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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by Robert Greayer 16 Jan '09

16 Jan '09

----- Original Message ---- From: John A. De Goes <john(a)n-brain.net> On Jan 15, 2009, at 9:31 AM, John Goerzen wrote: >> AFAIK, the only language where that sort of wheel reinvention is >> popular is Java. But then Java seems to encourage wheel reinvention >> anyhow ;-) > The Java reinventions look and feel like Java, because they're native implementations. > This is even more important in Haskell where the differences between > Haskell and C is about as large as you can get. The Java reinventions largely exist because of the huge deployment-time benefits you get from pure-Java code (cross-platform portability of compiled (byte) code, dynamic loading of compiled code over a network, etc.). Such reinventions are much less important for Haskell, since the typical deployment model for a Haskell program is much closer to that of a C program than a Java program or even a Python program.

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software correctness ... can we in FPL step up to the plate??
by Galchin, Vasili 16 Jan '09

16 Jan '09

http://www.businessweek.com/the_thread/techbeat/archives/2009/01/nsa_dhs_in… ... I think that http://www.galois.com is already doing as stated in the article/ ...... I sincerely think there is a segway for Haskell here with strong and static type checking.. ?? Vasili

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Arch Haskell News: Jan 11 2009
by Don Stewart 16 Jan '09

16 Jan '09

Arch Haskell News: Jan 11 2009 Hey all, welcome to the first Arch Haskell News of 2009. (News about Haskell on the Arch Linux platform). Arch now has 827 Haskell packages in AUR. That’s an increase of 93 new packages in the last 48 days, or 1.9 new Haskell apps and libraries a day over the holiday season. Well done everyone!. Noteworthy * lhc-0.6.20081127: “LHC Haskell Compiler” * cabal2arch-0.4.2: “Create Arch Linux packages from Cabal packages” * timberc-1.0.2: “The Timber Compiler.” * haskell-x11-1.4.5: “A binding to the X11 graphics library” * haskell-curl-1.3.2.2: “Haskell binding to libcurl” * alex-2.3.1: “Alex is a tool for generating lexical analysers in Haskel * haskell-bytestring-trie-0.1: “Efficient map from strings to values.” * haskell-llvm-0.5.0.1: “Bindings to the LLVM compiler toolkit” * tetris-0.27178: “A 2-D clone of Tetris” * haskell-network-bytestring-0.1.1.4: “Fast and memory efficient low-level networking” * haskell-hdbc-1.1.6: “Haskell Database Connectivity” http://archhaskell.wordpress.com/2009/01/11/arch-haskell-news-jan-11-2009/ Other news: * llvm support, http://archhaskell.wordpress.com/2009/01/11/llvm-haskell-bindings/ * new hdbc release, http://archhaskell.wordpress.com/2009/01/11/hdbc-haskell-database-suite-upd… * salvia, new web framework, supported http://archhaskell.wordpress.com/2009/01/11/new-haskell-web-framework-salvi… Bring on 2009 and the era of ubiquitous Haskell! -- Don

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IFL 2009: Call for Papers
by IFL 2009 16 Jan '09

16 Jan '09

Call for Papers IFL 2009 Seton Hall University SOUTH ORANGE, NJ, USA http://tltc.shu.edu/blogs/projects/IFL2009/ The 21st IFL symposium, IFL 2009, will be held for the first time in the USA. The hosting institution is Seton Hall University in South Orange, NJ, USA and the symposium dates are September 23-25, 2009. It is our goal to make IFL a regular event held in the USA. The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2009 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2009 will use a post-symposium review process to produce a formal proceedings which we expect to be published by Springer in the Lecture Notes in Computer Science series. All participants in IFL 2009 are invited to submit either a draft paper or and extended abstract describing work to be presented at the symposium. These submissions will be screened by the program committee chair to make sure they are within the scope of IFL and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full arcticle for the formal review process. These revised submissions will be reviewed by the program committee using prevailing academic standards to select the best articles that will appear in the formal proceedings. TOPICS IFL welcomes submissions describing practical and theoretical as well as submissions describing applications and tools. If you are not sure if your work is appropriate for IFL 2009, please contact the PC chair at ifl2009(a)shu.edu. Topics of interest include, but are not limited to: language concepts type checking contracts compilation techniques staged compilation runtime function specialization runtime code generation partial evaluation (abstract) interpretation generic programming techniques automatic program generation array processing concurrent/parallel programming concurrent/parallel program execution functional programming and embedded systems functional programming and web applications functional programming and security novel memory management techniques runtime profiling and performance measurements debugging and tracing virtual/abstract machine architectures validation and verification of functional programs tools and programming techniques PAPER SUBMISSIONS Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English, conform to the Springer-Verlag LNCS series format and not exceed 16 pages. The draft proceedings will appear as a technical report of the Department of Mathematics and Computer Science of Seton Hall University. IMPORTANT DATES Registration deadline August 15, 2009 Presentation submission deadline August 15, 2009 IFL 2009 Symposium September 23-25, 2009 Submission for review process deadline November 1, 2009 Notification Accept/Reject December 22, 2009 Camera ready version January 15, 2010 PROGRAM COMMITTEE Peter Achten University of Nijmegen, The Netherlands Jost Berthold Philipps-Universität Marburg, Germany Andrew Butterfield University of Dublin, Ireland Robby Findler Northwestern University, USA Kathleen Fisher AT&T Research, USA Cormac Flanagan University of California at Santa Cruz, USA Matthew Flatt University of Utah, USA Matthew Fluet Toyota Technological Institute at Chicago, USA Daniel Friedman Indiana University, USA Andy Gill University of Kansas, USA Clemens Grelck University of Amsterdam/Hertfordshire, The Netherlands/UK Jurriaan Hage Utrecht University, The Netherlands Ralf Hinze Oxford University, UK Paul Hudak Yale University, USA John Hughes Chalmers University of Technology, Sweden Patricia Johann University of Strathclyde, UK Yukiyoshi Kameyama University of Tsukuba, Japan Marco T. Morazán (Chair) Seton Hall University, USA Rex Page University of Oklahoma, USA Fernando Rubio Universidad Complutense de Madrid, Spain Sven-Bodo Scholz University of Hertfordshire, UK Manuel Serrano INRIA Sophia-Antipolis, France Chung-chieh Shan Rutgers University, USA David Walker Princeton University, USA Viktória Zsók Eötvös Loránd University, Hungary PETER LANDIN PRIZE The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 euros.

