- Haskell-Cafe - Haskell.org

Book?
by Gregory Guthrie 18 Jan '12

18 Jan '12

I've seen a book: "The Practice Of Monadic Interpretation" Dan Popa Nov. 2008 Or "Practical Monadic Interpretation" Dan Popa Which seem that they might be the same book? As reported on Haskell Wiki/books as published in 2008, but Don't find it available anywhere under either title. Any pointers or references?

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Need advice: Haskell in Web Client
by dokondr 17 Jan '12

17 Jan '12

Hi all, I hope to use Haskell for graphics (charts) programming in Web client. My current implementation in brief: Server side, Haskell modules: 1) collecting various statistics from Twitter 2) generating text data for Gnuplot (http://www.gnuplot.info/) 3) Gnuplot creates png files with charts Web client: GWT (Google Web Toolkit) web UI that allows user to enter queries and see resulting charts in Web browser. Charts are png files generated by Gnuplot on the server side. Ideally, on the server side instead of using Gnuplot I would like Haskell to generate Javascript to be downloaded to Web client and draw charts in the browser. Something, very approximately, similar to what GWT does :) This code will be specific of course to plotting Javascript framework, no problem. Looking at "Haskell in web browser - HaskellWiki:" http://www.haskell.org/haskellwiki/Haskell_in_web_browser#Haskell_web_toolk… I feel a little confused about current state of the available Haskell tools for this task. Any ideas? Thanks! Dmitri

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Please upgrade http-enumerator and/or http-conduit
by Michael Snoyman 17 Jan '12

17 Jan '12

Hi all, Thanks to Felipe for catching this issue[1]. Both http-enumerator and http-conduit will not properly reject invalid certificates. I've released updated versions of both packages: http-enumerator-0.7.2.4 http-conduit-1.1.2.2 All users are recommended to upgrade. Michael [1] https://github.com/snoyberg/http-conduit/pull/6

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Pruned Sparks in Original Strategies
by Burak Ekici 17 Jan '12

17 Jan '12

Dear List, I am trying to parallelize some number of codes with using Haskell's original and second generation strategies in order to be able to compare them. As far as I understood, in original strategies, ROOT garbage collection mechanism is used which does not allow any spark to be pruned. Since, they do not return any information back to its caller. (a type of void reference making fizzling of a spark impossible, is returned), However, in most of my measurements, spraks are being pruned. I wonder the reasons why this incident occurs? Do you have any idea? Many thanks, Burak.

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__GLASGOW_HASKELL__ for GHC 7.4
by Eugene Kirpichov 17 Jan '12

17 Jan '12

Hi, I'm fixing a build error in a package that depends on the RTS API, which changed in 7.4, using #if __GLASGOW_HASKELL__ >= 740. However, build log shows that __GLASGOW_HASKELL__ is still 704, not 740 as I'd expect. Is this a bug? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Re: [Haskell-cafe] decoupling type classes
by Yin Wang 17 Jan '12

17 Jan '12

On Sat, Jan 14, 2012 at 2:38 PM, Dominique Devriese <dominique.devriese(a)cs.kuleuven.be> wrote: >> I may or may not have thought about it. Maybe you can give an example >> of parametric instances where there could be problems, so that I can >> figure out whether my system works on the example or not. > > The typical example would be > > instance Eq a => Eq [a] where > [] == [] = True > (a : as) == (b : bs) = a == b && as == bs > _ == _ = False It can handle this case, although it doesn't handle it as a parametric instance. I suspect that we don't need the concept of "parameter instances" at all. We just searches for instances recursively at the call site: 1. If "g" has an implicit parameter "f", search for values which matches the name and instantiated type in the current scope. 2. If a value is found, use it as the argument. 3. Check if the value is a function with implicit parameters, if so, search for values that matches the name and type of the implicit parameters. 4. Do this recursively until no more arguments contain implicit parameters. > This coupling you talk about is not actually there for instance > arguments. Instance arguments are perfectly usable without records. > There is some special support for automatically constructing record > projections with instance arguments though. Cool. So it seems to be close to what I had in mind. > I am not sure about the exact workings of your system, but I want to > point out that alternative choices can be made about the workings of > inferencing and resolving type-class instances such that local > instances can be allowed. For example, in Agda, we do not infer > instance arguments and we give an error in case of ambiguity, but > because of this, we can allow local instances... Certainly it should report error when there are ambiguities, but sometimes it should report an error even there is only one value that matches the name and type. For example, foo x = let overload bar (x:Int) = x + 1 in \() -> bar x baz = in foo (1::Int) Even if we have only one definition of "bar" in the program, we should not resolve it to the definition of "bar" inside foo. Because that "bar" is not visible at the call site "foo (1::int)". We should report an error in this case. Think of "bar" as a "typed dynamically scoped variable" helps to justify this decision.

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fixity declaration in GHCi
by Takayuki Muranushi 17 Jan '12

17 Jan '12

Hello everyone, while I'm playing with fixity declaration in GHCi, I found a strange behavior. Can anyone explain this? Is this actually an expected behavior, or is it a glitch? ---------------------------------------------------------------- $ ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> minus --(1) <interactive>:1:1: Not in scope: `minus' Prelude> let minus x y = x-y; infixl 6 `minus` --(2) Prelude> 10^5 `minus` 2 --(3) 1000 Prelude> let infixl 6 `minus` --(4) <interactive>:1:14: The fixity declaration for `minus' lacks an accompanying binding Prelude> let subtract x y = x-y --(5) Prelude> 10^5 `subtract` 2 --(6) 1000 Prelude> let f x y = x-y; infixl 6 `f` in 10^5 `f` 2 --(7) 99998 Prelude> ---------------------------------------------------------------- (1) suggests that `minus` is not defined here. in (2), I try to define `minus` with precedence level 6. However, (3) suggests that precedence level of `minus` is stronger than ^ . (4) suggests that you cannot redefine fixity here. (5)(6) is consistent with Haskell 98 report section 4.4.2; "Any operator lacking a fixity declaration is assumed to be infixl 9." in (7), precedence level is set as expected. So, what is happening in (3)? Where did the fixity declaration in (2) went? Best, -- MURANUSHI Takayuki The Hakubi Center, Kyoto University : http://www.hakubi.kyoto-u.ac.jp/

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Network.Browser and Network.TLS
by Myles C. Maxfield 17 Jan '12

17 Jan '12

Hello! I am interested in extending the Network.HTTP code in the HTTP package to support HTTPS via TLS. A clear candidate is to use the Network.TLS module in the TLS library (because its TLS logic is written in pure Haskell, rather than any of the FFI libraries like Network.Curl or the OpenSSL package). It's simple enough to provide an implementation of the Network.Stream.Stream typeclass around a TLSCtx, and this works for the Network.HTTP.Stream functions. However, I am interested in using the functionality in the Network.Browser module. This module uses the Network.HTTP.HandleStream interface, which is implemented directly on top of the Handle datatype and the Network.BufferType.BufferOp functions. HandleStreams seem to be for allowing the user to pull out an arbitrary data type out of an HTTP stream, not for doing any stream processing the way TLS does. As far as I can tell, the current typeclass system does not allow a TLSCtx to piggypack off of a HandleStream. My assumption is that this interface is used for speed so the user doesn't have to convert some canonical type into the type that he/she desires in client code. TLS, however, must use a specific type to decode the bytes that it pulls out of the stream. I don't think it's reasonable to try to modify the TLS library to decode bytes from an arbitrary type. Decoding necessarily needs byte-level access to its input (and therefore output) streams in a manner extremely similar to the functions that ByteString provides. Perhaps I'm wrong about this, but the conclusion I've reached is that it doesn't make sense for TLS to use an arbitrary typeclass because the interface it requires is so similar to the existing ByteString datatype. If an application wants a specific type out of a TLS stream, it must necessarily convert the type in software. Any speed that might be gained by pulling your native type out of a network connection will be dwarfed anyway by the cost of decryption. The Network.Stream functions allow this by using String type for all data transfers (which is counterintuitive for binary data transfers). An implementation of Network.Stream.Stream using TLS would convert TLS's output ByteString into a String (possibly by doing something like ((map (toEnum . fromIntegral)) . unpack) which doesn't make a whole lot of sense and is fairly wasteful). A client program might even convert it back to a ByteString, so the client program must have knowledge about how the bytes are packed into the String. Network.Browser only seems to have one function which isn't simply a state accessor/mutator: 'request'. This function gives the connection a type of HStream ty => HandleStream ty. As stated before, the HandleStream directly uses the Handle type. This means that, as far as I can tell, there is no way to fit TLS into the Network.Browser module as it stands because the types don't allow for it. Supporting TLS in Network.Browser would have to change the type of 'request' and therefore break every program out there which uses the Network.Browser module. It would be possible to create something like a 'requestHTTPS' function which returns a different type, but this is quite inelegant - there should be one function that inspects the scheme of the URI it is handed. I am left with the conclusion that it is impossible to support TLS in Network.Browser without breaking many Haskell programs. It is obviously possible to fork the HTTP library, but I'd like to improve the state of the existing libraries. Likewise, it is possible to create a new module that supports HTTPS but has different typed functions and lots of code duplication with Network.Browser, but that is quite inelegant. I suppose this is mostly directed at the maintainers of the HTTP and TLS libraries, Sigbjorn Finne and Vincent Hanquez, but I'd be greatful for your input on what I can do to contribute to the Haskell community regarding Network.Browser and Network.TLS. Perhaps I should just use the Network.HTTP.Enumerator module and not deal with Network.Browser? Maybe I'm going about this in entirely the wrong way. Thanks, Myles C. Maxfield

