- Haskell-Cafe - Haskell.org

Control.Parallel missing
by Mitar 28 Dec '09

28 Dec '09

Hi! A friend of mine tried to learn Haskell and asked me for an advice because he had problems with Haskell in 5 steps: http://www.haskell.org/haskellwiki/Haskell_in_5_steps Under "Write your first parallel Haskell program" section there is "import Control.Parallel" which is simply missing in 6.12 and also it is not in documentation anymore: http://www.haskell.org/ghc/docs/latest/html/libraries/index.html I know that we have moved now to Haskell Platfrom so you can install it additionally (probably), but for a novice it is not really nice that example programs do not compile out of the box. Maybe we could extend this 4th step also with an introduction to Cabal? Mitar

2 1

ANNOUNCE: CPython / libpython bindings
by John Millikin 28 Dec '09

28 Dec '09

CPython, the primary implementation of the Python language, has a C API for embedding Python into applications and writing extension modules[1]. The cpython[2] package is a binding to this API. It's not a complete binding yet -- there's a couple areas where I haven't figured out how to represent the C-ish idioms -- so it's not yet possible to do things like implement extension modules[3], run code directly from a file[4], or compile Python strings to code[4]. Hackage can't build docs because it doesn't have the Python development libraries installed, so I've uploaded them to <http://ianen.org/haskell/bindings/cpython/api-documentation/>. If you encounter any build errors, or have wishes / ideas on which parts of the API to add to the binding, please say so. I will add parts as I figure them out, but wanted to get a working version out ASAP so people can play with it. [1] http://docs.python.org/3.1/c-api/ [2] http://hackage.haskell.org/package/cpython/ [3] http://docs.python.org/3.1/c-api/module.html#initializing-c-modules [4] http://docs.python.org/3.1/c-api/veryhigh.html

2 4

time of day
by Brian Denheyer 28 Dec '09

28 Dec '09

Hi All, What's the best way to get the year/month/day/hour/min/sec of the current time ? I've become mired in confusion with Time, Data.Time, DateTime and I think there is even an old-time. Might be a couple of calendar libraries in there, not quite sure... Thanks, Brian

3 2

install-dirs on Mac OS X
by Mark Lentczner 27 Dec '09

27 Dec '09

I have been thinking about the location of installed Haskell package files on Mac OS X. The choice of location affects: GHC & other Haskell implementations Haskell Platform Cabal & cabal-install Haddock If all those agreed on directory locations and layouts, I think the state of Haskell on Mac OS X would be nicer for users. In particular, my hope is for users to end up with an automatically complete Haddock documentation incorporating everything they install. 1) Packages the user installs --global I noticed that the default location for global installation with cabal is on Mac OS X is /usr/local: install-dirs global -- prefix: /usr/local This then intermingles all the Haskell package files along with all sorts of other things that live in /usr/local and environs. While this is arguably convenient for executables that end up in /usr/local/bin (in most people's PATH), it does make things less than tidy, and harder to clean up. Taking a cue from the various preinstalled language systems on Mac OS X, up over in /Library might be a better place: Python puts installed packages in: /Library/Python/<version>/site-packages Ruby puts installed packages in: /Library/Ruby/Gems/<version> /Library/Ruby/Site/<version> Java appears to use /Library/Java/Extensions and has a link to the packages that come the framework as: /Library/Java/Home Perl put installed packages in: /Library/Perl/<version> I suggest that the default place for global installs on Mac OS X be: /Library/Haskell/ Since cabal already by default interposes the compiler version into the lib dir, path, there doesn't appear to be a need to put a <version> dir level near the top. 2) Structure of package pieces I notice that cabal/cabal-install's default layout of where a package's pieces go, and GHC's layout of its base packages don't agree. Further, cabal/cabal-install's are not set up so that one can easily delete an installed package without hunting down its parts. cabal/cabal-install defaults the parts as follows: executables: --prefix--/bin libraries: --prefix--/lib/--pkgid--/--compiler-- data: --prefix--/share/--pkgid-- doc: --prefix--/share/doc/--pkgid--/ html: --prefix--/share/doc/--pkgid--/html That's at least four directories you need to hunt down if you want to clean out a package, and rummaging through bin to figure out which things to remove. (Not to mention libexec, which isn't used by any packages on my system, so I can't say where it goes...) GHC/Haskell Platform use a different layout: executables: --prefix--/bin libraries: --prefix--/lib/--compiler--/--pkgid-- data: --prefix--/share/--pkgid-- doc: --prefix--/share/doc/ghc/libraries/--pkgid--/ html: --prefix--/share/doc/ghc/libraries/--pkgid--/ I think it best if everything for a package is in one place - making removal very easy: executables: --prefix--/packages/--pkgid--/bin libraries: --prefix--/packages/--pkgid--/lib/--compiler-- data: --prefix--/packages/--pkgid--/share doc: --prefix--/packages/--pkgid--/doc html: --prefix--/packages/--pkgid--/doc I put the "packages" level at the top, so that other things, like a master Haddock index dir could be put easily directly under the prefix. 3) Symlinks for binaries This does suggest that it would be nice for the symlink-bindir facility (is that in cabal itself, or added by cabal-install?) to have a version for --global installs. Users could then either set something like: symlink-global-bindir: /usr/local/bin in .cabal/config. Or symlink-global-bindir: /Library/Haskell/bin and then put that in their PATH 4) Quick access to the Framework I like Java's convenience link 'Home', and suggest that /Library/Haskell/GHC be a symlink to where the framework's package files are stored: /Library/Frameworks/GHC.framework/Versions/Current/usr/ Other implementations could use the same idea. Having this here would also (I suspect) help in getting Haddock to be able to find all the bits needed to generate a comprehensive index. Thoughts? I'd be happy to help by supplying patches for various tools to normalize all this on some agreed upon layout. I admit that I'm a bit unclear where the directory choices are being made: Haskell Platform's build process, or GHC's? Cabal's defaults or cabal-install's? And then clearly parts of Haddock. Given the number of tools that need to agree, seems best that we hash it out (here or in the wiki) first, before making patches. - Mark (mtnviewmark) Mark Lentczner http://www.ozonehouse.com/mark/ mark(a)glyphic.com

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Video for Linux (v4l) library for Haskell
by Christopher Done 27 Dec '09

27 Dec '09

Hackage and Google turn up nothing¹, so I am asking here; has anyone written a v4l library for Haskell? I have been reading the v4l documentation² and I am ready to implement a Haskell interface for webcams, but it would be a waste of effort if someone's already got one knocking about. I could hack on theirs! Cheers! Christopher Done ¹ In fact, the only Google result is a posting to this mailing list asking if there is a v4l library to which the single response is "no, write an FFI interface". ² http://pages.cpsc.ucalgary.ca/~sayles/VFL_HowTo/ and http://pages.cpsc.ucalgary.ca/~sayles/VFL_HowTo/Video4LinuxAPI.html

