- Haskell-Cafe - Haskell.org

[Pipes] Can pipes solve this problem? How?
by Daniel Hlynskyi 16 Aug '12

16 Aug '12

Hello Cafe. Consider code, that takes input from handle until special substring matched: > matchInf a res s | a `isPrefixOf` s = reverse res > matchInf a res (c:cs) = matchInf a (c:res) cs > hTakeWhileNotFound str hdl = hGetContents hdl >>= return.matchInf str [] It is simple, but the handle is closed after running. That is not good, because I want to reuse this function. Code can be rewritten without hGetContent, but it is much less comprehensible: hTakeWhileNotFound str hdl = fmap reverse$ findStr str hdl [0] [] where findStr str hdl indeces acc = do c <- hGetChar hdl let newIndeces = [ i+1 | i <- indeces, i < length str, str!!i == c] if length str `elem` newIndeces then return (c : acc) else findStr str hdl (0 : newIndeces) (c : acc) So, the question is - can pipes (any package of them) be the Holy Grail in this situation, to both keep simple code and better deal with handles (do not close them specifically)? How?

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linking Haskell app with Curl on Windows
by Eugene Dzhurinsky 16 Aug '12

16 Aug '12

Hi! I'm facing strange issue with linking my application against Curl (using Haskell Curl binding curl-1.3.7): the application compiles well, but fails on linking stage: ===================================================================================== C:\haskell\bin\ghc.exe --make -o dist\build\imgpaste\imgpaste.exe -hide-all-packages -fbuilding-cabal-package -package-conf dist\package.conf.inplace -i -idist\build\imgpaste\imgpaste-tmp -i. -idist\build\autogen -Idist\build\autogen -Idist\build\imgpaste\imgpaste-tmp -optP-include -optPdist\build\autogen\cabal_macros.h -odir dist\build\imgpaste\imgpaste-tmp -hidir dist\build\imgpaste\imgpaste-tmp -stubdir dist\build\imgpaste\imgpaste-tmp -package-id base-4.5.0.0-597748f6f53a7442bcae283373264bb6 -package-id bytestring-0.9.2.1-df82064cddbf74693df4e042927e015e -package-id curl-1.3.7-ed08f87bd8c487f1e11a8c3b67bf4e51 -package-id directory-1.1.0.2-0270278088d4b2588b52cbec49af4cb7 -package-id hxt-9.2.2-e687550fbbb6ff367ee9c95584c3f0a0 -package-id hxt-xpath-9.1.2-4a15d34a0b66fa21832bb4bb0f68477f -package-id regex-pcre-0.94.4-f2f06ed579a684904354d97b04a74d9e -O -XHaskell98 .\Main.hs -llibcrypto -lssh2 -lssl -lz -lidn -LC:\curl\lib Linking dist\build\imgpaste\imgpaste.exe ... C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0xd2): undefined reference to `_imp__curl_easy_getinfo' C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0xee): undefined reference to `_imp__curl_easy_getinfo' C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0x10a): undefined reference to `_imp__curl_easy_getinfo' [ lots of error messages skipped ] C:\curl\lib/libcurl.a(md5.o):(.text+0x3b): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.text+0x4e): undefined reference to `MD5_Final' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x0): undefined reference to `MD5_Init' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x4): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x8): undefined reference to `MD5_Final' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x14): undefined reference to `MD5_Init' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x18): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x1c): undefined reference to `MD5_Final' cabal.EXE: Error: some packages failed to install: imgpaste-0.2 failed during the building phase. The exception was: ExitFailure 1 ===================================================================================== What may be wrong here? Curl itself was installed without errors. Looking for symbols 'MD5_Final' in directory C:\curl\lib results in 'libcrypto.a', 'libcrypto.dll.a' and 'libcurl.a' -- Eugene N Dzhurinsky

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Custom QuickCheck Gens without custom shrinks
by Niklas Hambüchen 16 Aug '12

16 Aug '12

I wrote this Gen to generate lines of texts without \NUL and \n: fullLinesProp = forAll linesGen ... linesGen = listOf . listOf $ arbitrary `suchThat` (`notElem` ['\NUL', '\n']) -- alternatively: linesGen = arbitrary `suchThat` (all (all (`notElem` ['\NUL', '\n']))) However, I just realized that QuickCheck disables shrinking when I use this instead of (linesGen = arbitrary :: Gen [String]). Why is that? Is there a way to get what I want + shrinking without defining a custom shrink (e.g. forAllShrink)? Thanks

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Haskell Weekly News: Issue 240
by Daniel Santa Cruz 16 Aug '12

