- Haskell-Cafe - Haskell.org

Existential Data Types
by Richard G. 04 Jul '08

04 Jul '08

Hello I've been doing some stuff with existential data types and came across a curious pattern. If anyone has thoughts, comments, or feedback, it would be appreciated. --- I'm trying to port and extend the Be messaging scheme to Haskell for use in distributed, heterogenous environments. BeOS was object oriented and pervasively threaded. That, and my background as an OOP programmer, has resulted in a very OOP approach. Because OOP allows for subclassing and the Be API was designed for it, I've used a wrapper approach. class HandlerC h where . . . data Handler = forall h . HandlerC h => Handler h instance HandlerC Handler where . . . data DefaultHandler = DefaultHandler ... instance HandlerC DefaultHandler where . . . newDefaultHandler = Handler (DefaultHandler ...) newHandler h = Handler h The HandlerC defines the interface. When Handler implements the interface, it mostly passes the functions to the inner Handler that actually has the data and the real functions. The advantage is that I can wrap anything that implements the HandlerC interface and have it behave in a known and simple way from the outside. ---- One area where this might have some application is in unit testing to make mock objects easier to use. If I had a Looper ADT that depended on the Handler, I would want test the Looper without having to worry about the complexity of the Handler interfering with the tests. I could do this with the following: data MockHandler = MockHandler ... instance HandlerC MockHandler where . . . h = newHandler (MockHandler ...) where h is passed to the Looper some how. ---- Overall, this seems very similar to data D = D1 | D2 | D3 | ... The big difference is that all the Dn constructors don't need to be known at compile time, which is very useful when writing libraries. Also, the wrapped data can include functions and state data, making it very easy to create applets. This is handy when doing message passing or request handling, such as in a web server, especially when using STM. Richard

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Wouter Swierstra style extensible records!
by Ron Alford 04 Jul '08

04 Jul '08

Having made over-use of Wouter's expression idioms, I decided to hack it into extensible records. It's not documented, it's probably got holes in its usability, but I thought I'd post it so people can play with it (and give suggestions before I rewrite my code to use it!). I know there are other extensible record systems, but I wondered how this stacks up. Notably, it looks like in HList the records are only extensible in one direction (tacking fields onto the end), although they have a much more developed syntax (if you're into that sort of thing). Current problems: * Defining a new field is rather bulky - currently it takes lines. Definitions are bulky in Wouter's extensible expressions, too, but I could probably shorten the get/set convenience functions to something more readable. * Construction of new records is only convenient by using the defaulting system. Maybe that makes sense, maybe it doesn't. A side benefit: Record access functions are reusable between record types (as long as they have the same type). Anyway, thanks again, Wouter! -Ron

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Re: [Haskell-cafe] Access to Oracle database from Haskell
by Henning Thielemann 03 Jul '08

03 Jul '08

On Thu, 26 Jun 2008, Alistair Bayley wrote: >> Try this version of configOracle in Setup.hs: >> >> configOracle verbose buildtools = do >> if not (sqlplusProgram `isElem` buildtools) >> then return Nothing >> else do >> path <- getEnv "ORACLE_HOME" >> info verbose ("Using Oracle: " ++ path) >> makeConfig path libDir "oci/include" >> where libDir = if isWindows then "bin" else "lib" >> >> >> You'll also need to add this import: >> >> import System.Environment (getEnv) > > Did you get a chance to try this? I'm quite keen to fix the > non-Windows parts of the installation process, if I can. I have replaced configOracle by configOracle verbose buildtools = do if not (sqlplusProgram `isElem` buildtools) then return Nothing else do path <- getEnv "ORACLE_HOME" info verbose ("Using Oracle: " ++ path) let (libDir, includeDir) = if isWindows then ("bin", "oci/include") else ("lib", "/usr/include/oracle/10.2.0.4/client") makeConfig path libDir includeDir This is obviously a hack. We should get the Oracle include path from the user, maybe via another environment variable or a custom Cabal option. Now I can start GHCi with the example program you gave me. However I have to start with -lclntsh, otherwise symbol OCIEnvCreate cannot by found. I thought I do not need this option, because the installed Takusen package contains the library name: $ grep clntsh dist/installed-pkg-config extra-libraries: clntsh clntsh I wonder why it appears twice, because in dist/setup-config it exists only once: extraLibs = ["clntsh"] Nevertheless I can start GHCi with the example program and I can see 'hello world'! Great - many thanks for the hints that led me to that state! Next step is to fetch real data from the database. But now I encounter new problems. If I write a real 'select' statement I get at best: Unexpected null in row 1, column 1. *** Exception: (unknown) and at worst: 50001752 *** glibc detected *** /usr/lib/ghc-6.8.2/ghc-6.8.2: free(): invalid pointer: 0x08d10065 *** ======= Backtrace: ========= /lib/libc.so.6[0xb7dba4b6] /lib/libc.so.6(cfree+0x89)[0xb7dbc179] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(lfvclose+0x1c)[0xb5df9c1e] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(SlfMunmap+0x3f)[0xb5e70c3b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(ldiutzd+0x37)[0xb5e49e7b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(kputerm+0x51)[0xb54c96b9] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(OCITerminate+0x1c)[0xb5596888] [0xb6a33801] /usr/lib/ghc-6.8.2/ghc-6.8.2[0x8a600de] ======= Memory map: ======== ... then GHCi quits. > BTW, the location of your header files is still a puzzle. Oracle's docs: > http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14250/ociabdem.htm… > > state that the header files shoulld be in $ORACLE_HOME/rdbms/public. > But perhaps things are different for the Instant Client. I don't know. I have just installed the RPMs and I expect that others will do so as well.

