February 2009 - Haskell-Cafe

parallelism or concurrency ? if they are different
by Paolino 20 Feb '09

20 Feb '09

When I came to haskell, I arrived with a small and only evolutionary background in programming. First monad I met was MonadState StdGen m. Everything was in someway acceptable, I had no problem in explicitating the need for the generator. The lesson was referential transparency. To me referential tranparency is still obscure as a feature. Not using the monad , my functions pass around a generator, then they are repeatable, same generator , same computation. Years pass by, now evolutionary algorithms need to scale multicores. But multicore can be used with threads or par, the difference is that par is pure, because it respects referential transparency. But threads not. They are always unrespectful ? Or it's an implementation issue of preemptive choice? Can I have a baton to pass around like I had for random generator, so that the computation ends without IO (unsafe performed) , without breaking tranparency, something like (runIOThreads :: ThreadsIO a -> ThreadsBaton -> a) ? >From Real World Haskell my algorithm have to be parallelized as they don't do some kind of IO, they don't deal with the world, but where is it stated that it is possible to write them with par (I couldn't) ? More , I'm not caring that my computation is unrepeatable, for me it's fine that the runtime system gives me the cached results for same arguments computation. The fact that it doesn't ,and recompute the function giving out something fuzzily different from before, is enough to coerce me to spit out IO values ? Finally, why and where the optimizer will substitute a value with its definition, so that it possibly get computed twice ? Thanks paolino

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Haskell.org GSoC
by Daniel Kraft 20 Feb '09

20 Feb '09

Hi, I noticed last year Haskell.org was a mentoring organization for Google's Summer of Code, and I barely noticed some discussion about it applying again this year :) I participated for GCC in 2008 and would like to try again this year; while I'm still active for GCC and will surely stay so, I'd like to see something new at least for GSoC. And Haskell.org would surely be a very, very nice organization. Since I discovered there's more than just a lot of imperative languages that are nearly all the same, I love to do some programming in Prolog, Scheme and of course Haskell. However, so far this was only some toy programs and nothing "really useful"; I'd like to change this (as well as learning more about Haskell during the projects). Here are some ideas for developing Haskell packages (that would hopefully be of general use to the community) as possible projects: - Numerics, like basic linear algebra routines, numeric integration and other basic algorithms of numeric mathematics. - A basic symbolic maths package; I've no idea how far one could do this as a single GSoC project, but it would surely be a very interesting task. Alternatively or in combination, one could try to use an existing free CAS package as engine. - Graphs. - Some simulation routines from physics, though I've not really an idea what exactly one should implement here best. - A logic programming framework. I know there's something like that for Scheme; in my experience, there are some problems best expressed logically with Prolog-style backtracking/predicates and unification. This could help use such formulations from inside a Haskell program. This is surely also a very interesting project. What do you think about these ideas? I'm pretty sure there are already some of those implemented, but I also hope some would be new and really of some use to the community. Do you think something would be especially nice to have and is currently missing? Thanks for your comments, Daniel

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Re: Looking for pointfree version (Kim-Ee Yeoh) (02/12)
by Gene Arthur 20 Feb '09

20 Feb '09

>Kim-Ee Yeoh said: >On the same note, does anyone have ideas >for the following snippet? Tried the pointfree >package but the output was useless. >pointwise op (x0,y0) (x1,y1) = (x0 `op` x1, y0 `op` y1) First sorry for the delay in getting to this.. been behind on projects so had some days of mail piled up. Here is what I came up with, using one arrow operator, so you would have to import: Control.Arrow ((***)) at minimum to use this solution: pointfree :: forall t t1 c. (t -> t1 -> c) -> (t, t1) -> (t, t1) -> (c, c) pointfree op = curry $ (\(a,b) -> a `op` b) *** (\(a,b) -> a `op` b) examples of use, that were executed using: ghci -fglasgow-exts -farrows Control.Arrow *>pointfree (*) (3,5) (12,12) (15,144) *> pointfree (++) ("This ","That") ("Old"," Man") ("This That","Old Man") Hope that helps. -- gene ============================== website: http://haskblog.cloud.prohosting.com ============================== If Helen Keller is alone in a forest and falls, does she make a sound? ____________________________________________________________ Receive Notifications of Incoming Messages Easily monitor multiple email accounts & access them with a click. Visit http://www.inbox.com/notifier and check it out!