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Comments from OCaml Hacker Brian Hurt
by Andrzej Jaworski 16 Jan '09

16 Jan '09

If you called your girlfriend "Kitten" and on a mountain trip she broke her leg would cry/phone for help to treat your Kitten or would you use a stupid high-brow term "woman"? Or perhaps you would rename "Washington Square" for " My Kitten Square" ? There are hundreds programming languages designed by hackers, that is why I have chosen Haskell...

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ANN: split-0.1.1 (doc bugfix; new functions wordsBy and linesBy)
by Brent Yorgey 15 Jan '09

15 Jan '09

Version 0.1.1 of the split library is now on Hackage, which provides a wide range of strategies and a unified combinator framework for splitting lists with respect to some sort of delimiter. This version: * fixes a couple Haddock bugs that were preventing the documentation from building on Hackage, and * adds two new convenience functions suggested by Neil Mitchell, wordsBy and linesBy, such that words === wordsBy isSpace lines === linesBy (=='\n'). Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/split darcs: http://code.haskell.org/~byorgey/code/split -Brent

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ANN: hs-dotnet, version 0.3.0
by Sigbjorn Finne 15 Jan '09

15 Jan '09

Hi, the first public release of hs-dotnet is now available - a pragmatic take on interoperating between Haskell (via GHC) and .NET. For downloads and (some) info, see: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hs-dotnet http://haskell.forkIO.com/dotnet Feedback most welcome, both on what's there and what you would like to see better handled/supported next. enjoy --sigbjorn

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Re: Issues with posix-realtime package
by Galchin, Vasili 15 Jan '09

15 Jan '09

Hi Manlio, I am the author of this package. Let me think about what you have said. Regards, Vasili

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Re: [Haskell-cafe] Looking for Haskellers on Windows
by Günther Schmidt 15 Jan '09

15 Jan '09

Hi Kyra, and again, thanks for taking the effort. Kyra I've tried any sort of values to any sort of columns. I tried "insert into somesinglecolumntable (someNumbercolumn) values (?)" [toSql 5] ... and so on. So I'm not certain at all the error message does actually give the right clue. It just blows no matter what. What does work though is this: run dbc "insert into sometable (someVarcharcolumn) values ('some string value')" [] This would be a workaround for my app, but I hope I can do better. Günther Am 10.01.2009, 17:01 Uhr, schrieb kyra <kyrab(a)mail.ru>: > Günther Schmidt wrote: >> Hi Kyra, >> thanks for your reply. >> The problem I was facing is using an MS-Access backend with HDBC-ODBC. >> I try to export data to MS-Access and I can't get it to work. Otherwise >> HDBC works fine in other parts of my app but with MS-Access it blows up. >> I did of course post this on the haskell-cafe list first, but no >> response and the HDBC package maintainer himself was unable to help too >> since he's not using Windows. > > I've looked at Your problem now. What is the type of that single field > in 'mytable'? Google shows numerous similar cases, e.g. for date or memo > fields, that Access don't accept varchars and require longvarchars for. > > I'm in no way an Access expert. > > Cheers, > Kyra -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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Stack Overflow, tail recursion and CPS
by Günther Schmidt 15 Jan '09

15 Jan '09

Hi all, I get a stack overflow when I want to insert a huge, lazy list into a Map. I have changed the insertion algo to use foldl to make it tail-recursive but still get a stack overflow as the "insert" remains lazy. Could CPS be a solution in these cases? Günther

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Re: [Haskell-cafe] Stack Overflow, tail recursion and CPS
by Günther Schmidt 15 Jan '09

15 Jan '09

Hi Eugene, tried that, but since the action to be evaluated is the insertion into a structure that won't work. The strictness here doesn't go deep enough, it stopps short. Günther Am 14.01.2009, 18:27 Uhr, schrieb Eugene Kirpichov <ekirpichov(a)gmail.com>: > Use foldl' ? > > 2009/1/14 Günther Schmidt <redcom(a)fedoms.com>: >> Hi all, >> >> I get a stack overflow when I want to insert a huge, lazy list into a >> Map. >> >> I have changed the insertion algo to use foldl to make it >> tail-recursive but >> still get a stack overflow as the "insert" remains lazy. >> >> Could CPS be a solution in these cases? >> >> Günther >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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HEADS UP: finalizer changes coming in GHC 6.10.2
by Simon Marlow 14 Jan '09