3 2

Installing Haskell on Windows 7
by Thomas Hühn 27 Dec '09

27 Dec '09

[Please excuse me if I seem to be a bit frustrated. I am. I have tried to transcribe a live Haskell platform installation session, but I don't remember details of my past attempts to get cabal working. If you have any questions: shoot.] Hi I've been trying to install GHC and some libraries on Windows 7 for some time now, to no avail. I'm on Windows 7 Home Premium here, and I'd like to install all Haskell-related stuff in C:\Weiteres\Haskell (cabal --user installations ending up in C:\Users\Thomas\AppData\Roaming\cabal is fine). Actually I would like to make system-wide package installations (maybe in C:\Weiteres\Haskell\cabal), but not even --user is working fine so far, so I postpone that. I see two basic options: * Haskell Platform (older release, but all relevant libs bundled with it) * ghc and libs installed by hand (newer release, but more hassle) For the first option I expect a nice installer that just does everything for me, for the second option I'm going along with the instructions in Real World Haskell's appendix. So far, none of it worked. ghc is fine (mostly), but cabal is a total mess. I've just tried the second option and actually I've got quite far, until the network library didn't install. I don't have any specifics anymore. And whichever option I choose I can only make --user installs with cabal (but I'd be willing to settle for that). Okay, let's skip all this "manual installation stuff", on to the beginner-friendly first option: Haskell platform I specify the destination folder "C:\Weiteres\Haskell\Platform" and just go with the default selection of checkboxes. Now I'm trying ghci from the start menu... okay, it works. Entering Windows' command prompt I now enter "cabal update". There is a newer version of cabal-install available. I'm ignoring that for now. I don't want to mess up a tested, working cabal-install version and deviate from the Haskell Platform release just yet. Now "cabal install hakyll --user": C:\Users\Thomas>cabal install hakyll --user Resolving dependencies... Downloading pandoc-1.3... [1 of 1] Compiling Main ( C:\Users\Thomas\AppData\Local\Temp\pandoc- 1.35032\pandoc-1.3\Setup.hs, C:\Users\Thomas\AppData\Local\Temp\pandoc-1.35032\p andoc-1.3\dist\setup\Main.o ) Linking C:\Users\Thomas\AppData\Local\Temp\pandoc-1.35032\pandoc-1.3\dist\setup\ setup.exe ... Configuring pandoc-1.3... Preprocessing library pandoc-1.3... Preprocessing executables for pandoc-1.3... Building pandoc-1.3... src\Text\Pandoc\Shared.hs:121:7: Could not find module `System.IO.UTF8': There are files missing in the `utf8-string-0.3.6' package, try running 'ghc-pkg check'. Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: hakyll-0.2 depends on pandoc-1.3 which failed to install. pandoc-1.3 failed during the building phase. The exception was: exit: ExitFailure 1 That went well. :-( "ghc-pkg check": C:\Users\Thomas>ghc-pkg check There are problems in package binary-0.5.0.2: import-dirs: C:\Users\Thomas\AppData\Roaming\cabal\binary-0.5.0.2\ghc-6.10.4 oesn't exist or isn't a directory library-dirs: C:\Users\Thomas\AppData\Roaming\cabal\binary-0.5.0.2\ghc-6.10.4 doesn't exist or isn't a directory file Data\Binary.hi is missing file Data\Binary\Put.hi is missing file Data\Binary\Get.hi is missing file Data\Binary\Builder.hi is missing cannot find libHSbinary-0.5.0.2.a on library path There are problems in package digest-0.0.0.8: import-dirs: C:\Users\Thomas\AppData\Roaming\cabal\digest-0.0.0.8\ghc-6.10.4 oesn't exist or isn't a directory library-dirs: C:\Users\Thomas\AppData\Roaming\cabal\digest-0.0.0.8\ghc-6.10.4 doesn't exist or isn't a directory file Data\Digest\CRC32.hi is missing file Data\Digest\Adler32.hi is missing cannot find libHSdigest-0.0.0.8.a on library path There are problems in package template-0.1.1.1: import-dirs: C:\Users\Thomas\AppData\Roaming\cabal\template-0.1.1.1\ghc-6.10. doesn't exist or isn't a directory library-dirs: C:\Users\Thomas\AppData\Roaming\cabal\template-0.1.1.1\ghc-6.10 4 doesn't exist or isn't a directory file Text\Template.hi is missing cannot find libHStemplate-0.1.1.1.a on library path There are problems in package utf8-string-0.3.6: import-dirs: C:\Users\Thomas\AppData\Roaming\cabal\utf8-string-0.3.6\ghc-6.10 4 doesn't exist or isn't a directory library-dirs: C:\Users\Thomas\AppData\Roaming\cabal\utf8-string-0.3.6\ghc-6.1 .4 doesn't exist or isn't a directory file Codec\Binary\UTF8\String.hi is missing file Codec\Binary\UTF8\Generic.hi is missing file System\IO\UTF8.hi is missing file System\Environment\UTF8.hi is missing file Data\String\UTF8.hi is missing file Data\ByteString\UTF8.hi is missing file Data\ByteString\Lazy\UTF8.hi is missing cannot find libHSutf8-string-0.3.6.a on library path There are problems in package zip-archive-0.1.1.4: import-dirs: C:\Users\Thomas\AppData\Roaming\cabal\zip-archive-0.1.1.4\ghc-6. 0.4 doesn't exist or isn't a directory library-dirs: C:\Users\Thomas\AppData\Roaming\cabal\zip-archive-0.1.1.4\ghc-6 10.4 doesn't exist or isn't a directory file Codec\Archive\Zip.hi is missing cannot find libHSzip-archive-0.1.1.4.a on library path There are problems in package haddock-2.4.2: import-dirs: c:/builds/slave/x86-win-stable/build/ghc-6.10.4\haddock-2.4.2 do sn't exist or isn't a directory library-dirs: c:/builds/slave/x86-win-stable/build/ghc-6.10.4\haddock-2.4.2 d esn't exist or isn't a directory file Distribution\Haddock.hi is missing file Haddock\DocName.hi is missing file Haddock\Types.hi is missing file Haddock\InterfaceFile.hi is missing file Haddock\Exception.hi is missing file Haddock\Utils.hi is missing file Haddock\GHC\Utils.hi is missing cannot find libHShaddock-2.4.2.a on library path The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. binary-0.5.0.2 digest-0.0.0.8 template-0.1.1.1 utf8-string-0.3.6 zip-archive-0.1.1.4 haddock-2.4.2 Excuse me? Broken packages right after installation? Okay, reinstalling them all (with --user --reinstall"). Except haddock: C:\Users\Thomas>cabal install haddock --user --reinstall Resolving dependencies... cabal: cannot configure haddock-2.6.0. It requires ghc >=6.12 && <6.14 There is no available version of ghc that satisfies >=6.12 && <6.14 *sigh* I'm starting to think about the manual installation of ghci and the libs again... but I won't do it, because it gives me all other kinds of pain (like "let's ship the mingw tool chain, but don't enter it into the PATH, the user will certainly know where to find ar.exe and how to make cabal use it when building a package"). I'm failing there just as much as here with the Platform. Okay, let's forget about haddock. Back to hakyll: "cabal install hakyll --user" Ends with: Linking dist\build\markdown2pdf\markdown2pdf.exe ... Created man\man1\pandoc.1 Created man\man1\hsmarkdown.1 Created man\man1\html2markdown.1 Created man\man1\markdown2pdf.1 setup.exe: CreateDirectory: permission denied (Zugriff verweigert) cabal: Error: some packages failed to install: hakyll-0.2 depends on pandoc-1.3 which failed to install. pandoc-1.3 failed during the final install step. The exception was: exit: ExitFailure 1 Okay, remember that cabal-install was out of date? "cabal install cabal-install" ends with: Linking dist\build\cabal\cabal.exe ... cabal: Error: some packages failed to install: cabal-install-0.6.4 failed during the final install step. The exception was: CreateDirectory: permission denied (Zugriff verweigert) Would be nice to know which directory cabal-install tried to create, don't you think? (This was just thrown in to show why I've only user user installations so far). Okay, the same with "--user" succeeds. Yay! Let's try pandoc again: Linking dist\build\markdown2pdf\markdown2pdf.exe ... Created man\man1\pandoc.1 Created man\man1\hsmarkdown.1 Created man\man1\html2markdown.1 Created man\man1\markdown2pdf.1 setup.exe: CreateDirectory: permission denied (Zugriff verweigert) cabal: Error: some packages failed to install: pandoc-1.3 failed during the final install step. The exception was: exit: ExitFailure 1 *sigh* What is cabal trying to do outside the battle-proven AppData-Cabal directory anyways? Let's revisit the phrase "beginner-friendly first option". :-/ Just for fun's sake, let's have a look into cabal's config file, since I seem to have some directory trouble: install-dirs user -- prefix: C:\Users\Thomas\AppData\Roaming\cabal -- bindir: $prefix\bin -- libdir: $prefix -- libsubdir: $pkgid\$compiler -- libexecdir: $prefix\$pkgid -- datadir: "C:\\Program Files (x86)\\Haskell" -- datasubdir: $pkgid -- docdir: $prefix\doc\$pkgid -- htmldir: $docdir\html -- haddockdir: $htmldir install-dirs global -- prefix: "C:\\Program Files (x86)\\Haskell" -- bindir: $prefix\bin -- libdir: $prefix -- libsubdir: $pkgid\$compiler -- libexecdir: $prefix\$pkgid -- datadir: "C:\\Program Files (x86)\\Haskell" -- datasubdir: $pkgid -- docdir: $prefix\doc\$pkgid -- htmldir: $docdir\html -- haddockdir: $htmldir I certainly don't have a "C:\\Program Files (x86)\\Haskell" folder. Might that result in problems? I don't know, because I have no clue what's supposed to be in the datadir. But then again I shouldn't have to meddle with some installed configuration file, especially since there has been no warning that I had to do that and very especially since I entered my destination folder upon installation. Maybe it's okay, but it kind of makes me nervous. And God knows what else I haven't stumbled upon yet. (I changed those directory entries to the ghc installation directory when I tried the second option and couldn't make it work either.) Has anyone who is *not* a Haskell/ghc/cabal expert been able to install Haskell satisfactorily on Windows 7? What are my options now? Any stupid mistakes I made? Waiting for the next release of the Haskell Platform (whenever that might happen)? Giving up, selling RWH on ebay and preparing New Year's Eve celebrations? Thomas