16 Aug '12

Welcome to issue 240 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 05 to 11, 2012. Quotes of the Week * johnw: Monads are an interface, all bets are off until you see what the interface does for a given Type * Evbn: I heard that Untyped and Well Typed tried to set up a corporate softball league, but they couldn't agree on a ruleset to use. * wuttf: i think i have to learn this language, the type systems i know just dont feel right Top Reddit Stories * Studying protein structures in Haskell Domain: haskellforall.com, Score: 53, Comments: 6 On Reddit: [1] http://goo.gl/94PA4 Original: [2] http://goo.gl/VJJD0 * Haskell Live Episode 2: Time To Refactor is up! Domain: youtube.com, Score: 52, Comments: 14 On Reddit: [3] http://goo.gl/BOmK7 Original: [4] http://goo.gl/5wTVu * Announcing Raskell - user ratings for Hackage Domain: ocharles.org.uk, Score: 43, Comments: 29 On Reddit: [5] http://goo.gl/wgK4E Original: [6] http://goo.gl/7q4RP * Yesod 1.1 Released Domain: yesodweb.com, Score: 38, Comments: 14 On Reddit: [7] http://goo.gl/jiwLx Original: [8] http://goo.gl/B5czq * Running Haskell on Raspberry Pi Domain: alenribic.com, Score: 37, Comments: 5 On Reddit: [9] http://goo.gl/zPcDA Original: [10] http://goo.gl/FsESS * Koka - a "function"-oriented language with side-effect inteference Domain: rise4fun.com, Score: 36, Comments: 18 On Reddit: [11] http://goo.gl/lyKhA Original: [12] http://goo.gl/n35n1 * Is Haskell liberal or conservative? (Response post to Yegge) Domain: blog.ezyang.com, Score: 34, Comments: 51 On Reddit: [13] http://goo.gl/AJ5Wp Original: [14] http://goo.gl/Juis3 * ANNOUNCE: basic-prelude Domain: haskell.org, Score: 34, Comments: 8 On Reddit: [15] http://goo.gl/iiDLq Original: [16] http://goo.gl/4Taz0 * ANNOUNCE: tardis Domain: haskell.org, Score: 31, Comments: 3 On Reddit: [17] http://goo.gl/e6yi7 Original: [18] http://goo.gl/590QR * Any Hackage mirrors? How do you deal with Hackage outages? Domain: self.haskell, Score: 30, Comments: 27 On Reddit: [19] http://goo.gl/1BOMF Original: [20] http://goo.gl/1BOMF * Is there a standard Haskell development setup for Vim? Domain: self.haskell, Score: 29, Comments: 44 On Reddit: [21] http://goo.gl/gSRWp Original: [22] http://goo.gl/gSRWp * O'Reilly Webinar this Thursday: Designing Type-Safe Haskell APIs Domain: yesodweb.com, Score: 28, Comments: 8 On Reddit: [23] http://goo.gl/GNogN Original: [24] http://goo.gl/ZRkrH * acid-state with Yesod Domain: meadowstalk.com, Score: 25, Comments: 13 On Reddit: [25] http://goo.gl/NM4Bc Original: [26] http://goo.gl/8MDXo * Philip Wadler: Faith, Evolution, and Programming Languages [InfoQ] Domain: infoq.com, Score: 25, Comments: 6 On Reddit: [27] http://goo.gl/WFxLq Original: [28] http://goo.gl/fBqx7 * Hiring Haskell Developers (Singapore) Domain: cufp.org, Score: 22, Comments: 13 On Reddit: [29] http://goo.gl/bhqrb Original: [30] http://goo.gl/umhh4 * Verifying Software Transactional Memory: bisimulation in Agda [pdf] Domain: cs.nott.ac.uk, Score: 22, Comments: On Reddit: [31] http://goo.gl/X1zvj Original: [32] http://goo.gl/vJ1gA * iterate (*2) 1 Domain: postimage.org, Score: 17, Comments: 13 On Reddit: [33] http://goo.gl/lwLv1 Original: [34] http://goo.gl/lfIab * Error Handling: How would you solve this? Domain: self.haskell, Score: 16, Comments: 11 On Reddit: [35] http://goo.gl/jHLZ8 Original: [36] http://goo.gl/jHLZ8 Top StackOverflow Questions * Advice defining a data structure in Haskell votes: 13, answers: 4 Read on SO: [37] http://goo.gl/xVxlx * What is the preferred way to combine two sinks? votes: 13, answers: 2 Read on SO: [38] http://goo.gl/QUczW * Why is numCapabilities a pure function? votes: 12, answers: 1 Read on SO: [39] http://goo.gl/4HU27 * Using generics to convert between types of the same “shape” votes: 10, answers: 1 Read on SO: [40] http://goo.gl/gx8ZL * Controlling parallel execution votes: 9, answers: 2 Read on SO: [41] http://goo.gl/zzvtU * Lists of lists of lists votes: 9, answers: 5 Read on SO: [42] http://goo.gl/HEicW * Positive integer type votes: 8, answers: 5 Read on SO: [43] http://goo.gl/xXY49 * Haskell bit arrays votes: 7, answers: 1 Read on SO: [44] http://goo.gl/g9TDQ * How to keep track of number of guesses in a simple guessing game (Haskell) votes: 6, answers: 2 Read on SO: [45] http://goo.gl/VQaWn * A haskell program that controls the mouse votes: 5, answers: 1 Read on SO: [46] http://goo.gl/rtveO Until next time, Daniel Santa Cruz References 1. http://www.haskellforall.com/2012/08/code-example-1.html 2. http://www.reddit.com/r/haskell/comments/y0wlw/studying_protein_structures_… 3. http://www.youtube.com/watch?v=6KkF5-_erns 4. http://www.reddit.com/r/haskell/comments/xy6gt/haskell_live_episode_2_time_… 5. http://ocharles.org.uk/blog/posts/2012-08-05-announcing-raskell.html 6. http://www.reddit.com/r/haskell/comments/xq5ap/announcing_raskell_user_rati… 7. http://www.yesodweb.com/blog/2012/08/announcing-yesod-1-1 8. http://www.reddit.com/r/haskell/comments/xujq6/yesod_11_released/ 9. http://alenribic.com/writings/post/running-haskell-on-raspberry-pi 10. http://www.reddit.com/r/haskell/comments/xs5hn/running_haskell_on_raspberry… 11. http://www.rise4fun.com/koka/tutorial 12. http://www.reddit.com/r/haskell/comments/xwe10/koka_a_functionoriented_lang… 13. http://blog.ezyang.com/2012/08/is-haskell-liberal-or-conservative/ 14. http://www.reddit.com/r/haskell/comments/xzukd/is_haskell_liberal_or_conser… 15. http://www.haskell.org/pipermail/haskell/2012-August/023461.html 16. http://www.reddit.com/r/haskell/comments/y0kld/announce_basicprelude/ 17. http://www.haskell.org/pipermail/haskell/2012-August/023455.html 18. http://www.reddit.com/r/haskell/comments/xt7yg/announce_tardis/ 19. http://www.reddit.com/r/haskell/comments/y1tgc/any_hackage_mirrors_how_do_y… 20. http://www.reddit.com/r/haskell/comments/y1tgc/any_hackage_mirrors_how_do_y… 21. http://www.reddit.com/r/haskell/comments/xxxyi/is_there_a_standard_haskell_… 22. http://www.reddit.com/r/haskell/comments/xxxyi/is_there_a_standard_haskell_… 23. http://www.yesodweb.com/blog/2012/08/webinar-oreilly 24. http://www.reddit.com/r/haskell/comments/xrebo/oreilly_webinar_this_thursda… 25. http://meadowstalk.com/post/migration-to-acid-state 26. http://www.reddit.com/r/haskell/comments/xs0t2/acidstate_with_yesod/ 27. http://www.infoq.com/presentations/Faith-Evolution-Programming-Languages 28. http://www.reddit.com/r/haskell/comments/xtusf/philip_wadler_faith_evolutio… 29. http://cufp.org/jobs/haskell-development-role-within-quant-development-team… 30. http://www.reddit.com/r/haskell/comments/xrpl4/hiring_haskell_developers_si… 31. http://www.cs.nott.ac.uk/~gmh/hu-thesis.pdf 32. http://www.reddit.com/r/haskell/comments/xw1p5/verifying_software_transacti… 33. http://postimage.org/image/ucqiynz17/ 34. http://www.reddit.com/r/haskell/comments/xwfbm/iterate_2_1/ 35. http://www.reddit.com/r/haskell/comments/y1gg0/error_handling_how_would_you… 36. http://www.reddit.com/r/haskell/comments/y1gg0/error_handling_how_would_you… 37. http://stackoverflow.com/questions/11833920/advice-defining-a-data-structur… 38. http://stackoverflow.com/questions/11844523/what-is-the-preferred-way-to-co… 39. http://stackoverflow.com/questions/11851005/why-is-numcapabilities-a-pure-f… 40. http://stackoverflow.com/questions/11887666/using-generics-to-convert-betwe… 41. http://stackoverflow.com/questions/11828654/controlling-parallel-execution 42. http://stackoverflow.com/questions/11850821/lists-of-lists-of-lists 43. http://stackoverflow.com/questions/11910143/positive-integer-type 44. http://stackoverflow.com/questions/11853306/haskell-bit-arrays 45. http://stackoverflow.com/questions/11856135/how-to-keep-track-of-number-of-… 46. http://stackoverflow.com/questions/11896870/a-haskell-program-that-controls…

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Improvement suggestions
by José Lopes 15 Aug '12

15 Aug '12

Hello everyone, I am quite new to monadic code so I would like to ask for improvement suggestions on the last line of the code below. I know I could do something like "do strs <- mapM ...; intercalate ..." etc, but I would like to avoid the use of "<-". Thank you, José data XmlState = XmlState Int type XmlM a = State XmlState a loop :: Document -> XmlM String someFn :: [Document] -> XmlM String someFn docs = return concat `ap` (sequence $ intersperse (return "\n") (map loop docs)) <--- improve this line -- José António Branquinho de Oliveira Lopes 58612 - MEIC-A Instituto Superior Técnico (IST), Universidade Técnica de Lisboa (UTL) jose.lopes(a)ist.utl.pt

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Upcoming resourcet 0.4
by Michael Snoyman 15 Aug '12

15 Aug '12

Hi all, I've been working with Aristid on an enhancement to resourcet[1]. Please see the issue for more details, this email isn't about that change. Instead, now that we're looking at a new breaking release, I was wondering if anyone had ideas of something they thought should be changed in resourcet. Thanks, Michael [1] https://github.com/snoyberg/conduit/issues/61

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For consistency; it would be better if the import statement matched the cabal install statement or :m form.
by KC 15 Aug '12

15 Aug '12

:m +Data.Array.Repa.Algorithms.Randomish cabal install repa.algrothms would be more consistent. -- -- Regards, KC

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Debian 6.0.5: 'cabal install curl' problem
by dokondr 15 Aug '12

15 Aug '12

Please help to solve a problem installing curl package ( http://hackage.haskell.org/package/curl/) on Debian 6.0.5. I am running the most recent Debian Haskell platform with GHC 6.12.1. I did: -> cabal update -> cabal install cabal-install It is interesting that in case you do again 'cabal update" you again get a suggestion from cabal to upgrade cabal-install, and this process never ends: """ -> cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install """ Anyway, then I did: -> apt-get install curl -> cabal install curl Cabal fails to install 'curl' because it can not find curl library. Please see detailed cabal output at the end of this message. So my questions: 1) What is missing to install curl package? 2) Should I upgrade Debian GHC 6.12.1 to the latest GHC 7.4.2? Maybe this should solve the problem with 'cabal install'? Thanks! ---------- cabal install output ---------- -> cabal install curl Resolving dependencies... Configuring curl-1.3.7... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ANSI C... none needed checking how to run the C preprocessor... gcc -E configure: error: curl libraries not found, so curl package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: curl-1.3.7 failed during the configure step. The exception was: ExitFailure 1

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I use "cabal install repa" but then WinGHCi says module Data.Array.Rep.Algorithms.Ramdomish not found.
by KC 15 Aug '12

15 Aug '12

-- -- Regards, KC

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Fixity declaration extension
by Евгений Пермяков 14 Aug '12

14 Aug '12

fixity declaration has form *infix(l|r)? [Digit]* in haskell. I'm pretty sure, that this is not enough for complicated cases. Ideally, fixity declarations should have form *infix(l|r)? [Digit](\.(+|-)[Digit])** , with implied infinitely long repeated (.0) tail. This will allow fine tuning of operator priorities and much easier priority selection. For example, it may be assumed, that bit operations like (.&.) operator have hightest priority and have priorities like 9.0.1 or 9.0.2, anti-lisps like ($) have lowest priority like 0.0.1, control operators have base priority 1.* and logic operations like (&&) have priority of 2.* and it will be possibly to add new operators between or above all (for example) control operators without moving fixity of other ones. Agda2 language supports wide priority range, but still without 'tails' to my knowledge. Is there any haskell-influenced language or experimental syntactic extension that address the issue?

