- Haskell-Cafe - Haskell.org

Gödel's System T
by Petr Pudlak 11 Nov '10

11 Nov '10

Hi, I was reading the paper "Total Functional Programming" [1]. I encountered an interesting note on p. 759 that primitive recursion in a higher-order language allows defining much larger set of function than classical primitive recursion (which, for example, cannot define Ackermann's function). And that this is studied in in Gödel's System T. It also states that this larger set of primitive functions includes all functions whose totality can be proved in first order logic. I was searching the Internet but I couldn't find a resource (a paper, a book) that would explain this in detail, give proofs etc. I'd be happy if someone could give me some directions. Thanks, Petr [1] http://www.jucs.org/jucs_10_7/total_functional_programming/jucs_10_07_0751_…

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ANN: network-2.2.3, merger with network-bytestring
by Johan Tibell 11 Nov '10

11 Nov '10

Hi all, I like to announce a new version of the network package, network-2.2.3. You can install the latest version by running: cabal update && cabal install network This version marks the end of the network-bytestring package, which has now been merged into the network package. This means that efficient and correct networking using ByteStrings is available as part of the standard network package. As part of the merger, two new modules have been added: Network.Socket.ByteString and Network.Socket.ByteString.lAzy This release is backwards compatible with the previous release. Johan

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build-type: configure on windows
by Nils Schweinsberg 11 Nov '10

11 Nov '10

Hi, I'm having a few problems with cabals build-type "configure" on windows, especially with the packages "curl" and "pcre-light". Both fail either with: * Missing C library: pcre

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Haskell Weekly News: Issue 158 - November 10, 2010
by Daniel Santa Cruz 11 Nov '10