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: cabal install SDL
by Lie Ryan 27 Dec '09

27 Dec '09

================================================================ C:\Windows\system32>cabal install SDL Resolving dependencies... [1 of 1] Compiling Main ( C:\Users\LIERYA~1\AppData\Local\Temp\SDL-0.5.93108\SDL-0.5.9\Setup.lhs, C:\Users\LIERYA~1\AppData\Local\Temp\SDL-0.5.93108\SDL-0.5.9\dist\setup\Main.o ) C:\Users\LIERYA~1\AppData\Local\Temp\SDL-0.5.93108\SDL-0.5.9\Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Linking C:\Users\LIERYA~1\AppData\Local\Temp\SDL-0.5.93108\SDL-0.5.9\dist\setup\setup.exe ... Warning: defaultUserHooks in Setup script is deprecated. Configuring SDL-0.5.9... setup.exe: sh: runGenProcess: does not exist (No such file or directory) cabal: Error: some packages failed to install: SDL-0.5.9 failed during the configure step. The exception was: exit: ExitFailure 1 ================================================================ I searched on google, it says some modules (e.g. SDL) requires a unix-shell to run the ./configure script. I do have Cygwin on the machine, but I installed GHC/Haskell Platform on the host machine (not on Cygwin). If I installed the SDL package inside cygwin using cabal, would cabal installs the SDL on the Cygwin environment or on the host machine? (i.e. can I use the resultant SDL-binding outside Cygwin?)

2 1

HList and Data Types a la carte?
by Günther Schmidt 27 Dec '09

27 Dec '09

Hi, is there a connection between HList and Data Types a la Carte? Are both approaches pursuing the same goal? Or is it more like trying to compare apples and pears? Günther

1 0

Mixing internal functions and datatypes with externally available ones
by DPX-Infinity 27 Dec '09

27 Dec '09

Hello. I'm writing a library - some kind a wrapper around SDL library, an engine. For example, I have a module Graphics.UI.SDL.XEngine.State and functions modifyState and setColor in it. These functions are using "global" variable of type IO (IORef XState). I want to use the first function in Graphics.UI.SDL.XEngine.General and Graphics.UI.SDL.XEngine.Drawing modules and I don't want this function to be accessible from a program which uses my library, but I need to make the function setColor visible from everywhere. Is there a simple way to do such thing? Or maybe I should restructure my library?

2 1

Generating AST using Parsec
by CK Kashyap 27 Dec '09

27 Dec '09

Hi All, I recently came across the paper titled "Monadic Parser Combinators" - After going through it a few times, I think I am beginning to understand monads. However, the parser developed in the paper does not generate an AST - I feel, I'd grasp the whole thing a lot better if I could go over a sample that generates an AST from a simple expression (or even a standard language such as C or Java) ... Can someone please point me to a sample that generates AST - preferably with the simple parser combinator given in the paper. Regards, Kashyap

4 3

Continuable and serializable parsers.
by Serguey Zefirov 27 Dec '09

27 Dec '09

A pair of problems: 1) How to write a parser that could be restarted? Like, it will be represented by a function that returns something along the lines data ParseStepResult input result = Success (Maybe (input -> ParseStepResult input result)) (Maybe result) | Failure (ie, parsers using stream combinators like Fudgets have that property) ie, either a continuation of parsing process and result or failure flag. 2) How to write parser like one above that could be saved into database? data ParserCont input result = ... deriving (Show, Read) data ParseStepResult input result = Success (Maybe (ParserCont input result)) (Maybe result) | Failure I could imagine that it is possible using parser generator like Happy. Is it possible to get right using parsing combinators library?

