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ANNOUNCE: SourceGraph-0.1 and Graphalyze-0.3
by Ivan Miljenovic 08 Oct '08

08 Oct '08

I've now uploaded my SourceGraph program to Hackage [1]. It's rather simple at the moment, but if you pass in the .cabal file as a parameter (e.g. run it as "SourceGraph Foo.cabal"), it will create in the same directory as the .cabal file a Directory called "SourceGraph" that contains an html report of some basic graph-theoretic analysis of your code. The output format isn't ideal, but it should serve it's purpose for now (I'll fix it up and actually make it usable once my Thesis has been handed in). What I'd appreciate if people could try it out and tell me if there's any code, etc. that it can't parse. At the moment, it ignores all Data-based functions (e.g. class and instance declarations as well as record functions) and only looks at "stand-alone" functions (i.e. normal functions). SourceGraph requires version 0.3 of my Graphalyze library (version 0.2 added the reports in, but had some bugs that 0.3 fixes). [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SourceGraph -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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[hmatrix] build error
by Xiao-Yong Jin 08 Oct '08

08 Oct '08

Hi, looks like I hit a bug, but I'm not sure which software it belongs to, gcc, ghc or atlas? ---------- error doing "runhaskell Setup build" ---------- Preprocessing library hmatrix-0.4.1.0... /usr/bin/ld: dist/build/Numeric/GSL/Special/Internal_hsc_make: hidden symbol `__powidf2' in /usr/lib/gcc/x86_64-unknown-linux-gnu/4.3.2/libgcc.a(_powidf2.o) isreferenced by DSO /usr/bin/ld: final link failed: Nonrepresentable section on output collect2: ld returned 1 exit status linking dist/build/Numeric/GSL/Special/Internal_hsc_make.o failed command was: /usr/bin/ghc -optl-lgsl -optl-llapack dist/build/Numeric/GSL/Special/Internal_hsc_make.o -o dist/build/Numeric/GSL/Special/Internal_hsc_make ---------- error doing "runhaskell Setup build" ---------- I believe `__powidf2' is a internal symbol in gcc and it is hidden. The system is archlinux x86_64 and the software I used are atlas-lapack 3.8.2-1 gcc 4.3.2-1 Using built-in specs. Target: x86_64-unknown-linux-gnu Configured with: ../configure --prefix=/usr --enable-shared --enable-languages=c,c++,fortran,objc,obj-c++,treelang --enable-threads=posix --mandir=/usr/share/man --infodir=/usr/share/info --enable-__cxa_atexit --disable-multilib --libdir=/usr/lib --libexecdir=/usr/lib --enable-clocale=gnu --disable-libstdcxx-pch --with-tune=generic Thread model: posix gcc version 4.3.2 (GCC) ghc 6.8.2-2 [("Project name","The Glorious Glasgow Haskell Compilation System") ,("Project version","6.8.2") ,("Booter version","6.8.2") ,("Stage","2") ,("Interface file version","6") ,("Have interpreter","YES") ,("Object splitting","YES") ,("Have native code generator","YES") ,("Support SMP","YES") ,("Unregisterised","NO") ,("Tables next to code","YES") ,("Win32 DLLs","") ,("RTS ways"," debug thr thr_p thr_debug") ,("Leading underscore","NO") ] I googled but no useful information. Best, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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Bluetooth libraries
by Creighton Hogg 08 Oct '08

08 Oct '08

Hello, In the interest of not duplicating any effort, I wanted to ask if anyone was aware of/working on libraries for doing bluetooth programming from Haskell. I didn't find any on hackage nor did The Google indicate that this was a filled niche. Cheers, Creighton

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Re: [Haskell] Catching error / making library functions monadic (in failure)
by Duncan Coutts 08 Oct '08

08 Oct '08

On Wed, 2008-10-08 at 18:07 +0200, Philip K.F. Hölzenspies wrote: > I think this is the core problem I have. Really, error should be implemented > as fail, rather than vice versa. I see why this doesn't work, because it > would type > > error :: Monad m => String -> m a > > even though the whole idea is that it's typed 'a'. Right, that's exactly why it would not work to give error that type. However error should only be being used for programming errors, not for expected runtime errors. > Nonetheless, I would like > it if either IO wasn't the only monad allowing catching, or standard > functions not only being available with 'error' stuff (like !!, head, etc.). Those functions do have safe counterparts for when you cannot guarantee the pre-condition. It typically involves pattern matching, eg instead of head use: case foo of [] -> (x:xs) -> Or for !! you could use take or splitAt and pattern matching. If you want more exact analogues of head, !! etc then see Neil's Safe library. > > If you can report them we can fix them. > > I think I spoke too soon. The error occur in other libraries' functions, so I > unjustly smudged your wonderful gtk2hs work (for which, btw: thanks very, > very much). ok :-) > To be honest; when I write a library, I also don't assume monadic > contexts for stuff that I do in purely functional terms. Sometimes errors > occur only in deeply nested code and "just putting in error" is the easy way > out. Especially, libraries that are not intended for "industrial strength" > production code seem to be prone to this type of coding, when they're not > already monadic. I think it's not as bad as you suggest. In haskell, error is like unchecked exceptions or assertions in Java. They are for cases that really indicate a programmer error, not for bad input or IO failures. Even these kinds of errors can be caught and contained in a sensible way using exception handlers in IO. For example supposing some web server page handler had some programming error and called a function in a way that violated the pre-condition and called error. That need not kill the entire server. The page handler just needs to catch all exceptions and return a 500 internal server error and write the error out to the logs. So it's actually a reasonably robust way of dealing with the inevitable programmer errors. Duncan

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monadic parser with Happy and Alex
by Manlio Perillo 08 Oct '08

