November 2010 - Haskell-Cafe

Is there a better way to convert from UTCTime to EpochTime ?
by David Virebayre 10 Nov '10

10 Nov '10

I want to set a file's modification time to the time I got from exif data. To get the time from exif, I found : Graphics.Exif.getTag :: Exif -> String -> IO (Maybe String) To set the file modification time, I found : System.Posix.Files.setFileTimes :: FilePath -> EpochTime -> EpochTime -> IO () Assuming I do find a Time in Exif, I need to convert a String to an EpochTime. - With parseTime I can get a UTCTime. - With utcTimeToPOSIXSeconds I can get a POSIXTime - With a POSIXTime I can more or less get an EpochTime To convert from a UTCTime to EpochTime this typechecks, but I'm not sure it's correct : fromIntegral . fromEnum . utcTimeToPOSIXSeconds $ etime This is part of a function getTime that will return the time from Exif data, if present, otherwise the file's modification time : getTime (path,stat) = do let ftime = modificationTime $ stat err (SomeException _) = return ftime time <- liftIO $ handle err $ do exif <- Exif.fromFile path let getExifTime = MaybeT . liftIO . Exif.getTag exif res <- runMaybeT $ do tmp <- msum . map getExifTime $ [ "DateTimeOriginal", "DateTimeDigitized", "DateTime" ] MaybeT . return . parseTime defaultTimeLocale "%Y:%m:%d %H:%M:%S" $ tmp case res of Nothing -> return ftime Just etime -> return . fromIntegral . fromEnum . utcTimeToPOSIXSeconds $ etime return (path,time) Questions : 1) is there a better way to convert the time ? 2) any general comments on getTime ? Thanks, David.

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HTTP 4000.1.0 release
by Ganesh Sittampalam 09 Nov '10

09 Nov '10

Hi, I've just released HTTP 4000.1.0 to hackage: - Fixed a bug that caused infinite loops for some URLs on some platforms (whether the URL was a trigger is probably related to the size of the returned data, and the affected platforms. Based on a patch by Daniel Wagner. - This is technically a breaking change (and hence there is an API version bump) as the fix introduces a new member to the HStream class which can be used to define a new payload type instead of String/ByteString, but I scanned hackage and noone actually seems to be doing that. - Applied a patch by Antoine Latter to fix a bug in the handling of 301 and 307 response codes. Here's his description of the change: *** The Network.Browser module is a convenience layer over the Network.Http modules offering many of the niceties one would expect in a web-browser - cookies, user authentication, and transparent handling of redirects. When using Network.Browser, previously (since version 4000.0.8) a redirect response code of 301 or 307 from the server would force the redirected request to be sent as a GET, even if the original request was of some other HTTP method. This behavior is only appropriate for a response code of 302 or 303 and has been fixed. *** The upstream repo is now on github: https://github.com/haskell/HTTP, though I may well switch back to darcs if I remain maintainer for long. I'd like to emphasise that I have no particular desire to maintain HTTP and if anyone else would like to take over they would be very welcome! Cheers, Ganesh

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ActionScript Byte Code backend ?
by Aaron Gray 09 Nov '10

09 Nov '10

Is there a Flash ActionScript Byte Code generating backend for Haskell ? I know there was an older SWF 3 backend :- http://www.n-heptane.com/nhlab/repos/haskell-swf/ But is there anything more up to date ? Aaron

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change in overlapping instance behavior between GHC 6.12 and GHC 7 causes compilation failure
by Jeremy Shaw 09 Nov '10

09 Nov '10

Hello, I have a module, XMLGenerator, which has some overlapping instances. I have a second module, Test, which imports that module and also adds some more overlapping instances. Both modules contain {-# LANGUAGE OverlappingInstances #-} at the top. Under some old version of 6.13 (and probably 6.12), if I put both modules in the same directory and try to load Test.hs, it gets the error: Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' at Test.hs:16:15-21 Matching instances: instance (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined at XMLGenerator.hs:16:10-68 instance (XML m ~ x, XMLGen m) => EmbedAsChild m x -- Defined at XMLGenerator.hs:19:10-51 In the first argument of `($)', namely `asChild' In the expression: asChild $ (genElement "foo") In the definition of `asChild': asChild b = asChild $ (genElement "foo") If I put the XMLGenerator module in a separate package, dummy-hsx, and the Test modules links against it, I still get the error. *but* if I add: Extensions: OverlappingInstances to the dummy-hsx.cabal file, then Test.hs compiles just fine! So, for starters, I do not understand why that happens. Under GHC 7.0rc1, modifying the .cabal file has no effect. Instead I always get the error: Test.hs:16:15: Overlapping instances for EmbedAsChild (M IO) (XMLGenT m (XML m)) arising from a use of `asChild' Matching instances: instance [overlap ok] (m1 ~ m, EmbedAsChild m c) => EmbedAsChild m (XMLGenT m1 c) -- Defined in XMLGenerator (The choice depends on the instantiation of `m' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) Adding the IncoherentInstances flag does make it compile -- but I have never enabled that flag and not regretted it. What changed between GHC 6.12 and GHC 7.0? Is there a some solution besides using IncoherentInstances in every module that imports XMLGenerator? I have attached XMLGenerator.hs, Test.hs, and dummy-hsx.cabal. thanks! - jeremy