14 Jan '09

By popular demand, GHC 6.10.2 will support finalizers that are actually guaranteed to run, and run promptly. There aren't any API changes: this happens for finalizers created using newForeignPtr as normal. However, there's a catch. Previously it was possible to call back into Haskell from a finalizer (finalizers are C functions), by using foreign import "wrapper" or foreign export. According to the FFI spec, whether this is allowed or not is "system dependent". In GHC 6.10.1 and earlier it was allowed, but in 6.10.2 and later it is not. The reason being that C finalizers are now executed directly by the GC, when the runtime is not in a position to handle callbacks. You can still have fully-fledged Haskell finalizers if you want: we have Foreign.Concurrent.newForeignPtr for that purpose. But those finalizers are not subject to the same promptness guarantees that you get with C finalizers - they run in a separate thread, and are not guaranteed to be run at program exit. We'll make sure this is documented prominently in the 6.10.2 release notes, but I thought a heads-up would be a good idea right now as it turns out that there are existing libraries (e.g. LLVM) that will be affected. Cheers, Simon

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Recursive modules, GHC /= Report?
by Mauricio 14 Jan '09

14 Jan '09

Hi, Here: http://www.haskell.org/onlinereport/modules.html I read: "Modules may reference other modules via explicit import declarations, each giving the name of a module to be imported and specifying its entities to be imported. Modules may be mutually recursive." However, I get this from GHC: --- Module imports form a cycle for modules: main:Main imports: Control.Concurrent.MVar GtkMostrarConfig GtkLerECG Graphics.UI.Gtk main:GtkMostrarConfig imports: Control.Concurrent.MVar Graphics.UI.Gtk Main --- Aren't cycles of modules just recursive modules? Why wasn't that allowed? Thanks, Maurício

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The problem with Monads...
by Rafael Gustavo da Cunha Pereira Pinto 14 Jan '09

14 Jan '09

Last night I was thinking on what makes monads so hard to take, and came to a conclusion: the lack of a guided tour on the implemented monads. Let's take the Writer monad documentation: all it says is: Inspired by the paper "Functional Programming with Overloading and Higher-Order Polymorphism", Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995. SO WHAT? The best approach is the Part II of the "All About Monads" tutorial. There you have the almost ideal approach, except that the examples are just thrown there, with no step-by-step explanation. Of course one could copy, paste and run it, but this gives pretty much a "is it right?' feeling. Questions like "if a Reader is an application, why don't use a regular function instead?" or "what bind means for a State monad?". I will try to work on a "Part II" extended version on my vacations... maybe a WikiMonad... or MonadPedia... :-) After all, it is my duty as a haskell noob to write another monad tutorial! :D Cheers! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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endian-ness ....
by Galchin, Vasili 14 Jan '09

14 Jan '09

Hello, I am used to network-neutral endianness (TCP/IP) As far as "persistent" store, endian neutralness relies on a convention in a particular marshalling/serializing situation??? Sorry ... probably dumb question .. and I think I know the answer ... but "what the hey". I like to get a consensus answer. Regards, Vasili

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posting newspaper article?
by Galchin, Vasili 14 Jan '09

14 Jan '09

Hello, I don't want to risk the ire of the Haskell Cafe community. Is there a moderator? I want to post an editorial by the Big Blue CEO which seems to me to be very interesting vis-a-vis the FPL community and Haskell in general. I have an online subscription and can post to stimulate discussion. ???? Vasili

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ANN: json-0.4.1
by Sigbjorn Finne 14 Jan '09

14 Jan '09

Hi, a new release of the 'json' package is now available via hackage, version 0.4.1 http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json [no claims that it represents rocket science, but a number of downstream codebases depend on this package for their operation, hence the announcement here.] New in this release is a generic JSON encoder contributed by Lennart Augustsson (Text.JSON.Generic ; thanks Lennart!) along with a number of other, smaller changes (including changes to the default encoding for constructors and prelude types - no longer down-cased; see release notes for more.) Enjoy --sigbjorn

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Real World Haskell: confusion
by Peter Verswyvelen 14 Jan '09

14 Jan '09

On page 102: "partial function application is named currying" I thought "currying" or "to curry" means converting f :: (a,b) ->c into g :: a -> b -> c by applying "curry" (mmm, are Asian people good at Haskell? :-) g = curry f

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Adding Authentication and Authorization to a service implemented in Haskell
by Daryoush Mehrtash 14 Jan '09

14 Jan '09

I am trying to figure out a clean way to add authentication and authorization in a webservice. The services of the web services are implemented as Haskell functions. The request to the service contains the user authentication information. I want to authenticate the user by verifying his authentication informaton, assign him roles, and then based on the roles decide on the functions and data that he can have access too. Is there a clean way to do this in Haskell? Thanks Daryoush

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Request for help testing HDBC-postgresql
by John Goerzen 14 Jan '09

14 Jan '09

Hi folks, I've pushed to the Git repo a bunch of new code for HDBC-postgresql. Specifically, it: 1) Removes autoconf in favor of Duncan's Setup.lhs that should work on all combinations of GHC 6.8, GHC 6.10, POSIX, and Windows 2) Adds support for UTF-8 encoding of strings 3) Adds support for translation of dates/times into a PostgreSQL format I would appreciate people testing it on GHC 6.8, 6.10, and Windows before I upload it to hackage. Please reply with thumbs up/thumbs down. Get it with: git clone git://git.complete.org/hdbc-postgresql Thanks! - John

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ByteString intercalate semantics??
by Galchin, Vasili 14 Jan '09

14 Jan '09

Hello, From Hoogle (my friend) .... *intercalate* :: ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>-> [ ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>] -> ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…> Source<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/src…> *O(n)* The intercalate<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>function takes a ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>and a list of ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>s and concatenates the list after interspersing the first argument between each element of the list. So intercalate doesn't do a simple concatenation? Vasili

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unfoldr [ANN: HLint 1.2]
by Andrew Coppin 14 Jan '09