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Happy holidays Haskell
by R J 17 Jan '12

17 Jan '12

hey Haskell I didn't believe this at all until I started http://www.news13wise.com

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Haskell Summers of Code retrospective (updated for 2011)
by Gwern Branwen 16 Jan '12

16 Jan '12

The Wheel turns, and months come and pass, leaving blog posts that fade into 404s; a wind rose in Mountain View, whispering of the coming Winter... Tonight I sat down and finally looked into the 2011 SoCs to see how they turned out and judge them according to my whimsically arbitrary and subjective standards: http://www.gwern.net/Haskell%20Summer%20of%20Code#results-1 They turned out pretty much as I predicted - but then I *would* say that, wouldn't I? (Also submitted to /r/haskell for those who swing that way: http://www.reddit.com/r/haskell/comments/n82ln/summer_of_code_2011_retrospe… ) -- gwern http://www.gwern.net

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ICFP 2012 Call for papers
by Wouter Swierstra 16 Jan '12

16 Jan '12

===================================================================== ICFP 2012: International Conference on Functional Programming Copenhagen, Denmark, September 9 - 15, 2012 http://www.icfpconference.org/icfp2012 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Monday Mar 12, 2012 14:00 UTC Author response: Monday May 07, 2012 14:00 UTC - May 9 14:00 UTC Notification: Monday May 28, 2012 Final copy due: Monday Jul 02, 2012 Scope ~~~~~ ICFP 2012 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * Language Design: concurrency and distribution; modules; components and composition; metaprogramming; interoperability; type systems; relations to imperative, object-oriented, or logic programming * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling * Foundations: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types * Analysis and Transformation: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation * Applications and Domain-Specific Languages: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia programming; scripting; system administration; security * Education: teaching introductory programming; parallel programming; mathematical proof; algebra * Functional Pearls: elegant, instructive, and fun essays on functional programming * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By March 12 2012, 14:00 UTC, submit a full paper of at most 12 pages (6 pages for an Experience Report), including bibliography and figures. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.acm.org/sigplan/republicationpolicy.htm * Authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to the ACM. Presentations will be videotaped and released online if the presenter consents. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. If this requirement is a hardship, make contact with the program chair at least one week before the deadline. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available: http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted only online via the ICFP website. Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 48-hour period, starting at 14:00 UTC on Monday May 7th 2012, to read reviews and respond to them. Special Journal Issue: There will be a special issue of the Journal of Functional Programming with papers from ICFP 2012. The program committee will invite the authors of select accepted papers to submit a journal version to this issue. Conference Chair: Peter Thiemann, University of Freiburg Program Chair: Robby Findler, Northwestern University Program Committee: Andreas Rossberg, Google Andrew Tolmach, Portland State University Brigitte Pientka, McGill University Colin Runciman, University of York Eijiro Sumii, Tohoku University Jan Midtgaard, Aarhus University John Hughes, Chalmers University of Technology and Quviq AB Lars Birkedal, IT University of Copenhagen Manuel Fähndrich, Microsoft Research Mariangiola Dezani-Ciancaglini, University of Turin Matthias Blume, Google R. Kent Dybvig, Cisco and Indiana University Sam Tobin-Hochstadt, Northeastern University Satnam Singh, Google Research Simon Marlow, Microsoft Research Zena M. Ariola, University of Oregon

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Second CfP --- 75 Years of the Lambda Calculus
by Conor McBride 16 Jan '12

16 Jan '12

[posted on behalf of Patricia Johann] [Please note that the submission deadline is 27 January. Flyer attached below. -patty] International Workshop on 75 Years of Lambda-Calculus University of St Andrews, Scotland 15th June, 2012 Call for papers http://msp.cis.strath.ac.uk/lambda2012 Sponsored by Scottish Informatics & Computer Science Alliance (SICSA) Overview -------- In 1936, Alonzo Church?s foundational 'An unsolvable problem of elementary number theory' introduced the Lambda-calculus which, with Turing machines, now underpins contemporary theoretical and practical Computer Science. To celebrate the 75th anniversary of the publication of this seminal work, papers are invited for presentation at a one day International Workshop, to be held in St Andrews on 15th June 2012, immediately following the International Symposium on Trends in Functional Programming (TFP). Topics ------ Church's life & work Origins/history Computability Programming language design & implementation Logic, proof & reasoning Guest Speakers -------------- Henk Barendregt, University of Nijmegen Chris Hankin, Imperial College Roger Hindley, University of Swansea Fairouz Kamareddine, Heriot-Watt University Workshop Chairs --------------- Greg Michaelson, Heriot-Watt University Patricia Johann, University of Strathclyde Programme Committee ------------------- Robert Atkey, University of Strathclyde Dan Dougherty, WPI Maribel Fernandez, Kings College London Philip Scott, University of Ottawa Dates (2012) ------------ 27th January: submission 2nd March: notification of acceptance for Workshop 18th May: deadline for draft proceedings 15th June: Workshop 31st August: deadline for HOSC special issue 30th November: notification of acceptance for HOSC Submissions ----------- Submissions will be 'light touch' refereed for relevance before the Workshop, and will appear in the draft proceedings, with full refereeing thereafter for publication in a special issue of Higher Order and Symbolic Computation. Submissions should be of 16 pages maximum length in the Springer format from http://cs.au.dk/~hosc/ Submission ---------- https://www.easychair.org/conferences/?conf=75ylc Registration ------------ There will be an attendance charge of around ?50 for refreshments, lunch, and proceedings. There will be no charge for PhD students from SICSA affiliated Universities. Registration will be through TFP: http://www.cs.st-andrews.ac.uk/~kh/TFP2012/TFP_2012/Home.html

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Patch combinators
by Emil Axelsson 16 Jan '12

16 Jan '12

Hi all! Based on ideas by Koen Claessen, I have made a small module for what might be called patch combinators: http://hpaste.org/56501 Examples are found as comments. Before I push this to Hackage, I just wanted to check if there is any package that already provides this sort of functionality. (We're planning to use this module for expressing compile-time constraints on Feldspar functions.) Thanks for any feedback! / Emil

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named pipe interface
by Serge D. Mechveliani 16 Jan '12

16 Jan '12

People, (I wonder: is this for beginners(a)haskell.org ?) I need to organize a string interface for a Haskell function Main.axiom and a C program fifoFromA.c via a pair of named pipes (in Linux, UNIX). The pipes are created before running, by the commands > mkfifo toA > mkfifo fromA Main.axiom outputs a string to toA and inputs the respond string from fromA as the result. fifoFromA inputs a string from toA, converts it to the string resStr, outputs resStr to fromA. Main.axiom must be able to be applied in a loop, and there must be avoided repeated opening of a file/channel in a loop. As an example, the string conversion in fifoFromA.c is put the conversion of each character to the lower case: -------------------------- fifoFromA.c ------------------------------ #include <string.h> #include <stdio.h> #include <fcntl.h> #define BOUND 64 int main() { int toA, fromA, i, numread; char buf[BOUND]; toA = open("toA", O_RDONLY); fromA = open("fromA", O_WRONLY); for (;;) { numread = read(toA, buf, BOUND); buf[numread] = '\0'; // printf("A: Read from toA: %s\n", buf); i = 0; // convert the string to the lower case while (i < numread) {buf[i] = tolower(buf[i]); i++;} write(fromA, buf, strlen(buf)); } } ----------------------------------------------------------------------- For the "to-A" part writen in C (instead of Haskell), this interface loop works all right. With Haskell, I manage to process only a single string in the loop, and then it ends with an error. Main.hs is given below. I never dealt with such an IO in Haskell. Can you, please, fix the code or give comments? Please, copy the response to mechvel(a)botik.ru (I am not in the list). Thank you in advance for your notes, ------ Sergei mechvel(a)botik.ru ------------------------------------------------------------------- import System.IO (IOMode(..), IO(..), Handle, openFile, hPutStr, hGetLine, hFlush) import System.IO.Unsafe (unsafePerformIO) dir = showString "/home/mechvel/ghc/axiomInterface/byLowerLevel/" toA_IO = openFile (dir "toA") WriteMode :: IO Handle fromA_IO = openFile (dir "fromA") ReadMode -- used as global values toA = unsafePerformIO toA_IO -- fromA = unsafePerformIO fromA_IO -- axiomIO :: String -> IO String axiomIO str = do hPutStr toA str hFlush toA hGetLine fromA axiom :: String -> String -> String axiom str = showString (unsafePerformIO $ axiomIO str) -- Examples of usage -------------------------------------------- -- main = putStr (axiom "ABC1" "\n") -- I -- putStr (axiom "ABC1" $ showChar '\n' $ axiom "ABC2" "\n") -- II {- III: putStr (shows resPairs "\n") where n = 9000 str0 = "ABC" strings = [str0 ++ (show i) | i <- [1 .. n]] resPairs = [(str, axiom str "") | str <- strings] -} ----------------------------------------------------------------- I use Glasgow Haskell ghc-7.01. Build: > gcc -o fifoFromA fifoFromA.c > ghc --make Main Running: first, command > ./fifoFromA on terminal-2, then command > ./Main on terminal-1. Now, the example (I) works -- in the sense that terminal-1 shows the result "abc1" after the program on terminal-2 is interrupted. II and III do not work. And the aim is the examples like III.

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Functor0?
by Evan Laforge 16 Jan '12

16 Jan '12

Often when I define some type that wraps something else, I find myself writing a function like the following: newtype Thing = Thing X liftThing f (Thing x) = Thing (f x) It's like a Functor, but I can't make it an instance because Functor requires that the type be parametric. So I've been using type families to make a kind of "0 argument functor": class Functor0 a where type Elem a :: * fmap0 :: (Elem a -> Elem a) -> a -> a instance Functor0 Thing where type Elem Thing = X fmap0 = liftThing Is there a name for this? A better way to do it?