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Lost global Contents of haddock
by Marco Túlio Pimenta Gontijo 14 Aug '12

14 Aug '12

Hi. I just noticed that my global Contents of the haddock documentation of modules ( file:///home/marcot/.cabal/share/doc/index.html ) lost most of the modules. My workflow is fairly straightforward, but I do some --reinstall --force-reinstalls with cabal install. For instance, the link for Prelude is not there, but Prelude.Unicode from base-unicode-symbols is present. I can still access documentation from the package's Contents page ( file:///usr/local/share/doc/ghc/html/libraries/base-4.5.1.0/index.html ). What could have caused this? Is there a way to recover all the links? I'm using Debian Squeeze with ghc-7.4.2 (installed with http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-x86_64-unknown-linux.tar.bz2 ), cabal install version 0.14.0, using version 1.14.0 of the Cabal library. Greetings. -- marcot http://marcot.eti.br/

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overloading integer literals
by Евгений Пермяков 14 Aug '12

14 Aug '12

During development some toy base library I found impossible to use Numeric literals. Quick search showed, that one need both fromInteger in scope (reasonable) and, as I understand, access to Integer type from 'base' package ('base' for clarity later). It is perfectly reasonable if we assume that every module must depend on 'base'. However, with ghc-prim and NoIplicitPrelude it is not necessary this way. One may wish to drop 'base' dependency entirely. Currently, the only workaround I found is to use cast from string literals (that works perfectly okay) or use unboxed literals like 0xBB## (of unboxed type). Both solutions look dirty, especially in pattern guards. Is there any better solution or movement to improve RebindableSyntax facilities ? for example, moving out classes for RebindableSyntax into ghc-prim package and tuning them so all syntax facilities in base could be defined in terms of RebindableSyntax would be great.

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ANNOUNCE: abacate and chuchu
by Marco Túlio Pimenta Gontijo 13 Aug '12

13 Aug '12

Hi. I'm pleased to announce the first release of Abacate and Chuchu. Chuchu is a port of Ruby's Cucumber to Haskell, and Abacate is a parser for the Gherkin language, used by Cucumber. According to their web site, "Cucumber lets software development teams describe how software should behave in plain text. The text is written in a business-readable domain-specific language and serves as documentation, automated tests and development-aid - all rolled into one format." For more information on Cucumber: http://cukes.info/ Example of usage for a stack calculator: calculator.feature: Feature: Division In order to avoid silly mistakes Cashiers must be able to calculate a fraction Scenario: Regular numbers Given that I have entered 3 into the calculator And that I have entered 2 into the calculator When I press divide Then the result should be 1.5 on the screen calculator.hs: import Control.Applicative import Control.Monad.IO.Class import Control.Monad.Trans.State import Test.Chuchu import Test.HUnit type CalculatorT m = StateT [Double] m enterNumber :: Monad m => Double -> CalculatorT m () enterNumber = modify . (:) getDisplay :: Monad m => CalculatorT m Double getDisplay = do ns <- get return $ head $ ns ++ [0] divide :: Monad m => CalculatorT m () divide = do (n1:n2:ns) <- get put $ (n2 / n1) : ns defs :: Chuchu (CalculatorT IO) defs = do Given ("that I have entered" *> number <* "into the calculator") enterNumber When "I press divide" $ const divide Then ("the result should be" *> number <* "on the screen") $ \n -> do d <- getDisplay liftIO $ d @?= n main :: IO () main = chuchuMain defs (`evalStateT` []) Both packages are available at Hackage at http://hackage.haskell.org/package/abacate http://hackage.haskell.org/package/chuchu I have written them after the specification from Felipe Almeida Lessa. Any comments are welcome. -- marcot http://marcot.eti.br/

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Haskell eXchange LDN October 10th // Feat. Simon Peyton Jones!
by Theo 13 Aug '12

13 Aug '12

*Skills Matter are putting on a fantastic 1-day conference coming up on October 10th- the Haskell eXchange. This fast-paced, interactive day will showcase a range of expert-led talks on all things Haskell. The conference will feature a spectrum of leading experts including a keynote from thefather of Haskell, Simon Peyton Jones. Whether you are an experienced Haskell user or a curious observer, join us on October 10th for this unique event. What: Haskell eXchange When: October 10th 2012 Where: The Crypt, London Featuring: Simon Peyton Jones, Simon Marlow, Lennart Augustsson, & Blake Rain Learn more and book online: http://bit.ly/RMSfuK*

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hGetContents Illegal byte sequence / ghc-pkg
by Benjamin Edwards 12 Aug '12

12 Aug '12

Hello café, I have a program that is crashing, and I have no idea why: module Main where import System.Process (readProcessWithExitCode) main :: IO () main = do _ <- readProcessWithExitCode "ghc-pkg" ["describe", "hoopl"] "" putStrLn "Should never get here" this is using the process package from hackage. The program crashes with minimal-test: fd:5: hGetContents: invalid argument (invalid byte sequence) minimal-test: thread blocked indefinitely in an MVar operation inspecting the source of readProcessWithExitCode yields an obvious explanation to the MVar problem, but I don't understand why hGetContents is so offended. For the lazy it is defined as follows: readProcessWithExitCode :: FilePath -- ^ command to run -> [String] -- ^ any arguments -> String -- ^ standard input -> IO (ExitCode,String,String) -- ^ exitcode, stdout, stderr readProcessWithExitCode cmd args input = do (Just inh, Just outh, Just errh, pid) <- createProcess (proc cmd args){ std_in = CreatePipe, std_out = CreatePipe, std_err = CreatePipe } outMVar <- newEmptyMVar -- fork off a thread to start consuming stdout out <- hGetContents outh _ <- forkIO $ C.evaluate (length out) >> putMVar outMVar () -- fork off a thread to start consuming stderr err <- hGetContents errh _ <- forkIO $ C.evaluate (length err) >> putMVar outMVar () -- now write and flush any input when (not (null input)) $ do hPutStr inh input; hFlush inh hClose inh -- done with stdin -- wait on the output takeMVar outMVar takeMVar outMVar hClose outh hClose errh -- wait on the process ex <- waitForProcess pid return (ex, out, err) Now having looked at the source of ghc-pkg it is dumping it's output using putStr and friends, so that should be using my local encoding on the system, right? and so should hGetContents in my program..? Now, for the curious: the reason I care is that this problem has effectively prevented me from using virthualenv. Sadness and woe.

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pattern matching vs if-then-else
by Maarten Faddegon 12 Aug '12

12 Aug '12

Hi there, I am writing a toy compiler in Haskell to further my skills in functional programming. One of the functions I wrote is to determine the iteration count of a loop. I have a number of different test that I want to do and I find myself now testing some of these using pattern matching and some properties using an if-then-else construction. I do not consider this very pretty. My question is: are there guidelines of when to use pattern matching and when to use if-then-else? Snippet of the function I mentioned: ---8<----------------------------------------------------------------------------------- itercount (ForLoop [ ( Assignment update_lcv (Op2 "+" (Content update_lcv') update_expr) ) ] [(Assignment init_lcv init_expr)] (TestStmt (Op2 "<" (Content test_lcv) test_expr)) bodyblock) = if -- All stmts use the same lcv test_lcv == init_lcv && test_lcv == update_lcv && test_lcv == update_lcv' -- And the lcv is not updated in the body && intersect [test_lcv] (blockkills bodyblock) == [] then Just $ simple_itercount init_expr test_expr update_expr else Nothing itercount _ = Nothing ---8<----------------------------------------------------------------------------------- Thanks, Maarten

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Hackage is down?
by hanjoosten 12 Aug '12

12 Aug '12

Hi, Hackage seems to be down. Is there anyone out here who knows how to get it online again? Thanks! -- View this message in context: http://haskell.1045720.n5.nabble.com/Hackage-is-down-tp5715912.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Fwd: hGetContents Illegal byte sequence / ghc-pkg
by Benjamin Edwards 11 Aug '12