14 Jan '09

Neil Mitchell wrote: > Hi Andrew, > > >>> HLint will automatically detect if you should have used a map, a foldr >>> or a foldl and suggest how to change your code. In the GHC, darcs and >>> Hoogle code bases there are no obvious map-like functions, which is a >>> good sign :-) >>> >>> >> ...What an intriguing idea. Clearly I'm going to have to play with this >> sometime soon. >> >> Does it suggest unfoldr too? >> > > No, but it could do in the future - I never use unfold's in my code, > so am not really familiar with them. But I will look into them, do you > have any examples where it should suggest unfoldr so I can play with > them? > Off the top of my head, try this: convert b 0 = [] convert b n = n `mod` b : convert b (n `div` b) (Takes a number and yields the radix-B representation of it. Backwards.) convert b = unfoldr (\n -> if n > 0 then Just (n `mod` b, n `div` b) else Nothing) It's also useful for converting a data structure into a list: heap_to_list = unfoldr (\h -> if heap_empty h then Nothing else Just (heap_top h, heap_delete_top h)) Those are the major uses I'm aware of. I'm sure somebody else will think of others. (IIRC, unstream is implemented as an unfold...?)

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Re: [Haskell] ANN: ghci-haskeline 0.1
by Andrew Hunter 14 Jan '09

14 Jan '09

On Mon, Jan 12, 2009 at 12:57:57PM -0800, Judah Jacobson wrote: > I'm pleased to announce the first release of ghci-haskeline. This > package uses the GHC API to reimplement ghci with the Haskeline > library as a backend. Haskeline is a library for line input in > command-line programs, similar to readline or editline, which is > written in Haskell and thus (hopefully) more easily integrated into > other Haskell programs. > Perhaps this has already been discussed at length, in which case I apologize but, well, why provide line input editing at all? A number of languages/programs (off the top of my head: sml, most Schemes) don't; the standard method to get line editing is rlwrap. And this works (in my limited experience) quite well. The disadvantage as I see it of using editline or Haskeline or whatever is that it's going to be sutbly different than other methods; presumably, people won't like the changes in behavior. It seems to me that from a UNIX-y separation of concern view, the right thing to do (as many languages have chosen) is to /not/ provide line editing, and just let the user do that with any number of convenient tools that focus on getting/that/ right (like rlwrap.) Is there a reason we've not taken that approach? Thanks, AHH

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errno handling in concurrent haskell
by Manlio Perillo 13 Jan '09

13 Jan '09

Hi. I have some doubts about errno handling in a Concurrent Haskell program. Let's suppose that GHC non threaded runtime is used, so that each Haskell thread is bound to an OS thread. Let's suppose there are two threads running (`A` and `B`). Thread `A` calls a function `f`, that, in turn, calls via FFI a C function `c_f`. Function `c_f` fails, settings errno; however the GHC scheduler suspends execution of thread `A` and switch to thread `B`, before the current value of errno is read. Now, let's suppose thread `B` calls a function `g`, that, in turn, calls via FFI a C function `c_g`. Function `c_g`, too, fails, setting errno. Is this possible? P.S.: I have found this is C.Foreign.Error.hs, in base package (not the latest version): throwErrnoIfRetry :: (a -> Bool) -> String -> IO a -> IO a throwErrnoIfRetry pred loc f = do res <- f if pred res then do err <- getErrno if err == eINTR then throwErrnoIfRetry pred loc f else throwErrno loc else return res This function calls getErrno two times. Is this safe? Why the throwErrno function does not accept errno as parameter? Thanks Manlio Perillo

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[Haskell-cafe] type error using ghc 6.10.1, not in previous versions
by Tim Bauer 13 Jan '09

13 Jan '09

Hi all. Under I have some old code that broke under ghc 6.10.1. Under (6.6.1), 6.8.1 (and I think 6.8.2), this compiles. import Prelude hiding(catch) import Control.Concurrent import Control.Exception(catch,throw,evaluate) async :: IO a -> IO (MVar a) async ioa = do mVar <- newEmptyMVar forkIO $ catch (ioa >>= putMVar mVar) (putMVar mVar . throw) return mVar Under 6.10. I now get a type error. TypeError.hs:13:55: Ambiguous type variable `e' in the constraint: `GHC.Exception.Exception e' arising from a use of `throw' at TypeError.hs:13:55-59 Probable fix: add a type signature that fixes these type variable(s) Prelude> :t catch catch :: IO a -> (IOError -> IO a) -> IO a Prelude> :t Control.Exception.catch Control.Exception.catch :: (GHC.Exception.Exception e) => IO a -> (e -> IO a) -> IO a What changed that causes this to break between 6.8 and 6.10? In fact, I don't even see the type error (the message is of little help). The function throw returns an `a', so it should unify with the required type for the handler. I tried simplifying the problematic line to: forkIO $ catch (ioa >>= putMVar mVar) (error "boom") (error "boom") should also unify with anything. Can anyone suggest other things I can try and perhaps what is going on? Thanks, - Tim

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ANN: bytestring-trie 0.1.4
by wren ng thornton 13 Jan '09

13 Jan '09

-------------------------------------------- -- bytestring-trie 0.1.4 -------------------------------------------- Another release for efficient finite maps from (byte)strings to values. Equal parts bugfix release and real release. This upgrade is suggested for all users and is what 0.1.0 should have been. -------------------------------------------- -- Changes (since 0.1.2) -------------------------------------------- * Fixed a number of obscure bugs when "" is used as a key. * Added keys and toListBy (which generalizes toList, keys,...). Also did some optimization of toListBy so it shouldn't be as egregiously inefficient as it was before. * Added fromListL, fromListR, and fromListS to Data.Trie.Convenience along with notes about when each is more efficient than the others. * Separated Data.Trie (the main module for users) from Data.Trie.Internal (gritty details, and core implementation). Data.Trie re-exports most of Data.Trie.Internal so this is primarily a cosmetic change. The showTrie and lookupBy_ functions are no longer exported from Data.Trie however. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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Functions with generic return types
by Stephen Hicks 13 Jan '09

