December 2010 - Haskell-Cafe

Looking for Haskell job at China.
by Andy Stewart 18 Dec '10

18 Dec '10

Hi all, I'm a Chinese haskeller, i'm looking for haskell job at *China*. Please contact me if any Chinese company interested me. Below is my skills: * Java : (2007-06 ~ 2008-09) Worked on a variety of commercial J2ME games, masterpiece <<Machine King>> : http://goo.gl/lcJF * Elisp : (2008-07 ~ 2009-05) Emacs expert, developed over a large number of Emacs extensions : http://goo.gl/DNLj My best-known Emacs extensions are: AutoInstall (automatic installation of Emacs extensions): http://goo.gl/ogBY MultiTerm (terminal emulator): http://goo.gl/PwpO Yaoddmuse (OddmuseWiki editor): http://goo.gl/cUrA ElispFormat (Elisp formatter): http://goo.gl/nVzb Irfc (RFC document reader): http://goo.gl/XZwi ... too much :) Other emacs extensions and elisp code, please see my EmacsWiki homepage: http://goo.gl/DNLj * Haskell : (2009-05 ~ 2010) Haskell/Gtk+ expert. Developer of gtk2hs (Haskell binding to GTK+) : http://goo.gl/AhJw Development of a large number of Haskell libraries: http://goo.gl/CoV0 I'm developing my own Haskell operating system : Video: http://www.youtube.com/watch?v=weS6zys3U8k Homepage: http://hackage.haskell.org/package/manatee Screenshots: http://goo.gl/MkVw Framework: http://goo.gl/otfb http://goo.gl/DMFW Source Repository: http://goo.gl/CoV0 I have finish below modules in my Haskell OS: Webkit-Browser: http://goo.gl/uoaX Editor: http://goo.gl/ouDR File Manager: http://goo.gl/x8Xi Picture Viewer: http://goo.gl/EYih PDF reader: http://goo.gl/8jCK Process Manager: http://goo.gl/ISOr Music Player: http://goo.gl/TMeD IRC client: http://goo.gl/kY6v News reader: http://goo.gl/BYVp Download-Manager: http://goo.gl/30tTN Terminal-Emulator: http://goo.gl/VMyaN Below are modules in developing: Mail-Client, IDE framework.... Thanks, -- Andy

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Layout styles and EclipseFP evolution.
by Scott Michel 17 Dec '10

17 Dec '10

Disclaimer: I'm not looking to start a favorite IDE flame war, or resurrect Emacs vs. VIM vs. Yi vs. <insert your favorite IDE here> discussions. I've been helping JP with EclipseFP, with the objective of evolving EclipseFP into a relatively high productivity IDE for Haskell. EclipseFP has the potential to mature into a reasonable platform for making routine coding easy, e.g., a HAppStack plugin that makes writing web apps in Haskell just as easy as it is for the Java community to write web services. I've just added Haskell code templates (*) to EclipseFP and, in the process, encountered the distinct lack of layout autoindentation. I'm sure it used to exist, but it doesn't exist today. I've looked at the Emacs haskell-mode.el indentation style, which I'm inclined to port over. Are there other layout autoindentation styles that other people prefer and believe should be supported? -scooter (*) The current development version supports templates like "let<Ctrl+Space>" and you get a let expression inserted. Eventually, if you were to type "case<Ctrl+Space>" and you tab out of the expression between "case" and "of", you would get the data type's alternatives inserted. (**) Don S told me that a similar "case" autocompletion exists in another IDE. Yes, EclipseFP is just catching up. (***) Wishlist: An incremental parser that can accept document changes/deltas and can look backward from the current point to give better context for completions, e.g., the previous token before the editor's point is "import".

