- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Need help with scrap-your-boilerplate
by Roman Cheplyaka 06 Oct '14

06 Oct '14

On 06/10/14 21:24, Michael Sperber wrote: > Would using a different generic-programming framework help me do this > without the unsafe stuff? Yes. For example, using my traverse-with-class library: {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, FlexibleInstances, ConstraintKinds, UndecidableInstances, ImplicitParams, OverlappingInstances #-} import Data.Generics.Traversable import Data.Generics.Traversable.TH import Data.Proxy data Foo a = Foo a deriveGTraversable ''Foo -- define a class around our generic operation class Bar b where bar :: b -> b -- default case instance Bar a where bar x = x -- Foo-specific case instance Bar (Foo a) where bar x = x -- deep traversal recursiveBar :: (Bar a, GTraversable (Rec Bar) a) => a -> a recursiveBar x = let ?c = Proxy :: Proxy Bar -- traversal with the Bar dictionary in everywhere bar x Roman

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New Functional Programming Job Opportunities
by Functional Jobs 06 Oct '14

06 Oct '14

Here are some functional programming job opportunities that were posted recently: Full Stack Functional Web Engineer at Front Row Education http://functionaljobs.com/jobs/8745-full-stack-functional-web-engineer-at-f… Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8744-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

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Cryptographically secure random numbers?
by Bardur Arantsson 06 Oct '14

06 Oct '14

Hi all, I'm in a situation where I have to generate cryptographically secure random UUIDs, i.e. ones that cannot feasibly be guessed/predicted. Does anyone know what options there are in this space? Ideally, I'd like to fulfill as many of these points as possible: - Few big/complicated dependencies. - Avoid native C if possible (ditto for non-base dependencies). - Avoid RDRAND if possible. - Non-opaque UUID type. - Cross-platform if possible, but the main platform would be Linux. (Generating a UUID from a random stream would be trivial, so obviously a library for secure random numbers would be a-okay too.) Any suggestions?

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Wreq: use basic auth and also change the response timeout
by Carlo Hamalainen 06 Oct '14

06 Oct '14

Hi, I am using Wreq with basic authentication. I also want to change the response timeout. The following code compiles but the user authentication fails: import Network.HTTP.Client (defaultManagerSettings, managerResponseTimeout) opts = defaults & manager .~ Left (defaultManagerSettings { managerResponseTimeout = Just 3000000000 } ) & auth .~ basicAuth "user" "pass" Perhaps I'm not using the lens onto the manager correctly? If I use just opts = defaults & auth .~ basicAuth "user" "pass" then my program runs fine. But I need the longer timeout for some large downloads. Thanks, -- Carlo Hamalainen http://carlo-hamalainen.net

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ANN: Aivika 1.4 - simulation library
by David Sorokin 05 Oct '14

05 Oct '14

Hi Cafe, I'm glad to announce version 1.4 of my simulation library Aivika. The main improvement is that the main package aivika[1] contains a new module now, which is destined for simplifying an export of simulation results. Also the module can print in a text form the results of simulation. It recognizes queues, servers, arrival timers as well as references, variables and ordinary simulation variables. All this can help you to work on the simulation model directly from terminal if you like it, although, as before, my library provides powerful charting components that can automatically plot charts, save the CSV files, gather statistics, build histograms and unite them all in a single HTML file that can be then observed in your favorite Internet browser. I believe that all this must simplify much the process of modeling, simulation and analysis. At least, the deviation charts (using the rule of 3 sigma) cannot be underestimated as a tool of sensitivity analysis for the Monte-Carlo simulation. Another improvement is that the main package aivika can be compiled to JavaScript with help of Haste. I didn't check yet how it works in the browser but the package itself can be installed with help of haste-inst. I think this is a very interesting direction as it allows creating online flight simulators using Aivika. I hope you will find this information helpful. Best regards, David Sorokin [1] http://hackage.haskell.org/package/aivika

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Unable to build a mingw cross compiler.
by Sven Bartscher 05 Oct '14

05 Oct '14

Greetings, I'm trying to build GHC to cross compile to windows. GCC and friends are in /usr/bin/x86_64-w64-mingw32-*, libraries and headers are in /usr/x86_64-w64-mingw32/{include,lib}. I configured with: ./configure --target=x86_64-w64-mingw32 \ --with-gcc=/usr/bin/x86_64-w64-mingw32-gcc # Needed because # otherwise /usr/bin/gcc # would be used Building fails with the following error: utils/hsc2hs/dist/build/Main.o: In function `s2nI_info': (.text+0x5c5): undefined reference to `GetModuleFileNameW' collect2: error: ld returned 1 exit status (The GHC invocation line is included in the attached file) So it seems that GHC either uses the wrong linker or invokes it with the wrong search path. However, the configure script detects the right linker (/usr/bin/x86_64-w64-mingw32). Is this a bug, or a feature? System data: OS: Debian jessie Linux/GNU i386 Installed GHC (for bootstrapping): version 7.6.3 GHC I'm trying to build: version 7.8.3 Regards Sven