11 Aug '12

Responding to the list.. ---------- Forwarded message ---------- From: Benjamin Edwards <edwards.benj(a)gmail.com> Date: 11 August 2012 17:37 Subject: Re: [Haskell-cafe] hGetContents Illegal byte sequence / ghc-pkg To: David McBride <toad3k(a)gmail.com> Thank you Gents, Most useful. One thing that had escaped me is that of course, even if hGetContents and friends are respecting *my* locale, I have a binary distro and the package database probably isn't in my locale. I will try and find that. Thanks, Ben On 11 August 2012 15:26, David McBride <toad3k(a)gmail.com> wrote: > I had this same problem a couple weeks ago when trying to install > virthualenv and I don't really understand it got into a bad state, but the > way I solved it was by fixing the locale settings on my gentoo machine so > that I'm using UTF8. That just involved a few changes in /etc and then the > problem went away. > > On Sat, Aug 11, 2012 at 7:13 AM, Benjamin Edwards <edwards.benj(a)gmail.com>wrote: > >> Hello café, >> >> I have a program that is crashing, and I have no idea why: >> >> module Main >> where >> >> import System.Process (readProcessWithExitCode) >> >> >> main :: IO () >> main = do _ <- readProcessWithExitCode "ghc-pkg" ["describe", "hoopl"] "" >> putStrLn "Should never get here" >> >> this is using the process package from hackage. The program crashes with >> >> minimal-test: fd:5: hGetContents: invalid argument (invalid byte sequence) >> minimal-test: thread blocked indefinitely in an MVar operation >> >> inspecting the source of readProcessWithExitCode yields an obvious >> explanation to the MVar problem, but I don't understand why hGetContents is >> so offended. >> >> For the lazy it is defined as follows: >> >> readProcessWithExitCode >> :: FilePath -- ^ command to run >> -> [String] -- ^ any arguments >> -> String -- ^ standard input >> -> IO (ExitCode,String,String) -- ^ exitcode, stdout, stderr >> readProcessWithExitCode cmd args input = do >> (Just inh, Just outh, Just errh, pid) <- >> createProcess (proc cmd args){ std_in = CreatePipe, >> std_out = CreatePipe, >> std_err = CreatePipe } >> >> outMVar <- newEmptyMVar >> >> -- fork off a thread to start consuming stdout >> out <- hGetContents outh >> _ <- forkIO $ C.evaluate (length out) >> putMVar outMVar () >> >> -- fork off a thread to start consuming stderr >> err <- hGetContents errh >> _ <- forkIO $ C.evaluate (length err) >> putMVar outMVar () >> >> -- now write and flush any input >> when (not (null input)) $ do hPutStr inh input; hFlush inh >> hClose inh -- done with stdin >> >> -- wait on the output >> takeMVar outMVar >> takeMVar outMVar >> hClose outh >> hClose errh >> >> -- wait on the process >> ex <- waitForProcess pid >> >> return (ex, out, err) >> >> Now having looked at the source of ghc-pkg it is dumping it's output >> using putStr and friends, so that should be using my local encoding on the >> system, right? and so should hGetContents in my program..? >> >> Now, for the curious: the reason I care is that this problem has >> effectively prevented me from using virthualenv. Sadness and woe. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Re: [Haskell-cafe] Data.Data and OverlappingInstances
by oleg＠okmij.org 11 Aug '12

11 Aug '12

Timo von Holtz wrote: > class Test a where > foo :: Monad m => m a > > instance Num a => Test a where > foo = return 1 > > instance Test Int where > foo = return 2 > > test constr = fromConstrM foo constr I'm afraid the type checker is right. From the type of fromConstrM fromConstrM :: forall m a. (Monad m, Data a) => (forall d. Data d => m d) -> Constr -> m a we see its first argument has the type (forall d. Data d => m d) If instead it had the type (forall d. Test d => m d) we would have no problem. As it is, when you pass 'foo' of the type (Test d, Monad m) => m d as the first argument of fromConstrM, which only assures the Data d constraint on 'd' and _nothing_ else, the compiler checks if it can get rid of (discharge) the constraint Test d. That is, the compiler is forced to choose an instance for Test. But there is not information to do that. Overlapping here is irrelevant. If you had non-overlapping instances > class Test a where > foo :: Monad m => m a > > instance Num a => Test [a] where > foo = return [1] > > instance Test Int where > foo = return 2 > > test constr = fromConstrM foo constr 'test' still causes the problem. The error message now describes the real problem: Could not deduce (Test d) arising from a use of `foo' from the context (Monad m, Data a) bound by the inferred type of test :: (Monad m, Data a) => Constr -> m a at /tmp/d.hs:16:1-36 or from (Data d) bound by a type expected by the context: Data d => m d at /tmp/d.hs:16:15-36 Possible fix: add (Test d) to the context of a type expected by the context: Data d => m d or the inferred type of test :: (Monad m, Data a) => Constr -> m a In the first argument of `fromConstrM', namely `foo' and it recommends the right fix: change the type of fromConstrM to be fromConstrM :: forall m a. (Monad m, Data a) => ( forall d. (Test d, Data d) => m d) -> Constr -> m a That will solve the problem. Alas, fromConstrM is a library function and we are not at liberty to change its type. > Right now I use a "case (typeOf x) of" kind of construct That is precisely the right way to use Data. SYB provides good combinators for building functions (generic producers) of that sort. But you never need unSafeCoerce: gcast is sufficient.

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Re: [Haskell-cafe] CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 11 Aug '12

11 Aug '12

Anthony Clayden wrote: > So three questions in light of the approach of abandoning FunDeps and > therefore not getting interference with overlapping: > A. Does TTypeable need to be so complicated? > B. Is TTypeable needed at all? > C. Does the 'simplistic' version of type equality testing suffer possible > IncoherentInstances? It is important to keep in mind that Type Families (and Data Families) are _strictly_ more expressive than Functional dependencies. For example, there does not seem to be a way to achieve the injectivity of Leibniz equality http://okmij.org/ftp/Computation/extra-polymorphism.html#injectivity without type families (and relaying instead on functional dependencies, implemented with TypeCast or directly). I'd like to be able to write data Foo a = Foo (SeqT a) where SeqT Bool = Integer SeqT a = [a] otherwise (A sequence of Booleans is often better represented as an Integer). A more practical example is http://okmij.org/ftp/Haskell/GMapSpec.hs http://okmij.org/ftp/Haskell/TTypeable/GMapSpecT.hs It is possible to sort of combine overlapping with associated types, but is quite ungainly. It is not possible to have overlapping associated types _at present_. Therefore, at present, TTYpeable is necessary and it has to be implemented as it is. You point out New Axioms. They will change things. I have to repeat my position however, which I have already stated several times. TTypeable needs no new features from the language and it is available now. There is no problem of figuring out how TTypeable interacts with existing features of Haskell since TTypeable is implemented with what we already have. New Axioms add to the burden of checking how this new feature interacts with others. There have been many examples when one feature, excellent by itself, badly interacts with others. (I recall GADT and irrefutable pattern matching.)

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Re: [Haskell-cafe] 3 level hierarchy of Haskell objects
by Patrick Browne 11 Aug '12

11 Aug '12

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resource post on Concurrent and Parallel Haskell
by Obscaenvs 10 Aug '12

10 Aug '12

Hi! I have recently gone through some tutorials on the subject of Concurrent and Parallel Haskell, most notably chapter 24 in Real World Haskell. In doing so I have collected some thoughts and resources on the subject in a post on my blog: http://monoid.se/haskell/a-resource-post-on-concurrent-and-parallel-haskell/ Is there some other resource that should really be included in such a list, or is there in your opinion anything else that should be pointed out for somebody starting out in this particular corner of the Haskell ecosystem? I would appreciate pointers and suggestions, even pertaining to English grammar :) /F

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unix package licensing
by Oleg Sidorkin 10 Aug '12

10 Aug '12

System/Posix/DynamicLinker/Module/ByteString.hsc and System/Posix/DynamicLinker/Prim.hsc sources in unix-2.5.1.0 package contains the following reference to GPL-2 c2hs package: -- Derived from GModule.chs by M.Weber & M.Chakravarty which is part of c2hs -- I left the API more or less the same, mostly the flags are different. Does it mean that GPL license should be applied to unix package and all dependent packages? unix package is included into Haskel Platform so does it mean that Haskell Platform should be GPL also?