13 Jan '09

Hi, I'm trying to write a small module for conveniently writing functions that can return any of a finite number of types. That is, I'd like to be able to write something like foo :: StringOrInt t => String -> IO t This is pretty easy to do if I hard-code the classes as above, but I run into difficulties making it more general. In particular, it's convenient to use incoherent instances so that I can insert the polymorphism at whatever level I choose (see the third instance declaration below, as well as the function foo). But when I try to make this work for monads, it fails - that is, my function bar (below) always prints both "str" and "int" because I can't make it lazy enough - in order to get even a thunk in the non-monadic type, it seems like the side effect has to happen (unsafeInterleaveIO + seq would sort of fix this here, but not generally), whereas the type alone should be sufficient in selecting which monad needs to be evaluated. I'm having a hard time explaining this abstractly, but I think the following concrete code explains pretty clearly what I'm trying to do, and I would appreciate any comments or suggestions for either a better way to go about this, or something already written that achieves something similar? Thanks, steve ==== {-# LANGUAGE TypeSynonymInstances, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, IncoherentInstances #-} class Pick c a where pick :: c -> a instance Pick (a,b) a where pick = fst instance Pick (a,b) b where pick = snd instance Pick (a,b) c => Pick (d -> a, d -> b) (d -> c) where pick (a,b) d = pick (a d,b d) -- This instance definition is broken... instance (Monad m,Pick (a,b) c) => Pick (m a,m b) (m c) where pick (ma,mb) = do { a <- ma; b <- mb; return $ pick (a,b) } foo :: Pick (String,Int) t => String -> t foo = pick (id :: String -> String, length :: String -> Int) toStr :: String -> IO String toStr s = putStrLn "str" >> return s toInt :: String -> IO Int toInt s = putStrLn "int" >> return (length s) bar :: Pick (String,Int) t => String -> IO t bar = pick (toStr,toInt)

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Re: [Haskell-cafe] Looking for Haskellers on Windows
by Günther Schmidt 13 Jan '09

13 Jan '09

Hi Bulat, I do :), but I was amazed that there was no response to a post with, what I thought, would be a rather common problem for an application developer. That post was about writing to an MS-Access database via HDBC-ODBC, which fails. When I then asked the HDBC maintainer himself it turned out that he doesn't use Windows either and thus can't help. Thus my cry for help explicitly to Haskellers on Windows. Günther Am 10.01.2009, 15:18 Uhr, schrieb Bulat Ziganshin <bulat.ziganshin(a)gmail.com>: > Hello Günther, > > Saturday, January 10, 2009, 5:03:18 PM, you wrote: > > there are lot of windows haskellers so you can just write your questions > in > haskell-cafe > >> Hi all, > >> I'm looking for Haskell programmer that use Windows as their >> OS-Plattform. >> I developing an application in Haskell on Windows and run into problems >> that seem to have a lower priority with the greater Haskell community as >> most of them are using Linux where the problems do not occur or are >> already solved. > >> So it'd be nice to get in touch with other Haskellers that face the same >> problems. > >> Günther > >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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System.CPUTime and picoseconds
by Manlio Perillo 13 Jan '09

13 Jan '09

Hi. Just out of curiosity, but why Haskell 98 System.CPUTime library module uses picoseconds instead of, say, nanoseconds? At least on POSIX systems, picoseconds precision is *never* specified. Thanks Manlio Perillo

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Gtk2Hs on Windows for GHC 6.10.1
by Peter Verswyvelen 13 Jan '09

13 Jan '09

Gtk2Hs 0.9.13 has an annoying bug on Windows that makes it impossible to run via GHCi. So to see if the latest development version works better, I tried to build it on Windows using GHC 6.10.1, but I failed to do so with both MSYS and Cygwin. Does anybody know how to build it on Windows? Thanks, Peter

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Data analysis with Haskell
by Daniel Kraft 13 Jan '09

13 Jan '09

Hi all, I'm going to start a project where I'll have to do some data analysis (statistics about product orders) based on database entries; it will mostly be some very basic stuff like grouping by certain rules and finding averages as well as summing up and such. It will however be more than what can be done directly in the database using SQL, so there will be some processing in my program. I'm thinking about trying to do this in Haskell (because I like this language a lot); however, it is surely not my most proficient language and I tried to do some number crunching (real one that time) before in Haskell where I had to deal with some 4 million integer lists, and this failed; the program took a lot more memory than would have been necessary and ran for several minutes (kept swapping all the time, too). A rewrite in Fortran did give the result in 6s and didn't run out of space. This was probably my fault at that time, because I surely did something completely wrong for the Haskell style. However, I fear I could run into problems like that in the new project, too. So I want to ask for your opinions, do you think Haskell is the right language to do data analysis of this kind? And do you think it is hard for still beginner Haskell programmer to get this right so Haskell does not use up a lot of memory for thunks or list-overhead or things like that? And finally, are there database bindings for Haskell I could use for the queries? Thanks a lot! Daniel

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A pattern type signature cannot bind scoped type variables `t'
by rodrigo.bonifacio 13 Jan '09

13 Jan '09

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Fw: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses
by Rob Hoelz 13 Jan '09