08 Oct '08

Hi. I have completed a draft of a CSS lexer, using Alex. http://hg.mperillo.ath.cx/haskell/webtools/file/tip/src/CSS/Lexer.x The lexer use the posn wrapper. Now I'm starting to write the parser with Happy, however for the final product I would like to: 1) Be able to do I/O in the lexer, for stylesheets inclusion (@import rule) 2) be able to keep state in the parser (or lexer?), for character transcoding (@charset rule) This should be possible with Happy (and there are some example), however I don't find examples that make use of a lexer written with Alex. Should I write a lexer using only the Alex basic interface (without wrappers)? Thanks Manlio Perillo

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ANNOUNCE: Data.IVar 0.1
by Luke Palmer 08 Oct '08

08 Oct '08

Several very elegant FRP approaches are emerging, most visibly FRP.Reactive, which rely on blocking on multiple variables at once, continuing when the *first* of them is available. . . inside an unsafePerformIO, so the beautiful STM "orElse" solution is not available. The current solution is to race threads against each other, and have the one that finishes first kill the other one. This is implemented, for example, in Data.Unamb. However, our empirical tests have shown that the GHC scheduler is not *quite* good enough to handle this efficiently, and ends up introducing too much latency and nondeterminacy. The Data.IVar<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-ivar-0.1>module, just uploaded to hackage, provides an alternative to thread racing as a solution to this problem. It provides *write-once* variables which can be blocked on in parallel, without forking any threads or using STM (so it is safe to use in unsafePerformIO). Example usage from the documentation: import qualified Data.IVar as IVar import Control.Concurrent main = do iv <- IVar.new iv' <- IVar.new forkIO $ threadDelay 10000000 >> writeIVar iv' "my spoon is too big" let merger = IVar.read iv `mplus` IVar.read iv' print =<< IVar.nonblocking merger -- most likely "Nothing" print =<< IVar.blocking merger -- waits a while, then prints writeIVar iv' "i am a banana" -- throws error "IVar written twice" Enjoy! Luke

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RE: I'll do USB in Haskell - tips?
by Galchin, Vasili 08 Oct '08

08 Oct '08

Hello, I am also interested in SCSI, PCI, etc. I think the main problem/issue vis-a-vis USB/SCSI/PCI et. al. for Haskell is does (or will there exist) a Posix API and a Windows API for these buses/protocols? I read one URL but it will only support Linux. Ok .. maybe there API will be implemented for other Posix OS's like solaris, AIX, HP-UX, etc. but I wouldn't "bet the farm on it". I am not trying to be pesimistic but just save Marco some time in his life. Maybe Marco should talk to the people doing the Linux implementation for the "Posix" API and see how viable that API is before spending a year for nothing. Or Marco maybe you can be a trailblazer and come up with a better "Posix/Win32 USB API" and implement in Haskell? ;^) Very kind regards, Vasili

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Call for Contributions - Haskell Communities and Activities Report, November 2008 edition
by Janis Voigtlaender 08 Oct '08

08 Oct '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 15th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 31 October 2008 (please send your contributions to hcar(a)haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2008/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2008/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should have received your old entry as a template already (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Do not worry about writing correct LaTeX, I will be able to handle your file. Please do not use HTML or even DOC. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Type classes question
by Roly Perera 08 Oct '08

08 Oct '08

Hi, I'm reasonably well versed in Haskell but fairly new to defining type classes. In particular I don't really understand how to arrange for all instances of X to also be instances of Y. It's quite possibly that my question is ill-posed, so I'll make it as concrete as possible: in the following code, I define a Stream class, with two instances, Stream1 and Stream2. How do I arrange for there to be one implementation of Functor's fmap for all Stream instances? I currently rely on delegation, but in the general case this isn't nice. I guess I'm either misunderstanding what it is I'm trying to achieve, or how to do this kind of thing in Haskell. Any help would be greatly appreciated. many thanks, Roly Perera {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ExistentialQuantification, FunctionalDependencies #-} module Test where ------------------------------------------------------------------------------- -- Just some helpers. ------------------------------------------------------------------------------- -- Product map. prod :: (a -> b) -> (c -> d) -> (a, c) -> (b, d) f `prod` g = \(a, c) -> (f a, g c) -- Diagonal. diag :: a -> (a, a) diag x = (x, x) -- Mediating morphism into the product. both :: (a -> b) -> (a -> c) -> a -> (b, c) both f g = prod f g . diag ------------------------------------------------------------------------------- -- "Abstract" stream. ------------------------------------------------------------------------------- class Stream s a | s -> a where first :: s -> a next :: s -> s fby :: a -> s -> s -- I want every Stream to be a Functor. fmap_ :: Stream s' b => (a -> b) -> s -> s' fmap_ f = uncurry fby . both (f . first) (fmap_ f . next) ------------------------------------------------------------------------------- -- Implementation 1. ------------------------------------------------------------------------------- data Stream1 a = a :< Stream1 a instance Functor Stream1 where fmap = fmap_ instance Stream (Stream1 a) a where first (x :< _) = x next (_ :< xs) = xs fby = (:<) ------------------------------------------------------------------------------- -- Implementation 2. ------------------------------------------------------------------------------- data Stream2 a = forall b . S b (b -> a) (b -> b) instance Functor Stream2 where fmap = fmap_ instance Stream (Stream2 a) a where first (S x c _) = c x next (S x c i) = S (i x) c i fby y s = S (y, s) fst (uncurry (,) . both first next . snd)

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Darcs / Git
by Dominic Steinitz 08 Oct '08

08 Oct '08

Not really a Haskell question but I'm not sure where else to go. What's the preferred method of converting a darcs repository to git? And is there a way of converting from git to darcs? The reason I ask is that my colleague cannot get darcs to work on his Windows box. Thanks, Dominic.

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