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Re: [Haskell-cafe] Re: [darcs-users] darcs and Google Summer of Code
by Wolfgang Jeltsch 20 Feb '09

20 Feb '09

Am Donnerstag, 19. Februar 2009 05:10 schrieben Sie: > Wolfgang Jeltsch wrote: > > I’ve put myself as a potential mentor on the old Haskell SoC ticket > > (<http://hackage.haskell.org/trac/summer-of-code/ticket/17>). However, > > one or two years ago I was told that mentoring is a very time-consuming > > thing (and to my knowledge, you don’t get any payments for that, at least > > for Haskell projects). So if some other person wants to mentor a > > Grapefruit-based GUI or patch viewer project, I’d be very happy and would > > support this person the best I can. > > Unless Google's changed things, the organization gets 500$ for each > project. What the Darcs organization does with that, who knows. Yes, who knows. The “Haskell orgainization” didn’t give a cent to the mentors, as far as I know. Am I wrong here? > It'd be a piddling amount as a stipend for the mentor, but... …it would be much better than nothing. :-) Best wishes, Wolfgang

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unresolved Liskell installation problems
by Benjamin L.Russell 20 Feb '09

20 Feb '09

I have been trying to install Liskell on top of GHC 6.10.1 on Windows XP Professional, Service Pack 2, since last month, but despite changes to the included Main.hs file and repeated exchanges on a related Weblog (see "CFruhwirth's Weblog: Liskell standalone" at http://blog.clemens.endorphin.org/2009/01/liskell-standalone.html) the problem is still unsolved. My last question was posted there on January 27, but there have been no further responses since then. In summary, here is a transcription of the related correspondence there: Posted by Clemens Fruhwirth on 9 January 2009 at 14:17: >[...] > >You can grab it with the usual > >darcs get http://code.haskell.org/liskell/ > > >This version has been tested with ghc 6.10.1 and should install like > >./Setup.lhs configure >./Setup.lhs build >./Setup.lhs install >cd LskPrelude >make install-inplace > >Optionally you can run make tests in the testsuite subdirectory. Posted by Benjamin Russell on 21 January 2009 at 23:27: >I just downloaded Darcs 2.2 and Liskell for Windows XP, Service Pack 2, but the following error message appeared when I ran: > >./Setup.lhs configure > >in Cygwin: > >Configuring liskell-0.1... >Setup.lhs: At least the following dependencies are missing: >ghc-paths -any > >How should I resolve this issue? Posted by ctnd on 23 January 2009 at 23:48: >[...] > >Perhaps install this package: >http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ghc-paths Posted by ctnd on 24 January 2009 at 00:04: > Okay, I got Liskell running and compiling here with a couple of changes: > >In Main.hs, change: > >-- GHC.runGhc (Just "/home/clemens/deploy/ghc-6.10.1/lib/ghc-6.10.1/") $ do >GHC.runGhc (Just "/home/chris/.GHC/lib/ghc-6.10.1/") $ do > >To the appropriate value as I have. > >./Setup.lhs configure --user >./Setup.lhs build >./Setup.lhs install > >$ cd LskPrelude/ >$ make >$ make install-inplace > >Couldn't really get the tests working but I don't much care about that right