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MIN_VERSION_base
by Kazu Yamamoto 10 Aug '12

10 Aug '12

Hello, The stable branch of the network library includes the following code: #if !(MIN_VERSION_base(4,6,0)) I cannot compile the stable branch with HP 2011.4.0.0 on Linux 2: Preprocessing library network-2.3.0.14... BSD.hsc:115:23: error: missing binary operator before token "(" BSD.hsc:115:23: error: missing binary operator before token "(" BSD.hsc:115:23: error: missing binary operator before token "(" compiling dist/build/Network/BSDhsc_make.c failed (exit code 1) command was: /usr/bin/gcc -c dist/build/Network/BSD_hsc_make.c -o dist/build/Network/BSD_hsc_make.o -fno-stack-protector -fno-stack-protector -DGLASGOW_HASKELL_=700 -Dlinux_BUILD_OS -Dlinux_HOST_OS -Di386_BUILD_ARCH -Di386_HOST_ARCH -Iinclude -DCALLCONV=ccall -I/ghc7.0.4/lib/ghc-7.0.4/unix-2.4.2.0/include -I/ghc7.0.4/lib/ghc-7.0.4/bytestring-0.9.1.10/include -I/ghc7.0.4/lib/ghc-7.0.4/base-4.3.1.0/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include/ "dist/build/autogen/cabal_macros.h" defines MIN_VERSION_base but "dist/build/Network/BSD_hsc_make.c" does not include it. If you guys know why this happens, please let me know. For more information, please refer to: https://github.com/haskell/network/issues/43 --Kazu

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[Wadler 89] Philip Wadler. "Theorems for free!": Can't get the same results in Haskell
by Stayvoid 10 Aug '12

10 Aug '12

Hi there, I was reading "Real World Haskell" and found this paper. [1] On the first page of the paper there is an example: inc* (odds_Int [1,2,3]) = [2,4] /= [4] = odds_Int (inc* [1,2,3]) I tried to implement it in Haskell: (I'm a newbie. I guess it's possible to write a better version.) module Param where import Prelude odds :: [Int] -> [Int] odds [] = [] odds [x] = if odd x then [x] else [] odds (x:xs) = if odds [x] == [] then odds xs else [x] ++ odds xs inc :: [Int] -> [Int] inc [] = error "Empty list" inc [x] = [succ x] inc (x:xs) = inc [x] ++ inc xs Looks fine: *Param> odds [1,2,3] [1,3] *Param> inc [1,2,3] [2,3,4] But my results differ from the paper's: *Param> inc (odds [1,2,3]) [2,4] *Param> odds (inc [1,2,3]) [3] I doubt that there is an error in the paper. So it seems that something is wrong with my code. But I can't find the error. Could you help me? (Or there is no error at all and I misunderstood something.) [1] http://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf Thanks

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parsec: parserFail & multiple error messages
by silly8888 10 Aug '12

10 Aug '12

I am trying to create a parsec parser that parses an integer and then checks if that integer has the right size. If not, it generates an error. I tried the following: 8<--------------------------------------------------------------- import Text.Parsec import Text.Parsec.String integer :: Parser Int integer = do s <- many1 digit let n = read s if n > 65535 then parserFail "integer overflow" else return n 8<--------------------------------------------------------------- The problem is that when I try this parse integer "" "70000" I get the following error: Left (line 1, column 6): unexpected end of input expecting digit integer overflow ie there are three error messages but I only want the last one. Is there something I can do about this?

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Parsec type error with Flexible Contexts
by Marco Túlio Gontijo e Silva 10 Aug '12

10 Aug '12

Hi. I wanted to generalize the definitions in Text.Parsec.Language to be able to use them with Parsec Text instead of Parsec String. I'm getting a type error with the following code snippet: http://hpaste.org/72844 I don't really understand the error, specially with regard to the fact that the if I comment one of the marked lines on the code pasted the program build. Any help is welcome. Greetings. -- marcot http://marcot.eti.br/

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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Data.Data and OverlappingInstances
by Timo von Holtz 09 Aug '12

09 Aug '12

Hi cafe, in my code I use Data.Data and OverlappingInstances. My problem now is, that I can't use functions using these instances as arguments for fromConstrM. This is what my problem looks like: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} import Data.Data class Test a where foo :: Monad m => m a instance Num a => Test a where foo = return 1 instance Test Int where foo = return 2 test constr = fromConstrM foo constr but when I compile I get: test.hs:15:26: Overlapping instances for Test d arising from a use of `foo' at test.hs:15:26-28 Matching instances: instance [overlap ok] (Num a) => Test a -- Defined at test.hs:9:9-23 instance [overlap ok] Test Int -- Defined at test.hs:12:9-16 (The choice depends on the instantiation of `d' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the first argument of `fromConstrM', namely `foo' In the expression: fromConstrM foo constr In the definition of `test': test constr = fromConstrM foo constr Failed, modules loaded: none. Is there a way out? Right now I use a "case (typeOf x) of" kind of construct, but it gets pretty messy and I even have to unsafeCoerce at one point. - Timo

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mutable arrays of tuples
by David Feuer 09 Aug '12

09 Aug '12

So I was thinking about a mutable array of tuples, but to avoid allocating tuples to modify their fields, I guess I really want an immutable array of tuples of STRefs. Just how much less efficient is this than a plain mutable array? might it even make sense to use parallel mutable arrays? The thought of that is disgusting to me, but at least one of the arrays could likely be made unboxed...

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Haskell Weekly News: Issue 239
by Daniel Santa Cruz 09 Aug '12

09 Aug '12

Welcome to issue 239 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 29 to August 4, 2012. Quotes of the Week * romm: i thought i knew programming. this is like discovering a new continent. * edwardk: i think the comonads are warm fuzzy, and the monads are warm sticky. its easy to get out of a comonad, its soft and you like wearing it. monads on the other hand stick to you and are icky Top Reddit Stories * Update on GHC's new code generator Domain: hackage.haskell.org, Score: 70, Comments: 39 On Reddit: [1] http://goo.gl/NkmqH Original: [2] http://goo.gl/4bUw1 * The first video in my new series of free Haskell live coding videos Domain: youtube.com, Score: 64, Comments: 61 On Reddit: [3] http://goo.gl/f1tmg Original: [4] http://goo.gl/ihwze * hbro, Haskell based browser, ideologically similar to uzbl Domain: github.com, Score: 48, Comments: 18 On Reddit: [5] http://goo.gl/apILX Original: [6] http://goo.gl/STSwc * Thinking Functionally with Haskell | PragProg Domain: pragprog.com, Score: 32, Comments: 6 On Reddit: [7] http://goo.gl/yL4Qj Original: [8] http://goo.gl/mHss0 * Two ways of representing perfect binary trees : Inside 374 Domain: blog.ezyang.com, Score: 30, Comments: 1 On Reddit: [9] http://goo.gl/1GfdH Original: [10] http://goo.gl/JJAl1 * Hoogle: from the command line Domain: self.haskell, Score: 28, Comments: 8 On Reddit: [11] http://goo.gl/Ob9Mw Original: [12] http://goo.gl/Ob9Mw * Free monad transformers Domain: haskellforall.com, Score: 27, Comments: 11 On Reddit: [13] http://goo.gl/XbQAj Original: [14] http://goo.gl/bJfrK * If someone asked you "why Haskell?", what would you say? Domain: self.haskell, Score: 26, Comments: 73 On Reddit: [15] http://goo.gl/LQUUh Original: [16] http://goo.gl/LQUUh * Feedback from someone who learnt Haskell as his first language Domain: blog.raynes.me, Score: 25, Comments: 50 On Reddit: [17] http://goo.gl/QsnfZ Original: [18] http://goo.gl/wFOLN * Why isn't `length` part of `Foldable`? Domain: self.haskell, Score: 23, Comments: 19 On Reddit: [19] http://goo.gl/L0WWX Original: [20] http://goo.gl/L0WWX Top StackOverflow Questions * Monoidal parsing — what is it? votes: 19, answers: 2 Read on SO: [21] http://goo.gl/bVGBo * Applying multiple functions to the same value point-free style in Haskell votes: 7, answers: 4 Read on SO: [22] http://goo.gl/PifBz * How do I get consistent criterion benchmarks, or interpret results across runs? votes: 7, answers: 1 Read on SO: [23] http://goo.gl/qwT5c * Why is this prime test so slow? votes: 7, answers: 4 Read on SO: [24] http://goo.gl/QehSC * Is there a theory that combines category theory/abstract algebra and computational complexity? votes: 7, answers: 1 Read on SO: [25] http://goo.gl/CeAFw * Haskell Polymorphic Tree Sum votes: 7, answers: 1 Read on SO: [26] http://goo.gl/NqYQW * reasonably efficient pure-functional matrix product in Haskell? votes: 7, answers: 3 Read on SO: [27] http://goo.gl/8H6o9 * Is access to internal strucutre of a monad required for monad transformer? votes: 7, answers: 1 Read on SO: [28] http://goo.gl/Yln18 * Haskell sub-typeclass requires UndecidableInstances? votes: 6, answers: 2 Read on SO: [29] http://goo.gl/u9FQh * Haskell Data.List.Class and syntax votes: 6, answers: 1 Read on SO: [30] http://goo.gl/wvKEk Until next time, Daniel Santa Cruz References 1. http://hackage.haskell.org/trac/ghc/blog/newcg-update 2. http://www.reddit.com/r/haskell/comments/xmfes/update_on_ghcs_new_code_gene… 3. http://www.youtube.com/watch?v=ScS8Q32lMxA 4. http://www.reddit.com/r/haskell/comments/xfcmk/the_first_video_in_my_new_se… 5. https://github.com/k0ral/hbro 6. http://www.reddit.com/r/haskell/comments/xe0w3/hbro_haskell_based_browser_i… 7. http://pragprog.com/magazines/2012-08/thinking-functionally-with-haskell 8. http://www.reddit.com/r/haskell/comments/xmip7/thinking_functionally_with_h… 9. http://blog.ezyang.com/2012/08/statically-checked-perfect-binary-trees/ 10. http://www.reddit.com/r/haskell/comments/xov6n/two_ways_of_representing_per… 11. http://www.reddit.com/r/haskell/comments/xijsb/hoogle_from_the_command_line/ 12. http://www.reddit.com/r/haskell/comments/xijsb/hoogle_from_the_command_line/ 13. http://www.haskellforall.com/2012/07/free-monad-transformers.html 14. http://www.reddit.com/r/haskell/comments/xg75g/free_monad_transformers/ 15. http://www.reddit.com/r/haskell/comments/xlb1g/if_someone_asked_you_why_has… 16. http://www.reddit.com/r/haskell/comments/xlb1g/if_someone_asked_you_why_has… 17. http://blog.raynes.me/blog/2011/11/27/the-clojure-community-and-me/ 18. http://www.reddit.com/r/haskell/comments/xk7fz/feedback_from_someone_who_le… 19. http://www.reddit.com/r/haskell/comments/xo05l/why_isnt_length_part_of_fold… 20. http://www.reddit.com/r/haskell/comments/xo05l/why_isnt_length_part_of_fold… 21. http://stackoverflow.com/questions/11808539/monoidal-parsing-what-is-it 22. http://stackoverflow.com/questions/11709350/applying-multiple-functions-to-… 23. http://stackoverflow.com/questions/11712234/how-do-i-get-consistent-criteri… 24. http://stackoverflow.com/questions/11718067/why-is-this-prime-test-so-slow 25. http://stackoverflow.com/questions/11725899/is-there-a-theory-that-combines… 26. http://stackoverflow.com/questions/11742405/haskell-polymorphic-tree-sum 27. http://stackoverflow.com/questions/11768656/reasonably-efficient-pure-funct… 28. http://stackoverflow.com/questions/11792275/is-access-to-internal-strucutre… 29. http://stackoverflow.com/questions/11707171/haskell-sub-typeclass-requires-… 30. http://stackoverflow.com/questions/11770733/haskell-data-list-class-and-syn…