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OT: Monad co-tutorial: the Compilation Monad
by Gregg Reynolds 17 Dec '10

17 Dec '10

This is a little off-topic, since it isn't specifically about Haskell, but since Haskell is the home of the monad tutorial it isn't completely irrelevant. The monad tutorial genre is well-known; it's often written somebody who has just learned the concept trying to explain it, often in highly imaginative terms (e.g. "a monad is just like a giant squid, with special tentacles"). I propose "monad co-tutorial" for attempts, often by an amateur (e.g. me) to work out how a monad appears in an unexpected place. In particular, I recently suffered through a few days of learning to deal with Ant and Maven2, which got me thinking about build systems in general, which got me thinking about the (meta-)language used in such systems, which got me thinking about What It's Really All About. Lo and behold - a monad! I think, anyway. The basic idea is that program texts (source or object code) are to programs as sentences are to propositions. Compilation transforms one representation into another. This can be viewed in terms of the unit natural transformation of a monad of transformations over program representations. The multiplication component captures the fact that program text transformations can be chained, as in macro expansion, phased optimization, etc. I'm not sure if this works, but it fits my somewhat limited idea of monads, so I wrote an article <http://blog.mobileink.com/> about it and I'd appreciate any corrections. I expect if it does work somebody has already done something like it, so I would appreciate any pointers to more info. My real goal is to think about better language for software build systems, since what we have now is pretty weak, in my view. One thing that follows from thinking in terms of monads is that we can discard the notion of dependency of object code of source code in favor of a notion of equivalence (i.e. unit transform). My hunch is that thinking along such lines might lead to an improved metalanguage for build DSLs. Thanks, Gregg

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(Read r) => IO (Maybe r)
by Jacek Generowicz 17 Dec '10

17 Dec '10

Hi, What are some interesting, idiomatic ways of writing something similar to the following, using a) Only standard utilities b) Non-standard utilities getValidatedInteger = do maybeInt <- maybeGet case maybeInt of Just int -> return int Nothing -> do putStrLn "That doesn't seem to be an integer. Try again." getValidatedInteger maybeGet :: (Read r) => IO (Maybe r) maybeGet = getLine >>= return . maybeReadS maybeReadS text = case reads text of [(int, rest)] | all (== ' ') rest -> Just int _ -> Nothing

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bus error trying to use Network
by Marcelo Sousa 17 Dec '10

17 Dec '10

Hi, I'm getting a bus error with Network library on my Mac OS X 10.6.5. I have ghc 6.12.3, network 2.3. As an example: main = withSocketsDo $ do sock <- listenOn $ PortNumber 4244 (handle,host,port) <- accept sock hPutStr handle "Hi!" sClose sock Any idea of what the problem could be? If I use the Network.Socket library it works fine: main = withSocketsDo $ do sock <- socket AF_INET Stream 0 setSocketOption sock ReuseAddr 1 bindSocket sock (SockAddrInet 4244 iNADDR_ANY) listen sock 1 (csock,_) <- accept sock send csock "Hi!\n" sClose csock sClose sock Cheers, Marcelo

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What is NoPush module?
by Magicloud Magiclouds 17 Dec '10

17 Dec '10

Hi, When I compiling happstack with ghc 7, I got an error: src/Happstack/Server/Internal/Handler.hs:29:8: Could not find module `NoPush': Use -v to see a list of the files searched for. I looked around, there is no clue for the module NoPush, except it has a function call withNoPush. What is it? -- 竹密岂妨流水过山高哪阻野云飞

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syb-with-class problem.
by Magicloud Magiclouds 16 Dec '10