13 Jan '09

Forwarding to Haskell Cafe per Eric's suggestion. Begin forwarded message: Date: Sun, 11 Jan 2009 23:01:31 -0600 From: Rob Hoelz <rob(a)hoelzro.net> To: darcs-devel(a)darcs.net Subject: [darcs-devel] "Inferred type is less polymorphic than expected" and type witnesses Hello again, Darcs users and developers, As I mentioned in my last e-mail, I'm working on http://bugs.darcs.net/issue291. It's actually gone pretty well, and I feel I'm just about finished (I've done all but sorting out the changes after leaving the editor), only I've encountered the compiler error you see in the subject of this message. This error only appears when compiling with witnesses. Here's the source for the function that it's complaining about: compare_changes_with_old (x :>: xs) (y :>: ys) = case compare_changes_with_old xs ys of nx :> ny -> if unsafeCompare x y then ((x :>: nx) :> ny) else (NilFL :> (y :>: ys)) compare_changes_with_old NilFL NilFL = (NilFL :> NilFL) compare_changes_with_old NilFL ys@(_ :>: _) = (NilFL :> ys) compare_changes_with_old x@(_ :>: _) NilFL = (NilFL :> NilFL) Now, I have two questions: 1) What exactly does this error mean, and how do I get around it? 2) What are witness types, and what are they used for? I will gladly accept links to fine manuals as answers to either question, but simple explanations would be nice. =D I thought I understood Haskell pretty well, but existentially qualified types have thrown me for a loop. Thanks much, Rob Hoelz _______________________________________________ darcs-devel mailing list (AUTOMATIC POSTINGS ONLY PLEASE!) darcs-devel(a)darcs.net http://lists.osuosl.org/mailman/listinfo/darcs-devel

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haskell-platform mailing list
by Duncan Coutts 13 Jan '09

13 Jan '09

Hi all, If you are interested in helping out with the haskell platform project then we invite you to subscribe to the haskell-platform mailing list. http://projects.haskell.org/cgi-bin/mailman/listinfo/haskell-platform This mailing list is for discussing practical stuff. We expect to discuss policy questions on the libraries mailing list. Issues like which packages to pick, what package QA standards to use etc would be discussed on the libraries mailing list. Those issues are for the community as a whole. Building and coordinating the actual releases will be done via the haskell-platform mailing list. Duncan

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Re: [Haskell] HaskellWiki Upgrade Aborted
by Duncan Coutts 13 Jan '09

13 Jan '09

On Sat, 2009-01-10 at 19:02 -0800, Ashley Yakeley wrote: > I have given up attempting to upgrade HaskellWiki, and resumed the > site's previous normal operation. > > I wrote: > > I will also need up upgrade both PHP and MySQL. > > It turns out it would be hard to upgrade MySQL, as various things depend > on the old version: For example the gtk2hs website is using mysql and php too. I think we'd need to upgrade that to have it work with mysql-5. Perhaps gtk2hs should move to projects.haskell.org. > $ sudo rpm -U MySQL-server-community-5.1.30-0.rhel3.i386.rpm > warning: MySQL-server-community-5.1.30-0.rhel3.i386.rpm: V3 DSA > signature: NOKEY, key ID 5072e1f5 > error: Failed dependencies: > libmysqlclient.so.10 is needed by (installed) mod_auth_mysql-20030510-2.ent > libmysqlclient.so.10 is needed by (installed) perl-DBD-MySQL-2.1021-4.EL3 > libmysqlclient.so.10 is needed by (installed) php-mysql-4.3.2-48.ent > > I don't know what mod_auth_mysql and perl-DBD-MySQL are being used for, > so I don't believe I can upgrade them safely. > > The machine is running RHEL AS 3 update 9 with Linux 2.4.21. We really need to upgrade the whole thing. Not an easy job given the range of stuff being run on there by lots of different people. Duncan

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Generalized partial application (again)
by Peter Verswyvelen 13 Jan '09

13 Jan '09

Please correct me if I'm wrong in any of the reasoning below. Haskell provides the ability the perform partial application on the rightmost arguments. I learned from some pretty smart guys that partial application cannot be emulated with lambas, because they behave differently operationally, e.g. e = trace "e" op = (+) a1 = let f = (op (e 1)) in (f 10, f 100) a2 = let f = (\x -> op (e 1) x) in (f 10, f 100) a1 and a2 are operationally not the same: a1 will evaluate (e 1) once, a2 will evaluate it twice. However, Haskell also allows to partially apply the second argument of binary functions (using sections) b1 = let f = (`op` (e 1)) in (f 10, f 100) b2 = let f = (\x -> (e 1) `op` x) in (f 10, f 100) Again, b1 evaluates (e 1) once, b2 twice. b1 is actually the same as c1 = let f = ((flip op) (e 1)) in (f 10, f 100) So for ternary functions we could write more permutations of flip: flip3_12 f a1 a2 a3 = f a2 a1 a3 flip3_13 f a1 a2 a3 = f a3 a2 a1 flip3_23 f a1 a2 a3 = f a1 a3 a2 And use these to partially apply any argument e1 x = trace "e1" x e2 x = trace "e2" x op2 :: Int -> Int -> Int -> Int op2 x y z = x+10*y+100*z z1 = let f = (flip3_12 op2) (e1 1) (e2 2) in (f 3, f 4) z2 = let f = (flip3_13 op2) (e1 1) (e2 2) in (f 3, f 4) z3 = let f = (flip3_23 op2) (e1 1) (e2 2) in (f 3, f 4) This could be extended to N-ary functions. Haskell doesn't give any special syntactic support for this, but it surely is possible. Am I missing something here? Is there any easier way to perform "generalized partial application"?