now, just want to play with Liskell. > >Hope you guys figure it out! Posted by Benjamin Russell on 25 January 2009 at 20:51: >[...] > >Thanks for the advice, but: > >> In Main.hs, change: >> >> -- GHC.runGhc (Just "/home/clemens/deploy/ghc-6.10.1/lib/ghc-6.10.1/") $ do >> GHC.runGhc (Just "/home/chris/.GHC/lib/ghc-6.10.1/") $ do > >Done. However, the building step still fails: > >> $ ./Setup.lhs build >> Preprocessing library liskell-0.1... >> Setup.lhs: The program happy is required but it could not be found > >But "happy" is installed and listed in the PATH! > >> $ which happy >> /cygdrive/c/Program Files/Haskell/bin/happy > >> Perhaps install this package: >> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ghc-paths > >Already installed; _viz_: > >> $ cabal install ghc-paths >> Resolving dependencies... >> No packages to be installed. All the requested packages are already installed. >> If you want to reinstall anyway then use the --reinstall flag. > >How should I resolve this issue? Posted by ctnd on 27 January 2009 at 07:28: >[...] > >I had the same problem: > >Configuring liskell-0.1... >Setup.lhs: At least the following dependencies are missing: >ghc-paths -any > >I solved it by configuring as --user. I should've done this from the start as my GHC and cabal packages are all --user installed anyway. > >./Setup.lhs configure --user >./Setup.lhs build >./Setup.lhs install > >Hopefully it's the same for you. > >RE the `happy` problem, no idea. > [...] > >P.S. I saw your question on the mailing list but I'm unable to post to the list (despite asking for help from the admins) so I'll have to respond here. Posted by Benjamin Russell on 27 January 2009 at 21:59: >[...] > >Thanks again for the response. > >This time, a parse error occurs in the building phase: > >> Main.hs:218:0: parse error (possibly incorrect indentation) > >These are lines 218-19 of my Main.hs file: > >> partition_args :: [String] -> [(String, Maybe Phase)] -> [String] >> -> ([(String, Maybe Phase)], [String]) > >I tried merging line 219 into line 218, but the same error recurs. > >In addition, the following error occurs during the install phase: > >>Setup.lhs: dist\build\liskell\liskell.exe: copyFile: does not exist (No such file or directory) > >Sorry for the repeated questions, but would you happen to know how to resolve these issues? Does anybody know how to install Liskell successfully? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Overloading functions based on arguments?
by Daniel Kraft 20 Feb '09

20 Feb '09

Hi, I just came across a problem like this: Suppose I've got two related functions that do similar things, and I want to call them the same... Like in: foobar :: String -> Int -> Int foobar :: Int -> String -> Int (Bad example, but I hope you got the point.) Is this kind of overloading (instead of the polymorphism based overloading) possible in Haskell? Namely to have two functions with the same name but different signatures so they could be distinguished by a call's parameters? I fear not... So I guess I have to name the functions differently, right? Thanks, Daniel

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garbage collector woes
by Chris Waterson 20 Feb '09