16 Dec '10

Hi, I am trying to make happstack work in my box, which is using ghc 7. While compiling HStringTemplate, which I did a little modification to make it work, I found that it does not work with template-haskell-2.5.0.0. I changed it to use template-haskell-2.4.0.1. This step worked fine. Then I got another error: $ cabal install Resolving dependencies... Configuring syb-with-class-0.6.1.1... Preprocessing library syb-with-class-0.6.1.1... Building syb-with-class-0.6.1.1... [1 of 4] Compiling Data.Generics.SYB.WithClass.Context ( Data/Generics/SYB/WithClass/Context.hs, dist/build/Data/Generics/SYB/WithClass/Context.o ) [2 of 4] Compiling Data.Generics.SYB.WithClass.Basics ( Data/Generics/SYB/WithClass/Basics.hs, dist/build/Data/Generics/SYB/WithClass/Basics.o ) [3 of 4] Compiling Data.Generics.SYB.WithClass.Derive ( Data/Generics/SYB/WithClass/Derive.hs, dist/build/Data/Generics/SYB/WithClass/Derive.o ) Data/Generics/SYB/WithClass/Derive.hs:23:1: Bad interface file: /home/magicloud/.cabal/lib/template-haskell-2.4.0.1/ghc-7.0.1/Language/Haskell/TH.hi Something is amiss; requested module template-haskell-2.4.0.1:Language.Haskell.TH differs from name found in the interface file template-haskell:Language.Haskell.TH cabal: Error: some packages failed to install: HStringTemplate-0.6.3 depends on syb-with-class-0.6.1.1 which failed to install. syb-with-class-0.6.1.1 failed during the building phase. The exception was: ExitFailure 1 How to resolve this? -- 竹密岂妨流水过山高哪阻野云飞

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Printing of asynchronous exceptions to stderr
by Mitar 16 Dec '10

16 Dec '10

Hi! I have been spend some time to debug this in my program. ;-) Why is ThreadKilled not displayed by RTS when send to thread (and unhandled), but any other exception is? For example, I am using custom exceptions to signal different kinds of thread killing. Based on those my threads cleanup in different ways. But after they cleanup they keep "running". Not really running as their computations were interrupted. But simply hanging there. And if then new custom exception arrives it is not handled anymore (as it is after my normal exception handlers already cleaned everything) and it is printed to stderr. Now I added "`finally` throwIO ThreadKilled" at the end of my cleanup computations to really kill the thread. So that possible later exceptions are not delivered anymore. Why it is not so that after all computations in a thread finish, thread is not killed/disposed of? Because it can be reused by some other computation? But then my exceptions could be delivered to wrong (new) thread while they were meant for old one? Mitar

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Re: [Haskell-cafe] [Haskell] Functor => Applicative => Monad
by Tillmann Rendel 16 Dec '10

16 Dec '10

Hi, John Smith wrote: > I would like to formally propose that Monad become a subclass of > Applicative A lot of code would break because of this change, but all problems should be reported at compile time, and are easy to fix. In most of the cases, either adding obvious Functor and Applicative instances to a library; or deleting such instances from a client, I would expect. This kind of clean-up would actually increase the quality of the library, however, and might therefore be acceptable. > The change is described on the wiki at > http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal There you write, among other things: > fail should be removed from Monad; a failed pattern match could error in the same way as is does for pure code. How is this part of the proposal related to Functor and Applicative? Since code depending on the current behavior can not be detected at compile time, this is a more serious change in a way: code keeps compiling but changes its meaning. Can you estimate how much code would break because of this change? Would it be possible to keep user-defined failure handling in do notation without keeping fail in Monad? Tillmann

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TLS package server interface do not seem to allow for asyncrhonious communication
by Mads Lindstrøm 16 Dec '10

16 Dec '10

Hi Haskeleers, I got a working SSL server using the TLS package. Great. However, I really intended to use SSL for an asynchronous server. That is, the server must constantly listen for a client message, which may result in zero or more messages send _to_ the client. And the server will, without being waken up by a client message, send messages to the client. And this interface http://hackage.haskell.org/packages/archive/tls/0.3.1/doc/html/Network-TLS-… do not seem to allow for an asynchronous server. That is, whenever I call: recvPacket :: Handle -> TLSServer IO (Either TLSError [Packet]) or recvData :: Handle -> TLSServer IO ByteString The server is stuck until the client sends a message. Or am I missing something? Contrast this to Network package socket implementation. Here I can spawn two threads and simultaneously listen for client messages and send messages to the client. It seems to me that a general SSL implementation should not preclude asynchronous servers. I know, the TLS package is not more than a few months old and one can already use it for SSL servers and clients, which is quite impressive. So do not read this as a negative comment, more as suggestions for a better interface. Regards, Mads Lindstrøm

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