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porting feed to xml-enumerator
by Simon Michael 16 Jan '12

16 Jan '12

Hi Sigbjorn (and Don), I'm back for another reason. feed leaks and uses a lot of memory due to the xml package. Rather than fix xml I ported feed to xml-enumerator, which is used by yesod and more actively maintained than xml. This seems to have fixed the problem so I'm thinking of uploading this version to hackage as feed-1.0 (which I'll use for rss2irc and hackagebot.) Please let me know whether you agree. Also you might be interested in moving your repo (http://code.galois.com/cgi-bin/gitweb?p=feed.git;a=summary) to github ? This would make it easier to publish my changes, either to the main repo or a branch or fork. Otherwise I'll need to get them to your repo somehow. Thanks again, -Simon On Feb 21, 2011, at 5:14 PM, Simon Michael wrote: > thanks for feed. I'm just investigating a bug with rss2irc, and I think I'm seeing problems in the current feed on hackage. It lookas as if there's no way to get item updated date as opposed item published date, and getItemPublishDate actually gets the updated date in the case of an atom feed (http://hackage.haskell.org/packages/archive/feed/0.3.8/doc/html/src/Text-Fe…) > > I'd like to contribute a fix. Would you be able to make feed's repo public, eg on darcsden.com or github ?

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HaskellWiki upload failed
by Henk-Jan van Tuyl 15 Jan '12

15 Jan '12

L.S., I am trying to upload a file of about 12 Mbyte to the HaskellWiki; the upload fails without error message. Is the file too big? Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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hackage trac broken
by Joachim Breitner 15 Jan '12

15 Jan '12

Hi, a Debian user tried to report a bug against cabal-install (http://bugs.debian.org/655752) but the trac instance linked from the Cabal homepage seems to be down: http://hackage.haskell.org/trac/hackage/ Internal Server Error TracError: IOError: [Errno 13] Permission denied: '/srv/trac/hackage/VERSION' Is this known and will it be fixed? Thanks, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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SBLP - call for papers
by Simon Thompson 15 Jan '12

15 Jan '12

============================================================ CALL FOR PAPERS 16th BRAZILIAN SYMPOSIUM ON PROGRAMMING LANGUAGES Natal, Rio Grande do Norte, Brazil September 24-28, 2012 http://www.cbsoft.dimap.ufrn.br ============================================================ IMPORTANT DATES Paper abstract submission (15 lines): April 20nd, 2012 Full paper submission: April 27th, 2012 Notification of acceptance: May 28th, 2012 Final papers due: June 29th, 2012 INTRODUCTION The 16th Brazilian Symposium on Programming Languages, SBLP 2012, will be held in Natal, Brazil, on September 24-28, 2012. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 3nd Brazilian Conference on Software: Theory and Practice, CBSoft 2012, http://www.cbsoft.dimap.ufrn.br, which will host four traditional, well-established symposia: * XXVI Brazilian Symposium on Software Engineering (SBES) * XVI Brazilian Symposium on Programming Languages (SBLP) * XV Brazilian Symposium on Formal Methods (SBMF) * VI Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2012 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain- specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. All papers should be prepared using the SBC template. In particular, we encourage the submission of short papers reporting on partial results of on-going master dissertations or doctoral theses. All accepted papers will be published in the conference proceedings. Submissions should be done using SBLP 2012 installation of the EasyChair conference mangement system at http://www.easychair.org/conferences/?conf=sblp2012. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. GENERAL CO-CHAIRS Nélio Cacho, UFRN Gibeon Aquino, UFRN Martin Musicante, UFRN PROGRAMME CHAIRS Francisco Heron de Carvalho Junior, UFC Luis Soares Barbosa, Univ. do Minho PROGRAMME COMMITTEE * Alberto Pardo, Univ. de La Republica * Alex Garcia, IME * Alvaro Freitas Moreira, UFRGS * André Rauber Du Bois, UFPel * Andre Santos, UFPE * Carlos Camarao, UFMG * Christiano Braga, UFF * Fernando Castor Filho, UFPE * Fernando Quintão Pereira, UFMG * Francisco Heron de Carvalho Junior, UFC (co-chair) * Joao Saraiva, Universidade do Minho * João F. Ferreira, Teeside University * Jonathan Aldrich, Carnegie Mellon Univ. * Jose Luiz Fiadeiro, Univ. of Leicester * Lucilia Figueiredo, UFOP * Luis Soares Barbosa, Univ. do Minho * Manuel António Martins, Univ. de Aveiro * Marcelo A. Maia, UFU * Marcello Bonsangue, Leiden Univ/CWI * Marcelo d'Amorim, UFPE * Marco Tulio Valente, UFMG * Mariza A. S. Bigonha, UFMG * Martin A. Musicante, UFRN * Noemi Rodriguez, PUC-Rio * Paulo Borba, UFPE * Peter Mosses, Swansea University * Qiu Zongyang, Beijing University * Rafael Dueire Lins, UFPE * Renato Cerqueira, PUC-Rio * Ricardo Massa, UFPE * Roberto S. Bigonha, UFMG * Roberto Ierusalimschy, PUC-Rio * Sandro Rigo, UNICAMP * Sergio Soares, UFPE * Simon Thompson, Univ. of Kent * Varmo Vene, Univ. de Tartu Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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function composition
by TP 15 Jan '12

15 Jan '12

Hi, I have a basic question concerning function composition. I have used http://www.haskell.org/tutorial/functions.html to write a composition function: Prelude> let f°g = f g Prelude> let p = (*2) Prelude> let q = (+3) Prelude> p°q 4 14 Prelude> :t (°) (°) :: (t1 -> t) -> t1 -> t If I understand well, this means that the infix operator "°" takes a function of type t1, i.e. g in f°g, and applies f on it, which takes a type t1 as input and returns f(g) which is of type t. The final result is of type t. So the first argument is represented above by "(t1->t)", and the second by "t1", the final result being of type t. However, I am not able to get the type of p°q Prelude> :t p°q <interactive>:1:3: Couldn't match expected type `Integer' with actual type `Integer -> Integer' In the second argument of `(°)', namely `q' In the expression: p ° q Prelude> What's the problem here? How can I obtain the type of p°q? Thanks in advance, TP

3 2

Installing dph-examples in Mac OS X Version 10.7.2
by mukesh tiwari 15 Jan '12

15 Jan '12

Hello all I am trying to install dph-examples on Mac OS X. First I have Xcode 4.2.1 and during the installation i got this <http://hpaste.org/56445> error Installing library in /Users/mukesh/.cabal/lib/dph-seq-0.5.1.1/ghc-7.2.1Registering dph-seq-0.5.1.1...Downloading dph-examples-0.5.1.2...Configuring dph-examples-0.5.1.2...Building dph-examples-0.5.1.2...Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2...[1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o )Error (fd:12: hGetLine: end of file)Warning: Couldn't figure out LLVM version!Make sure you have installed LLVMghc: could not execute: optcabal: Error: some packages failed to install:dph-examples-0.5.1.2 failed during the building phase. The exception was:ExitFailure 1 On stackoverflow<http://stackoverflow.com/questions/8864696/installing-dph-examples-in-mac-o…> , i got suggestion to remove Xcode 4.2 and install Xcode 3.2. After installing Xcode 3.2 , I am still getting this error. Macintosh:~ mukesh$ cabal install dph-examples Resolving dependencies... Configuring dph-examples-0.5.1.2... Building dph-examples-0.5.1.2... Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2... [1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o ) Error (fd:11: hGetLine: end of file) Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc: could not execute: opt cabal: Error: some packages failed to install: dph-examples-0.5.1.2 failed during the building phase. The exception was: ExitFailure 1 Macintosh:~ mukesh$ uname -a Darwin Macintosh 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:56:15 PDT 2011; root:xnu-1699.24.8~1/RELEASE_I386 i386 Macintosh:~ mukesh$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.2.1 Xcode 3.2 ( 64 bit ) Could some one please tell me how to resolve this issue. Regards Mukesh Tiwari

1 0

Is it possible to create an instance of MonadBaseControl for PropertyM (QuickCheck)/continuations?
by Oliver Charles 15 Jan '12

15 Jan '12

Hi I'm working on a little experiment at the moment: using monadic QuickCheck to test the integration of my code and the database. I see some of my functions having properties like - given a database in this state, selectAll should return all rows, and so on. My initial attempt has worked nicely, and now I'm trying to test some more complicated properties, but I'm hitting a problem with overlapping primary keys, and this is because I'm not correctly cleaning up after each check. The simplest solution to this is to bracket property itself, and for that I turned to Control.Monad.Trans.Control in lifted-base, but I am struggling to actually write an instance for MonadBaseContol IO (PropertyM IO). It seems that PropertyM is a continuation monad transformer: newtype PropertyM m a = MkPropertyM { unPropertyM :: (a -> Gen (m Property)) -> Gen (m Property) } Given this, is it possible to even write an instance of MonadBaseControl? From the lifted-base documentation, it explicitly calls out ConT as *not* having instances, so I wonder if it can't be done. Sadly, I also lack intuition as to what MonadBaseControl really means - I think it means 'capture the state of this monad, so later we can go back to exactly the same state (or sequence of actions?)', but this is very flakey. So... is this possible, and if so how can I move forward? Thanks for any help or advice! - Ollie