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Re: [Haskell-cafe] Error installing hp2any-graph on Snow Leopard
by John Lato 09 Nov '10

09 Nov '10

> > From: Mark Spezzano <mark.spezzano(a)chariot.net.au> > > Hi, > > I get the following error when doing a 'cabal install hp2any-graph' (the > profiling tool) > > Does anyone know why? (I'm running Snow Leopard and I've tried using other > flags suggested...but with no success). GLUT is already installed via > MacPorts and via cabal. > Have you tried to build any Haskell programs using GLUT? Do they work? John

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Combining wl-pprint and ByteString?
by Mark Spezzano 09 Nov '10

09 Nov '10

Hi all, I want to do the following tasks in this order: 1. Read text from standard input (should this be stored internally in my program as a ByteString or as a String?) 2. Process the text via left justifying it and making it word-wrap (again, internally, should I store this as a ByteString or String?) 3. Format the text using the Wadler-Leijen Pretty Printer (to the best of my knowledge this only processes Strings, but not ByteStrings) 4. Print to Standard output the formatted (i.e. colourised, and justified) text (again, should this be as a ByteString or as a String?) I thought that the reason that most people use ByteString was for _performance_, but wl-pprint can only process Strings to the best of my knowledge. I would, ideally, like to use ByteStrings everywhere...all throughout my code. Is this possible with coloured text? I want to boost up the performance of my application dramatically, so in theory I could process everything as a String, then as the last second pack everything processed into a ByteString and print it out...I think that this might work. Then again it might not because of the types expected. Any help with this would be appreciated. Cheers, Mark Spezzano

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ANNOUNCE: Haskell Communities and Activities Report (19th ed., November 2010)
by Janis Voigtländer 09 Nov '10

09 Nov '10

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (19th edition, November 2010) is now available in PDF and HTML formats: http://haskell.org/communities/11-2010/report.pdf http://haskell.org/communities/11-2010/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2011: target deadline for contributions to the May 2011 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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iMAP library
by A Smith 09 Nov '10

09 Nov '10

I'm looking for a Haskell library to allow me to access an IMAP mailbox. Anyone know of one I could use ? -- Andrew

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Francisco Vieira Souza deixou uma mensagem para você no Badoo!
by Badoo 08 Nov '10

08 Nov '10

Você tem uma nova mensagem em Badoo! Francisco Vieira Souza deixou uma mensagem pra você. Siga este link para abrir: http://us1.badoo.com/01121329442/in/nYfwm9FNeVQ/?lang_id=61 Mais gente que espera pacientemente por você: Douglas Levita (Teresina, Brasil) Naikely (Teresina, Brasil) Paula Hullek (Teresina, Brasil) http://us1.badoo.com/01121329442/in/nYfwm9FNeVQ/?lang_id=61 Se os links desta mensagem não funcionam, copie e cole-os na barra de endereços do seu browser. Este email é parte do sistema de entrega de mensagens enviadas por Francisco Vieira Souza. Se você recebeu este email por engano, por favor, ignore. Em breve a mensagem será removida do sistema. Divirta-se! A Badoo Equipe Você recebeu este email, porque um membro do Badoo deixou uma mensagem para você no Badoo. Esta é uma mensagem de somente envio. Respostas a esta mensagem não são monitoradas ou respondidas. Se você não quer receber mais mensagens do Badoo, por favor nos notifique: http://us1.badoo.com/impersonation.phtml?lang_id=61&mail_code=21&email=hask…

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Decoupling type classes (e.g. Applicative)?
by Dominique Devriese 08 Nov '10

08 Nov '10

Hi all, I have a problem with the design of the Applicative type class, and I'm interested to know people's opinion about this. Currently, the Functor and Applicative type class are defined like this: class Functor f where fmap :: (a -> b) -> f a -> f b class Functor f => Applicative f where pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b My problem is that in the grammar-combinators library [1], the pure combinator is too general for me. I would propose a hierarchy like the following: class Pointed f where pure :: a -> f a class ApplicativeC f where (<*>) :: f (a -> b) -> f a -> f b The original type class Applicative can then be recovered as follows, and the applicative laws can be specified informally in this class's definition. class (Pointed f, ApplicativeC f, Functor f) => Applicative f where This would allow me to restrict injected values to stuff I can lift into Template Haskell later on: class LiftablyPointed f where pureL :: (Lift a) -> a -> f a pureL' :: a -> Q Exp -> f a class (LiftablyPointed f, ApplicativeC) => LiftablyApplicative f where This problem currently makes it impossible for me to use the (<*>) combinator and I have to redefine it under a different name (I currently use (>>>)). To me the problem seems similar to the well known example of the inclusion of the fail primitive in the monad class, where the general opinion seems to be that it was a bad idea to include fail in the Monad class (see e.g. the article on the haskell wiki about handling failure [2]). I've been thinking about the following type class design principles: * Only include two functions in the same design class if both can be implemented in terms of each other. * Only introduce a dependency from type class A to type class B if all functions in type class B can be implemented in terms of the functions in type class A or if type class A is empty. (Disclaimer: I currently do not follow these principles myself ;)) I would like to know people's opinions about this. Are there any issues with this advice that I don't see? Have other people encountered similar problems? Any interesting references? Thanks, Dominique Footnotes: [1] http://projects.haskell.org/grammar-combinators/ [2] http://www.haskell.org/haskellwiki/Failure

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