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Lazy evaluation/functions
by michael rice 27 Dec '09

27 Dec '09

I've seen the terms "lazy evaluation" and "lazy function." Is this just lazy language or are both these terms valid? Michael

3 2

Deconstruction
by haskell＠kudling.de 26 Dec '09

26 Dec '09

Hi, while this works: data Foo a = Foo a unwrapFoo :: Foo a -> a unwrapFoo (Foo x) = x this: {-# LANGUAGE ExistentialQuantification #-} class BarLike a where doSomething :: a -> Double data Bar = forall a. BarLike a => Bar a unwrapBar :: Bar -> a unwrapBar (Bar x) = x gives me: Couldn't match expected type `a' against inferred type `a1' `a' is a rigid type variable bound by the type signature for `unwrapBar' at test.hs:8:20 `a1' is a rigid type variable bound by the constructor `Bar' at test.hs:9:11 In the expression: x In the definition of `unwrapBar': unwrapBar (Bar x) = x How can i deconstruct the enclosed value of type a? Thanks, Lenny

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Does the TMVar and TChan really obey STM rules?
by Andrey Sisoyev 26 Dec '09

26 Dec '09

Hi everyone, >isEmptyTMVar :: TMVar a -> STM Bool Source > >Check whether a given TMVar is empty. > >Notice that the boolean value returned is just a snapshot >of the state of the TMVar. By the time you get to react on its result, >the TMVar may have been filled (or emptied) - so be extremely careful >when using this operation. Use tryTakeTMVar instead if possible. When I read this in the haddock to Control.Concurrent.STM.TMVar, I started to suspect that the behavior of TMVar and TChan might be worse than I imagined. Few questions on TMVar and TChan: (1) If 2 threads are sleeping-waiting for the output of TChan, and it gets filled, do they both wakeup, or just one? (2) Similar question about reading/writing TMVar. (3) If a thread is sleeping-waiting for the output of TChan, but transaction wants to restart due to the change in any of touched TVar, then does the thread wakeup and restart the transaction? (4) Similar question about TMVar. Also, if there is some paper on this, please, give me a link on it. Please help, Andrey -- View this message in context: http://old.nabble.com/Does-the-TMVar-and-TChan-really-obey-STM-rules--tp269… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 3

Haskell type system and Barendregt's Lambda Cube classification
by Vladimir Ivanov 26 Dec '09

26 Dec '09

Dear All, Recently, I've been playing with self-application and fixed-point combinators definition in Haskell. It is possible to type them in Haskell using recursive types. I took Y & U combinators: > newtype Rec a = In { out :: Rec a -> a } > u :: Rec a -> a > u x = out x x > y :: (a -> a) -> a > y f = let uf = f . u in uf (In uf) Recursive types are part of System F-omega, which corresponds to lambda omega in Barendregt's Lambda Cube. For all type systems in Lambda Cube it is proven that they are strongly normalizing. But using "y" I can easily construct a term even without a normal form. So, I got a contradition and conclude that type system implementation in Haskell contains something, that is absent from System F-omega. My question is: what's particular feature in Haskell type system, that breaks strong normalization property? Or am I totally wrong classifying it in terms of Lambda Cube? Best regards, Vladimir Ivanov

2 2

Fwd: [Haskell-cafe] Deconstruction
by Alberto G. Corona 26 Dec '09

26 Dec '09

Cast uses unsafeCoerce. but well, it´s more elegant ;) 2009/12/26 Miguel Mitrofanov <miguelimo38(a)yandex.ru> > On 26 Dec 2009, at 12:45, Alberto G. Corona wrote: > > You can also make use of Typeable and unsafeCoerce (Data.Dynamic style) >> >> >> data Bar = forall a. (BarLike a, Typeable a) => Bar a >> >> unwrapBar :: Bar -> a >> unwrapBar (Bar x) = case typeof x== typeof a of >> True -> unsafeCoerce x >> False -> error $ "casting error for >> type: "++ typeOf a >> > > That's gross. Don't use unsafe functions unless you really need one. > > > data Bar = forall a. (BarLike a, Typeable a) => Bar a > unwrapBar :: Typeable a => Bar -> a > unwrapBar (Bar x) = fromMaybe (error $ "casting error for type: " ++ show > (typeOf x)) $ cast x > > > > > >> >> >> 2009/12/26 Miguel Mitrofanov <miguelimo38(a)yandex.ru> >> >> On 26 Dec 2009, at 11:58, haskell(a)kudling.de wrote: >> >> class BarLike a where >> doSomething :: a -> Double >> >> data Bar = forall a. BarLike a => Bar a >> >> unwrapBar :: Bar -> a >> unwrapBar (Bar x) = x >> >> How can i deconstruct the enclosed value of type a? >> >> You can't write a function with a type that mentions existentially >> quantified "a". Period. >> >> But you can deconstruct the enclosed value temporarily: >> >> getSomething :: Bar -> Double >> getSomething b = >> case b of >> Bar a -> doSomething a >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

1 0

ANNOUNCE: graphviz-2999.8.0.0
by Ivan Lazar Miljenovic 26 Dec '09

26 Dec '09

I am pleased to announce the latest release of my graphviz [1] library, which provides bindings and helper functions for use with the Graphviz [2] suite of graph visualisation programs. [1] http://hackage.haskell.org/package/graphviz [2] http://www.graphviz.org/ For the most part, this release has a compatible API with the 2999.7.0.0 version; the only exceptions to this are the runGraphviz and runGraphvizCommand functions in Data.GraphViz.Commands; I released after the last release that it might be useful to have these return the filenames of the files created rather than just indicating success, especially when using addExtension (from the same module) to add the correct extension to a provided filename. The rest of the changes to this release are: * Added support for generalised DotGraphs; this optional representation removes the restriction of ordering of Dot statements. This allows for greater flexibility of how to specify Dot code. As an offshoot from this, most relevant functions now utilise a new DotRepr class that work with both DotGraphs and the new GDotGraphs; this shouldn't affect any code already in use. * With the prompting of Noam Lewis, the augmentation functions have been revamped in two ways: - Now contains support for multiple edges. - The ability to perform "manual" augmentation if greater control is desired. * Add a preview function to quickly render and visualise an FGL graph using the Xlib canvas. * Added a pseudo-inverse of the FGL -> Dot functions (a graph is created, but the original node and edge labels are unrecoverable). * The Printing and Parsing modules have been moved (from Data.GraphViz.Types to Data.GraphViz). In the near future, this library is also likely to contain support for record shapes for graph nodes thanks to Minh Thu. If you want/require any additional features in graphviz, you too can do what countless (well, OK, not actually countless per-se) people like Noam and Thu have done: bug me to do it or send me a patch! ;-) (I hope that I don't have to make yet another release soon fixing an obvious flaw as soon as I actually start using this release myself...) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Re: [Haskell-cafe] Performance of functional priority queues
by Louis Wasserman 25 Dec '09