1 0

parse error in pattern, and byte code interpreter
by TP 15 Jan '12

15 Jan '12

Hi everybody, I want to test a higher level language than C/C++ (type inference, garbage collection), but much faster than Python. So I am naturally led to Haskell or OCaml. I have read and tested tutorials for the two languages. For the time being, my preference goes to Haskell, first because it looks more like Python, and also because some things appear not so clean in OCaml, at least for a beginner (Put a line termination or not? Why have I to put "rec" in the definition of a recursive function? Etc.). I have two questions. 1/ Inspiring from tutorials out there, I have tried to write a small formal program which is able to distribute n*(x+y) to n*x+n*y. The OCaml version is working (see Post Scriptum). However, I have difficulties to make the Haskell version work. This is my code: {----------------------------------------------------} data Expr = Plus Expr Expr | Minus Expr Expr | Times Expr Expr | Divide Expr Expr | Variable String deriving ( Show, Eq ) expr_to_string expr = case expr of Times expr1 expr2 -> "(" ++ ( expr_to_string expr1 ) ++ " * " ++ ( expr_to_string expr2 ) ++ ")" Plus expr1 expr2 -> "(" ++ ( expr_to_string expr1 ) ++ " + " ++ ( expr_to_string expr2 ) ++ ")" Variable var -> var distribute expr = case expr of Variable var -> var Times expr1 Plus( expr2 expr3 ) -> Plus ( Times ( expr1 expr2 ) Times ( expr1 expr3 ) ) main = do let x = Times ( Variable "n" ) ( Plus ( Variable "x" ) ( Variable "y" ) ) print x print ( expr_to_string x ) {----------------------------------------------------} When I try to run this code with "runghc", I obtain: pattern_matching_example.hs:28:24: Parse error in pattern: expr2 Thus it does not like my pattern "Times expr1 Plus( expr2 expr3 )". Why? How can I obtain the right result, as with the OCaml code below? 2/ It seems there is no possibility to generate bytecode, contrary to OCaml. Is it correct? Is there an alternative? What is interesting with bytecode run with "ocamlrun" is that the process of generating the bytecode is very fast, so it is very convenient to test the program being written, in an efficient workflow. Only at the end the program is compiled to get more execution speed. Thanks a lot in advance. TP PS: --- To test the OCaml tutorial, type: $ ocamlc -o pattern_matching_example pattern_matching_example.ml $ ocamlrun ./pattern_matching_example (*************************************************************) (* from OCaml tutorial, section 'data_types_and_matching.html' *) (* This is a binary tree *) type expr = Plus of expr * expr | Minus of expr * expr | Times of expr * expr | Divide of expr * expr | Value of string ;; let v = Times ( Value "n", Plus (Value "x", Value "y") ) let rec to_string e = match e with Plus ( left, right ) -> "(" ^ (to_string left ) ^ " + " ^ (to_string right) ^ ")" | Minus ( left, right ) -> "(" ^ (to_string left ) ^ " - " ^ (to_string right) ^ ")" | Times ( left, right ) -> "(" ^ (to_string left ) ^ " * " ^ (to_string right) ^ ")" | Divide ( left, right ) -> "(" ^ (to_string left ) ^ " / " ^ (to_string right) ^ ")" | Value value -> value ;; (* by type inference, ocaml knows that e is of type expr just below *) let print_expr e = print_endline ( to_string e );; print_expr v;; let rec distribute e = match e with Times ( e1, Plus( e2, e3 ) ) -> Plus (Times ( distribute e1, distribute e2 ) , Times ( distribute e1, distribute e3 ) ) | Times ( Plus( e1, e2 ), e3 ) -> Plus (Times ( distribute e1, distribute e3 ) , Times ( distribute e2, distribute e3 ) ) | Plus ( left, right ) -> Plus ( distribute left, distribute right ) | Minus ( left, right ) -> Minus ( distribute left, distribute right ) | Times ( left, right ) -> Times ( distribute left, distribute right ) | Divide ( left, right ) -> Divide ( distribute left, distribute right ) | Value v -> Value v ;; print_expr ( distribute v );; (*************************************************************)

2 1

Haskell Propaganda needed
by Victor Miller 15 Jan '12

15 Jan '12

I'm a research mathematician at a research institute with a bunch of other mathematicians (a number of whom are also expert programmers). I recently (starting three months ago) have been learning Haskell. I'm going to give a talk to the staff about it. Most of the audience are pretty experienced programmers in C/C+/Python, but with little or no exposure to functional languages. I'm looking for talks from which I can cannibalize good selling points. I was led to Haskell by a somewhat circuitous route: at our place, as with most of the world, parallel programs (especially using GPUs) are becoming more important. A little googling lead me a few interesting projects on automatic mapping computations to GPUs, all of which were based on Haskell. I feel that this will be the way to go. There's one guy on the staff who's a demon programmer: if someone needs something to be adapted to GPUs they go to him. Unfortunately I find reading his code rather difficult -- it's rather baroque and opaque. Thus, I'd like something more high level, and something amenable to optimization algorithms. In my former life I worked at IBM research on one of the leading edge compiler optimization projects, and learned to appreciate the need for clear semantics in programs, not just for developing correct programs, but also to allow really aggressive optimizations to be performed. This is another reason that I'm interested in functional languages. I know that I'll get peppered with questions about efficiency. We (our staff) is interested in *very* large scale computations which must use the resources as efficiently as possible. One of our staff members also opined that he felt that a lazy language like Haskell wouldn't be acceptable, since it was impossible (or extremely difficult) to predict the storage use of such a program. So, any suggestions are welcome. Victor

2 1

STM atomic blocks in IO functions
by Rob Stewart 14 Jan '12

14 Jan '12

Hi, My question is regarding the correct us of STM in Haskell. I have put to together a simple example, which represents more verbose code in a library that I am writing. The question is a simple one. Must all operations on a TVar happen within *the same* atomically block, or am I am I guaranteed thread safety if, say, I have a number of atomically blocks in an IO function. Here's my example: http://pastebin.com/Hit5vKmk . Is this example permissible? thanks, -- Rob Stewart

6 6

decoupling type classes
by Yin Wang 14 Jan '12

14 Jan '12

Hi all, I have an idea about type classes that I have been experimenting. It appears to be a generalization to Haskell’s type classes and seems to be doable. It seems to related the three ideas: type classes, implicit parameters, and (typed) dynamic scoping. But I don't know whether it is good or not. I hope to get some opinions before going further. Basically, Haskell’s type classes passes dictionaries around. Each dictionary contains one or more “methods”. When “names” which belong to a dictionary are called, we invoke functions that match its principle type in the call site's scope. I have an experimental system which “decouples” the dictionary. Instead of passing on a dictionary, it passes individual “implicit parameters" around. Those implicit parameters are type inferenced and they can contain type parameters just as methods in a type class. Similarly, they are resolved by their types in the call site's scope. The convenience of this approach compared to Haskell’s type classes is that we no longer require a user of a type class to define ALL the methods in a type class. For example, a user could just define a method + without defining other methods in the Num class: -, *, … He can use the method + independently. For example, if + is defined on the String type to be concatenation, we can use + in another function: weirdConcat x y = x + y + y This has a utility, because the methods in the Num class don’t “make sense” for Strings except +, but the current type class design requires us to define them. Note here that weirdConcat will not have the type (Num a) => a -> a -> a, since we no longer have the Num class, it is decoupled into separate methods. There is another benefit of this decoupling: it can subsume the functionality of MPTC. Because the methods are no longer grouped, there is no “common” type parameter to the methods. Thus we can easily have more than one parameter in the individual methods and conveniently use them as MPTC methods. SOME IMPLEMENTATION DETAILS Here is how it can be implemented. When we see an “undefined” variable in a function definition which has been declared as “overloaded function”, we store the function name, and the type variables that are associated with it. For example, overload g — (explicitly declare g as an overloaded function) f x y (String s) = … … let z = g x s y in … … We don’t know what x and y are, but we know from the body of f that their types satisfy this pattern: g ’a String ’b. Thus we store this pattern constraint as an extra (implicit) argument in the type of f: f :: a → b → String (exist g: g a String b) We may have multiple such arguments. At the call sites of f, we look for a function g in the scope that satisfies the pattern g ‘a String ’b, but we don’t pass on the substitution, so they remain polymorphic. Once found, the function is passed as an extra parameter to f. This is essentially dictionary passing, but without grouping. It can be also more efficient because the parameters may be stored in registers. Here g is explicitly declared as “overloaded”, although my experimental system doesn’t need this. Any undefined variable inside function body automatically becomes overloaded. This may cause unintended overloading and it catches bugs late. That’s why we need the “overload” declarations. But the automatic overloading of the undefined may be useful in certain situations. For example, if we are going to use Haskell as a shell language. Every “command” must be evaluated when we type them. If we have mutually recursive definitions, the shell will report “undefined variables” either way we order the functions. The automatic overloading may solve this problem. The undefined variables will temporarily exist as automatic overloaded functions. Once we actually define a function with the same name AND satisfies the type constraints, they become implicit parameters to the function we defined before. If we call a function whose implicit parameters are not associated, the shell reports error very similar to Haskell’s “type a is not of class Num …” RELATIONSHIP TO DYNAMIC SCOPING It seems to be helpful to think of the “method calls” as referencing dynamically scoped variables. They are dispatched depending on the bindings we have in the call site's scope (and not the scope where the method is defined!). So it is very much similar to the much-hated dynamic scoping. But the dispatching is more disciplined — it doesn't just match the name. It must match both the name and the inferred principle type. This intuition also explains why local instances shouldn't be allowed, because if we capture the variables at the definition site, the method call becomes "statically scoped". -- yin

2 2

[ANNOUNCE] JuicyPixels 1.0 - image loading library
by Vincent Berthoux 14 Jan '12

