29. http://stackoverflow.com/questions/7506684/question-about-the-and-operators… 30. http://stackoverflow.com/questions/7464208/folding-across-maybes-in-haskell 31. http://stackoverflow.com/questions/7487455/how-is-foldl-lazy 32. http://stackoverflow.com/questions/7504012/avoid-explicit-passing-of-lookup… 33. http://stackoverflow.com/questions/7510078/why-is-char-based-input-so-much-…

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instance Enum Double considered not entirely great?
by Evan Laforge 28 Sep '11

28 Sep '11

So I've been excising occurrences of the [n..m] syntax from my code. The reason is that I had been using [0..5] as a shorthand for [0, 1, 2, 3, 4, 5], mostly in tests. This works for integral types and it appears to work for floating point types. Then I tried switching to a fixed point format, and discovered my mistake. Enum is supposed to enumerate every value between the two points, and the result is memory exhaustion. I had already been switching from enumFrom because they accumulate floating point errors, in favor of a couple of more appropriate functions: -- | Enumerate an inclusive range. Uses multiplication instead of successive -- addition to avoid loss of precision. -- -- Also it doesn't require an Enum instance. range :: (Num a, Ord a) => a -> a -> a -> [a] range start end step = go 0 where go i | val > end = [] | otherwise = val : go (i+1) where val = start + (i*step) -- | Like 'range', but includes the end. range_end :: (Num a, Ord a) => a -> a -> a -> [a] range_end start end step = go 0 where go i | val >= end = [end] | otherwise = val : go (i+1) where val = start + (i*step) -- | Infinite range. range_ :: (Num a) => a -> a -> [a] range_ start step = go 0 where go i = start + (i*step) : go (i+1) So I think the Enum instances for Float and Double are sort of a trap. They misled me into thinking the .. syntax is the range function defaulting to a step of 1, led to precision errors I had to debug then fix (and I've noticed lead to the odd stack overflow question), then led to more disaster when I switched to an Enum instance that lived up to Enum's real purpose. The problem is that the [..] syntax is convenient and tempting, especially for tests, wanting a numeric range (rather than enumeration) is very common, and as far as I know the 'range' function isn't in the standard library. So it seems like one of these things that seem convenient in the short term but come around and bite you later. Using the right functions from the start avoids all that. Is there any support for the idea of removing Enum instances for floating point numbers?

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Much faster complex monad stack based on CPS state
by Nicu Ionita 28 Sep '11

28 Sep '11

Hello list, Starting from this emails (http://web.archiveorange.com/archive/v/nDNOvSM4JT3GJRSjOm9P) I could refactor my code (a UCI chess engine, with complex functions, in which the search has a complex monad stack) to run twice as fast as with even some hand unroled state transformer! So from 23-24 kilo nodes per second it does now 45 to 50 kNps! And it looks like there is still some improvement room (I have to play a little bit with strictness annotations and so on). (Previously I tried specializations, then I removed a lot of polimorphism, but nothing helped, it was like hitting a wall.) Even more amazingly is that I could program it although I cannot really understand the Cont & ContT, but just taking the code example from Ryan Ingram (newtype ContState r s a = ...) and looking a bit at the code from ContT (from the transformers library), and after fixing some compilation errors, it worked and was so fast. I wonder why the transformers library does not use this kind of state monad definition. Or does it, and what I got is just because of the unrolling? Are there monad (transformers) libraries which are faster? I saw the library kan-extensions but I did not understand (yet) how to use it. Nicu

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network package Windows co-maintainer/buildbot needed (Was: network 2.3.05 on windows 7 with ghc-7.2.1)
by Johan Tibell 28 Sep '11

28 Sep '11

Let me take this opportunity to ask for a co-maintainer that can help me keep the network package working on Windows. I don't have easy access to a Windows machine (or even VM) anymore so testing on Windows is hard. What I'd really like is a buildbot that builds the following Jenkins job as a "slave" node: http://ci.johantibell.com/job/network/ config.xml: <?xml version='1.0' encoding='UTF-8'?> <matrix-project> <actions/> <description></description> <keepDependencies>false</keepDependencies> <properties/> <scm class="hudson.plugins.git.GitSCM"> <configVersion>2</configVersion> <userRemoteConfigs> <hudson.plugins.git.UserRemoteConfig> <name>origin</name> <refspec>+refs/heads/*:refs/remotes/origin/*</refspec> <url>https://github.com/haskell/network.git</url> </hudson.plugins.git.UserRemoteConfig> </userRemoteConfigs> <branches> <hudson.plugins.git.BranchSpec> <name>stable</name> </hudson.plugins.git.BranchSpec> </branches> <recursiveSubmodules>false</recursiveSubmodules> <doGenerateSubmoduleConfigurations>false</doGenerateSubmoduleConfigurations> <authorOrCommitter>false</authorOrCommitter> <clean>false</clean> <wipeOutWorkspace>false</wipeOutWorkspace> <pruneBranches>false</pruneBranches> <remotePoll>false</remotePoll> <buildChooser class="hudson.plugins.git.util.DefaultBuildChooser"/> <gitTool>Default</gitTool> <browser class="hudson.plugins.git.browser.GithubWeb"> <url>https://github.com/haskell/network/</url> </browser> <submoduleCfg class="list"/> <relativeTargetDir></relativeTargetDir> <excludedRegions></excludedRegions> <excludedUsers></excludedUsers> <gitConfigName></gitConfigName> <gitConfigEmail></gitConfigEmail> <skipTag>false</skipTag> <scmName></scmName> </scm> <canRoam>true</canRoam> <disabled>false</disabled> <blockBuildWhenDownstreamBuilding>false</blockBuildWhenDownstreamBuilding> <blockBuildWhenUpstreamBuilding>false</blockBuildWhenUpstreamBuilding> <triggers class="vector"> <hudson.triggers.SCMTrigger> <spec>*/5 * * * *</spec> </hudson.triggers.SCMTrigger> </triggers> <concurrentBuild>false</concurrentBuild> <axes> <hudson.matrix.TextAxis> <name>compiler</name> <values> <string>ghc-6.10.4</string> <string>ghc-6.12.3</string> <string>ghc-7.0.3</string> </values> </hudson.matrix.TextAxis> </axes> <builders> <hudson.tasks.Shell> <command>cabal install -w $compiler --only-dependencies --enable-tests cabal clean autoreconf cabal configure -w $compiler --enable-tests cabal build cabal test cabal sdist</command> </hudson.tasks.Shell> </builders> <publishers> <hudson.tasks.Mailer> <recipients>johan.tibell(a)gmail.com</recipients> <dontNotifyEveryUnstableBuild>false</dontNotifyEveryUnstableBuild> <sendToIndividuals>false</sendToIndividuals> </hudson.tasks.Mailer> </publishers> <buildWrappers/> <runSequentially>false</runSequentially> </matrix-project>

