June 2015 - Haskell-Cafe

Do people use visitor pattern in Haskell...
by Nicholls, Mark 22 Jun '15

22 Jun '15

My initial guess would be no...its built into the language But.... I find myself tempted to use it (in order to match on a type that's the instance of a typeclass)...which makes me think I've missed something. So... > data Foo = Foo > data Bar = Bar Ooo...this looks like an OO design pattern...surely wrong? > class Wibble a where > visit :: (Foo -> b) -> (Bar -> b) -> a -> b > instance Wibble Foo where > visit f _ x = f x > instance Wibble Bar where > visit _ f x = f x I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a] so I package it up? > data WibblePackage where > WibblePackage :: (Wibble a) => a -> WibblePackage lets try this now...so pointless function across Wibbles in a list. > fizzBuzz :: [WibblePackage] -> Integer > fizzBuzz [] = 0 > fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs) > help :: Integer > help = fizzBuzz [WibblePackage Foo,WibblePackage Bar] That works! OO nerds will get uptight about "closing" the Wibble typeclass....but I've managed to create a list of different types and have a mechanism for recovering the type. How would a Haskell nerd do this? (I have looked, but they look more complicated....maybe to my OO eye). CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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"Universal" constraints?
by Michael Walker 21 Jun '15

21 Jun '15

Hi all, I was wondering about constraints in class declarations. Suppose we have something like this: class Monad m => Store m where type Var m :: * -> * new :: a -> m (Var m a) read :: Var m a -> m a write :: Var m a -> a -> m () Then some reasonable implementations would be IO and ST, using IORefs and STRefs, instance Store IO where type Var IO = IORef new = newIORef read = readIORef write = writeIORef instance Store (ST s) where type Var (ST s) = STRef s new = newSTRef read = readSTRef write = writeSTRef Now, both IORefs and STRefs have equality, which doesn't require equality of the contained type, instance Eq (IORef a) where ... instance Eq (STRef s a) where ... This corresponds to pointer equality and, because reading the values is a monadic action, is as good an Eq instance as we can get. Given this, it would be nice to be able to specify that all instances of our Store class have this property. Something like, class (Monad m, Eq (Var m a)) => Store m where ... But we can't write this! The `a` isn't in scope! What I really want to do is to be able to write class constraints like this, class (forall a. C (T a)) => D x where ... Or why not get really crazy: class (forall a. C a => D (T a)) => E x where ... Is there some extension or combination of extensions that would make this work without completely sacrificing type safety? FlexibleContexts allows it (the simpler form, at least) for function constraints, but not for class constraints. If not, has anyone thought about this sort of thing before? -- Michael Walker (http://www.barrucadu.co.uk)

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Troubles with HStringTemplate (compilation/runtime errors)
by Ilya Seleznev 21 Jun '15

21 Jun '15

Hello Haskellers, I hope I chose proper mailing list, if not I apologize. And thanks for reading this! I run into a problem with HStringTemplate ( http://hackage.haskell.org/package/HStringTemplate) that I have never seen before. Code, similar to {-# LANGUAGE OverloadedStrings, DeriveDataTypeable, NoImplicitPrelude #-} module Main where import Data.Data import qualified Data.Text as T import qualified Text.StringTemplate as Template renderTextTemplate :: (Template.ToSElem a) => T.Text -> a -> IO T.Text renderTextTemplate name input = do templatesGroup <- Template.directoryGroup "templates" case Template.getStringTemplate (T.unpack name) templatesGroup of Nothing -> return $ "Internal Error: template '" `mappend` name `mappend` "' can not be rendered" Just template -> return $ Template.render (Template.setAttribute "it" input (template:: Template.StringTemplate T.Text)) data Foo = Foo {value:: T.Text} deriving (Data, Typeable) main :: IO () main = do content <- renderTextTemplate "test" $ Foo {value = "oh hi there!"} putStrLn content with a simple template like this: Just a test $it.value$! started to cause complaints from GHC (7.8.2 and 7.8.4 on Linux x86_64): No instance for (Template.ToSElem Foo) Problem is - it didn't do that before. What even more confusing, older project[0] uses same approach, compiles (same machine, same compiler) and works just fine! I've exhausted all means to fix the problem that I could think off: - Adding 'instance Template.ToSElem Foo' allows code to compile but it doesn't work and prints message in console like this: Main.hs:30:10-29: No instance nor default method for class operation Text.StringTemplate.Classes.toSElem - Tried to use same version of HStringTemplate deps (syb was different) - All kinds of desperate messing around (exporting records from modules, changing field names...) - Updated ghc from 7.8.2 to 7.8.4 - Googled error messages to no avail. I hope almighty All would give me some ideas/directions because I have none left. [0] Function: https://github.com/itsuart/fdc_archivist/blob/master/src/HttpUtils.hs#L59 Data: https://github.com/itsuart/fdc_archivist/blob/master/src/ViewModels/HomeVie… Usage: https://github.com/itsuart/fdc_archivist/blob/master/src/HomeViewFeature.hs… -- With best regards, Ilya Seleznev

