October 2008 - Haskell-Cafe

is there a way to pretty print a module?
by Anatoly Yakovenko 02 Nov '08

02 Nov '08

is there a way to pretty print a module? like: module Main where import Language.Haskell.TH main = do print $ pprint Main

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Newbie on instance of Monad
by Mauricio 02 Nov '08

02 Nov '08

Hi, After a lot of thinking, I can't get what I am doing wrong in this code: ------ data ( RandomGen g ) => RandomMonad g a = RandomMonad (g -> a) instance Monad (RandomMonad g) where return = RandomMonad . const RandomMonad f1 >>= f2 = RandomMonad f3 where f3 a = f2f1 a (next a) RandomMonad f2f1 = f2 . f1 ------ I get this error message: Could not deduce (RandomGen g) from the context (Monad (RandomMonad g)) arising from a use of `RandomMonad' at src/encherDB.hs:10:11-21 Possible fix: add (RandomGen g) to the context of the type signature for `return' In the first argument of `(.)', namely `RandomMonad' In the expression: RandomMonad . const In the definition of `return': return = RandomMonad . const but I'm not smart enough to understand what it means. Thanks a lot, Maurício

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Memory efficiency questions for real-time graphics
by T Willingham 01 Nov '08

01 Nov '08

As my first Haskell exposure, I've been working through Real World Haskell. I am considering converting some of my C++ graphics libraries to Haskell. I've done a fair amount of googling on the subject, however I haven't quite been able to find clear answers to some of following issues. (1) Using an OpenGL vertex array (a contiguous chunk of memory which is handed to the graphics card) is important. I see the source code of Frag does this, so it looks like we're good. Check. (2) In-place modification of the vertex array is important. Every vertex changes on each frame update. And we always want more vertices. (3) Growing and shrinking the vertex array efficiently is important. Here the behavior of C++ std::vector happens to be ideal. When growing, extend the array in-place if possible (using reserved space if any), otherwise allocate a new chunk ("amortized constant time"). When shrinking, do nothing but record the smaller size; the unused memory chunk is reserved for possible future growth. (4) Doing little to no allocations each frame update is important. In the current C++ version, zero allocations occur during a normal frame update. When the vertex array changes, that is the only allocation which happens. To give a context for all of this, I am applying a non-linear transformation to an object on every frame. (Note: non-linear, so a matrix transform will not suffice.) The object is described by a certain function, and is generated from a 3D domain grid. The user can change the resolution of the grid on the fly, while the object is moving. (Hence the need for grow/shrink efficiency.) Given that (1) is out of the way, what's the best I expect from Haskell concerning (2)-(4)? Thanks, T. Willingham

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An irritating Parsec problem
by Andrew Coppin 01 Nov '08

01 Nov '08

I like Parsec. I use it for everything. But it does have one irritating problem. Consider the following parser: expressions = many1 expression Suppose this is the top-level parser for my language. Now suppose the user supplies an expression with a syntax error half way through it. What I *want* to happen is for an error to be raised. What *actually* happens is that Parsec just ignores all input after that point. So if "+" is not a valid token, but the user writes x = 1; y = 2; z = 3 + z; w = 4; then what my program receives back is "x = 1; y = 2; z = 3", as if everything parsed successfully. But actually it has ignored half the input! o_O Does anybody know how to fix this irratiting quirk? I can see why it happens, but not how to fix it.

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Why 'round' does not just round numbers ?
by L.Guo 31 Oct '08

31 Oct '08

Hi all: I just read about definitions of Prelude [1], and noticing that. In 6.4.6 Coercions and Component Extraction, it discribes like this: "round x returns the nearest integer to x, the even integer if x is equidistant between two integers." I think this is unresonable. then try it in GHC 6.8.3. Prelude> round 3.5 4 Prelude> round 2.5 2 Is there any explanation about that ? [1] The Haskell 98 Report: Predefined Types and Classes http://haskell.org/onlinereport/basic.html Regards -------------- L.Guo 2008-10-27

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darcs hacking sprint #1 (report)
by Eric Y. Kow 31 Oct '08

