November 2008 - Haskell-Cafe

Calling Haskell from other languages?
by Colin Paul Adams 14 Nov '08

14 Nov '08

Is there a way to call Haskell code from other languages? I have looked on the wiki, and as far as I can see, it only talks about the other way round (when Haskell is the main program). -- Colin Adams Preston Lancashire

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ANN: bustle-0.1, a D-Bus activity charting tool
by Will Thompson 14 Nov '08

14 Nov '08

Hi, For the (probably small) set of people who are interested both in D-Bus and in Haskell, I'm pleased to announce the first release of Bustle, a tool to show diagrams of D-Bus traffic for profiling purposes. It consists of a small C executable to log traffic, and a Gtk+ application which draws diagrams using Cairo. There's no single web page for it yet. http://resiak.livejournal.com/59590.html has slightly more information and context, the release is at http://people.collabora.co.uk/~wjt/bustle/releases/bustle-0.1.tar.gz and the source at git://git.collabora.co.uk/git/user/wjt/bustle.git . Unfortunately, it's not on Hackage, as I haven't written the necessary glue to get Cabal to compile the C executable, but I hope to have time to work on getting Cabal to invoke make appropriately for the next release. Thanks, -- Will

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TimeDiff to Int?
by Lyle Kopnicky 14 Nov '08

14 Nov '08

Hi folks, I had some code using the oldtime package, and want to convert it to use the time package. One of the things I need to do is calculate the number of seconds since midnight. The easy part is getting a TimeDiff result: utc <- getCurrentTime tz <- getCurrentTimeZone let td = timeOfDayToTime $ localTimeOfDay $ utcToLocalTime tz utc Now td is a TimeDiff representation of the number of seconds since midnight. It prints nicely, but I'm having a heck of a time figuring out how to truncate it to an Int. The floor function is only supported by the RealFrac class. Although TimeDiff has an instance of RealFrac and Fractional, it doesn't have an instance of RealFrac. I looked up the various to* and from* functions and have come up short. fromEnum yields an Int but it's the wrong value. I know I could use show and then use readS to get an Integer, or use formatTime (and reparse that), but that's a hack. I can convert it to a TimeOfDay which gives me hours, minutes, and seconds, but then I have to do arithmetic on it, and the seconds are of type Pico, which I can't call floor on either. I can convert it to a Rational via timeOfDayToDayFraction, but a TimeDiff is already a Rational those don't have floor. What am I missing? There has got to be an easy way to count seconds! Thanks, Lyle

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[magnus@galois.com: Re: [Haskell-cafe] Monads for Incremental computing]
by Don Stewart 14 Nov '08

14 Nov '08

Magnus writes: Thanks to Peter Jonsson, the source is now on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Adaptive Cheers, Magnus Donnie Jones wrote: > Hello sanzhiyan, > > I believe this is the same paper, the pdf is available here: > http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.8.3014 > > Cheers. > __ > Donnie > > On Thu, Nov 13, 2008 at 9:02 PM, Don Stewart <dons(a)galois.com > <mailto:dons@galois.com>> wrote: > > I sit next to the author, CC'd. > > -- Don > > sanzhiyan: > > I'm looking for the source code of the library for adaptive > computations > > exposed in Magnus Carlsson's "Monads for Incremental > Computing"[1], but > > the link in the paper is broken. > > So, does anyone have the sources or knows how to contact the > author? > > > > [1] http://portal.acm.org/citation.cfm?id=581482 > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > http://www.haskell.org/mailman/listinfo/haskell-cafe > > ----- End forwarded message -----

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ghci start-up banner
by Cetin Sert 14 Nov '08

14 Nov '08

Hi all, after reading this announcement on the Galois blog: http://www.galois.com/blog/2008/11/13/tech-talk-mechanically-verified-lisp-… I installed Hol4 to play around with it a little bit. I made 2 obvious observations ^_^: 1) It looks like the Hol4 system is just a ML module that gets loaded into the Moscow ML interpreter with the help of a script 2) it prints a custom banner after loading And now I have the following questions: == about proof assistants in general == 1) how many other proof assistants are built the same way as Hol4: as a module loaded into some interpreter? 2) is there a paper or even better a funny but detailed comparison of proof assistants? == about custom banners at module load time == 1) is there a way to do it in ghci? Regards, CS

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openal & alut troubles
by Stephen 14 Nov '08