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shakespeare >= 2.0.2 fails to install in OS X 10.6.8, Haskell-platform 2014.2.0.0; cabal-install 1.22.4.0
by Miguel A. Santos 20 Jun '15

20 Jun '15

(I already sent a comment on the Shakespeare site but couldn't get too far. So, if this isn't the right place for this issue, I'd appreciate you point me to the right one.) Hi, I can't install yesod. Doing a cabal install yesod-bin or cabal install yesod fails because it fails to build shakespeare. I tried with all versions of shapeare >= 2.0, as required by yesod-bin. Below are all outputs. My environment: OS X 10.6.8, Xcode 3.2 Haskell-platform 2014.2.0.0 ghc-7.8.3 cabal-install 1.22.4.0 using version 1.22.3.0 of the Cabal library. This corresponds to a fresh install of the HP, ghc, and cabal I didn't before trying to install yesod. The developer of Shakespeare suspects that be a bug of HP and suggested I try a clean install of GHC as explained in http://www.stackage.org/install#mac-os-x However, the bindist available there doesn't work with my OS 10.6.8 I have a large screen output from those trials that you can check on the Shakespeare github page corresponding to the thread I started there https://github.com/yesodweb/shakespeare/issues/156 If you prefer, I can paste it here as well. Thanks in advance. MA -- Public key ID: E8FE60D7 Public key server: see, e.g., hkp://keys.gnupg.net

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Re: [Haskell-cafe] [Haskell-beginners] Haskell -> JavaScript -> nodejs
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 20 Jun '15

20 Jun '15

Better try on the cafe. cc-ing. On 20 June 2015 at 15:26, Martin Vlk <martin(a)vlkk.cz> wrote: > I made some progress - in order for Haskell modules to work with ghcjs > you need to use cabal with the --ghcjs flag to install them. > > Martin > > Martin Vlk: > > Hi, > > I can successfully compile a basic Hello World into JavaScript with > > ghcjs and run it with nodejs. > > But what I ultimately need to do is write a nodejs module in Haskell and > > compile down to JavaScript with ghcjs. I need to be able to require > > other nodejs modules as well. > > > > Could somebody point me in the right direction? E.g. what approach and > > libraries to use... > > > > I have tried a simple thing with jsaddle/lens, but when I try to compile > > with ghcjs it is not finding the imported modules, even though when > > compiling with ghc it does find them (they are installed using cabal). > > > > Many Thanks > > Martin > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Regards Sumit Sahrawat

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Haskell and Spark(Hadoop)
by Vasili I. Galchin 20 Jun '15

20 Jun '15

> Hello, > > There was a posting from a Polish guy(Wojcieh .,,,, been trying to > contact him) on implementing a library/wrapper for Spark. Does anybody know > the status? > > Vasili >

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Question about terminateProcess
by Niklas Hambüchen 19 Jun '15

19 Jun '15

Hi, http://hackage.haskell.org/package/process-1.2.0.0/docs/System-Process.html… says: " Note: on Windows, if the process was a shell command created by createProcess with shell, or created by runCommand or runInteractiveCommand, then terminateProcess will only terminate the shell, not the command itself. On Unix systems, both processes are in a process group and will be terminated together. " I don't see that happening, at least not in ghci. When I do import System.Process (_,_,_,p) <- createProcess $ (shell "sleep 10") -- wait a second or so terminateProcess p in ghci, then in `ps aux | grep "sleep 10"`, the sleep is still running; same when I'm using `(shell "sleep 10"){ create_group = True }` (`shell` defaults to `create_group = False`). What am I missing?