25 Dec '09

>> One of the correspondents in that thread claims that it is >> provably impossible to have an efficient priority queue implementation >> without mutability. I think he's cuckoo. But I'd like to have some >> numbers to back me up. He is cuckoo, and here's proof: http://www.brics.dk/RS/96/37/BRICS-RS-96-37.pdf <http://www.brics.dk/RS/96/37/BRICS-RS-96-37.pdf>This demonstrates a functional priority queue that performs every desired operation in asymptotically optimal time. Granting that its constant factor could be significantly improved, realize that there are more complicated functional data structures with similar asymptotic guarantees. (Observation: I'm pretty sure that the Fibonacci heap is actually surprisingly functional.) Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

2 1

Why does HXT use Arrows?
by Gregory Crosswhite 25 Dec '09

25 Dec '09

Something I have been wondering for a while is: Why is HXT built on arrows, rather than monads? I understand that arrows are a generalization of monads, but they also add extra complexity that is often unnecessary. Furthermore, as I understand it they only give you additional power when your arrow cannot implement ArrowApply, since otherwise your arrow is equivalent to a monad anyway, and it looks like the result of parsing an XML document gets you an arrow which implements ArrowApply. Is there some circumstance I am missing where the fact that HXT uses arrows rather than monads lets one do something that one couldn't have done with monads? Cheers, Greg

4 3

problem with editline install
by Andrew U. Frank 25 Dec '09

25 Dec '09

i tried to install editline (because i wanted to install djinn, which depends on it): with cabal install and with downloading and runghc Setup.lhs configure i got the same error: checking for completion_matches... no configure: error: editline not found, so this package cannot be built See `config.log' for more details. in the log i did not see anything helpful (at my level of understanding). i could then install with synaptic because a deb package existed for ubuntu). what did i wrong? thanks for helpful comments andrew ps. in this case a wiki would have been helpful. it would have been a less intruding form to communicate than sending an email to the developer. especially in the case i assume i made some error.

3 2

Haskell type system and Barendregt's Lambda Cube classification
by Vladimir Ivanov 25 Dec '09

25 Dec '09

Dear All, Recently, I've been playing with self-application and fixed-point combinators definition in Haskell. It is possible to type them in Haskell using recursive types. I took Y & U combinators: > newtype Rec a = In { out :: Rec a -> a } > u :: Rec a -> a > u x = out x x > y :: (a -> a) -> a > y f = let uf = f . u in uf (In uf) Recursive types are part of System F-omega, which corresponds to lambda omega in Barendregt's Lambda Cube. For all type systems in Lambda Cube it is proven that they are strongly normalizing. But using "y" I can easily construct a term even without a normal form. So, I got a contradition and conclude that type system implementation in Haskell contains something, that is absent from System F-omega. My question is: what's particular feature in Haskell type system, that breaks strong normalization property? Or am I totally wrong classifying it in terms of Lambda Cube? Best regards, Vladimir Ivanov

1 0

Memory-aware Haskell?
by Roman Cheplyaka 25 Dec '09

25 Dec '09

Imagine some system with hard memory bound (e.g. 64M of physical memory, no swap). I don't want some accidental laziness (maybe even not in my code, but in some used package) to crash my program. So, let's think what we can do at runtime. Suppose RTS takes the parameter -- upper limit of consumed memory. When it sees that memory consumption is close to upper bound, it can: 1. force garbage collection 2. apply some heuristics to find and reduce some chunks which will benefit from reduction in terms of size 3. if nothing helps, throw an exeption. It can be caught in IO and memory-aware program can make apropriate decision -- e.g. abort opening a large file and gracefully warn the user. (And there still is a problem of foreign code whose memory consumption we know nothing about...) What do you think? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

5 4

Haskell Users in Leipzig
by Matthias-Christian Ott 25 Dec '09

25 Dec '09

Hi, I just wanted to know whether there are any Haskell users and enthusiasts in the Leipzig area who are interested in meeting with other users and perhaps interested in creating a user group. Users of other functional programming languages are of course also welcome. Regards, Matthias-Christian

3 4

pointfree-trouble
by slemi 24 Dec '09

24 Dec '09

hello everybody, i'm a newbie this is my first post here.. i have trouble making a function pointfree: data RealFrac a => Matrix a = Matr [[a]] | Scalar a deriving (Show, Eq) unMatr :: RealFrac a => Matrix a -> [[a]] unMatr = (\(Matr a) -> a) reMatr :: RealFrac a => ([[a]] -> [[a]]) -> (Matrix a -> Matrix a) reMatr a = Matr . (flip (.) unMatr) a this works fine, but if i leave the 'a' in the last function's definition like this: reMatr = Matr . (flip (.) unMatr) it gives an error. can anybody tell me why? (i'm using ghci) -- View this message in context: http://old.nabble.com/pointfree-trouble-tp26881661p26881661.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANNOUNCE: repr-0.3.2
by Bas van Dijk 24 Dec '09

24 Dec '09

Hello, Some months ago I uploaded a little package called 'repr' to hackage. I've now updated the package to work with ghc-6.12.1 and its new base library 4.2.0.0. Back then I forgot to make a proper announcement so I will do that now: 'repr' allows you to render overloaded expressions to their textual representation. For example: *Repr> let rd = 1.5 + 2 + (3 + (-4) * (5 - pi / sqrt 6)) :: Repr Double *Repr> show rd "fromRational (3 % 2) + 2 + (3 + negate 4 * (5 - pi / sqrt 6))" See: http://hackage.haskell.org/package/repr-0.3.2 regards, Bas

3 3

trace
by michael rice 24 Dec '09

24 Dec '09

Can someone provide a simple example of tracing a function. Michael

2 3

Improved message for GHC "No instance for" Errors
by Victor Mateus Oliveira 24 Dec '09

24 Dec '09

Hi, I would like to propose a modification in the ghc message for "no instance for" errors. In the message, the ghc tells that there's no instance for the type you are using and it give you a hint that you can fix it adding a new instance for your type. But it don't tell me any instance alternative that already exists. So, if I don't know the correct type, I need to look at the documentation. At least for me, I get this message when I'm using the function in the wrong way or when I forget to specify the some function signature. It's quite rare the I want to add a new instance. Of course that it's not a problem to look at the documentation, but improving it can save a significant time and help the beginers to guess the error and how fix it. In C++ and Java, when I use g++ or javac and I make a wrong overloading, the compiler show me some alternatives. I think it help more than tell me to add a new overload to the function. It's possible to do it in ghc? I'm new haskell programmer and I enjoying a lot this language. []s Victor -- GNU/Linux user #446397 - http://counter.li.org