14 Nov '08

The link to the OpenAL documentation [ http://haskell.org/ghc/docs/latest/html/libraries/OpenAL/Sound-OpenAL.html ] from here [ http://www.haskell.org/haskellwiki/OpenAL ] seems to be down. Also, I tried to use cabal to install ALUT and got the following checking for unistd.h... yes checking AL/alut.h usability... no checking AL/alut.h presence... no checking for AL/alut.h... no checking OpenAL/alut.h usability... no checking OpenAL/alut.h presence... no checking for OpenAL/alut.h... no configure: error: no ALUT header found, so this package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: ALUT-2.1.0.0 failed during the configure step. The exception was: exit: ExitFailure 1 :/

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Monads for Incremental computing
by sanzhiyan＠gmail.com 13 Nov '08

13 Nov '08

I'm looking for the source code of the library for adaptive computations exposed in Magnus Carlsson's "Monads for Incremental Computing"[1], but the link in the paper is broken. So, does anyone have the sources or knows how to contact the author? [1] http://portal.acm.org/citation.cfm?id=581482

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What is Haskell used for?
by Jason Dagit 13 Nov '08

13 Nov '08

I was looking over the Hackage categories today to figure out what Haskell is *really* used for? I just really quickly sorted the categories by number of packages. It's really interesting to see System, Graphics and Network so high on the list. Clearly, next to data types and text processing, people are doing systems, graphics and network programming. C had better watch out! Jason Categories: Data (89) Text (84) System (83) Graphics (50) Network (48) Development (45) Web (41) Control (37) Language (36) Math (36) Database (33) Sound (31) Codec (26) Game (26) User Interfaces (23) Unclassified (22) Parsing (19) Compilers/Interpreters (16) Data Structures (16) Distribution (15) Testing (15) Algorithms (12) XML (12) Natural Language Processing (10) Monads (9) Pugs (9) Bioinformatics (8) Generics (8) GUI (8) Hardware (3) Foreign (7) FRP (6) Concurrency (6) Reactivity (5) Distributed Computing (5) Source-tools (5) Music (4) Cryptography (4) Editor (4) Interfaces (4) Physics (4) AI (3) Code Generation (3) Codecs (3) Other (3) Search (3) List (2) Data Mining (2) Combinators (2) Console (2) Debug (2) Numerical (2) Theorem Provers (2) Utils (2) Comonads (1) Composition (1) Desktop (1) FFI Tools (1) Finance (1) Help (1) Middleware (1) Monadic Regions (1) Numeric (1) Screensaver (1) Scripting (1) System.Console (1) Test (1) Trace (1) User-interface (1)

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The Gtk Clutter API
by Darrin Thompson 13 Nov '08

13 Nov '08

http://www.clutter-project.org/ What is the degree-of-difficulty for creating Haskell bindings for Clutter? Is it simple to extend gtk2hs to cover it? Or has it been done and I just don't see it? Thanks. -- Darrin

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Getting rid of functional dependencies with type proxies
by Tobias Bexelius 13 Nov '08

13 Nov '08

Hi, some time ago I asked about a problem I had with functional dependencies conflicting when using overlapping instances in a code like this: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-incoherent-instances #-} data V2 a = V2 a a class Vec v a where dot :: v a -> v a -> a instance Num a => Vec V2 a where V2 a1 a2 `dot` V2 b1 b2 = a1*b1+a2*b2 class Mult a b c | a b -> c where (*.) :: a -> b -> c instance (Num x) => Mult x x x where (*.) = (*) instance (Vec a x) => Mult (a x) (a x) x where (*.) = dot {- Functional dependencies conflict between instance declarations: instance [overlap ok] (Num x) => Mult x x x instance [overlap ok] (Vec a x) => Mult (a x) (a x) x -} The problem here is that the dependency check is occuring too soon. Removing the dependency would solve the problem, but then you would need to put type annotations everywhere you use the multiplication operator, and that's not acceptable. However, I did found a solution I would like to share with you: Creating a "type proxy"! I find it strange that such a simple solution to such a (for me at least) common problem hasn't been documented more. This is the working code: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-incoherent-instances #-} data V2 a = V2 a a class Vec v a where dot :: v a -> v a -> a instance Num a => Vec V2 a where V2 a1 a2 `dot` V2 b1 b2 = a1*b1+a2*b2 class Mult a b c | a b -> c where (*.) :: a -> b -> c class MultProxy a b c where (*..) :: a -> b -> c instance MultProxy a b c => Mult a b c where (*.) = (*..) instance (Num x) => MultProxy x x x where (*..) = (*) instance (Vec a x) => MultProxy (a x) (a x) x where (*..) = dot Thanks to Emil Axelsson that pointed me to http://haskell.org/haskellwiki/GHC/AdvancedOverlap, where similar problems where discussed. Again, I haven't found links to this page in any of the Haskell tutorials I've read so far. /Tobias

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