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ANN: The Disciplined Disciple Compiler - alpha 1.1
by Ben Lippmeier 03 Jul '08

03 Jul '08

Hi All, I'm pleased to announce version 1.1 of the Disciplined Disciple Compiler (DDC) Disciple is an explicitly lazy dialect of Haskell which supports: - first class destructive update of arbitrary data. - computational effects without the need for state monads. - type directed field projections. - allied functional goodness. All this and more through the magic of effect typing. New in this version: - support for x86_64 under linux and darwin, thanks to Jared Putnam. - the -make flag now does a full dependency driven build/rebuild. - constructor classes. - irrefutable patterns. - partial support for monadic do notation. - an unboxed Bool# type with true# and false# literals. - field projection punning. - lots more example code. Project page with full release notes and download at: http://www.haskell.org/haskellwiki/DDC DDC: more than lambdas. Ben.

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parallel_map_reduce, in easy one liner mod
by jinjing 03 Jul '08

03 Jul '08

Hi haskellers, So.. the type says it all p_map_reduce :: ([a] -> b) -> (b -> b -> b) -> [a] -> b so the idea is to write your computation extensive funciton as a map_reduce function, simplest example: sum ( can be seen as map id then reduce (+) ) then instead of calling: sum xs just call: p_map_reduce sum (+) xs the function in the background will split the xs into 16 parts, and use all your cores to process the computation. Enless the computation from the map_reduce function is too trivial or the list is too small, it should bump all your cores to 100% usage, and linearly increase overall performance. You can customize the number of parts by calling the helper function p_map_reduce_to I hope this can be useful to someone besides me :) here is the code warning: i'm using a very annoying coding style, by redefining the (.) operater to be reverse application. Please forgive me and metally transform the order, or just pretend you are reading Java / Python / Ruby or whatever :) module Main where import Data.List import Control.Parallel import Prelude hiding ((.)) -- for my poor oo mind (.) :: a -> (a -> b) -> b a . f = f a infixl 9 . (...) :: (b -> c) -> (a -> b) -> a -> c (...) f g x = f (g x) -- helpers reduce = foldl1 in_group_of n [] = [] in_group_of n xs = xs.take(n) : xs.drop(n).in_group_of(n) split_to n xs = xs.in_group_of(size) where size = if xs.length < n then n else xs.length `div` n -- parallel processing p_eval' xs = xs.pseq(xs.reduce(par)) p_reduce' op xs = xs.p_eval'.reduce(op) p_map_reduce_to n m r xs = xs.split_to(n).map(m).p_reduce'(r) p_map_reduce m r xs = p_map_reduce_to 16 m r xs -- test fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) fibs xs = xs.map(fib).sum test_list = replicate 50 30 s_fibs = test_list.fibs p_fibs = test_list.p_map_reduce fibs (+) main = p_fibs.show.putStrLn

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database 101 question
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I have installed the mysql server (mysqld) on my Ubuntu machine. What are the Haskell libraries/components that I will need to write a mysql client?? Kind regards, Vasili