11 Nov '10

Welcome to issue 158 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 31 - November 06. I'd like to start things off with a quote from Jeremy O'Donoghue this past week on a thread that produced plenty of laughs: Haskell was developed by Isaac Newton in the 15th Century as a tool to help his investigations into the Alchemic arts. It was rediscovered in the 1980s by three Cambridge undergraduates who were browsing through Newton's laboratory notebooks looking for smutty jokes in the margins, and has since developed into an elaborate joke perpetrated by elite computer scientists who believe that predictable order of execution is contrary to natural law. The current version of Haskell is Haskell 1714, which adds syntactic sugar for Zygohistomorphic Premorphisms to the original language definition of 1693. This week we had 378 messages posted to Haskell-Cafe (up 76 from last week!), 30 stories posted to Reddit (1 more than last week), 40 questions posted to StackOverflow (15 more than last week), and 96 updates to Hackage (11 more than last week). Busy busy week! From all the noise in the intertubes, it sounds like BelHac was awesome. There were plenty of kudos to Jasper Van der Jeugt for how well things went. Jasper posted a [2]summary blog, and we also have some [3]pictures from the event. Announcements Will Sonnex [4]updated Zeno, a fully automated inductive theorem proving tool for Haskell programs. Gregory D. Weber [5]updated Sifflet, a visual, functional programming language and support system for students learning about recursion. Mario BlaÅ¾eviÄ/= [6]updated monad-coroutine and SCC to version 0.6. The monad-coroutine package exports a generic monad transformer: Coroutine: Functor s => MonadTrans (Coroutine s). Johan Tibell informed us that he has [7]merged network-bytestring into the network package. This means that efficient and correct networking using ByteString is available as part of the standard network package. By far, the most popular thread this week in -cafe was a thread entitled [8]"Haskell is a scripting language inspired by Python." As you can imagine, many could not resist! The thread diverted into some interesting historical notes about Haskell. Top Reddit Stories * Learn You a Haskell: Zippers Domain: learnyouahaskell.com Score: 91, Comments: 44 On Reddit: http://www.reddit.com/r/haskell/comments/e0qqz/learn_you_a_haskell_zippers/ Original: http://learnyouahaskell.com/zippers * Neat Haskell commenting trick Domain: self.haskell Score: 78, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dyvsi/neat_haskell_commenting_tric… Original: /r/haskell/comments/dyvsi/neat_haskell_commenting_trick/ * Haskell Halloween! Domain: i.imgur.com Score: 53, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dyznc/haskell_halloween/ Original: http://i.imgur.com/t7qWf.jpg * Circular wavefronts Domain: claudiusmaximus.goto10.org Score: 39, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/e02vp/circular_wavefronts/ Original: http://claudiusmaximus.goto10.org/cm/2010-11-02_circular_wavefronts.html * Optimizing Haskell Loops With LLVM Domain: dmpots.com Score: 35, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/e1yfn/optimizing_haskell_loops_wit… Original: http://www.dmpots.com/blog/2010/11/05/optimizing-haskell-loops-with-llvm.ht… * Obtaining the name of a function in Haskell Domain: mainisusuallyafunction.blogspot.com Score: 24, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/e1i3q/obtaining_the_name_of_a_func… Original: http://mainisusuallyafunction.blogspot.com/2010/11/obtaining-name-of-functi… * Generative Type Abstraction and Type-level Computation Domain: cis.upenn.edu Score: 17, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/dzx80/generative_type_abstraction_… Original: http://www.cis.upenn.edu/~sweirich/newtypes.pdf * Relations rather than functions - where to learn about them? Domain: self.haskell Score: 17, Comments: 17 On Reddit: http://www.reddit.com/r/haskell/comments/e1520/relations_rather_than_functi… Original: /r/haskell/comments/e1520/relations_rather_than_functions_where_to_learn/ * Haskellers.com: Special Interest Groups feature active Domain: haskellers.com Score: 16, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/dzkel/haskellerscom_special_intere… Original: http://www.haskellers.com/news/6/ * The Utrecht Haskell Compiler now in github Domain: github.com Score: 16, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/e26ar/the_utrecht_haskell_compiler… Original: https://github.com/uhc/uhc Top StackOverflow Questions * how to do this in haskell? Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4106247/how-to-do-this-in-haskell * Haskell define Merge using mapReduce Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4099627/haskell-define-merge-using-mapre… * Real GHC RTS memory usage Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4088351/real-ghc-rts-memory-usage * reading file and accessing in haskell (functional programming language) Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4074342/reading-file-and-accessing-in-ha… * Converting tree types Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4073091/converting-tree-types * Printing Binary Tree per Layer in a List Question (votes: 0, answers: 3): Read on SO: http://stackoverflow.com/questions/4068681/printing-binary-tree-per-layer-i… * Comparing Bool Value in Matrix Question (votes: 0, answers: 2): Read on SO: http://stackoverflow.com/questions/4063553/comparing-bool-value-in-matrix * Converting Int to Integer Question (votes: 0, answers: 3): Read on SO: http://stackoverflow.com/questions/4061777/converting-int-to-integer * Integrating Haskell with .Net? Question (votes: 1, answers: 3): Read on SO: http://stackoverflow.com/questions/4111467/integrating-haskell-with-net * Putting existentials into monad Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4106793/putting-existentials-into-monad Quotes of the week * Luke Palmer:Also they "don't scale well", which I guess means that they don't make it inconvenient to design badly. * merijn: <dankna> someday we'll need meta-software that converts from obsolete languages into modern ones <merijn> Don't we call that compilers? * dolio: Let's spawn a thread every time we want to and two booleans. * ray: how about a language where you program with unit vectors: orientation oriented programming * Axman6: Haskell is easy, you just apply functions to arguments, and then you apply them to other arguments. Axman6's girlfriend: I don't like conflict * monochrom: in soviet haskell, functions run you * geheimdienst: in soviet russia, i remember lambdabot's quotes * Funktorsalat: sum is just the free monoid functor applied to the mediating arrow of the terminality of () * geheimdienst: ah, yes. now we have to make a lambdacat captioned "kittehs no bery gud at speling. we says leksah fur lexer and monad fur moniod in catagory of ennobunktors" * Vanadium: That tripped my C++ highlight. * jmcarthur: what you just typed isn't even well typed About the Haskell Weekly News New editions are posted to [9]the Haskell mailing list as well as to [10]the Haskell Sequence and [11]Planet Haskell. [12]RSS is also available, and headlines appear on [13]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://jaspervdj.be/posts/2010-11-09-belhac-summary.html 3. http://www.flickr.com/groups/1566670@N23/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18277 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18279 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18281 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/82798 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/82995 9. http://www.haskell.org/mailman/listinfo/haskell 10. http://sequence.complete.org/ 11. http://planet.haskell.org/ 12. http://sequence.complete.org/node/feed 13. http://haskell.org/

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Fwd: Questions about lambda calculus
by Patrick LeBoutillier 11 Nov '10