20 Feb '09

I'm at wits end with respect to GHC's garbage collector and would very much appreciate a code review of my MySQL driver for HDBC, which is here: <http://www.maubi.net/~waterson/REPO/HDBC-mysql/Database/HDBC/MySQL/Connecti… > In particular, the problem that I'm having is that my "statements" (really, just iterators over a SQL query result set) are getting garbage collected prematurely. I've included the source for the relevant functions below. The first is newStatement, which creates a new HDBC Statement record (an elided version, below): > newStatement :: ForeignPtr MYSQL -> String -> IO Types.Statement > newStatement mysql__ query = withForeignPtr mysql__ $ \mysql_ -> do > stmt_ <- mysql_stmt_init mysql_ > ... > stmt__ <- newForeignPtr mysql_stmt_close stmt_ > ... > return $ Types.Statement > { > , Types.fetchRow = fetchRow mysql__ stmt__ results > } The mysql_stmt_init function is the MySQL C API that creates a new statement record. Its result must be freed by mysql_stmt_close, and therefore is wrapped with a ForeignPtr appropriately. The fetchRow function is part of the HDBC Statement API, and its signature is: > fetchRow :: IO (Maybe [SqlValue]) I.e., it retrieves the next row from the result set and returns the column values as a list. The "results" parameter is not relevant to this discussion: it maintains the buffers used for storage of the column values. The next function of interest is my implementation of fetchRow, copied in its entirety, below: > fetchRow :: ForeignPtr MYSQL -> ForeignPtr MYSQL_STMT -> [MYSQL_BIND] > -> IO (Maybe [Types.SqlValue]) > fetchRow mysql__ stmt__ results = > withForeignPtr mysql__ $ \_ -> > withForeignPtr stmt__ $ \stmt_ -> do > rv <- mysql_stmt_fetch stmt_ > case rv of > 0 -> row > #{const MYSQL_DATA_TRUNCATED} -> row > #{const MYSQL_NO_DATA} -> return Nothing > _ -> statementError stmt_ > where row = mapM cellValue results >>= \cells -> return $ Just cells It unwraps the ForeignPtrs and invokes the mysql_stmt_fetch C API with the native MYSQL_STMT structure, causing the column values for the next row to get bound into storage areas that we can convert to Haskell values. Typically, this code gets invoked in the generic HDBC layer by the fetchAllRows routine, which looks like this: > fetchAllRows :: Statement -> IO [[SqlValue]] > fetchAllRows sth = unsafeInterleaveIO $ > do row <- fetchRow sth > case row of > Nothing -> return [] > Just x -> do remainder <- fetchAllRows sth > return (x : remainder) The sth argument is simply an instance of the Statement record that I've returned from low-level MySQL driver. This all works, except... The behavior that I'm seeing is that the ForeignPtr to the native MYSQL_STMT struct has become unreachable after some number of iterations of the above loop. This causes mysql_stmt_close to be invoked, and subsequent calls to mysql_stmt_fetch fail with an error. Curiously, if I replace > stmt__ <- newForeignPtr mysql_stmt_close stmt_ with > stmt__ <- newForeignPtr_ stmt_ in my newStatement routine (i.e., so that mysql_stmt_close is never called), then fetchAllRows will merrily fetch all the rows from my statement handle. The down side, of course, being that I'll leak statement handles. So, I'm completely puzzled as to how I can continue to extract values from the apparently unreachable pointer! Using System.Mem.Weak, I've verified that: * The Statement record that I return from newStatement becomes unreachable. * The applied value of fetchRow becomes unreachable; e.g.: let r = fetchRow mysql__ stmt__ results addFinalizer r $ putStrLn "r is unreachable!" I'm not sure how this can be, given that fetchAllRows appears to be explicitly maintaining a reference to sth, which in turn ought to reference my fetchRow application. I've created a test program that directly invokes my driver code the same way that the generic HDBC layer does, and have verified that the lack of "-fno-cse" with unsafeInterleaveIO in the generic HDBC module is not relevant. I was similarly able to verify that this problem occurs with "-O0", so it's not due to any particular GHC optimization. Any thoughts on how to debug this would be greatly appreciated! Thanks in advance, chris

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Re: [Haskell-cafe] How to create an online poll
by gregg reynolds 19 Feb '09

19 Feb '09

See also www.surveymonkey.com Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: >Hello haskell-cafe, > >http://zohopolls.com/ > >-- >Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANNOUNCE: Cabal-1.6.0.2 and cabal-install-0.6.2
by Duncan Coutts 19 Feb '09