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foreign import and gmp
by Lars Kuhtz 09 Aug '12

09 Aug '12

Hi all, There have been rumors that recent versions of GHC may allow foreign imports of objects that link against gmp without interfering with the gmp imports in integer-gmp. What is the current state of that issue? What is the currently recommended way to deal with that problem? Thanks, Lars

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'let' keyword optional in do notation?
by Martijn Schrage 08 Aug '12

08 Aug '12

Hi cafe, For a while now, I've been wondering why the 'let' keyword in a do block isn't optional. So instead of do ... let x = exp1 y = exp2 z <- exp3 ... you could simply write do ... x = exp1 y = exp2 z <- exp3 ... Where each sequence of let-less bindings is put in a separate binding group. I'm no parsing wizard, but I couldn't come up with any situations in which this would cause ambiguity. To me, the let-less version is easier on the eyes, more consistent with <- bindings, and also makes it less of a hassle to move stuff around. The above probably also holds for list/monad comprehensions, but the explicit let has never really bothered me there. Cheers, Martijn Schrage -- Oblomov Systems (http://www.oblomov.com)

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Data structure containing elements which are instances of the same type class
by Daniel Trstenjak 08 Aug '12

08 Aug '12

Hi all, it should be possible a call a function on all elements of the data structure, to add and remove elements. What I currently have: the type class: class Foo a where hasId :: a -> Int -> Maybe a a few instances: data A = A deriving Show instance Foo A where hasId a 1 = Just a hasId _ _ = Nothing data B = B deriving Show instance Foo B where hasId a 2 = Just a hasId _ _ = Nothing data C = C deriving Show instance Foo C where hasId a 3 = Just a hasId _ _ = Nothing the data structure holding any instance of Foo, which itself is a instance of Foo: data Foos l r = Foos l r | FooL l | FooR r | NoFoos deriving Show instance (Foo l, Foo r) => Foo (Foos l r) where hasId (Foos l r) id = case (hasId l id, hasId r id) of (Just l, Just r) -> Just $ Foos l r (Just l, _ ) -> Just $ FooL l (_ , Just r) -> Just $ FooR r _ -> Nothing combinator for Foos: (+++) :: l -> r -> Foos l r l +++ r = Foos l r infixr 5 +++ Now I can write: *Main> A +++ B +++ C +++ A Foos A (Foos B (Foos C A)) *Main> (A +++ B +++ C +++ A) `hasId` 1 Just (Foos A (FooR (FooR A))) Doesn't seem that nice. For every operation I would have to extend the type class. After some operations the data structure contains many dummy nodes (FooR, FooL). Is there some nicer way? Greetings, Daniel

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Re: [Haskell-cafe] HUnit and table-driven tests
by Matthew 07 Aug '12

07 Aug '12

On Tue, Aug 7, 2012 at 12:51 AM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > At 4:30 PM -0700 8/5/12, Matthew wrote: >> >> On Sun, Aug 5, 2012 at 12:32 AM, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> >> wrote: >>> >>> On Sun, 05 Aug 2012 03:21:39 +0200, Matthew <wonderzombie(a)gmail.com> >>> wrote: >>> >>>> I've got a function which takes in two chars, describing a playing >>>> card and a suit. An example would be 4C or TH for a 4 of Clubs or a >>>> Ten of Hearts. I need to be able to compare the ranks of a card (e.g. >>>> a King is 13), so a Card is a tuple of rank and suit. The function >>>> which parses a Card is type String -> Maybe Card. >>>> >>>> I'm writing unit tests for this using HUnit, and ideally I'd go with a >>>> table-driven[1] approach, where each test case is a tuple of the input >>>> and the expected output. (Possibly I could expand this to a triple, or >>>> simply a list, to allow for an error message for each test case.) Then >>>> all the test function has to do is run through each case and assert as >>>> necessary. Example: [("TH", Just (Hearts, 10)), ("XH", Nothing)]. >>> >>> >>> >>> A simple solution: >>> >>>> parseCard :: String -> Maybe Card >>>> parseCard string = <your function to test> >>>> test :: Bool >>>> test = all testEqual [("TH", Just (Hearts, 10)), ("XH", Nothing)] >>>> where >>>> testEqual (input, output) = parseCard input == output >>> >>> >>> >>> For a description of 'all', see: >>> >>> >>> http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… >> >> >> Thanks for the response. The one problem I have with this is that it >> will not be at all obvious which test case (or cases!) failed. >> >> That said, maybe I could do something similar, with a Writer? A passed >> test writes "", but a failed one writes a test-specific failure >> message. Then the test itself uses this string as the assert message. > > > > Let HUnit tell you about the failing test cases. Here's one way to do it. > > > import Test.HUnit > import Data.Char (isDigit) > > data Suit = Spades | Hearts | Diamonds | Clubs > deriving (Show, Read, Eq, Ord) > type Rank = Int -- 2 .. 14 (jack=11, queen=12, king=13, ace=14) > type Card = (Suit, Rank) > > > parseCard :: String -> Maybe Card > parseCard [rankChar, suitChar] = do suit <- suitFrom suitChar; rank <- > rankFrom rankChar; return (suit, rank) > parseCard _ = Nothing > > suitFrom char = lookup char [('S', Spades), ('H', Hearts), ('D', Diamonds), > ('C', Clubs)] > > rankFrom dig | isDigit dig = let v = read [dig] in if v >= 2 then Just v > else Nothing > rankFrom char = lookup char [('T', 10), ('J', 11), ('Q', 12), ('K', 13), > ('A', 14)] > > makeTest :: (String, Maybe Card) -> Test > makeTest (string, result) = string ~: result ~=? parseCard string > > tests = [("TH", Just (Hearts, 10)), ("XH", Nothing)] > > main = (runTestTT . TestList . map makeTest) tests ...it seems so obvious now. This is *exactly* what I was looking for; clearly I was over-thinking this. Thanks, Dean! > > > Dean

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Benchmark of DFT libraries in Haskell
by Takayuki Muranushi 07 Aug '12