1 0

Cabal's License type: MIT commented out?
by Tom Tobin 24 Dec '09

24 Dec '09

I noticed on Hackage that packages that are MIT licensed show up as "OtherLicense". I took a peek inside the Cabal code, and noticed that the License type has lines for MIT, but commented out [1]: > -- -- | The MIT license, similar to the BSD3. Very free license. > -- | MIT Why is this? Both the BSD3 and BSD4 get entries in this module. It would be nice to know up-front that a license is indeed liberal (BSD/MIT) rather than having to open up the package and look at the LICENSE file. (I also use the MIT license for my code, and would like to avoid confusing people once I start distributing Haskell libraries.) [1] http://hackage.haskell.org/packages/archive/Cabal/latest/doc/html/src/Distr…

2 2

Happstack with XML-RPC
by Michael Hartl 24 Dec '09

24 Dec '09

Hello, has anybody ever used Happstack as XML-RPC-Server? Perhaps in conjunction with HaXR? I like the architecture of Happstack and would like to use it in a project where I have to use HTML as well as XML-RPC, now to me it would make sense to integrate XML-RPC abilities into Happstack, since HaXR only provides a CGI-based server. Any experiences? BTW, what's the status of HaXR? Is it being actively developed? regards Mike

2 1

Instances of `IsString`.
by Jason Dusek 24 Dec '09

24 Dec '09

A quick check on Hayoo! and in my interpreter shows that there are basically no instances of `IsString`. Is it really so little used? -- Jason Dusek

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ANN: Hemkay, the 100% Haskell MOD player
by Patai Gergely 24 Dec '09

24 Dec '09

Hello all, I just uploaded the fruit of a little side project. Hemkay [1] is an oldschool module music [2] player that performs all the hard work in Haskell. If there was any goal, it was to express the transformation from the song structure to the output of the mixer as a series of function compositions, maintaining a style that one might call idiomatic Haskell. Considering the dirtiness of the format in question, I'm quite pleased with the initial version. Still, I'd be curious to see how the overall quality of the code could be improved. In particular, retrieving and updating record fields is somewhat inconvenient. Also, the actual mixing (limited to the mixChunk function) is embarrassingly slow, and I wonder how much it could be improved without leaving the pure world. The program uses Portaudio for playback, but that might easily change in the future. The problem is that I couldn't get sound to work smoothly when producing samples in batches, so I'm sending them off one by one (!) at the moment, which doesn't help with performance either. I'm open to suggestions as to what library to use to push data to the sound card. Gergely [1] http://hackage.haskell.org/package/hemkay [2] http://en.wikipedia.org/wiki/MOD_(file_format) -- http://www.fastmail.fm - Or how I learned to stop worrying and love email again

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Space Leak with semi-implicit parallelization and the nasty Garbage collector
by Michael Lesniak 24 Dec '09

24 Dec '09

Hello haskell-cafe (and merry christmas!), I have a strange problem with the garbage collector / memory which I'm unable to find a solution for. I think the source of my problems has to do with lazy evaluation, but currently I'm unable to find it. Using the attached program and threadscope I see that the GC is using a lot of time and the program comes (more or less) to a halt (see exa-1.png). When I increase the heap the program takes much longer and the GC is running more or less all the time (see exa-2.png). Some more detailled information: * I can see the described behaviour under both GHC 10.4 and GHC 12.1 * Linux kernel 2.6.31-16 on a dualcore * Program compiled with ghc --make -O2 -threaded -eventlog Example.hs -o exa * Started with exa +RTS -ls and one of { -N, -N1, -N2 } Any help (pointing into the right direction, mention possibly helpful blog articles or paper, constructive critic in general) is appreciated! Best wishes, Michael PS: 1. I hope the graphs are good enough to see the problem 2. Is it a known bug that threadscope eats 100%+ CPU when I just view an eventlog? ------------------------------------------------------------------------------- -- -- Minimal test example for problems with garbage collection (both under -- GHC6.10.4 and GHC6.12.1). We calculate Pi to an arbitrary length (here -- 50.000 + n) for n times using Machin's formula[1]. -- -- [1] http://en.literateprograms.org/Pi_with_Machin's_formula_(Haskell) ------------------------------------------------------------------------------- module Main where import Control.Parallel import Control.Parallel.Strategies main = do -- 20 tasks, length 50000 + [1..20] let values = implicit (initTasks 20 50000) -- Dirty trick to force evaluation: mapM_ (print . (`mod` 10)) values ------------------------------------------------------------------------------- -- Using semi-implicit parallelization here. implicit :: [PiTask] -> [Integer] implicit tasks = parMap rdeepseq (\(PiTask k) -> calcPiPure k) tasks ------------------------------------------------------------------------------- -- Definition of a task to calculate \pi to an arbitrary length and its -- implementation. data PiTask = PiTask Integer instance Show PiTask where show (PiTask i) = "PiTask <" ++ show i ++ ">" -- Returns @number@ tasks of length @len@ up to @len+number@. We add one for -- each successive task to prevent additional compiler or runtime optimizations -- through referential transparency. In dimensions of 10^5 this should not make a -- measureable difference. initTasks :: Int -> Int -> [PiTask] initTasks number len = let len' = toEnum len num' = toEnum number in map PiTask [len'..len'+num'] -- Not used here calcPi :: Integer -> IO () calcPi digits = calcPiPure digits `pseq` return () calcPiPure :: Integer -> Integer calcPiPure digits = pi' `div` (10 ^ (10 :: Integer)) where unity = 10 ^ (toInteger digits + 10) pi' = 4 * (4 * arccot 5 unity - arccot 239 unity :: Integer) arccot :: Integral t => t -> t -> t arccot x unity = arccot' x unity 0 start 1 1 where start = unity `div` x arccot' x' u sm xpower n sign | xpower `div` n == 0 = sm | otherwise = arccot' x' u (sm + sign*term) (xpower `div` (x'*x')) (n+2) (-sign) where term = xpower `div` n

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Generating Haskell From a XSD
by jonathanGfischoff＠gmail.com 24 Dec '09

24 Dec '09

I would like to generate Haskell data types and xml serialization code from xsd. I know of DtdToHaskell but unfortunately I yet to be able to generate a valid dtd from my xsd. Is there a way to generate Haskell from a xsd? -Jonathan

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ANNOUNCE: regions-0.1.0.1
by Bas van Dijk 23 Dec '09