14 Jan '12

Hello, I'd like to announce the first release of JuicyPixels, an image loading library written entirely in Haskell. The focus of this library is ease of use and correct loading. In this first version the library is capable of the following actions : - PNG (.png) * Reading - 1,2,4,8 bits loading, Grayscale, 24bits, 24 bits with alpha, interleaved & filtered (fully compliant with the standard, tested against png suite). * Writing - 8bits RGB (non interleaved) - 8bits RGBA (non interleaved) - 8bits greyscale (non interleaved) - Bitmap (.bmp) (mainly used as a debug output format) * Reading - 24bits (RGB) images * Writing - 32bits (RGBA) per pixel images - 24bits (RGB) per pixel images - 8 bits greyscale (with palette) - Jpeg (.jpg, .jpeg) * Reading non-interlaced baseline DCT image, seems to be OK * Writing : none yet I'm interested on any feedback regarding ease of use, performance and bugs. GitHub : https://github.com/Twinside/Juicy.Pixels Hackage : http://hackage.haskell.org/package/JuicyPixels Thanks Vincent Berthoux

2 1

named pipes interface
by Serge D. Mechveliani 13 Jan '12

13 Jan '12

I thank people for the notes. People write that the example without unsafePerformIO is desirable. So I present the improved question and improved, "pure" code. ---------------------------------------------------------------------- I need to organize a string interface for a Haskell function Main.axiom and a C program fifoFromA.c via a pair of named pipes (in Linux, UNIX). The pipes are created before running, by the commands > mkfifo toA > mkfifo fromA Main.axiom outputs a string to toA and inputs the respond string from fromA as the result. fifoFromA inputs a string from toA, converts it to the string resStr, outputs resStr to fromA. Main.axiom must be able to be applied in a loop, and there must be avoided repeated opening of a file/channel in a loop. As an example, the string conversion in fifoFromA.c is put the conversion of each character to the lower case: -------------------------- fifoFromA.c ------------------------------ #include <string.h> #include <stdio.h> #include <fcntl.h> #define BOUND 64 int main() { int toA, fromA, i, numread; char buf[BOUND]; toA = open("toA", O_RDONLY); fromA = open("fromA", O_WRONLY); for (;;) { numread = read(toA, buf, BOUND); buf[numread] = '\0'; // printf("A: Read from toA: %s\n", buf); i = 0; // convert the string to the lower case while (i < numread) {buf[i] = tolower(buf[i]); i++;} write(fromA, buf, strlen(buf)); } } ----------------------------------------------------------------------- For the "to-A" part writen in C (instead of Haskell), this interface loop works all right. With Haskell, I manage to process only the first string in the loop, and then it ends with an error. Main.hs is given below. Can you, please, fix the code? Please, copy the response to mechvel(a)botik.ru Thanks, ------ Sergei mechvel(a)botik.ru ------------------------------------------------------------------------ import System.IO (IOMode(..), IO(..), Handle, openFile, hPutStr, hGetLine, hFlush) dir = showString "/home/mechvel/ghc/axiomInterface/byLowerLevel/" toA_IO = openFile (dir "toA") WriteMode :: IO Handle -- used as global values fromA_IO = openFile (dir "fromA") ReadMode -- axiomIO :: String -> IO String axiomIO str = do toA <- toA_IO fromA <- fromA_IO hPutStr toA str hFlush toA hGetLine fromA ---------------------------------------------- Example of usage -- main = do putStr "str1 --> " str1' <- axiomIO str1 putStr (str1' ++ "\n") putStr "str2 --> " str2' <- axiomIO str2 putStr (str2' ++ "\n") where n = 9 str0 = "ABC" strings = [str0 ++ (show i) | i <- [1 .. n]] str1: str2: _ = strings ------------------------------------------------------------------ I use Glasgow Haskell ghc-7.01. Build: > gcc -o fifoFromA fifoFromA.c > ghc --make Main Running: first, command > ./fifoFromA on terminal-2, then command > ./Main on terminal-1. And it processes only the first string: terminal-1: m> ./Main str1 --> abc1 str2 --> Main: /home/mechvel/m/toA: openFile: does not exist (No such device or address)

1 0

Code generation and optimisation for compiling Haskell
by Steve Horne 13 Jan '12

13 Jan '12

Although I'm far from being an expert Haskell programmer, I think I'm ready to look into some of the details of how it's compiled. I've a copy of Modern Compiler Design (Grune, Bal, Jacobs and Langendoen) - I first learned a lot of lexical and parsing stuff from it quite a few years ago. Today, I started looking through the functional languages section - I've read it before, but never absorbed much of it. Graph reduction, lambda lifing, etc - it seems pretty simple. Far too simple. It's hard to believe that decent performance is possible if all the work is done by a run-time graph reduction engine. Simon Peyton Jones has written a couple of books on implementing functional languages which are available for free download. At a glance, they seem to covers similar topics in much more detail. However, they're from 1987 and 1992. Considering SPJs period "of despair" when he couldn't get practical performance for monadic I/O, these seem very dated. Some time ago, I made a note to look up the book "Functional Programming and Parallel Graph Rewriting" (I forget why) but again that's from the early 90's. I've also got a note to look up Urban Boquists thesis. SPJ also has some papers on compilation - http://research.microsoft.com/en-us/um/people/simonpj/papers/papers.html#co… - and the papers on optimisation by program transformation have caught my eye. Are there any current text-books that describe the techniques used by compilers like GHC to generate efficient code from functional languages? It's OK to assume some knowledge of basic compiler theory - the important stuff is code generation and optimisation techniques for lazy functional languages in particular. Also, what papers should I read? Am I on the right lines with the ones I've mentioned above?

4 5

LDAP hackage problem.
by Magicloud Magiclouds 13 Jan '12

13 Jan '12

Hi, I have a Windows AD server. And ldapsearch tool works to get someone's information. But when I turned it into Haskell code. It does not work as expected. Here is the ldapsearch and result. You can see that sn/givenName have values that are encoded utf-8. Next to it is the code I am using. Running this code returns the similar thing. The problem is that only sn/givenName/displayName are empty. I have tried a few diag ways trying to find out why. No luck. $ ldapsearch -h vancloa.cn -D CN=wangshida.admin,OU=admin_accounts,DC=vancloa,DC=cn -w ******** -b "DC=vancloa,DC=cn" -x '(&(sAMAccountName=chengtao)(&(objectClass=user)(!(ou=Recycle_Bin))))' # extended LDIF # # LDAPv3 # base <DC=vancloa,DC=cn> with scope subtree # filter: (&(sAMAccountName=chengtao)(&(objectClass=user)(!(ou=Recycle_Bin)))) # requesting: ALL # # 0000000634, Employees_Accounts, vancloa.cn dn: CN=0000000634,OU=Employees_Accounts,DC=vancloa,DC=cn objectClass: top objectClass: person objectClass: organizationalPerson objectClass: user cn: 0000000634 sn:: 56iL c: CN title:: 5oqA5pyv5pSv5oyB5bel56iL5biI description: 0000000634 physicalDeliveryOfficeName:: 5oiQ6YO95LuT5YKo givenName:: 5rab distinguishedName: CN=0000000634,OU=Employees_Accounts,DC=vancloa,DC=cn instanceType: 4 whenCreated: 20101231073752.0Z whenChanged: 20120106005157.0Z displayName:: 56iL5rab # search reference ref: ldap://ForestDnsZones.vancloa.cn/DC=ForestDnsZones,DC=vancloa,DC=cn # search reference ref: ldap://DomainDnsZones.vancloa.cn/DC=DomainDnsZones,DC=vancloa,DC=cn # search reference ref: ldap://vancloa.cn/CN=Configuration,DC=vancloa,DC=cn # search result search: 2 result: 0 Success # numResponses: 5 # numEntries: 1 # numReferences: 3 6|main :: IO () 7|main = do 8| args <- getArgs 9| let alias = head args 10| putStrLn alias 11| entries <- failLDAP $ do 12| ldap <- ldapOpen "vancloa.cn" 389 13| ldapSimpleBind ldap "CN=wangshida.admin,OU=admin_accounts,DC=vancloa,DC=cn" "********" 14| ldapSearch ldap (Just "DC=vancloa,DC=cn") LdapScopeSubtree (Just ("(&(sAMAccountName=" ++ alias ++ ")(&(objectClass=user)(!(ou=Recycle_Bin))))"\ )) LDAPAllUserAttrs False --(LDAPAttrList ["sn", "givenname"]) False 15| let LDAPEntry _ attrs = head entries 16| sn = head $ snd $ head $ filter (\(a, _) -> a == "sn") attrs 17| givenName = head $ snd $ head $ filter (\(a, _) -> a == "givenName") attrs 18| print entries -- 竹密岂妨流水过山高哪阻野云飞

1 0

Unboxed Rationals?
by Victor Miller 12 Jan '12

12 Jan '12

I have a program which does a lot of computations with Array's of rationals. In order to get better performance I'd like to use unboxed arrays, but, since (Ratio Int) isn't a primitive type this doesn't work. It occured to me that I could rewrite this using -fglasgow-exts where I represent a rational by (# Int, Int #). It isn't too hard to do this, but I'd like to know it it's already in some library out there, so that I can use something already existing. This is a more general problem that I have (I'm pretty new to Haskell) of finding a library to "do X". Victor