07 Aug '12

Dear everyone, I'm always grateful to your help. I have been assigned a complicated and growing task in which I'll perform a lot of discrete Fourier transforms, so I have measured performance of several DFT libraries in Haskell: http://en.pk.paraiso-lang.org/Hackage/what-is-the-fastest-dft-in-haskell/ma… The raw result: http://paraiso-lang.org/html/bench-dft-in-haskell.html I'll share the result in hope that some of you will also find this result useful. Also, please let me know any possible flaws or improvements in the benchmark process! My observations are as follows: * vector-fftw with wisdom was more than 1/2 times faster than fftw in C with wisdom (and with communication overhead.) * vector-fftw without wisdom was significantly _faster_ than fftw in C without wisdom. I wonder why. * vector-fftw over vector was faster than fft over CArray. * any library that doesn't use fftw is much slower than those that does. Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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ANNOUNCE: tardis
by Dan Burton 07 Aug '12

07 Aug '12

I'm pleased to announce that I've uploaded the *tardis* package to hackage. http://hackage.haskell.org/package/tardis-0.2.0.0 *TardisT* is a monad transformer that combines the (regular, "forwards") State monad with the "backwards State monad", allowing you to communicate on two channels of state: one is "backwards travelling" state, while the other is "forwards travelling" state. Its primitives are comparable to the State monad's "get" and "put", but the names disambiguate which channel they operate on: getPast :: Monad m => TardisT bw fw m fw getFuture :: Monad m => TardisT bw fw m bw sendPast :: Monad m => bw -> TardisT bw fw m () sendFuture :: Monad m => fw -> TardisT bw fw m () These primitives allow you to write some very amusing timey-wimey code, but it only works in the presence of sufficient laziness. If you run into an infinite loop, try adding laziness annotations everywhere, and make sure that what you are trying to do is not a time paradox. I've provided both *transformers* and *mtl* style modules: Control.Monad.Trans.Tardis contains the transformer, while Control.Monad.Tardis.Class contains the MonadTardis class, and Control.Monad.Tardis re-exports both as well as the TardisT instance for MonadTardis. See the readme on github for more details: https://github.com/DanBurton/tardis#readme As a side note, since the code base is relatively small, it can also serve as a simple demonstration of how to use a cabal flag in conjunction with CPP to selectively include swaths of code (see Control/Monad/Tardis.hs and tardis.cabal). I'm not entirely sold on the naming conventions for the package (including the package name), and am open to suggestions. -- Dan Burton

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HUnit and table-driven tests
by Matthew 07 Aug '12

07 Aug '12

I've got a function which takes in two chars, describing a playing card and a suit. An example would be 4C or TH for a 4 of Clubs or a Ten of Hearts. I need to be able to compare the ranks of a card (e.g. a King is 13), so a Card is a tuple of rank and suit. The function which parses a Card is type String -> Maybe Card. I'm writing unit tests for this using HUnit, and ideally I'd go with a table-driven[1] approach, where each test case is a tuple of the input and the expected output. (Possibly I could expand this to a triple, or simply a list, to allow for an error message for each test case.) Then all the test function has to do is run through each case and assert as necessary. Example: [("TH", Just (Hearts, 10)), ("XH", Nothing)]. The problem, of course, is that I've no clue how to make this work. I still don't *quite* grok do notation well enough to understand how I can bind a monadic value (in this case a Maybe Card), pattern match on such as the Integer in there, and then assertEqual. I suspect I'm just thinking about this all wrong. Any suggestions? Thanks again! [1]: http://code.google.com/p/go-wiki/wiki/TableDrivenTests

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ANNOUNCE: system-time-monotonic-0.1
by Joey Adams 06 Aug '12

06 Aug '12

system-time-monotonic [1] provides access to the system's monotonic clock. Usage looks like this: * Use 'newClock' to create a new monotonic clock * Use 'clockGetTime' to see how much time has elapsed since the clock was created. This package currently supports Linux and Windows. It might also work on BSD, but I haven't tested it there. Mac OS X support is currently not implemented, but patches are welcome. GHC uses mach_absolute_time and mach_timebase_info; see ticket #5865 [2]. I also added a handy utility function 'delay', a variant of threadDelay taking a DiffTime instead of Int microseconds. Thus: delay 1.5 Waits 1.5 seconds. Note that since 'delay' simply calls 'threadDelay' in a loop, it is disrupted by system clock changes (again, see ticket #5865). [1]: http://hackage.haskell.org/package/system-time-monotonic [2]: http://hackage.haskell.org/trac/ghc/ticket/5865 --- The rest of this email describes various hurdles involved in implementing this package, and how they were addressed. ## GetTickCount The most obvious one is that GetTickCount has a short wraparound (49.7 days). Two ways to address this: * Don't use GetTickCount; use QueryPerformanceCounter (or similar) instead. This is currently how it's done in GHC HEAD. * Use GetTickCount, but avoid comparing times that are far apart by tracking the total difference each time we look at the clock. I took the second approach, because I found out that QueryPerformanceCounter is actually less accurate in the long run than GetTickCount. In particular, QueryPerformanceCounter stops ticking (or maybe ticks slower, I forget) when the computer is asleep. Here's the trick I use to work around GetTickCount's wraparound (pseudocode): st1 :: Word32 t1 :: DiffTime newClock: st1 := GetTickCount() t1 := 0 clockGetTime: st2 := GetTickCount() t2 := t1 + (Int32)(st2 - st1 modulo 2^32) st1 := st2 t1 := t2 return t2 This workaround only works if clockGetTime is called at least once every 24.8 days. It's important that st2 - st1 be done modulo 2^32, to compensate for wraparound. However, the result should be converted to a signed 32-bit integer, in case st2 was recorded earlier than st1 (which can easily happen in a concurrent context). In particular, here's what you should *not* do (which GHC currently does, when QueryPerformanceCounter is not available): st1 = (bigger_type) GetTickCount(); ... st2 = (bigger_type) GetTickCount(); return (st2 - st1) This will return a bogus value if a wraparound occurred between st1 and st2. system-time-monotonic tests, at runtime, if GetTickCount64 is available. If so, it uses it. Otherwise, it falls back to GetTickCount. Here's the code I used to do the run-time system call lookup: /* cbits/dll.c */ typedef ULONGLONG (WINAPI *GetTickCount64_t)(void); GetTickCount64_t system_time_monotonic_load_GetTickCount64(void) { return (GetTickCount64_t) GetProcAddress(GetModuleHandle(TEXT("kernel32.dll")), "GetTickCount64"); } -- System/Time/Monotonic/Direct.hsc type C_GetTickCount64 = IO #{type ULONGLONG} foreign import ccall system_time_monotonic_load_GetTickCount64 :: IO (FunPtr C_GetTickCount64) foreign import stdcall "dynamic" mkGetTickCount64 :: FunPtr C_GetTickCount64 -> C_GetTickCount64 Did I do this right? In particular: * Can I assume the kernel32.dll HMODULE won't be pulled out from under me? * Is `foreign import stdcall "dynamic"` the right incantation for using a pointer to a WINAPI function? ## CLOCK_MONOTONIC is not actually monotonic on Linux CLOCK_MONOTONIC is subject to NTP adjustments. Worse, CLOCK_MONOTONIC stops when the computer is put to sleep (unlike GetTickCount, which does the right thing). Two clocks were introduced very recently in Linux: * CLOCK_MONOTONIC_RAW * CLOCK_BOOTTIME I'd like to support CLOCK_BOOTTIME at some point. I'm not sure if it's subject to NTP adjustments or not, since the announcement [3] says: CLOCK_BOOTTIME is identical to CLOCK_MONOTONIC, except it also includes any time spent in suspend (as currently measured by read_persistent_clock()). This allows applications to get a suspend aware monotonic clock. One idea would be to hard-code the clockid_t of CLOCK_BOOTTIME and CLOCK_MONOTONIC_RAW, and try calling clock_gettime with each of these. If one of these succeeds, use it. Otherwise, fall back to CLOCK_MONOTONIC. Thus, the compiled binary can test, at runtime, if a new enough kernel is available. For now, system-time-monotonic simply uses CLOCK_MONOTONIC. [3]: http://lwn.net/Articles/428176/

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Re: [Haskell-cafe] CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 06 Aug '12

06 Aug '12

> I think instead you should have: > - abandoned FunDeps > - embraced Overlapping more! Well, using TypeCast to emulate all FunDeps was demonstrated three years later after HList (or even sooner -- I don't remember when exactly the code was written): http://okmij.org/ftp/Haskell/TypeClass.html#Haskell1 We demonstrate that removing from Haskell the ability to define typeclasses leads to no loss of expressivity. Haskell restricted to a single, pre-defined typeclass with only one method can express all of Haskell98 typeclass programming idioms including constructor classes, as well as multi-parameter type classes and even some functional dependencies. The addition of TypeCast as a pre-defined constraint gives us all functional dependencies, bounded existentials, and even associated data types. Besides clarifying the role of typeclasses as method bundles, we propose a simpler model of overloading resolution than that of Hall et al. > So here's my conjecture: > 1. We don't need FunDeps, except to define TypeCast. > (And GHC now has equality constraints, which are prettier.) > 2. Without FunDeps, overlapping works fine. > ... I agree on point 1 but not on point 2. The example of incoherence described in Sec `7.6.3.4. Overlapping instances' of the GHC User Guide has nothing to do with functional dependencies.

