October 2013 - Haskell-Cafe

The rmonad package is mysterious
by Simon Marechal 03 Dec '13

03 Dec '13

I tried to do something simple using the rmonad package (attached). I followed the example that's in haddocks (it is wrong, constraints has no arguments), grabbed the list of extensions from rmonad's source, but this program doesn't typecheck: Could not deduce (Hashable a) arising from a use of `HM.singleton' This seems to be an obvious use of this package ... am I missing something ?

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Product Profunctor and Contravariant
by Tom Ellis 20 Nov '13

20 Nov '13

Does anyone recognise these typeclasses: import Data.Profunctor (Profunctor) import Data.Functor.Contravariant (Contravariant) class Profunctor p => ProductProfunctor p where empty :: p () () (***!) :: p a b -> p a' b' -> p (a, a') (b, b') class Contravariant f => ProductContravariant f where point :: f () (***<) :: f a -> f b -> f (a, b) They are both a bit like Applicative, and ProductProfunctor is basically Arrow without the Category part. I'm finding ProductProfunctor useful for marshalling data from a database into Haskell, and both of them come up a lot inside my database library. Has anyone ever seen these before? Has Edward Kmett written a library for these already? Thanks, Tom

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Best practices for Arrows?
by Tom Ellis 13 Nov '13

13 Nov '13

I feel I may be doing a lot of programming with Arrows in the near future. Currently I'm delighted that Arrow notation[1] exists. It makes using Arrows much less painful. Are there any best-practices I should be aware of with Arrows? Or is it just a case of getting on with it? Tom 1. http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html

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a tiny HTTP server.. that doesn't work
by Corey O'Connor 08 Nov '13

08 Nov '13

I attempted to write a tiny HTTP server using only Haskell Platform packages: * https://github.com/coreyoconnor/tiny-http-hp Which works... Unless the threaded runtime is used. When compiled using the threaded runtime and run with +RTS -N this server fails to reply correctly to ~3% of requests. The expectation is that the reply will be exactly the request body. However, 3% of the time the HTTP request fails to be parsed due to an "ErrorClosed" The server then executes: * https://github.com/coreyoconnor/tiny-http-hp/blob/master/TinyHttp.hs#L23 which responds to the client. The client receives the response correctly most of the time. My current hypothesis is that some aspect of lazy IO is not playing nice. The connection is being closed before the request can be completely parsed. Is this correct? What am I missing? I know there are other HTTP server packages that could be used. However, the exercise was to build a HTTP server using only Haskell Platform packages. Below is the main body of the code: main = withSocketsDo $ do http_socket <- listenOn $ PortNumber 9090 dispatch_on_accept http_socket $ either handle_failed_request handle_valid_request sClose http_socket handle_failed_request failure = return $ Response (4,0,0) "Bad Request" [mkHeader HdrConnection "close"] (encodeUtf8 $ pack $ show failure) handle_valid_request request = do let request_body = rqBody request return $ Response (2,0,0) "OK" [mkHeader HdrConnection "close"] (encodeUtf8 $ pack $ show request_body) dispatch_on_accept http_socket handler = forever $ accept http_socket >>= forkIO . httpHandler . fst where httpHandler client_socket = bracket (socketConnection "client" 0 client_socket) Network.HTTP.close client_interact client_interact :: HandleStream BS.ByteString -> IO () client_interact byte_stream = receiveHTTP byte_stream >>= handler >>= respondHTTP byte_stream -Corey O'Connor coreyoconnor(a)gmail.com http://corebotllc.com/

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Cabal: --enable-shared and executables
by Magnus Therning 05 Nov '13

05 Nov '13

We've had a discussion about DSOs on the arch-haskell mailing list[1] and it made me realise one thing: --enable-shared will cause the generation of DSOs for libraries, but it won't cause executables to be dynamically linked Is the only way to cause these executables to be dynamically linked against their dependencies to pass the -dynamic flag to ghc (e.g. via the --ghc-options flag when configuring the package)? That then raises the issue of finding the DSOs. Something that is discussed in the Ghc documentation[2]. However, the generated DSOs are correctly linked to their dependencies. I guess what I'm trying to get to is that it seems to me that Cabal only provides half the solution to DSOs; it solves generation, but not usage. Is that an observation that people agree with? Are there any plans to solve this (maybe it's already solved in 1.18)? /M [1]: http://www.haskell.org/pipermail/arch-haskell/2013-October/004599.html [2]: http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/using-shared-libs.ht… -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus The day after tomorrow is the third day of the rest of your life.

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How do I lift Control.Monad.Reader.local?
by David Fox 01 Nov '13

01 Nov '13

I can write a stack push function in the reader monad like so: import Control.Monad.Reader push :: String -> Reader [String] a -> Reader [String] a push s action = local (s :) action How can I write a push that works inside a monad transformer? push' :: MonadTrans t => String -> t (Reader [String]) a -> t (Reader [String]) a push' s action = ???

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Cabal sandboxes over cabal-dev
by Graham Berks 01 Nov '13

01 Nov '13

Can someone point me in the direction of why you would use one over the other ?? Thanks Graham

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cabal-install sandbox with happy and alex
by Badi' Abdul-Wahid 31 Oct '13

31 Oct '13

Hi I've run into difficulty building happy and alex using the sandbox feature of cabal-install 1.18.0.2. Installation of dependencies succeeds but both happy and alex fail with: setup: The program happy is required but it could not be found My workaround is to use the --only-dependencies flag, install the dependencies, and manually install the packages. This is on Mac OS X 10.8.4 with GHC 7.6.3. I've copied the steps demonstrating the problem on lpaste: http://lpaste.net/95027 Summary to reproduce: cabal sandbox init; cabal install happy alex Below is the script for my workaround. [0 :)] badi@olorin /tmp/sb ~> cat sandbox-install.sh #!/usr/bin/env bash packages="$@" cabal sandbox init cabal install --only-dependencies $packages for p in $packages; do cabal unpack $p pushd $p-* cabal sandbox --sandbox ../.cabal-sandbox init cabal configure cabal build runhaskell Setup.lhs install popd done -- Badi' Abdul-Wahid

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xmonad install problem
by Никитин Лев 31 Oct '13

31 Oct '13

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Call for Contributions - Haskell Communities and Activities Report, November 2013 edition
by Janis Voigtlaender 31 Oct '13

31 Oct '13

[Transition in editorship of HCAR in progress!] Dear all, We would like to collect contributions for the 25th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 Nov 2013 (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 Janis Voigtlaender (for this edition) 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/11-2013/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2013/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.

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