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POSIX AIO (asych I/O) ...
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I am also testing my aio support. The aio_write binding seems to work ok .. as well as aio_error, Aio_return is a problem child. I think I wrote a really simple binding. I always receive nbytes as 0. I have been staring at the code hoping to catch a stupid mistake. I put putStrLn's in the code. .....Here is the code ... aioReturn :: AIOCB -> IO (AIOCB, ByteCount) aioReturn aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb count <- throwErrnoIfMinus1 "aioReturn" (c_aio_return p_aiocb) aiocb <- peek p_aiocb return (aiocb, fromIntegral count) foreign import ccall safe "aio.h aio_return" c_aio_return :: Ptr AIOCB -> IO CInt Maybe someone can spot something that I haven't. Thanks, Vasili

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GHC ARM Hackathon (Re: Haskell on ARM )
by Jeremy Apthorp 02 Jul '08

02 Jul '08

2008/7/3 Greg Fitzgerald <garious(a)gmail.com>: >> Perhaps we should have some sort of GHC on the ARM hackathon >> when 6.10 comes out > > Would anybody be interested in a "GHC on ARM Hackathon" in San Diego this > year? I'd be interested, but I live in Sydney, Australia. San Diego's a bit of a stretch :) > Jeremy Apthorp said: >> it'll also require that I significantly strip down the runtime system, as >> the current RTS won't fit in 4M > > Do you have a plan for how to do this? Maybe we can invite a Simon to teach > us the innards of the RTS and help guide us? I've discussed it briefly with chak (who'll be supervising the project), but as I said, I haven't looked too deeply into the code yet. Jeremy -- - Jem

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calling setWMHints (expects a CInt, I have an Int)
by Martin DeMello 02 Jul '08

02 Jul '08

Could someone give me an example of calling setWMHints from Graphics.X11.Xlib.Extras? The signature is setWMHints :: Display -> Window -> WMHints -> IO Status and WMHints is defined as data WMHints = WMHints { wmh_flags :: CLong wmh_input :: Bool wmh_initial_state :: CInt wmh_icon_pixmap :: Pixmap wmh_icon_window :: Window wmh_icon_x :: CInt wmh_icon_y :: CInt wmh_icon_mask :: Pixmap wmh_window_group :: XID } I can't figure out how to convert an Int to a CInt to construct the WMHints martin

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Haskell Weekly News: Issue 75 - July 2, 2008
by Brent Yorgey 02 Jul '08