11 Nov '10

Hi all, Sorry for cross-posting this, but I didn't get anything on the beginners list so I thought I'd give it a try here. Thanks, Patrick ---------- Forwarded message ---------- From: Patrick LeBoutillier <patrick.leboutillier(a)gmail.com> Date: Thu, Nov 4, 2010 at 2:02 PM Subject: Questions about lambda calculus To: beginners <beginners(a)haskell.org> Hi all, I've been reading this article: http://perl.plover.com/lambda/tpj.html in which the author talks about the lambda calculus and uses examples in Perl (here's a link directly to the code: http://perl.plover.com/lambda/lambda-brief.pl) I'm a total newbie with respect to the lambda calculus. I tried (naively) to port these examples to Haskell to play a bit with them: fTRUE = \a -> \b -> a fFALSE = \a -> \b -> b fIF = \b -> \x -> \y -> b x y fPAIR = \a -> \b -> \f -> f a b fFIRST = \p -> p fTRUE fSECOND = \p -> p fFALSE fZERO = fPAIR fTRUE fTRUE fSUCC = \x -> fPAIR fFALSE x fIS_ZERO = \x -> fFIRST x fPRED = \x -> fSECOND x fONE = fSUCC fZERO fTWO = fSUCC fONE fADD = \m -> (\n -> fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n))) but I couldn't get fADD to compile: Occurs check: cannot construct the infinite type: t = (t1 -> t1 -> t1) -> t Probable cause: `fADD' is applied to too many arguments In the third argument of `fIF', namely `(fADD (fPRED m) (fSUCC n))' In the expression: fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n)) I think its because in these Perl examples all functions are treated as being of the same type (number or type of args doesn't matter), but this is not the case in Haskell. Is there any way to create code similar to this in Haskell? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Typeable state map
by Vladimir Matveev 10 Nov '10

10 Nov '10

Hi, I want to use something like (Control.Monad.StateT (Data.Map.Map String Dynamic) m). So I wrote VarStateT datatype and corresponding classes (Control/Monad/VarState.hs in [1]). Then I tried to use it in my program (Math/Eigenvalues.hs in [1]). But compiler refused to compile it with many type errors. I thought that it would be sufficient to use ScopedTypeVariables and write all type annotations. It helped slightly, many messages were gone, but now compiler throws an error I don't know how to solve. ghc --make -o out/eigenvalues -outputdir build Main.hs [3 of 4] Compiling Math.Eigenvalues ( Math/Eigenvalues.hs, build/Math/Eigenvalues.o ) Math/Eigenvalues.hs:14:30: Could not deduce (MutableMatrix a e m) from the context (MutableMatrix a2 e2 m, Typeable a2, Typeable e2, Floating e2, Ord e2) arising from a use of `getPhi' at Math/Eigenvalues.hs:14:30-35 Possible fix: add (MutableMatrix a e m) to the context of the type signature for `compute' In the second argument of `(>>)', namely `getPhi' In the first argument of `(>>)', namely `maxElem a >> getPhi' In the first argument of `(>>)', namely `maxElem a >> getPhi >> buildB' Math/Eigenvalues.hs:14:40: Could not deduce (MutableMatrix a1 e1 m) from the context (MutableMatrix a2 e2 m, Typeable a2, Typeable e2, Floating e2, Ord e2) arising from a use of `buildB' at Math/Eigenvalues.hs:14:40-45 Possible fix: add (MutableMatrix a1 e1 m) to the context of the type signature for `compute' In the second argument of `(>>)', namely `buildB' In the first argument of `(>>)', namely `maxElem a >> getPhi >> buildB' In the first argument of `(>>=)', namely `maxElem a >> getPhi >> buildB >> buildA' How to sort this out? [1] http://dl.dropbox.com/u/1415321/prog/eigenvalues.tar.gz

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Finding the contents of haskell platform?
by Magnus Therning 10 Nov '10

10 Nov '10

I know there's a .cabal file for the latest version of HP somewhere, but I can't coerce Google into finding me a link that actually works. Furthermore, the following page: http://hackage.haskell.org/platform/contents.html does list all the contents, but to my big surprise it doesn't link to the specific versions of the packages for HP, instead it links to the latest version found on Hackage. Would someone with the power to make changes on the HP pages *please* make it as easy as possible to find the *exact* specification of what HP contains? Please, pretty please with sugar on top. No, a changelog entry (http://hackage.haskell.org/platform/changelog.html) is not very helpful (why the HP front page links to it I can't understand). Going via the Haskell wiki (http://www.haskell.org/haskellwiki/Haskell_Platform#What.27s_in_the_platform) to find a link to the .cabal (http://code.haskell.org/haskell-platform/haskell-platform.cabal) is not that user friendly. It's even worse that the latter link doesn't seem to work at all at the moment. (The short irritated tone in this email accurately shows my desperation with the situation: I thought I would be able to find this information with only 5 minutes to spare before my next meeting.) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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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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