19 Feb '09

I'm pleased to announce point-releases of the Cabal library and the cabal-install command line tool. If you are already using cabal-install then you can upgrade both using: $ cabal update $ cabal install Cabal cabal-install New users you can get it from: http://haskell.org/cabal/download.html For users on unix systems there is a source tarball with a bootstrap script. For users on Windows systems there is a pre-compiled cabal.exe. Release notes ============= Cabal-1.6.0.2 ------------- This is a point-release in the stable 1.6.x series. It is expected that this release will be included with ghc-6.10.2. It has a number of fixes and minor improvements. The most noticeable change is that Cabal now checks at configure time that C libraries and header files are available. This is known to break a small number of packages. The maintainers of these packages have been notified. Changes since 1.6.0.1: * New configure-time check for C headers and libraries * Added language extensions present in ghc-6.10 * Added support for NamedFieldPuns extension in ghc-6.8 * Fix in configure step for ghc-6.6 on Windows * Fix warnings in Path_pkgname.hs module on Windows * Fix for exotic flags in ld-options field * Fix for using pkg-config in a package with a lib and an executable * Fix for building haddock docs for exes that use the Paths module * Fix for installing header files in subdirectories * Fix for the case of building profiling libs but not ordinary libs * Fix read-only attribute of installed files on Windows * Ignore ghc -threaded flag when profiling in ghc-6.8 and older cabal-install-0.6.2 ------------------- This release contains a number of incremental improvements, including some new features. We expect to do another 0.6.x release in the not too distant future focusing on improvements in the way package dependencies are selected. Changes since 0.6.0: * The upgrade command has been disabled in this release * The configure and install commands now have consistent behaviour * Reduce the tendency to re-install already existing packages * The --constraint= flag now works for the install command * New --preference= flag for soft constraints / version preferences * Improved bootstrap.sh script, smarter and better error checking * New cabal info command to display detailed info on packages * New cabal unpack command to download and untar a package * HTTP-4000 package required, should fix bugs with http proxies * Now works with authenticated proxies. * On Windows can now override the proxy setting using an env var * Fix compatibility with config files generated by older versions * Warn if the hackage package list is very old * More helpful --help output, mention config file and examples * Better documentation in ~/.cabal/config file * Improved command line interface for logging and build reporting * Minor improvements to some messages Changes since 0.5.2: * Constraint solver can now cope with base 3 and base 4 * Allow use of package version preferences from hackage index * More detailed output from cabal install --dry-run -v * Improved bootstrap.sh Bugs and feature requests ========================= This is an excellent opportunity to make sure your favourite bug or feature request is properly described in our bug tracker: http://hackage.haskell.org/trac/hackage/ To help us guide development priorities please add yourself to the ticket's cc list and describe why that bug or feature is important to you. Credits ======= On behalf of the Cabal hackers and the community generally I'd like to thank the people who have contributed patches for these releases: * Gleb Alexeyev * Samuel Bronson * Thomas Schilling * Ganesh Sittampalam * Andrea Vezzosi Get involved ============ There's still a lot to do! We have big plans for Cabal-2.0, cabal-install and the Hackage website. So if you're interested in helping out with this core infrastructure project then: * subscribe to the cabal-devel mailing list: http://www.haskell.org/mailman/listinfo/cabal-devel * grab the source: http://haskell.org/cabal/code.html * read the guide to the source code: http://hackage.haskell.org/trac/hackage/wiki/SourceGuide * take a look at our list of bugs and feature requests: http://hackage.haskell.org/trac/hackage/report/12 especially the easy tickets: http://hackage.haskell.org/trac/hackage/report/13 Duncan

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Uses of `fix' combinator
by Khudyakov Alexey 19 Feb '09

19 Feb '09

Hello, While browsing documentation I've found following function > -- | @'fix' f@ is the least fixed point of the function @f@, > -- i.e. the least defined @x@ such that @f x = x@. > fix :: (a -> a) -> a > fix f = let x = f x in x I have two questions. How could this function be used? I'm unable to imagine any. Naive approach lead to nothing (no surprise): Prelude Data.Function> fix (^^2) <interactive>: out of memory (requested 2097152 bytes) Second question what does word `least' mean?`a' isn't an Ord instance. -- Khudyakov Alexey

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