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Question about touchForeignPtr
by Patrick Perry 13 Jan '09

13 Jan '09

I have the following code: IOVector n e = IOVector !ConjEnum !Int (ForeignPtr e)! (Ptr e)! Int! newtype Vector n e = IOVector n e unsafeAtVector :: Vector n e -> Int -> e unsafeAtVector (Vector (IOVector c _ f p inc)) i = let g = if c == Conj then conjugate else id in inlinePerformIO $ do e <- peekElemOff p (i*inc) io <- touchForeignPtr f let e' = g e e' `seq` io `seq` return e' {-# INLINE unsafeAtVector #-} The Ptr, 'p' is derived from the ForeignPtr, 'f'. For some offset, 'o', it is defined as: p = unsafeForeignPtrToPtr f `advancePtr` o The "touchForeignPtr" is there to keep the garbage collector from deallocating the memory before we have a chance to read 'e'. My question is the following: Is the `seq` on `io` necessary (from a safety standpoint)? Or am I just being paranoid? Thanks in advance for any help, Patrick

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data, util, and lang packages
by rodrigo.bonifacio 13 Jan '09

13 Jan '09

2 1

version conflict on Hackage
by Henning Thielemann 12 Jan '09

12 Jan '09

I repeatedly encounter the following versioning problem on Hackage: There is a package A with version 1.0. I upload a package B which imports A. Thus B is bound to A-1.0 Now a new version of A is uploaded, say 1.0.1. then I upload package C which depends both on A and B. However C is bound to the new A-1.0.1, which gives conflicts with the interface of B. All packages are sane. Both B and C work with either A-1.0 or A-1.0.1, but they must use the same version. Currently I'm forced to upload many packages with different versions, although nothing really changed. Maybe the most easiest fix to this problem is to let maintainers of a package allow to trigger re-configuration and re-compilation of a package. I would also like to flag package versions that people can safely ignore, because there was a flaw in package dependencies. On the other hand there are packages that are useful, but can no longer be compiled on Hackage, e.g. compatibility packages for older versions of GHC.

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Error building Sdf2Haskell
by rodrigo.bonifacio 12 Jan '09

12 Jan '09

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Computer time, independent of date
by Mauricio 12 Jan '09

12 Jan '09

Hi, I'm writing a program where I need to know elapsed times between some events, but I can't depende on Data.Time since ntpd or user can change that while my program is running. Is there an alternative? Something like how much time has passed since the program has started would be great. Thanks, Maurício

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[ANNOUNCE] First release candidate of darcs 2.2.
by Eric Kow 12 Jan '09

12 Jan '09

Dear Haskellers, I would like to forward this announcement on behalf of Petr Ročkai (the current darcs Release Manager). Darcs 2.2 is scheduled for release in 2009-01-15, only three days from now! This is the first of our biannual time-based releases. We hope you'll like it, and to help us ensure that you will like it, please help us test our the release candidate. Thanks! Eric Petr's words follow: ----------------8<------------------------------------------------------------- I am pleased to announce that darcs 2.2 is coming along nicely. I would like to ask everyone to give a ride to darcs 2.2, release candidate 1. This release again comes in two flavours: 1) The source tarball, http://repos.mornfall.net/darcs/darcs-2.2.0rc1.tar.gz, which can be built using the traditional autoconf-based buildsystem. This is the fully supported version. After downloading and unpacking, you can issue: $ ./configure $ make $ ./darcs --version or # make install More detailed instructions inside the tarball (file README). 2) Cabalised source. You can either download a tarball from http://repos.mornfall.net/darcs/darcs-2.1.99.0.tar.gz and build manually (see the build instructions in README inside the tarball), or, alternatively, you can use cabal-install to obtain a copy (the release candidate is now available on hackage): $ cabal update $ cabal install darcs This should give you a darcs binary in ~/.cabal/bin -- you should probably add that to your PATH. This is a preliminary changelog since version 2.1.2 (released last November): * In interactive record, it is now possible to get a list of currently selected hunks (command 'l'). (Christian Kellermann) * It is now possible to specify --in-reply-to when using darcs send, to generate correct references in the mail header. (Pavel Shramov) * New repositories with --no-pristine-tree can no longer be created. This only has effect on legacy darcs-1 repositories. * Improvements in Windows support. (Salvatore Insalaco) * Performance improvements in `darcs repair` and robustness improvements in `darcs check`. (Petr Ročkai) * Extensive manual and online help improvements. (Trent W. Buck) * Support for GHC 6.10. * Overhaul of the make-based build system. (Trent W. Buck) * Cabal is now supported as a build method for darcs. (Duncan Coutts, Petr Ročkai, Gwern Branwen) * First stab at libdarcs -- when building through Cabal, all of darcs implementation is now exposed in modules. No API guarantees whatsoever. (Eric Kow) * Additions to Haddock documentation of the existing darcs modules for improved development experience. * Improvements in the testing infrastructure. (Christian Kellermann, Gwern Branwen) * Low-level optimisations in filesystem code. (Ganesh Sittampalam) * Numerous major and minor bug fixes, refactorings and cleanups by David Roundy, Eric Kow, Jason Dagit, Dmitry Kurochkin, Thorkil Naur, Salvatore Insalaco, Christian Kellerman, Florent Becker, Duncan Coutts, Reinier Lamers, Ganesh Sittampalam, Petr Ročkai. Preliminary list of issues that have been fixed in darcs since version 2.1.2: 1223 sporadic init.sh test failure (2.1.1rc2+472) 525 amend-record => darcs patches show duplicate additions 1247 make TAGS is broken 1273 renameFile: does not exist (No such file or directory) 1165 get should print last gotten tag 1249 2.1.2 (+ 342 patches) local drive detection on Windows error 1238 wish: darcs help setpref should list all prefs 1199 Backup files darcs added after external merge 1043 pull => mergeAfterConflicting failed in geteff (2.0.2+) 1117 Whatsnew should warn on non-recorded files 1101 darcs send --cc recipient not included in success message Thanks to Thorkil Naur for compiling this list. I would like to thank all contributors -- developers, testers, bystanders -- for helping darcs get along further. It's been hard times recently for darcs, as many of you probably know. Nevertheless, we are regaining confidence in future darcs development. No way are we going to leave darcs fall by the road. I am sure that this one time, I speak for everyone in our developer and user community. Yours, Petr. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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concurrent haskell: thread priorities
by Neal Alexander 12 Jan '09