23 Dec '09

Hello, As explained in my previous email, I generalized the DeviceRegion monad transformer from my usb-safe package and put it in its own package: http://hackage.haskell.org/package/regions-0.1.0.1 Hackage does not seem to build documentation at the moment so if you're interested please look at the source code which you can get using: darcs get http://code.haskell.org/~basvandijk/code/regions The main goal of regions is to allow opening of scarce resoures and operating on them but to prevent operating on closed resources. The primary technique used in this package is called "Lightweight monadic regions" which was invented by Oleg Kiselyov and Chung-chieh Shan. See: http://okmij.org/ftp/Haskell/regions.html#light-weight The package provides the monad transformer: newtype RegionT resource s (pr ∷ * → *) α And the following operation for opening scarce resources (like files, memory pointers, database connections, USB devices, etc.): open ∷ (Resource resource, MonadCatchIO pr) ⇒ resource → RegionT resource s pr (RegionalHandle resource (RegionT resource s pr)) So you give it the resource you wish to open and it yields a computation in the region monad that returns a handle to the opened resource. Note that this handle is parametrized by the region itself. Finally there's the ability to run the region: runRegionT ∷ (Resource resource, MonadCatchIO pr) ⇒ (∀ s. RegionT resource s pr α) → pr α 'runRegionT' runs the given region and finally closes all opened resources automatically. Note that the 's' is quantified over the region but not over the return value. So all values that have this 's' in their type can not be returned from the given region. Remember that the handle to a resource, which we got from 'open', was parametrized by the region and so has this 's' in it. This prevents us from returning a handle to a closed resource from this function. So it's never possible to accidentally perform I/O with a closed resource. There's also the ability to concurrently run a region inside another region using: forkTopRegion ∷ (Resource resource, MonadIO pr) ⇒ TopRegion resource s () → RegionT resource s pr ThreadId Alo note the packages: http://hackage.haskell.org/package/regions-monadsfd-0.1.0.1 http://hackage.haskell.org/package/regions-monadstf-0.1.0.1 Which defines instances for the monads classes in monads-fd and monads-tf respectively. Finally, if you want to open your own scarce resource in a region, the only thing you have to do is define an instance for: class Resource resource where data Handle resource ∷ * openResource ∷ resource → IO (Handle resource) closeResource ∷ Handle resource → IO () That's it! I'm looking forward to see instances for file Handles, memory Ptrs, database connections, etc. regards, Bas

1 0

ANNOUNCE: usb-safe-0.4.1
by Bas van Dijk 23 Dec '09

23 Dec '09

Hello, I just released usb-safe-0.4.1: http://hackage.haskell.org/package/usb-safe-0.4.1 Changes compared to 0.3 (I didn't make an announcement for 0.4): * Tested with ghc-6.12.1 and base-4.2.0.0. * It turned out that an Interface can also be regarded as a scarce resource: it needs to be claimed before use and it needs to be released when you're done with it. So just as with USB devices I had to create a new region monad that supported claiming of interfaces. So now I had two region monads: a DeviceRegion and an InterfaceRegion which both were implemented exactly the same and supported the exact same operations. So the next thing I did was to generalize the region code and put it in its own package: 'regions'. This package will be announced separately. * Removed the FilteredEndpoint type. Specific endpoints can now be retrieved from the alternate directly using: 'getEndpoints'. * Added missing language extension: OverlappingInstances (The package was unusable without it) * Added convenience functions 'setConfigWhich', 'withInterfaceWhich' and 'setAlternateWhich' for quickly setting the first setting that satisfies the given predicate on its descriptor. * Hidden the ReadEndpoint and WriteEndpoint type classes so users can't write their own instances for it and thereby potentially messing up my safety guarantees. * Lots of little refactorings (is that a real word?): documentation updates, explicit imports of all used symbols, Unicode syntax and symbols (using the base-unicode-symbols package) * Finally I packaged up some example code in: darcs get http://code.haskell.org/~basvandijk/code/usb-safe-examples/ regards, Bas

1 0

ANNOUNCE: explicit-iomodes-0.1.2
by Bas van Dijk 23 Dec '09

23 Dec '09

Hello, I just released a minor update to my explict-iomodes package: http://hackage.haskell.org/package/explicit-iomodes-0.1.2 Changes: * Tested with ghc-6.12.1 and base-4.2.0.0 * Re-exported 'isEOF' from System.IO which I forgot in the previous version. * Internal changes: Explicit imports of all used symbols and Unicode syntax and symbols everywhere. regards, Bas

1 0

Is it just me... or is cabal/hackage a little broken?
by Bardur Arantsson 23 Dec '09

23 Dec '09

Hi all, Sorry about the inflammatory title, but I just got this message from an uploaded package ("hums"): Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. The thing is, I got the same message while trying to compile locally and it turned out that all I had to do was to $ cabal install <PKG-X> on all the packages that cabal complained about. So why doesn't hackage do this automagically when I upload a package? How am I supposed to know which versions of my package's dependencies (or their dependencies) are the most recently compiled by hackage? For the record: I did do a "Check package" upload first. It didn't complain. Is this an intractable problem? Am I being overly demanding (probably)? Cheers, Bárður

3 2

simple type?
by Jamie Morgenstern 23 Dec '09

23 Dec '09

Hello; I am writing some vectorized code, and I wondered exactly which types can be used in DPH? Is it true one cannot use data constructors and recursive types? Thanks, -Jamie

1 0

Haskell in image processing
by Ralf Laemmel 23 Dec '09

23 Dec '09

Disclaimer: this is an Xmas gift as opposed to serious stuff. http://professor-fish.blogspot.com/2009/12/major-breakthrough-in-image-proc… Greetings, Ralf

1 0

Missing symbol when trying to use OpenCLRaw
by Dan Piponi 23 Dec '09

23 Dec '09

Can anyone tell me what command line options I need to build a simple example using OpenCLRaw on MacOSX (ghc 6.10.4)? Using "-framework OpenCL" reduces the missing symbols down to just _clGetProgramInfo but I can't see how to eliminate that last one. Has anyone else played with OpenCLRaw? -- Dan

1 0

ANN: hnn-0.1, a haskell neural network library
by Alp Mestan 23 Dec '09

23 Dec '09

Hi, I just released the first version of my Haskell Neural Network library. It provides the very minimal features anyone would need in a neural network library. It has yet to be completed (regarding the features) and there are some ways for opitmizations. Though, I'd be very glad to hear from Haskellers about this library, get feedback, etc. Some links to get started with HNN : http://www.haskell.org/haskellwiki/HNN http://hackage.haskell.org/package/hnn -- hackage page http://mestan.fr/haskell/hnn/ -- online documentation http://alpmestan.wordpress.com/2009/12/23/hnn-0-1-has-been-released/ Please don't hesitate to try it and play with it and give as much feedback as you want ! Haskelly yours. -- Alp Mestan http://alpmestan.wordpress.com/ http://alp.developpez.com/