4 6

Spurious pattern match warnings with GADTs
by Tom Hawkins 12 Jan '12

12 Jan '12

Let's say I have: data T0 data T1 data T a where A :: T T0 B :: T T1 Then I can write the following without getting any non-exhaustive pattern match warnings: t :: T T0 -> String t a = case a of A -> "A" However, if I use type classes to constrain the constructors, instead of using the phantom types directly: class C0 a instance C0 T0 class C1 a instance C1 T1 data T a where A :: C0 a => T a B :: C1 a => T a Then I get a non-exhaustive pattern match warning on 't'. How come? (I'm using GHC 7.0.4) It appears that the intelligent pattern analysis of the first example is a relatively recent addition [1]. -Tom [1] http://hackage.haskell.org/trac/ghc/ticket/3476

3 2

How to terminate the process group of a process created with createProcess?
by André Scholz 12 Jan '12

12 Jan '12

Hello, (on unix) creating a process A which spawns itself a subprocess B and terminating process A before it finishes leaves process B as a process on its own. This is because "terminateProcess" sends the sigterm signal to the process only and not to its process group. Is there a way to terminate the process group of process A? Or alternatively is there a way to get he Posix ProcessId of process A? The following is a minimal example. The process "sleep 60" survives the termination of both "sleep" and of the main program. <code> module Main where import Control.Concurrent import System.Process main = do h@(_, _, _, hProc) <- createProcess (proc "time" ["sleep", "60"]) threadDelay $ 5 * 1000000 terminateProcess hProc waitForProcess hProc </code> Kind regards, André

3 6

ANNOUNCE: quickcheck-instances
by Antoine Latter 12 Jan '12

12 Jan '12

Hello! I'd like to announce the first release of the quickcheck-instances package, which aims to consolidate commonly needed class instances for use with QuickCheck. These instances are appropriate when your tests don't have strong requirements on the nature of the input data - for example, if you are testing a network-protocol parser, the ByteString instance provided in this package might not be what you want to use. Other limitations: * I do not have full coverage of the Haskell Platform - I have been adding instances as I need them. Patches are warmly welcomed, however. * I only have instances of the 'Arbitrary' class, however I would be happy to accept patches for instances of CoArbitrary and other classes that would be useful in quickcheck-based testing. On Hackage: http://hackage.haskell.org/package/quickcheck-instances Ordinarily I frown on orphaned instances in library packages, but the fact that quickcheck properties should never appear in a library Cabal package, most of the downsides of orphans are mitigated. Thanks, Antoine

1 0

Well-Typed are hiring: Haskell consultant
by Andres Löh 12 Jan '12

12 Jan '12

In order to keep up with customer demand, we are looking to hire a Haskell expert to work with us at Well-Typed (http://www.well-typed.com/) as a Haskell consultant. This is an exciting opportunity for someone who is passionate about Haskell and who is keen to improve and promote Haskell in a professional context. The role is quite general and could cover any of the projects and activities that we are involved in as a company. The tasks may involve: * working on the Haskell compilers, libraries and tools; * Haskell application development; * working directly with clients to solve their problems. Well-Typed has a variety of clients. For some we do proprietary Haskell development and consulting. For others, much of the work involves open-source development and cooperating with the rest of the Haskell community: the commercial, open-source and academic users. At the moment, we are running the Parallel GHC Project (http://www.haskell.org/haskellwiki/Parallel_GHC_Project/) It is likely that initial tasks will have some connection with parallel and/or concurrent programming in Haskell. We are also doing quite a bit of GHC maintenance, and some knowledge or interest in compiler internals, operating systems, the foreign language interface, and/or deployment issues would be welcome. Our ideal candidate has excellent knowledge of Haskell, whether from industry, academia, or personal interest. Familiarity with other languages, low-level programming, and good software engineering practices are also useful. Good organisation and ablity to manage your own time, and reliably meet deadlines, is important. You are likely to have a batchelor's degree or higher in computer science or a related field, although this isn't a requirement. Experience of consulting, or running a business, is also a bonus. The position is initially as a contractor for one year with a salary of 150 GBP per day. We offer flexible hours and work from home. Living in England is not required. In the longer term there is the opportunity to become a member of the partnership with a full stake in the business: being involved in business decisions, and fully sharing the risks and rewards. If you are interested, please apply via info(a)well-typed.com. Tell us why you are interested and why you would be a good fit for the job, and attach your CV. Please also indicate when you might be able to start. We are more than happy to answer informal enquiries. Contact Duncan Coutts, Ian Lynagh or Andres Löh (http://www.well-typed.com/who_we_are/) for further information, either by email or IRC. The deadline for applications is Friday 27th January 2012. == About Well-Typed Well-Typed LLP is a Haskell services company, providing consultancy services, writing bespoke applications, and offering commercial training in Haskell and related topics. -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

1 0

Re: [Haskell-cafe] How to terminate the process group of a process created with createProcess?
by John Lato 12 Jan '12

12 Jan '12

> From: Brandon Allbery <allbery.b(a)gmail.com> > > On Wed, Jan 11, 2012 at 16:26, Andr? Scholz <andre.scholz(a)uni-bremen.de>wrote: > >> (on unix) creating a process A which spawns itself a subprocess B and >> terminating process A before it finishes leaves process B as a process on >> its >> own. This is because "terminateProcess" sends the sigterm signal to the >> process only and not to its process group. >> > > terminateProcess passes on the semantics of kill(2); on SVID-compliant (and > I think POSIX.1-compliant) systems, the negative of the process group > leader's process ID is used to signal the process group. Note that you may > need to arrange for your initial child process to invoke setpgrp() to > insure that the parent program is not part of the process group. System.Process.terminateProcess takes a ProcessHandle as input, which is an opaque type which essentially can only be created via System.Process.createProcess or similar; it's not possible to pass arbitrary pid's to it. However, on Posix systems it is currently possible (ghc-7.2.1) to import System.Process.Internals, which would allow you to either create a new ProcessHandle from an arbitrary pid or retrieve the pid from a ProcessHandle. My answer on SO now shows how to do so. Of course this approach won't be portable, and it may change depending on your compiler version. (link to SO: http://stackoverflow.com/questions/8820903/haskell-how-to-timeout-a-functio… ) John L.

1 0

GHCi with wxhaskell-dev
by Dave Tapley 12 Jan '12

12 Jan '12

I'm working on a dev branch of wxHaskell [1] and one of the new features is that you can use wxHaskell in GHCi (again), but I seem to have broken something in the process: Firstly: wxHaskell works in GHCi insomuch as: You can load the hello world sample [2] in GHCi, do "> main", see the window spawn, close it and be returned to the GHCi prompt. What I'm interested in is the case where you call wxHaskell's start function [3] from GHCi. The result is becoming stuck with no way back to the GHCi prompt without resorting to sending kill signals. I made a screen cast to illustrate my findings [4]: 1. Once you've done "> start $ return ()" you can press ^C all you want, there's no way back to the GHCi prompt. 2. If you send three (less has no effect) kills to the GHCi process it will exit, but it will exit GHCi as well (instead of putting you back at the GHCi prompt). 3. If you do "> start $ return ()" but *do not* try and ^C, you can send a single kill to GHCi and be returned to the GHCi prompt. My questions: 1. Why doesn't GHCi respond to the ^C? 2a. Why does entering ^C from within GHCi then prevent you from getting back to the GHCi prompt? 2b. Why doesn't any other input (i.e. entering anything but ^C) cause this behaviour? 3. If you have sent ^C from GHCi, why do you then have to kill three times to get out of GHCi? Feel free to pull the repo [1] and try it out, you'll need the latest dev release of wxWidgets [5] installed. Thanks! Dave, [1] http://darcsden.com/DukeDave/wxhaskell-dev [2] http://darcsden.com/DukeDave/wxhaskell-dev/browse/samples/wx/HelloWorld.hs [3] http://darcsden.com/DukeDave/wxhaskell-dev/browse/wx/src/Graphics/UI/WX.hs#… [4] http://youtu.be/sEI48ultdS0 [5] http://sourceforge.net/projects/wxwindows/files/2.9.3/

2 1

How to catch all exceptions that could be caught?
by Magicloud Magiclouds 12 Jan '12

12 Jan '12

Hi, With Prelude.catch, I could write "catch () $ \_ -> return Nothing". But with Control.Exception.catch, I must specify a type for the "_". What should I do? PS: In Java document, a function declaration would tell us both the incoming args and outgoing, AND what exceptions would it throw. But it does not seem to exist in Haskell document? And the most awful part is that even when the program failed with error message, I still did not know what exception was this. Take "recv: does not exist (Connection refused)" for example. -- 竹密岂妨流水过山高哪阻野云飞

4 9

A simple telnet client using Conduit
by Erik de Castro Lopo 12 Jan '12

12 Jan '12

Hi all, I've written a simple telnet client using Michael Snoyman's Conduit library and was looking for comments as to whether I'm doing it right. In particular, is my usage of a ResourceT to track a thread a good idea, necessary or waste of time. The code is here: https://gist.github.com/1596792 Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

2 5

GHC exceeding command line length limit with split-objs - and a possible fix
by Eugene Kirpichov 11 Jan '12

11 Jan '12

Hi, I'm building gtk2hs on a mac with -fsplit-objs, and during compilation of the largest file, which produces about 7000 split objects, the linking phase fails. I'm assuming that's because the command line length has been exceeded, because other files compile fine, without -fsplit-objs it compiles fine too, and it compiles fine *with* -fsplit-objs on other OS - so perhaps the reason is in mac's tempfile name generation (they're longer than on other platforms) or something. Anyway. I think a nice fix would be to employ gcc's ability to read options from a file - gcc @file - and write overly long option strings into temp files. It'd be fun for me to implement this as my first contribution to ghc; is this a good idea in general? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

2 4

Re: [Haskell-cafe] GHC exceeding command line length limit with split-objs - and a possible fix
by John Lato 11 Jan '12