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Binding a socket to all interfaces
by Michael Snoyman 28 Sep '11

28 Sep '11

Hi, One of the recurring issues that comes up in Warp is binding to IPv4 versus IPv6 hosts. Our current code is available at [1]. It was updated to look like that in this commit [2] in order to support both IPv4 and IPv6 hosts by default. However, now it seems than on Debian and FreeBSD, it *only* responds to IPv6 by default[3][4]. I'm frankly stumped at this point on how to have our cake and eat it too. Does anyone have an idea of the correct incantation to get Warp to do the Right Thing(tm) here? And if not, is there any advice on sensible default behavior? I'm considering allowing a few special host values: * "*" (default, what we have now): Make this bind to IPv4 * "ipv4": Again, bind to IPv4. Guaranteed not to change in the future * "ipv6": Bind to IPv6. Michael [1] https://github.com/yesodweb/wai/blob/master/warp/Network/Wai/Handler/Warp.h… [2] https://github.com/snoyberg/warp/commit/02c1396c86e3fceb48cbe7df58cb631c804… [3] https://github.com/snoyberg/warp/issues/9 [4] http://stackoverflow.com/questions/7486257/yesod-devel-server-only-listenin…

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Parsing binary data question
by Michael Oswald 28 Sep '11

28 Sep '11

Hello all, I am currently working on parser for some packets received via the network. The data structure currently is like that: data Value = ValUInt8 Int8 | ValUInt16 Int16 | ValUInt32 Int32 -- more datatypes data Parameter = Parameter { paramName :: String, paramValue :: Value } | ParameterN { paramName :: String, paramValue :: Value }deriving (Show) data TCPacket = TCPacket { tcAPID :: Word16, tcType :: Word8, tcSubType :: Word8, tcParameters :: [Parameter] } The output should a parsed packet (I am using cereal for this). The packet structure can vary depending on the type and the configuration, so I have a function which takes a TCPacket as "input template" which has already the correct list of parameters which are then parsed: parseTCPacket :: Word16 -> Word8 -> Word8 -> ByteString -> TCPacket -> TCPacket parseTCPacket apid t st pktData tmplate = TCPacket apid t st params where tmplParams = (tcParameters tmplate) params = zipWith (\p v -> p {paramValue = v} ) tmplParams values' values = map paramValue tmplParams values' = binValues values (pktData pusPkt) getBinGet :: Value -> Get Value getBinGet (ValInt8 _) = getWord8 >>= \x -> return $ ValInt8 $ fromIntegral x getBinGet (ValInt16 _) = getWord16be >>= \x -> return $ ValInt16 $ fromIntegral x -- many more datatypes getBinValues :: [Value] -> Get [Value] getBinValues inp = mapM getBinGet inp binValues :: [Value] -> ByteString -> ([Value], B.ByteString) binValues inp bytes = case runGet (getBinValues inp) bytes of Left err -> throw $ DecodeError ("binValues: " ++ err) Right x -> x This works quite well and does what I want. Now I have the problem that there are some parameters, which could be so-called "group repeaters" (the ParameterN constructor above). This means, that if such a parameter N is encountered during parsing (it has to be an int type), all following parameters are repeated N times with possible nesting. So for example if the template (where the values are all 0) is like this: [Parameter "Param1" (ValUInt32 0), ParameterN "N1" (ValUInt8 0), Parameter "Param2" (ValUint16 0), ParameterN "N2" (ValUint8 0), Parameter "Param3" (ValUint8 0)] Which means there is a group for the last 3 parameters which is repeated N1 times which contains another group which is repeated N2 times. If binary data based on the template above would be like this (datatypes omitted): 10, 2, 439, 2, 12, 13, 65535, 2, 22, 23 then a valid packet after parsing would be: [Parameter "Param1" (ValUint32 10), ParameterN "N1" (ValUint8 2), Parameter "Param2" (ValUint16 439), ParameterN "N2" (ValUint8 2), Parameter "Param3" (ValUint8 12), Parameter "Param3" (ValUint8 13), Parameter "Param2" (ValUint16 65535), ParameterN "N2" (ValUint8 2), Parameter "Param3" (ValUint8 22), Parameter "Param3" (ValUint8 23)] Now I am a bit lost on how to implement such a parser. It would be much easier if the structure would be already encoded in the binary data, but I have to stick to this template approach. I have some C++ parser which does this but of course it's very imperative and a little bit quirky implemented, so if anybody has an idea on how to proceed (cereal, attoparsec whatever), please tell me. lg, Michael

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