12 Jan '09

From what i understand (correct me if I'm wrong): The threaded RT creates an OS thread for each CPU/core on the system and uses them to multiplex userland threads. These are context switched whenever they block/yield/gc and no priorities can be assigned. It seems like we could get some priority based scheduling (and still be slackers) if we allow marked green threads to be strictly associated with a specific OS thread (forkChildIO?). Its not a very good solution, but it seems easy to implement and would be nice for the few problems that really need priority based scheduling. How many of those problems involve having 1 thread running at high/realtime priority? Maybe most?

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how to link to external documentation with haddock ?
by minh thu 12 Jan '09

12 Jan '09

Hi, When generating haddock documention (I use runhaskell Setup.lhs haddock), haddock warns me : "could not find link destinations for:" and lists for instance GHC.Show.Show or Data.Either.Either. Is it possible to instruct Haddock to links against http://www.haskell.org/ghc/docs/latest/html/libraries/ for those unfound destinations ? Thanks, Thu

2 1

Understanding type synonym families
by Martijn van Steenbergen 12 Jan '09

12 Jan '09

Hello everybody, I think I'm finally beginning to understand how type synonym families [1] work, in the way that works best for me: running into a problem to which type synonyms offer a solution. I wrote down my train of thoughts [2] and I was hoping you could give me some feedback: are there any mistakes in my conclusions? Did I miss any obvious corollaries? Are the points I make valid? Also, I'm hoping it will be valuable to read for those wanting to understand type synonym families. Thank you in advance, Martijn. [1] http://www.haskell.org/haskellwiki/GHC/Type_families#Detailed_definition_of… [2] http://martijn.van.steenbergen.nl/journal/type-synonym-families/

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git mirrors for GHC boot libraries
by Manlio Perillo 12 Jan '09

12 Jan '09

Hi. Where can I find git mirrors for GHC boot libraries? From http://hackage.haskell.org/trac/ghc/wiki/Design/VersionControlSystem it is not clear if the mirrors are already available. Thanks Manlio Perillo

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Space leak with Data.Binary and decodeFile
by Maxime Henrion 12 Jan '09

12 Jan '09

Hello all, I've been observing a huge space leak with some code using Data.Binary that I cannot make sense of and I hope someone here can shed some light on this, so I'll try to explain my problem as clearly as possible. I qualify the space leak as huge because if I let the program run, it will soon consume the whole memory available (~3G) and finally will get killed by the system. The code I'm writing implements a search algorithm using an inverted index. This index is built from a Trie [Int] (from the bytestring-trie package) and an Array Int ByteString. The trie maps each referenced word to an integer list that is a list of indices into the array. Here is the code for the Index datatype and the obvious Binary instance: data Index = Index { entries :: Array Int ByteString , invidx :: Trie [Int } instance Binary Index where put (Index dirs idx) = put dirs >> put idx get = liftM2 get get I have no problems creating and seralizing this data structure to a file. The huge leak appears instead when I'm reading this data structure from a file and try to do something with it. This is the smallest test case I came up with that can reproduce the problem : main = do idx <- decodeFile "list.idx"; mapM_ (B.putStrLn . snd) (assocs (entries idx)) The space leak also appears when I try to touch the trie instead of the array. I've been trying tons of combinations involving adding or removing strictness annotations and seq calls in various places with no luck. I have also been adding SCC annotations and tried to profile the code. This seemed to suggest the space leak happens in the get method of the Array instance of Binary : instance (Binary i, Ix i, Binary e) => Binary (Array i e) where [...] get = do bs <- get n <- get -- read the length xs <- replicateM n get -- now the elems. return (listArray bs xs) The output of the profiler tells me that all the space gets allocated from the "replicateM n get" expression. Now for the really weird part: if I load my code in GHCi and type "main", I can observe the space leak. However, if I copy paste the definition of main instead, the code runs fine! This is the only circumstance I've seen this code work instead of eating all the RAM... I have been using GHC 6.10.1, binary 0.4.4 and bytestring-trie 0.1.2. If there's anything else that I can do to understand what's going on, I would gladfully hear about it. Please also tell me if I should provide more information. Thanks, -- Maxime Henrion

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Declaring each instance of a typeclass to be also an instance of another typeclass
by Mikhail Glushenkov 12 Jan '09

12 Jan '09

Hi, Is it possible to write something like this: > {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} > > import Control.Monad (liftM) > > instance (Monad a) => Functor a where > fmap = liftM without having to use UndecidableInstances (and preferably, other type system extensions too)?

2 1

Propositional logic implementation
by Andrew Wagner 11 Jan '09

11 Jan '09

All, Is there some better way to do this? It seems like a lot of boilerplate. Thanks! http://hpaste.org/13807#a1

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ANN: Haskell BLAS bindings version 0.7
by Patrick Perry 11 Jan '09

11 Jan '09

Here's the haddock documentation; I'm not sure if Hackage honors {-# OPTIONS_HADDOCK hide #-} when it displays the exposed modules: http://quantile95.com/blas/ Patrick

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