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[ANN] wai-middleware-metrics package
by Sebastian 19 Jun '15

19 Jun '15

Hello everyone, I'm announcing the release of the wai-middleware-metrics package. http://hackage.haskell.org/package/wai-middleware-metrics https://github.com/Helkafen/wai-middleware-metrics Full example here with scotty: https://hackage.haskell.org/package/wai-middleware-metrics-0.2.1/docs/Netwo… It registers some ekg metrics in WAI web servers (scotty, servant, yesod...) using a middleware. Right now the metrics are: - wai.request_count - wai.server_error_count - wai.latency_distribution Time distributions will benefit from future developments <http://blog.johantibell.com/2014/05/ekg-04-released.html> in ekg and become more descriptive. Other metrics could be added. Ideas, contributions and comments are welcome! Cheers -- Sebastian de Bellefon

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OCL 2015: Second Call for Papers - Only Four Weeks Left
by Achim D. Brucker 19 Jun '15

19 Jun '15

(Apologies for duplicates) CALL FOR PAPERS 15th International Workshop on OCL and Textual Modeling Tools and Textual Model Transformations Co-located with ACM/IEEE 18th International Conference on Model Driven Engineering Languages and Systems (MODELS 2015) September 28th, 2015, Ottawa, Canada http://ocl2015.lri.fr Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- meta-modeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing tools that support - in a very broad sense - textual modeling languages (if you have implemented OCL.js to run OCL in a web browser, this is the right workshop to present your work) as well as textual model transformations. Venue ===== The workshop will be organized as a part of MODELS 2015 Conference in Ottawa, Canada. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, 2013 in Miami, and 2014 in Valencia, Spain. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Three types of papers will be considered: * short papers (between 6 and 8 pages) describing ideas, * tool papers (between 6 and 8 pages), and * full papers (between 12 and 16 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://easychair.org/conferences/?conf=ocl20150) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Important Dates =============== Submission of papers: July 17, 2015 Notification: August 21, 2015 Workshop date: September 28, 2015 Organizers ========== Achim D. Brucker, SAP SE, Germany Marina Egea, Indra Sistemas S.A., Spain Martin Gogolla, University of Bremen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Programme Committee =================== Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Achim D. Brucker, SAP SE, Germany Fabian Buettner, Inria - Ecole des Mines de Nantes, France Jordi Cabot, Inria - Ecole des Mines de Nantes, France Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Spain Carolina Dania, IMDEA Software Institute, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Indra Sistemas S.A., Spain Geri Georg, Colorado State University, USA Martin Gogolla, University of Bremen, Germany Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ. Paris-Sud - LRI, France Steffen Zschaler, King's College, UK -- Dr. Achim D. Brucker, SAP SE, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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Re: [Haskell-cafe] Haskell users on Chile
by Anupam Jain 19 Jun '15

19 Jun '15

Also, I am a part of the Startup Chile community. I would be happy to promote this group on their mailing list. On Friday, June 19, 2015, Ismael Figueroa <ismael.figueroa(a)ucv.cl> wrote: > Hi all, > > I definitely love the idea of having a chilean Haskell user group :-) > Even more so if it is in the V Region ;-) > > -- > Ismael > > On 18/06/15 18:14, Éric Tanter wrote: > >> I’m a CS professor at Universidad de Chile, doing research in PL, some >> of it in Haskell. I also use Haskell for part of my PL courses, so there >> might be some Haskellers that came out of these experiences. >> >> Also, Ismael Figueroa (in cc) graduated from his PhD under my >> supervision last year. His PhD thesis was entirely around Haskell >> (monadic embedding of aspects, compositional reasoning about >> interference), and we’ve done some other stuff on Haskell too (effect >> capabilities). Ismael is now professor at Universidad Católica de >> Valparaíso. >> >> -- Éric >> >> >> On Jun 18, 2015, at 2:44 PM, Roman Dzvinkovsky <romand.ne(a)gmail.com >>> <mailto:romand.ne@gmail.com>> wrote: >>> >>> Living in Viña del Mar, would love to participate. >>> >>> 2015-06-09 9:46 GMT-03:00 Anupam Jain <ajnsit(a)gmail.com >>> <mailto:ajnsit@gmail.com>>: >>> >>> Great idea! Though not a native Chilean, I lived in Chile (Santiago) >>> for the better part of a year in 2014-2015, where my favourite >>> activity was infiltrating JS meetups and evangelizing Haskell :) >>> >>> It would be great to have a dedicated Haskell community there. >>> >>> -- Anupam >>> >>> On Tue, Jun 9, 2015 at 5:44 PM, Ruben Astudillo >>> <ruben.astud(a)gmail.com <mailto:ruben.astud@gmail.com>> wrote: >>> > Hi all >>> > >>> > I would like to see how many haskellers are living on Chile with >>> hopes of >>> > forming a user group. If you are in here, you should reply this >>> thread and >>> > see >>> > how many of us there are. You can also contact me to my mail. >>> > >>> > -- >>> > Ruben Astudillo. pgp: 0x3C332311 , usala en lo posible :-) >>> > Crear un haiku, en diecisiete silabas, es complica... >>> > _______________________________________________ >>> > Haskell-Cafe mailing list >>> > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> >> >> > -- > Ismael > -- Sent from my hyper-communicator

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