31 Oct '08

Dear Haskellers, I thought some of you might be interested in our report from the darcs hacking sprint, held on 25-26 October. See also the blog version of it at http://blog.darcs.net/2008/10/darcs-hacking-sprint-1-report.html (it has photos). So how did that darcs hacking sprint go? Well, we had a lot of fun and we got a lot of work done, so I think we can safely say that it was a great success! What next? The sprint participants found themselves with a lot of good work started that we simply couldn't put down, so we have continued hacking into the week, turning many of our good starts into more concrete progress. There is still a lot of work to do so we want to make sure we keep that momentum going! The big challenge right now is to get all of the patches reviewed and merged back into darcs.net. Unfortunately, this is creating a delay in the usual patch review process as we clear our backlog. In the meantime, please continue sending your patches! Thanks very much to David Roundy for his advice before the sprint and cheerful handling of this patch influx; Galois, the University of Brighton and Université de Paris 7 for graciously hosting our three venues; and especially to everybody who participated or joined #darcs to cheer us on :-) The next sprint will be held in conjunction with the Haskell Hackathon, March 2009 in Utrecht, Netherlands. So see you in six months! Overview -------- Total sprint patches : 234 Patches produced - during the sprint itself : 178 - as follow-up work : 56 Patches sent and - accepted into darcs.net : 43 - awaiting review : 191 ("during" defined as 25-27 October 2008 at 08:00 UTC) Site reports ------------ * Team Portland: - report: http://lists.osuosl.org/pipermail/darcs-users/2008-October/015082.html - pictures: http://blog.codersbase.com/2008/10/28/darcs-hacking-sprint-summary-from-por… * Team Brighton: - report: http://lists.osuosl.org/pipermail/darcs-users/2008-October/015122.html - pictures: http://koweycode.blogspot.com/2008/10/darcs-hacking-sprints-some-pictures.h… - pictures: http://koweycode.blogspot.com/2008/10/darcs-hacking-sprint-team-brighton-da… * Team Paris: - report: http://lists.osuosl.org/pipermail/darcs-users/2008-October/015215.html What we worked on ----------------- Warm up tasks ============= * Warn the user when the patch name looks like a command line option - http://bugs.darcs.net/issue395 - Participants: Reinier Lamers (Team Paris) - Status: Accepted! * Warn when tags are too short (length name <2) - http://bugs.darcs.net/issue1000 - Participants: Christian Kellerman - Status: Accepted! Optimisations ============= * Solved stack overflow when doing whatsnew on very large lines - http://bugs.darcs.net/issue1017 - Participants: Ganesh Sittampalam (Team Brighton) - Status: awaiting review * Smarter slurpies for faster directory lookups - http://bugs.darcs.net/issue711 - Participants: Ganesh Sittampalam (Team Brighton) - Status: awaiting review * Avoiding lstats in darcs whatsnew - http://bugs.darcs.net/issue390 - Participants: Reinier Lamers (Team Paris) - Status: needs tweaking * Bytestring optimisations - Participants: Don Stewart (Team Portland) - Status: awaiting review Performance-related features ============================ * Filecache to improve darcs annotate and darcs changes performance - Participants: Benedikt Schmidt - Status: still hacking! * Packs - http://wiki.darcs.net/index.html/Packs_Specification - Participants: Nicolas Pouillard, Florent Becker (Team Paris) - Status: plan is fleshed out; implementation started in branch * "Chunky" hunk representation - Participants: Ian Lynagh (Team Brighton) - Status: needs volunteer to start porting from camp to darcs * Global cache: enabled by default and more portable - http://bugs.darcs.net/issue839 - Participants: Simon Michael - Status: Accepted! * Networking performance (Team Paris) - Used tcpdump to examine network performance problems - New issues filed: * http://bugs.darcs.net/issue1167 - darcs-transfer mode versions * http://bugs.darcs.net/issue1168 - ssh pipelining * http://bugs.darcs.net/issue1170 - Nagle's algorithm - Participants: Team Paris - Status: needs implementation! Improved Windows support ======================== * Bugs fixed: - newline endings on Windows for HTTP module. - http://bugs.darcs.net/issue774 - fix character echo on win32. - http://bugs.darcs.net/issue1023 - bracket file writing to prevent windows permission errors. - http://bugs.darcs.net/issue784 - fix file handle leak and check for exceptions on process running. * Participants: Salvatore Insalaco * Status: Accepted! Infrastructure and cleanups =========================== * Code cleanups - Added language pragmas in all files - Removed OldFastPackedString - Replaced FastPackedStrings api in favor of Data.ByteString api - Participants: Don Stewart and Jason Dagit (Team Portland) - Status: awaiting review * Documentation cleanups - Use .lhs only for truly literate files - Migrate .tex to reST in preparation for user manual overhaul - Participants: Trent Buck - Status: awaiting review * Cabalisation - Participants: Duncan Coutts (Team Portland) * Makefile simplifications and cleanups (for doc) - Participants: Trent Buck * Makefile improvements (make ci, make darcs_p) - Participants: Simon Michael, Eric Kow (Team Brighton) - Status: make ci is accepted! make darcs_p awaiting review * Benchmarking suite - Participants: Eric Kow (Team Brighton) - Status: usable as of Wednesday, needs visualisation Camp ==== * http://projects.haskell.org/camp * Progress... - developed "chunky" hunk representation and tests! - got libcurl working under Windows with Cabal - primitive interactive patch selection implemented - Repository type rather than FilePath for repos * Participants: Ian Lynagh (Team Brighton) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Re: [darcs-users] Poll: Do you need to be able to build darcs from source on GHC 6.6?
by Juliusz Chroboczek 31 Oct '08