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Printing telnet stream to wx widget with conduit
by Tilmann 05 Oct '14

05 Oct '14

Hi, I have this little program that works 'sometimes'. It is supposed to connect to a server via telnet and print the incoming text to a wx textwidget. Usually I get an error message: ChessGui: <socket: 22>: hGetBufSome: illegal operation (handle is closed), but I acutally saw it working a few times... The whole thing is a bit out of my Haskell-league, so I´m a bit lost now. Any help is very appreciated! Best regards, Tilmann module Main where import Control.Concurrent (forkIO, killThread) import Control.Monad.IO.Class (MonadIO, liftIO) import Control.Monad.Trans.Resource import Data.Conduit import qualified Data.Conduit.Binary as CB import qualified Data.ByteString.Char8 as BS import Network (connectTo, PortID (..)) import System.IO import Graphics.UI.WX import Graphics.UI.WX.Types import Graphics.UI.WXCore.WxcDefs main = start gui gui = do f <- frame [] t <- textCtrlEx f (wxTE_MULTILINE .+. wxTE_RICH2) [font := fontFixed] e <- entry f [] set f [layout := boxed "console" (grid 5 5 [[floatLeft $ expand $ hstretch $ widget t] ,[expand $ hstretch $ widget e]])] telnet "freechess.org" 5000 t telnet :: String -> Int -> TextCtrl() -> IO () telnet host port t = runResourceT $ do (releaseSock, hsock) <- allocate (connectTo host $ PortNumber $ fromIntegral port) hClose liftIO $ mapM_ (`hSetBuffering` LineBuffering) [ hsock ] liftIO $ forkIO $ CB.sourceHandle hsock $$ (sink' t) return () sink' :: TextCtrl () -> Sink BS.ByteString IO () sink' textCtrl = do mstr <- await case mstr of Nothing -> return () Just str -> do text' <- liftIO $ (get textCtrl text) liftIO $ set textCtrl [text := text' ++ BS.unpack str ] sink' textCtrl

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makeFields without lenses
by Nicola Gigante 05 Oct '14

05 Oct '14

Good morning to everyone. I really love what makeFields from Control.Lens.TH does: generating a typeclasse HasXXX for each field _typeXXX and instances of the class for each type that has a record field with the same name. However, it generates lenses, which at this time I don’t use in my little project. I'll learn to use them well sooner or later, I promise! In the meantime, is there some module out there than provides a template haskell function that do the same thing but generating simple accessor functions instead of lenses? Or can I configure the parameters of makeFieldsWith to generate simple functions instead of lenses? Another related question: if I use makeFields in more than one module, and different records have fields with the same name, the generated typeclasses will have the same name but will be different, because declared twice in different modules, am I right? Is there a way to use a unique typeclass in the whole project for every generated field of the same name? Thank you very much, Nicola

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Several default implementations for instance methods
by Andreas Abel 04 Oct '14

04 Oct '14

Consider the following class for an overloaded pretty printer. For atomic data (numeric types, String etc.) one would implement prettyShow, for complex data either pretty or prettyPrec. > import Text.PrettyPrint > > class Pretty a where > pretty :: a -> Doc > prettyShow :: a -> String > prettyPrec :: Int -> a -> Doc Implementing one of these methods should be enough, giving default implementations for the other two. It is easy to get prettyShow and prettyPrec from pretty. > prettyShow = render . pretty > prettyPrec = const . pretty However, to define pretty from one of the others, I need two default implementations. > pretty = text . prettyShow > pretty = prettyPrec 0 Is there a way to get this to work? Workarounds (not entirely satisfactory): Technically, one could define a cycle of default implementations. Alternative 1: > pretty = prettyPrec 0 > prettyShow = render . pretty > prettyPrec _ = text . prettyShow Problem: Here, if pretty is defined, > prettyPrec _ = text . render . pretty instead of just > prettyPrec _ = pretty and (text . render) is not the identity (destroys inner document structure). Alternative 2: > pretty = text . prettyShow > prettyShow = render . prettyPrec 0 > prettyPrec _ = pretty Problem: Here, if prettyPrec is defined, > pretty = text . render . prettyPrec 0 instead of just > pretty = prettyPrec 0 I guess alternative 2 is worse than alternative 1, as one would usually define prettyPrec to get pretty, and not the otherway round. But none of these two alternatives really does the job. -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Call for Contributions - Haskell Communities and Activities Report, November 2014 edition
by Mihai Maruseac 04 Oct '14

04 Oct '14

Dear all, We would like to collect contributions for the 27th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 Nov 2014 (please send your contributions to hcar at 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 an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we 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 previous edition --- you will find interesting projects 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, Mihai Maruseac and Alejandro Serrano Mena FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2014/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2014/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 soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. 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 up entries. There is no minimum length of an entry! The report aims for 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 about 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 simply ask us. 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. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically 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. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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