11 Jan '12

I used https://github.com/kennethreitz/osx-gcc-installer/downloads to get a real gcc on Lion. Biggish download, but it worked. I've also seen reports of success by self-compiling gcc, or by installing XCode 4 on top of an existing XCode 3 installation. John L. > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > > Oh well... looks like building ghc won't be easy, as it doesn't build with > llvm-gcc and it's not easy to get a "real" gcc on Lion. But I don't stop > trying :)

4 3

Where is the pairing-with-monoid monad instance?
by Conal Elliott 11 Jan '12

11 Jan '12

Is the standard pair-with-monoid monad instance in some standard place? I see the Applicative instance in Control.Applicative, and the pair-with-monoid Functor instance in Control.Monad.Instances, and the (->) e and Either e monad instances also in Control.Monad.Instances. I'm looking for something like instance Monoid o => Monad ((,) o) where return a = (mempty,a) (o,a) >>= f = (o `mappend` o', b) where (o',b) = f a Where is it hiding from me? Thanks, - Conal

2 1

debugging snap app
by Eric Wong 11 Jan '12

11 Jan '12

Hi, I'm trying to get the debugging messages in snap-server in order to find the socket leaking in my snap app. I find the doc on snapframework.com says that setting an environment variable DEBUG=1 will enable the app to output the messages in stderr. But I don't get any. I installed the snap-server package using -fopenssl flag but without -fnodebug, so I think it's normal. Any idea on this? Eric

1 0

When will constraint kinds roll out?
by Nicholas Tung 10 Jan '12

10 Jan '12

Just a quick, logistical question: I see constraint kinds didn't make it to GHC 7.2.2; does anyone have guesses when the extension will roll out in an official release? Our research team is interested in using Haskell for an EDSL, and having both constraint kinds and access to all of the libraries on Hackage would be optimal. (I've been writing an EDSL with monad transformers and the rmonad package myself, but think it is too difficult and cumbersome if we are to use Haskell EDSL approaches more broadly). Thanks in advance, Nicholas — https://ntung.com — 4432-nstung

2 1

ANNOUNCE: sloth-0.0.1
by Jan Christiansen 10 Jan '12

10 Jan '12

Hi, I have just released a prototype implementation of a tool (http://hackage.haskell.org/package/sloth) for checking whether a function is unnecessarily strict. It is inspired by Olaf Chitils StrictCheck tool (http://www.cs.kent.ac.uk/people/staff/oc/) but does not propose improvements that lead to non-sequential function definitions. For more details there is a short explanation with some examples at http://www.informatik.uni-kiel.de/fileadmin/arbeitsgruppen/ca_prog_develop/…. If you discover a bug or have any questions please use the human bug tracking system at jan_christiansen[at]gmx[dot]de. Cheers, Jan

1 0

Simple type-class experiment turns out not so simple...
by Steve Horne 10 Jan '12

10 Jan '12

I was messing around with type-classes (familiarization exercises) when I hit a probably newbie problem. Reducing it to the simplest case... module BinTree ( WalkableBinTree, BT (Branch, Empty) ) where -- n : node type -- d : data item type wrapped in each node class WalkableBinTree n where wbtChildren :: n -> Maybe (n, n) wbtData :: n -> Maybe d -- Simple tree type, mostly for testing data BT x = Branch x (BT x) (BT x) | Empty instance WalkableBinTree (BT x) where wbtChildren (Branch d l r) = Just (l, r) wbtChildren Empty = Nothing wbtData (Branch d l r) = Just d wbtData Empty = Nothing Loading this code into GHCi, I get... Prelude> :load BinTree [1 of 1] Compiling BinTree ( BinTree.hs, interpreted ) BinTree.hs:16:39: Couldn't match type `x' with `d' `x' is a rigid type variable bound by the instance declaration at BinTree.hs:12:32 `d' is a rigid type variable bound by the type signature for wbtData :: BT x -> Maybe d at BinTree.hs:16:5 In the first argument of `Just', namely `d' In the expression: Just d In an equation for `wbtData': wbtData (Branch d l r) = Just d Failed, modules loaded: none. Prelude> I've tried varying a number of details. Adding another parameter to the type-class (for the item-data type) requires an extension, and even then the instance is rejected because (I think) the tree-node and item-data types aren't independent. In any case, I can't understand why those types can't match.

7 10

How to "show" a utf8 string?
by Magicloud Magiclouds 10 Jan '12

10 Jan '12

Hi, I am using LDAP hackage to do some ldap searching. I am not sure if this is its problem. All Chinese chars returned like "\29579". How to convert it to the actual Chinese char? I thought it was my terminal's fault, so tried to use System.IO.UTF8 to put the result into a file and viewed by firefox, no luck. -- 竹密岂妨流水过山高哪阻野云飞

3 3

Re: [Haskell-cafe] typeclass and functional dependency problem
by John Lato 10 Jan '12

10 Jan '12

> From: Martin DeMello <martindemello(a)gmail.com> > Subject: [Haskell-cafe] typeclass and functional dependency problem > > I'm writing a Gtk2hs app, and I have several custom widgets that are > composite objects represented by records, one field of which is a > container widget. I am trying to write a replacement for gtk2hs's > boxPackStart > > boxPackStart :: (BoxClass self, WidgetClass child) => self -> child -> > Packing -> Int -> IO () > > that will accept either a gtk widget or one of my custom widgets to > place in the box, and do the right thing. Here's my attempt at it; > what I want to know is why the commented out bit didn't work and I had > to individually add instances of widgets instead: > > --------------------------------------------- > -- packable objects > class WidgetClass w => Packable a w | a -> w where > widgetOf :: a -> w > > --instance WidgetClass w => Packable w w where > -- widgetOf = id > > instance Packable Button Button where > widgetOf = id > > instance Packable Entry Entry where > widgetOf = id > > instance Packable Label Label where > widgetOf = id > > instance Packable Notebook Notebook where > widgetOf = id > > instance Packable HBox HBox where > widgetOf = id > > -- add widget to box > boxPackS :: (BoxClass b, WidgetClass w, Packable a w) => b -> a -> > Packing -> Int -> IO () > boxPackS box child p i = boxPackStart box (widgetOf child) p i > > --------------------------------------------- > > If I try to use > > instance WidgetClass w => Packable w w where > widgetOf = id > > instead, I get the compilation error > > Editor.hs:23:10: > Functional dependencies conflict between instance declarations: > instance Packable PairBox VBox -- Defined at Editor.hs:23:10-30 > instance WidgetClass w => Packable w w > -- Defined at GuiUtils.hs:13:10-38 > > even though PairBox does not belong to WidgetClass. This is a very common problem. The line > instance WidgetClass w => Packable w w where means "Every type is an instance of Packable by this declaration", not "Every type that has a WidgetClass instance is a Packable by this declaration", which is what you wanted. So you end up with two Packable instances for PairBox, "Packable PairBox PairBox" and "Packable PairBox Container" (or whatever type you used), which causes this conflict. This is a consequence of Haskell type classes being open. Even if you don't have a WidgetClass instance for a type (PairBox) in scope where you define this instance, one could be defined elsewhere and be in scope at a call site. Unfortunately Haskell doesn't provide a way to write what you want. I can think of a few solutions, none of which are ideal: 1. Create separate instances for each type. Writing some generating code to do this is probably the best available option. 2. Convert everything to a Container. 3. Make boxPackS a TH splice, and use a naming convention to differentiate your objects from the built-in widgets. I don't like 3) because it's fragile. Pretty much everything else is a variant of either 1 (lots of instance decls) or 2 (another function call/separate functions). John L.