31 Oct '08

> Also, note that Lenny has 6.8, and it is scheduled to become stable Real > Soon Now. That's irrelevant. Lenny going stable will not cause my servers to automatically get upgraded. FWIW, the experimental server is scheduled to switch to lenny in the summer of 2009. There is no ETA for the production servers, which tend to be managed more conservatively still. Juliusz

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Cabal packaging problem - Build-Depends name for Network.Curl.Download?
by Brian Troutwine 30 Oct '08

30 Oct '08

Hello all, I'm having a small bit of trouble using cabal to package a program I'm calling aule. I cannot build the program using cabal but can compile it using ghc directly. Here's my aule.config: Name: Aule Version: 0.2.0 Cabal-Version: >= 1.2 Build-type: Simple Author: Brian L. Troutwine Maintainer: goofyheadedpunk(a)gmail.com Stability: alpha Executable aule Build-Depends: base, network, bytestring, download-curl Main-Is: Main.hs Hs-Source-Dirs: src ghc-options: -Wall -O2 If I decend into src/ the result is as follows. blt@doritos:~/aule/src $ ghc -Wall -O2 --make Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... If I attempt to use cabal the result is the following. blt@doritos:~/aule $ runhaskell Setup.hs configure --prefix=$HOME Configuring Aule-0.2.0... Setup.hs: At least the following dependencies are missing: download-curl -any I've attempted variations on the download-curl name, as well. Also, download-curl was install using the nifty cabal-install tool. I run Ubuntu 8.04 with ghc 6.8.2. Thanks, Brian

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Re: Problems with Windows
by Felix Martini 30 Oct '08

30 Oct '08

Hi John, You don't have to compile hsc2hs yourself. It is part of the standard GHC installation. Cabal should be able to find it. You can check if it does by running runghc Setup.hs configure -v when configuring a package. Compiling network-2.2.0.0 against parsec-3.0.0 with msys/mingw worked fine for me. So if Cabal cannot find hsc2hs you could reinstall GHC or otherwise try to compile network-2.2.0.0 with msys. Regards, Felix

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LAST CALL: Haskell Communities and Activities Report
by Janis Voigtlaender 30 Oct '08

30 Oct '08

Dear Haskellers, It is not yet too late to contribute to the 15th edition of the HC&A Report. In fact, I will wait for a few more days. If you haven't already, please write an entry for your new project or update your old entry. Please mail your entries to hcar(a)haskell.org in plain text or LaTeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2008-October/020651.html I look forward to receiving your contributions. Thanks a lot, Janis (current editor) -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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