1 0

Control.Parallel and the Reader monad
by Paul Brauner 23 Dec '09

23 Dec '09

Hi, I'm facing the following problem. I've got come computation c :: a -> Reader e b that i'm running on several as: mapM c xs A natural optimisation of this program would to be to take advantage of Control.Parallel to run these computation in parallel, which seems sound since the Reader monad is commutative. Unfortunately, I didn't manage to do so. Has this situation been tackled with before ? Is there some library or function I've missed involving commutative monads and Control.Parallel ? Regards, Paul

1 0

Children elements with HXT
by Tony Morris 23 Dec '09

23 Dec '09

I am trying to parse XML using HXT following http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_X… Here is my XML file (way.xml): <way id="27776903" visible="true" timestamp="2009-05-31T13:39:15Z" version="3" changeset="1368552" user="Matt" uid="70"> <tag k="access" v="private"/> <tag k="highway" v="service"/> </way> The problem is when parsing, by reading the <tag> entries into the list held by the Way data structure, I cannot get anything but an empty list. Here is my parsing code: import Text.XML.HXT.Arrow newtype Way = Way { tags :: [Tag] } deriving (Eq, Show) xpWay :: PU Way xpWay = xpElem "way" (xpWrap (Way, tags) (xpList xpTag)) data Tag = Tag { k :: String, v :: String } deriving (Eq, Show) xpTag :: PU Tag xpTag = xpElem "tag" (xpWrap (uncurry Tag, k &&& v) (xpPair (xpAttr "k" xpText) (xpAttr "v" xpText))) When I run, I get the following result: Main> run = runX (xunpickleDocument xpWay [] "way.xml") [Way {tags = []}] Why is the tags list empty instead of holding two entries? -- Tony Morris http://tmorris.net/

3 3

profiling problem
by Ketil Malde 23 Dec '09

23 Dec '09

Hi, I guess I should report this? Previously, this program got a signal 11, and that happened somewhat later in the process, so I'm not sure how reproducible it is. Source and data is available, if it is of any interest. (This is using the GHC currently shipped with Ubuntu 9.10.) xml2x3prof: internal error: traverseWeakPtrList: not WEAK (GHC version 6.10.4 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Command terminated by signal 6 3577.55user 48.81system 1:00:27elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+1306970minor)pagefaults 0swaps -k -- If I haven't seen further, it is by standing in the footprints of giants

1 0

Are functional dependencies around to stay?
by Günther Schmidt 22 Dec '09

22 Dec '09

Hi everyone, I'm wondering if there is any chance that functional dependencies will not be around in the future. I do not actually understand the subject yet as such, but I'd like to make sure before I get deeper into it that it's something that will be around in the time to come. What I understand of Haskell is that it's not so much "engineered" like other languages but more "derived" from Math, well the very foundations anyway. But some things I can not determine whether or not they are derived from math and thus will stay, or are engineered and might go away, functional dependencies for instance. Günther

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what happend with the docs of base-libs in 6.10.4
by Günther Schmidt 22 Dec '09

22 Dec '09

Hi, I was just trying to consult my html-documentation of ghc-6.10.4 libraries on Windows. Almost all of the Standard Monads are missing, State, Writer etc. What happened, how do I get them back? Günther

2 1

Visible space with lhs2tex
by Felipe Lessa 22 Dec '09

22 Dec '09

Hello! Does anyone know if it is possible to turn spaces inside strings to visible spaces. That is, format " test" as "\textvisiblespace{}test" With the 'listings' package there's the option 'showspaces'. However, unfortunately this and obvious variations don't work: %format " " = "\textvisiblespace" Thanks in advance! :) -- Felipe.

1 0

Re: [Haskell-cafe] sizeOf on a type
by Patai Gergely 22 Dec '09

22 Dec '09

Hi, > I would like to write a routine like > > nextPtr :: Storable a => Ptr a -> Ptr a > nextPtr = (`plusPtr` sizeOf (undefined :: a)) > > which increments a pointer by the correct amount, but > GHC complains that the type variable "a" is ambiguous. Maybe Foreign.Marshal.Array.advancePtr is what you really need in this case. Gergely -- http://www.fastmail.fm - The professional email service

3 2

An old package and the new Cabal.
by Jason Dusek 22 Dec '09

22 Dec '09

I'm trying to update CEDICT to work with the new Cabal (after many years of neglect). http://github.com/jsnx/cedict/tree/40d426c6d7db65948cef5c357633628a5275d877 Right now, it can't seem to find the data header `c/data.h` no matter what I put in the Cabal file. I'd think these lines would do the job: includes : c/data.h c-sources : c/data.c extra-lib-dirs : c include-dirs : c The message I get is: Resolving dependencies... Found existing header file at c/data.h Configuring cedict-0.2.6... setup: Missing dependency on a foreign library: * Missing header file: c/data.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. It takes like a minute to err out, too. This isn't urgent or anything but it would be nice to know how to get this build process -- C generation followed by compilation -- to work again. -- Jason Dusek

2 2

powering of new types
by slemi 22 Dec '09

22 Dec '09

hey there and thanks for the replies to my earlier question, here is the next one: i have defined both the (*) and (/) operators for my new type data Matrix a = Matr {unMatr :: [[a]]} | Scalar a deriving (Show, Eq) (*) being matrix multiplication and (/) being multiplication with inverse i also defined them for scalars this allows me to use the (^^) powering operator, which works fine with non-zero exponents. however to my surprise when i try (^^ 0) the answer is (Scalar 1), and not the identity matrix as expected. does this mean that (a ^^ 0) is not defined as (a ^^ 1 * a ^^ (-1)) (or better yet (a / a)) in the prelude? if so, can i redefine it so that it gives the right answer? i am also very interested in how ghci got the answer (Scalar 1), it seems quite magical:) here are my operator definitions: instance RealFrac a => Num (Matrix a) where (Matr as) * (Matr bs) = Matr $ timesH as bs (Scalar a) * (Matr b) = Matr $ timesSH a b (Matr a) * (Scalar b) = Matr $ timesSH b a (Scalar a) * (Scalar b) = Scalar (a * b) instance RealFrac a => Fractional (Matrix a) where (Scalar a) / (Scalar b) = Scalar (a / b) (Matr a) / (Scalar b) = Matr $ map (map (/ b)) a a / b | a == b = idM a | otherwise = a * (inv b) liftMatr = (Matr .) . (. unMatr) transH = foldr (zipWith (:)) (repeat []) timesH :: RealFrac a => [[a]] -> [[a]] -> [[a]] timesH as bs = map (\p -> map (foldr1 (+) . zipWith (*) p) (transH bs)) as timesSH :: RealFrac a => a -> [[a]] -> [[a]] timesSH a = map (map (a *)) idM :: RealFrac a => Matrix a -> Matrix a idM = liftMatr idMH idMH :: RealFrac a => [[a]] -> [[a]] idMH as = let n = length as in take n . map (take n) . iterate (0 :) $ 1 :repeat 0 -- View this message in context: http://old.nabble.com/powering-of-new-types-tp26891202p26891202.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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