1 0

SMP parallelism increasing GC time dramatically
by Tom Thorne 10 Jan '12

10 Jan '12

I am having some strange performance issues when using SMP parallelism, that I think may be something to do with GC. Apologies for the large readouts below but I'm not familiar enough to know what is and isn't relevant! I have a pure function that is mapped over a list of around 10 values, and this happens several times for each iteration of my program. It does some fairly intensive calculations using hmatrix, generating intermediate matrices along the way. The computation is significantly more complex for some values, so the work done by each call is not spread equally. I did some profiling and it seems like the program is spending about 50% of its time in that function. First of all, without any attempts at parallelism, I see this from ./Main +RTS -s 67,142,126,336 bytes allocated in the heap 147,759,264 bytes copied during GC 109,384 bytes maximum residency (58 sample(s)) 354,408 bytes maximum slop 3 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 104551 collections, 0 parallel, 1.13s, 1.11s elapsed Generation 1: 58 collections, 0 parallel, 0.01s, 0.01s elapsed Parallel GC work balance: -nan (0 / 0, ideal 1) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 67.06s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 67.09s) 0.00s ( 0.00s) Task 2 (bound) : 66.95s ( 67.09s) 1.14s ( 1.12s) SPARKS: 0 (0 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 66.95s ( 67.09s elapsed) GC time 1.14s ( 1.12s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 68.09s ( 68.21s elapsed) %GC time 1.7% (1.6% elapsed) Alloc rate 1,002,835,517 bytes per MUT second Productivity 98.3% of total user, 98.2% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0 This looks ok to me... Then if I try to use Control.Parallel to parallelise my code, simpy replacing a map with parMap (rdeepseq), on a 12 core machine using +RTS -N12 -s I get this: 66,065,148,144 bytes allocated in the heap 197,202,056 bytes copied during GC 181,312 bytes maximum residency (251 sample(s)) 387,240 bytes maximum slop 12 MB total memory in use (3 MB lost due to fragmentation) Generation 0: 37592 collections, 37591 parallel, 245.32s, 26.67s elapsed Generation 1: 251 collections, 251 parallel, 3.12s, 0.33s elapsed Parallel GC work balance: 2.41 (24219609 / 10058220, ideal 12) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 2 (worker) : 0.00s ( 17.97s) 0.00s ( 0.00s) Task 3 (worker) : 0.00s ( 19.35s) 0.00s ( 0.00s) Task 4 (worker) : 0.00s ( 40.28s) 0.00s ( 0.00s) Task 5 (worker) : 0.00s ( 45.08s) 0.00s ( 0.00s) Task 6 (worker) : 0.00s ( 47.06s) 0.00s ( 0.00s) Task 7 (worker) : 18.30s ( 49.73s) 16.24s ( 1.71s) Task 8 (worker) : 0.00s ( 51.22s) 0.00s ( 0.00s) Task 9 (worker) : 0.00s ( 53.75s) 0.00s ( 0.00s) Task 10 (worker) : 0.00s ( 54.17s) 0.00s ( 0.00s) Task 11 (worker) : 5.65s ( 54.30s) 0.70s ( 0.08s) Task 12 (worker) : 0.00s ( 54.41s) 0.41s ( 0.04s) Task 13 (worker) : 4.34s ( 54.58s) 4.50s ( 0.48s) Task 14 (worker) : 5.82s ( 54.76s) 5.91s ( 0.64s) Task 15 (worker) : 6.50s ( 55.01s) 3.37s ( 0.38s) Task 16 (worker) : 7.60s ( 55.21s) 8.56s ( 0.94s) Task 17 (worker) : 11.05s ( 55.21s) 9.04s ( 0.96s) Task 18 (worker) : 11.75s ( 55.21s) 12.94s ( 1.43s) Task 19 (worker) : 16.02s ( 55.21s) 13.32s ( 1.43s) Task 20 (worker) : 26.98s ( 55.23s) 7.43s ( 0.77s) Task 21 (worker) : 7.36s ( 55.23s) 7.47s ( 0.83s) Task 22 (worker) : 16.08s ( 55.23s) 10.25s ( 1.12s) Task 23 (worker) : 7.04s ( 55.23s) 4.99s ( 0.57s) Task 24 (worker) : 28.47s ( 55.23s) 8.78s ( 0.94s) Task 25 (worker) : 7.43s ( 55.23s) 1.62s ( 0.18s) Task 26 (worker) : 6.33s ( 55.23s) 11.42s ( 1.23s) Task 27 (worker) : 9.80s ( 55.23s) 8.72s ( 0.95s) Task 28 (worker) : 4.88s ( 55.26s) 8.92s ( 0.99s) Task 29 (worker) : 0.00s ( 55.26s) 0.00s ( 0.00s) Task 30 (bound) : 5.59s ( 55.26s) 0.59s ( 0.06s) Task 31 (worker) : 41.16s ( 55.26s) 3.48s ( 0.38s) Task 32 (worker) : 17.03s ( 55.26s) 3.90s ( 0.42s) Task 33 (worker) : 14.89s ( 55.26s) 5.29s ( 0.58s) Task 34 (worker) : 6.30s ( 55.26s) 1.99s ( 0.21s) Task 35 (worker) : 16.13s ( 55.26s) 13.95s ( 1.50s) Task 36 (worker) : 16.70s ( 55.26s) 13.02s ( 1.41s) Task 37 (worker) : 11.68s ( 55.26s) 15.45s ( 1.68s) Task 38 (worker) : 7.65s ( 55.26s) 13.61s ( 1.50s) Task 39 (worker) : 14.18s ( 55.26s) 12.48s ( 1.35s) Task 40 (worker) : 5.51s ( 55.26s) 11.92s ( 1.31s) Task 41 (worker) : 3.10s ( 55.26s) 8.17s ( 0.93s) SPARKS: 42525 (37707 converted, 519 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 365.55s ( 55.26s elapsed) GC time 248.45s ( 27.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 614.01s ( 82.26s elapsed) %GC time 40.5% (32.8% elapsed) Alloc rate 180,724,403 bytes per MUT second Productivity 59.5% of total user, 444.4% of total elapsed gc_alloc_block_sync: 1339708 whitehole_spin: 36 gen[0].sync_large_objects: 197621 gen[1].sync_large_objects: 6396 It seems like suddenly GC is taking a lot longer. I also tried using Control.Monad.Par, with runPar $ do parMap, and using +RTS -N12 -s I get this: 71,480,561,920 bytes allocated in the heap 275,752,912 bytes copied during GC 198,704 bytes maximum residency (343 sample(s)) 394,232 bytes maximum slop 12 MB total memory in use (3 MB lost due to fragmentation) Generation 0: 37575 collections, 37574 parallel, 169.08s, 16.67s elapsed Generation 1: 343 collections, 343 parallel, 6.24s, 0.61s elapsed Parallel GC work balance: 2.56 (33928871 / 13275450, ideal 12) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.70s ( 46.46s) 0.00s ( 0.00s) Task 1 (worker) : 2.94s ( 46.46s) 0.00s ( 0.00s) Task 2 (worker) : 0.10s ( 46.46s) 0.00s ( 0.00s) Task 3 (worker) : 0.06s ( 46.47s) 0.00s ( 0.00s) Task 4 (worker) : 1.70s ( 46.47s) 0.00s ( 0.00s) Task 5 (worker) : 0.07s ( 46.47s) 0.00s ( 0.00s) Task 6 (worker) : 2.75s ( 46.47s) 0.00s ( 0.00s) Task 7 (worker) : 1.21s ( 46.47s) 0.00s ( 0.00s) Task 8 (worker) : 0.43s ( 46.47s) 0.00s ( 0.00s) Task 9 (worker) : 10.44s ( 46.47s) 0.00s ( 0.00s) Task 10 (worker) : 0.06s ( 46.47s) 0.00s ( 0.00s) Task 11 (worker) : 1.36s ( 46.47s) 0.00s ( 0.00s) Task 12 (worker) : 0.00s ( 46.50s) 46.69s ( 4.63s) Task 13 (worker) : 0.00s ( 46.50s) 0.00s ( 0.00s) Task 14 (bound) : 0.05s ( 46.50s) 0.56s ( 0.06s) Task 15 (worker) : 24.84s ( 46.50s) 2.24s ( 0.21s) Task 16 (worker) : 28.00s ( 46.50s) 1.95s ( 0.19s) Task 17 (worker) : 35.30s ( 46.50s) 2.79s ( 0.27s) Task 18 (worker) : 29.03s ( 46.50s) 4.06s ( 0.39s) Task 19 (worker) : 32.71s ( 46.50s) 7.04s ( 0.69s) Task 20 (worker) : 32.74s ( 46.50s) 11.47s ( 1.09s) Task 21 (worker) : 23.35s ( 46.50s) 15.16s ( 1.49s) Task 22 (worker) : 36.00s ( 46.50s) 7.94s ( 0.76s) Task 23 (worker) : 0.00s ( 46.50s) 35.84s ( 3.63s) Task 24 (worker) : 24.70s ( 46.50s) 21.26s ( 2.06s) Task 25 (worker) : 23.07s ( 46.50s) 18.33s ( 1.82s) SPARKS: 0 (0 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 307.74s ( 46.50s elapsed) GC time 175.32s ( 17.29s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 483.06s ( 63.79s elapsed) %GC time 36.3% (27.1% elapsed) Alloc rate 232,274,501 bytes per MUT second Productivity 63.7% of total user, 482.4% of total elapsed gc_alloc_block_sync: 1619540 whitehole_spin: 258 gen[0].sync_large_objects: 232661 gen[1].sync_large_objects: 6378 This seems slightly better, but the GC time is still much bigger than I would expect it to be. I don't think it is solely because of parallel GC, since running the parallel code with +RTS -N1 gives results similar to the non-parallel version. Am I simply misunderstanding the way the GC time is represented in this output, or is there something amiss? (I also noticed that there seem to be a lot more than 12 threads running if I look at top while the program is running, but I assume this is something to do with the runtime spawning more threads for GC etc)

14 23

typeclass and functional dependency problem
by Martin DeMello 10 Jan '12

10 Jan '12

I'm writing a Gtk2hs app, and I have several custom widgets that are composite objects represented by records, one field of which is a container widget. I am trying to write a replacement for gtk2hs's boxPackStart boxPackStart :: (BoxClass self, WidgetClass child) => self -> child -> Packing -> Int -> IO () that will accept either a gtk widget or one of my custom widgets to place in the box, and do the right thing. Here's my attempt at it; what I want to know is why the commented out bit didn't work and I had to individually add instances of widgets instead: --------------------------------------------- -- packable objects class WidgetClass w => Packable a w | a -> w where widgetOf :: a -> w --instance WidgetClass w => Packable w w where -- widgetOf = id instance Packable Button Button where widgetOf = id instance Packable Entry Entry where widgetOf = id instance Packable Label Label where widgetOf = id instance Packable Notebook Notebook where widgetOf = id instance Packable HBox HBox where widgetOf = id -- add widget to box boxPackS :: (BoxClass b, WidgetClass w, Packable a w) => b -> a -> Packing -> Int -> IO () boxPackS box child p i = boxPackStart box (widgetOf child) p i --------------------------------------------- If I try to use instance WidgetClass w => Packable w w where widgetOf = id instead, I get the compilation error Editor.hs:23:10: Functional dependencies conflict between instance declarations: instance Packable PairBox VBox -- Defined at Editor.hs:23:10-30 instance WidgetClass w => Packable w w -- Defined at GuiUtils.hs:13:10-38 even though PairBox does not belong to WidgetClass. martin

1 0