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Knight Capital debacle and software correctness
by Vasili I. Galchin 06 Aug '12

06 Aug '12

Hello Haskell Group, I work in mainstream software industry. I am going to make an assumption .... except for Jane Street Capital all/most "Wall Street" software is written in an imperative language. Assuming this why is Wall Street not awaken to the dangers. As I write, Knight Capital may not survive the weekend. Regards, Vasili

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Talk about Haskell at Ubucon in Berlin
by Robert Clausecker 05 Aug '12

05 Aug '12

Hello Haskellers! I'm going to give a talk on Haskell at Ubucon 2012 in Berlin (In German). I want to give an introduction about the concept of functional programming and the special language concepts of Haskell. Any ideas what to focus on? Yours, Robert Clausecker

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Sorting efficiency
by David Feuer 05 Aug '12

05 Aug '12

I'm writing a toy program (for a SPOJ problem--see https://www.spoj.pl/problems/ABCDEF/ ) and the profiler says my performance problem is that I'm spending too much time sorting. I'm using Data.List.sort on [Int32] (it's a 32-bit architecture). Others, using other languages, have managed to solve the problem within the time limit using the same approach I've taken (I believe), but mine is taking too long. Any suggestions? Do I need to do something insane like sorting in an STUArray? David Feuer

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do vs. pattern matching
by Matthew 05 Aug '12

05 Aug '12

I'm a somewhat experienced coder but I am relatively new to Haskell. I've got a question about whether a usage of do notation is idiomatic, or whether it's better to use pattern matching. I've got two functions which take an input and return Maybe SomeType. If either returns Nothing, I also want to return Nothing. If they both return something, then I'll return something unrelated. With do notation, I can write something like this: do foo <- callFoo x bar <- callBar x return (baz) Alternatively, there's a straightforward pattern match. After binding foo, bar in a couple of where clauses: case (foo,bar) of (Just x, Just y) -> baz _ -> Nothing So which approach is more idiomatic, do you think? Thanks, - Matthew

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Deprecating packages on Hackage
by Andrey Chudnov 04 Aug '12

04 Aug '12

Hello. What are the best practices in deprecating packages on Hackage? I've seen packages marked "DEPRECATED" in the synopsis field on Hackage, and one could add GHC "deprecated" pragmas for every module, but is that the best one can do? Thank you in advance, Andrey Chudnov

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cabal-dev + haskell mode (vim)
by Benjamin Edwards 04 Aug '12

04 Aug '12

Hello Café, I am struggling to get ctags and / or haskell mode to work with cabal-dev. This is quite annoying. Has anyone worked around this? Ben

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Why is the the transpose function in Data.List more complicated?
by KC 04 Aug '12

04 Aug '12

Transpose in Data.List is the following: transpose :: [[a]] -> [[a]] transpose [] = [] transpose ([] : xss) = transpose xss transpose ((x:xs) : xss) = (x : [h | (h:_) <- xss]) : transpose (xs : [ t | (_:t) <- xss]) Yet this seems to work. transp :: [[b]] -> [[b]] transp ([]:_) = [] transp rows = map head rows : transp (map tail rows) Why is the the transpose function in Data.List more complicated? -- -- Regards, KC

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Re: [Haskell-cafe] What Haskell Records Need
by Andrew Butterfield 03 Aug '12

03 Aug '12

On 2 Aug 2012, at 09:25, Erik Hesselink wrote: > Isn't this exactly the problem solved by all the lens packages? > Current popular ones are fclabels [0] and data-lens [1]. > > [0] http://hackage.haskell.org/package/fclabels > [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: > >> Take the following types from a contrived >> example. >> >>> type Salary = Integer >>> >>> data Job = Job >>> { title :: String >>> , salary :: Salary >>> } Rather than setTitle :: String -> Job -> Job we lift the first argument and define setfTitle :: (String -> String) -> Job -> Job setfTitle f jobrec = jobrec{ title = f $ title jobrec } then setTitle = setfTitle . const This is all just boilerplate, so we continue setfSalary :: (Salary -> Salary) -> Job -> Job setfSalary f jobrec = jobrec{ salary = f $ salary jobrec } >>> >>> data Person = Person >>> { name :: String >>> , job :: Job >>> } >> setfName :: (String -> String) -> Person -> Person setfName f prec = prec{ name = f $ name prec } setfJob :: (Job -> Job) -> Person -> Person setfJob f prec = prec{ job = f $ job prec } Now we can use function composition to do two levels setfTitleInPerson :: (String -> String) -> Person -> Person setfTitleInPerson = setfJob . setfTitle setTitleInPerson :: String -> Person -> Person setTitleInPerson = setfTitleInPerson . const Simple function composition works to put these together... I was frustrated by this problem a while back, and decided to approach it formally (I write literate Haskell/LateX documents), and went to work, doing the math with the intention of writing a suitable combinator, until I discovered I didn't need one .... lifting from X -> R -> R to (X -> X) -> R -> R gave me all I needed... >> Since I've used record syntax, I get >> getter/accessor functions (title, salary, >> name, job) for free. Now suppose I want to >> create an aggregate getter function: return >> the salary of a given person. Piece of cake, >> it's just function composition >> >>> getSalary :: Person -> Salary >>> getSalary = salary . job >> >> Done! Now suppose I want to write a >> setter/mutator function for the same nested >> field >> >>> setSalaryMessy :: Salary -> Person -> Person >>> setSalaryMessy newSalary person = >>> person { >>> job = (job person) { >>> salary = newSalary >>> } >>> } >> >> Ouch! And that's not even very deeply nested. >> Imagine 4 or 5 levels deep. It really makes >> Haskell feel clunky next to `a.b.c.d = val` >> that you see in other languages. Of course >> immutability means that the semantics of >> Haskell are quite different (we're creating >> new values here, not updating old ones) but >> it's still common to model change using these >> kinds of updates. >> >> What if along with the free getters that >> the compiler generates when we use record >> syntax, we also got semantic editor >> combinator (SEC) functions[0] that could be >> used as follows? >> >>> setSalary newSalary = job' $ salary' (const newSalary) >>> >>> giveRaise amount = job' $ salary' (+amount) >>> >>> givePercentRaise percent = job' $ salary' (*(1+percent)) >> >> For each field x, the compiler generates a >> function x' (the tic is mnemonic for change). >> These little functions aren't hard to write, >> but they're classic boilerplate. >> >>> job' :: (Job -> Job) -> Person -> Person >>> job' f person = person {job = f $ job person} >> >>> salary' :: (Salary -> Salary) -> Job -> Job >>> salary' f job = job { salary = f $ salary job} >> >> These type of utility functions are a dream >> when working with any reference type or >> State Monad. >> >>> modify $ givePercentRaise 0.25 >> >> The compiler could also generate polymorphic >> SEC functions for polymorphic fields. >> Further, the compiler could disallow using >> old-style update syntax for fields whose SEC >> update function is not in scope, giving us >> fine-grained control over access and update. >> On the other hand we currently have to create >> new functions to achieve this (exporting the >> getter means exporting the ability to update >> as well, currently). >> >> Of course this doesn't address the >> namespacing issues with records, but it is >> likely nicely orthogonal to other proposals >> which do. >> >> Also note that there's a package on hackage [1] >> that will generate SEC functions using TH. >> It's nice, but I prefer the style of field >> names used above for updaters (field' vs >> editField). >> >> Let me know what you think. I'll write up an >> official proposal if there's a bit of >> general interest around this. >> >> Thanks for reading, >> >> --Jonathan >> >> [0] - http://conal.net/blog/posts/semantic-editor-combinators >> [1] - >> http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-Semanti… >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ -------------------------------------------------------------------- -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

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Re: [Haskell-cafe] [Haskell-beginners] vector indexing time
by KC 03 Aug '12

03 Aug '12

Thank you for that insight. :) On Fri, Aug 3, 2012 at 4:50 AM, Heinrich Apfelmus <apfelmus(a)quantentunnel.de > wrote: > > Creating the vector still takes time proportional to the length of the > vector. In fact, it appears that in your example, the vector packages > optimizes the creation time to create only up to the element that you > actually demand. > > The linear time you're seeing is not the result of an inefficiency of > vector indexing, but the result of an efficiency in vector creation. > > > Best regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > -- -- Regards, KC

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