02 Jul '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080702 Issue 75 - July 02, 2008 --------------------------------------------------------------------------- Welcome to issue 75 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Anglo Haskell 2008. Matthew Sackman [2]announced [3]AngloHaskell 2008, a gathering of all people Haskell-related from beginners, to seasoned hackers to academic giants. All and more are welcomed by large fuzzy green lambdas. The proposed dates and location are Friday the 8th and Saturday the 9th of August, at Imperial College, London. CFP - Special Issue of Fundamenta Informaticae on Dependently Typed Programming. Wouter Swierstra [4]announced a call for papers for a special issue of [5]Fundamenta Informaticae on [6]Dependently Typed Programming. The deadline for submissions is October 1. Gtk2Hs 0.9.13. Peter Gavin [7]announced the release of Gtk2Hs 0.9.13, including bindings for Gnome VFS and GStreamer, a new Gtk+ tutorial adapted by Hans van Thiel, cairo image stride support, and more. Hasim. Jochem Berndsen [8]announced Hasim, a [9]small project to create a library to do discrete event simulation in Haskell, using monads to define a domain-specific language for "actions" of a process. Galois move. Don Stewart [10]announced that Galois has completed the move of its data center. Expect speedier response times for hackage.haskell.org and darcs.haskell.org. Google Summer of Code Progress updates from participants in the 2008 [11]Google Summer of Code. Hoogle 4. Neil Mitchell (ndm) is working on [12]Hoogle 4. [13]This week, Neil worked on better Haddock database generation, lazy name searching, and a snazzy --info flag for Hoogle. Next up: type search! DPH physics engine. Roman Cheplyaka (Feuerbach) is working on a [14]physics engine using [15]Data Parallel Haskell. [16]This week, he worked on implementing Mirtich's V-Clip algorithm for collision detection (and [17]got it to work), cabalized his project and added documentation. He also ran into an interesting QuickCheck puzzle. Generic tries. Jamie Brandon is working on a library for efficient maps using generalized tries. [18]This week, he created a generic framework for automatically running QuickCheck tests at a number of different types. This week he plans to synthesize the many suggestions from the [19]discussion on the libraries list into a stable API design. Language.C. Benedikt Huber (visq) is [20]working on Language.C, a standalone parser/pretty printer library for C99. [21]This week he worked on a better representation for declarators, and abstracted the notion of an InputStream over both String and ByteString, among other accomplishments. GHC plugins. Max Bolingbroke is working on dynamically loaded plugins for GHC. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a [22]make-like dependency analysis framework for Cabal. GHC API. Thomas Schilling (nominolo) is working on [23]improvements to the GHC API. Officials at HWN headquarters have released a statement reversing their previous position regarding the existence of Thomas, citing regrettably faulty information to explain their previous misapprehensions. Expect to hear more from Thomas soon, now that he has finished graduating and moving. Libraries Proposals and extensions to the [24]standard libraries. GetOpt formatting improvements. Duncan Coutts [25]proposed some modifications to make the output of the System.Console.GetOpt library more readable, resulting in quite a bit of discussion. HughesPJ improvements. Benedikt Huber [26]proposed a patch with some bug fixes, performance improvements, and QuickCheck test suite for the Text.PrettyPrint.HughesPJ pretty-printing library. Discussion A Monad for on-demand file generation?. Joachim Breitner [27]asked about a monad for transparently tracking files which may need to be regenerated due to dependencies, leading to an interesting discussion of incremental computation, strict vs. lazy I/O, and other issues. New mailing list proposal: Haskell-Edu. Benjamin L. Russell sent out a message [28]proposing a new mailing list hosted at haskell.org, "Haskell-Edu: The Haskell Educational Mailing List." The new mailing list would be guided by the principle that Haskell is useful not just in research, but also in teaching programming as part of a liberal arts education. Comments and discussion welcomed. Learning GADT types to simulate dependent types. Paul Johnson is trying to use GADTs to simulate aspects of a dependently typed system, and [29]asks for help improving his Oleg rating. Call graph tool?. C.M.Brown [30]asked whether there is a tool for visualizing the call graph for a collection of source files, leading to a discussion of various tools. Jobs Formal methods and automated reasoning at Rockwell Collins. Janis Voigtlaender [31]passed on an opening for a Senior Systems Engineer at [32]Rockwell Collins. The opening is for a computer scientist or engineer to develop and apply automated analysis to computer systems and to pursue research in formal methods and automated reasoning. Contact: rmgatto at rockwellcollins.com. Blog noise [33]Haskell news from the [34]blogosphere. * Roman Cheplyaka: [35]V-Clip seems to work!. * Benedikt Huber: [36]Last week on Language.C (1). An update on Benedikt's Google Summer of Code project. * Jamie Brandon: [37]Week 3 progress. An update on Jamie's Google Summer of Code project. * Philip Wadler: [38]Welcome to Scotland, Neil, Patricia, and Conor!. * >>> codders: [39]Coding style, Haskell. codders likes how Real World Haskell gives some hints about Haskell coding style and culture in addition to teaching the language itself. * >>> zoo: [40]Haskell plug-in for Eclipse. zoo explains how to install the [41]Haskell Eclipse plugin. * Dan Piponi (sigfpe): [42]A blessed man's formula for holey containers. Dan descries an enlightening derivation of the [43]combinatorial form of Faa di Bruno's formula from the perspective of derivatives of types. * Roman Cheplyaka: [44]Status report: week 5. An update on Roman's Google Summer of Code project. * >>> codders: [45]More Haskell fun. * >>> Marco Tulio Gontijo e Silva: [46]Rank 2 Types. Marco describes a practical use for GHC's rank-2 types. * Edward Kmett: [47]Memoizing Context. * >>> JP Moresmau: [48]Deserializing JSON to Haskell Data objects. * >>> codders: [49]Getting started with Haskell... still. codders is learning Haskell by reading the beta version of [50]Real World Haskell. * Neil Mitchell: [51]GSoC Hoogle: Week 5. * Arnar: [52]Parsing JSON with Haskell. A nice example of using [53]Parsec to parse JSON. * Thomas Hartman: [54]HAppS Tutorial. Quotes of the Week * quicksilver: [on what OS sjanssen uses] sjanssen runs haskell programs in his head; much more efficient. * EvilTerran: "We were somewhere around Barstow, on the edge of the desert, when the phantom types began to take hold." * audreyt: o/~ the phantom of the typesystem is here / inside my mind! o/~ * dmwit: No, no, no, ($) isn't right-assoc, it's wrong-assoc. * solrize: this would never happen in haskell: i sent in a search query to a certain python program, but left the query field empty, expecting to get back an error message. instead it found a bunch of books written by the diet doctor Gary Null. * heatsink: We're all inside do-blocks in the IO monad if you think about it. * djsiegel: [upon having a question answered by dons] oh my, I'm talking to the man * mar77a: the first computers were big because they were actually cupboards with fast humans inside About the Haskell Weekly News New editions are posted to [55]the Haskell mailing list as well as to [56]the Haskell Sequence and [57]Planet Haskell. [58]RSS is also available, and headlines appear on [59]haskell.org. Headlines are available as [60]PDF. To help create new editions of this newsletter, please see the information on [61]how to contribute. Send stories to byorgey at seas dot upenn dot edu. The darcs repository is available at darcs get [62]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/16272 3. http://www.haskell.org/haskellwiki/AngloHaskell/2008 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16269 5. http://fi.mimuw.edu.pl/ 6. http://sneezy.cs.nott.ac.uk/darcs/DTP08/journal.html 7. http://www.haskell.org//pipermail/haskell-cafe/2008-June/044805.html 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41881 9. http://huygens.functor.nl/hasim 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41847 11. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 12. http://code.haskell.org/hoogle/ 13. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-5.html 14. http://haskell.org/haskellwiki/Hpysics 15. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 16. http://physics-dph.blogspot.com/2008/06/status-report-week-5.html 17. http://physics-dph.blogspot.com/2008/07/v-clip-seems-to-work.html 18. http://jamiiecb.blogspot.com/2008/06/week-3-progress.html 19. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9259 20. http://www.sivity.net/projects/language.c/wiki/ 21. http://hsbene.blogspot.com/2008/07/last-week-on-languagec-1.html 22. http://code.haskell.org/~Saizan/cabal 23. http://hackage.haskell.org/trac/ghc/wiki/GhcApiStatus 24. http://haskell.org/haskellwiki/Library_submissions 25. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9415 26. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9377 27. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41868 28. http://thread.gmane.org/gmane.comp.lang.haskell.general/16271 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41848 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41786 31. http://article.gmane.org/gmane.comp.lang.haskell.general/16266 32. http://www.rockwellcollins.com/ 33. http://planet.haskell.org/ 34. http://haskell.org/haskellwiki/Blog_articles 35. http://physics-dph.blogspot.com/2008/07/v-clip-seems-to-work.html 36. http://hsbene.blogspot.com/2008/07/last-week-on-languagec-1.html 37. http://jamiiecb.blogspot.com/2008/06/week-3-progress.html 38. http://wadler.blogspot.com/2008/06/welcome-to-scotland-neil-patricia-and.ht… 39. http://talkingcode.co.uk/2008/06/30/coding-style-haskell/ 40. http://imonad.com/blog/2008/06/haskell-plug-in-for-eclipse/ 41. http://sourceforge.net/projects/eclipsefp/ 42. http://sigfpe.blogspot.com/2008/06/blessed-mans-formula-for-holey.html 43. http://en.wikipedia.org/wiki/Fa%C3%A0_di_Bruno's_formula#Combinatorial_form 44. http://physics-dph.blogspot.com/2008/06/status-report-week-5.html 45. http://talkingcode.co.uk/2008/06/28/more-haskell-fun/ 46. http://marcotmarcot.blogspot.com/2008/06/rank-2-types.html 47. http://comonad.com/reader/2008/memoizing-context/ 48. http://jpmoresmau.blogspot.com/2008/06/deserializing-json-to-haskell-data.h… 49. http://talkingcode.co.uk/2008/06/27/getting-started-with-haskell-still/ 50. http://www.realworldhaskell.org/ 51. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-5.html 52. http://www.hvergi.net/2008/06/parsing-json-with-haskell/ 53. http://legacy.cs.uu.nl/daan/parsec.html 54. http://ramblings.tumblr.com/post/39794244/happs-tutorial 55. http://www.haskell.org/mailman/listinfo/haskell 56. http://sequence.complete.org/ 57. http://planet.haskell.org/ 58. http://sequence.complete.org/node/feed 59. http://haskell.org/ 60. http://code.haskell.org/~byorgey/code/hwn/archives/20080702.pdf 61. http://haskell.org/haskellwiki/HWN 62. http://code.haskell.org/~byorgey/code/hwn/

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