September 2012 - Haskell-Cafe

in the vector library, why define unbox tuples > 2?
by Jeff Shaw 25 Sep '12

25 Sep '12

I'm looking at storing a data type with 7 fields in an unboxed vector, which means that I'll have to use GenUnboxTuple to create an instance for Unbox (a,b,c,d,e,f,g), but I was thinking that another solution is to use "instance (Unbox a, Unbox b) => Unbox (a,b)" recursively to create "instance Unbox (a,(b,(c,(d,(e,(f,g))))))". Is there any reason I shouldn't say something like, import qualified Data.Vector.Unboxed as Vector data X = X Int Int Int Int Int Int Int toTuple (X a b c d e f g) = (a,(b,(c,(d,(e,(f,g)))))) toVector = Vector.fromList . map toTuple I guess I'm wondering if there is a technical reason why I should use an explicit Unbox instance for 7-tuples, or if it's just a convenience to have Unbox defined for tuples > 2? Thanks, Jeff

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Fwd: [Haskell] ANNOUNCE: MFlow-0.1.5
by Alberto G. Corona 25 Sep '12

25 Sep '12

Oh, I´m stupid. You mean web pages with multiple tabs.... I have not tested it. but each tab can be handled easily by a different server process.. or it can be handled in a single server process, like in a menu. For example, this code present different options, and the process renders different things depending on the response. The last option is a link to a different process, while the others( wlinks) are links that return back to the same process. The operator <|> is the applicative operator. a breakline is prepended to each link: data Ops= Ints | Strings | Actions | Ajax | Opt deriving(Typeable,Read, Show) mainf= do r <- ask $ wlink Ints (bold << "increase an Int") <|> br ++> wlink Strings (bold << "increase a String") <|> br ++> wlink Actions (bold << "Example of a string widget with an action") <|> br ++> wlink Ajax (bold << "Simple AJAX example") <|> br ++> wlink Opt (bold << "select options") <++ (br +++ linkShop) -- this is an ordinary XHtml link case r of Ints -> clickn 0 Strings -> clicks "1" Actions -> actions 1 Ajax -> ajaxsample Opt -> options mainf where linkShop= toHtml $ hotlink "shop" << "shopping" . Alberto 2012/9/18 Alberto G. Corona <agocorona(a)gmail.com>: > Hi Jake > > I don´t know what you mean with multiple tabs. The user management is > simple, anonymous clients are identified with a cookie. if the user > is logged (MFlow has widgets for logging-validation) the user is the > identifier. > > The state of a process is associated to the client identifier and to > the path invoked in the url requested. > > I don´t know if this answer your question.... > > Alberto > > 2012/9/18 Jake McArthur <jake.mcarthur(a)gmail.com>: >> This sounds really cool. >> >> How do you handle users having multiple tabs? >> >> On Tue, Sep 18, 2012 at 11:26 AM, Alberto G. Corona <agocorona(a)gmail.com> wrote: >>> Hi haskellers and specially the web developers. >>> >>> http://hackage.haskell.org/package/MFlow-0.1.5.3 >>> >>> MFlow is a is a Web framework with some unique, and I mean unique, >>> characteristics that I find exciting: >>> >>> - It is a Web application server that start and restart on-demand >>> stateful web server processes (not request.-response) >>> This means that all the page navigation can be coded in a single >>> procedure. This increases readability of the programmer code. I woul >>> call it >>> a anti-node.js. Buit usual request-response (stateless) server >>> processes are also allowed >>> >>> - When the process is invoqued as result of an URL request, the Web >>> app server not only restart the process but also recover its execution >>> state. The enclosing Workflow monad provides the thread state >>> persistence. There are state timeouts and process timeouts defined by >>> the programmer. Processes with no persistent state (transient) are >>> possible. >>> >>> -The user interface is made of widgets. They are formlets with added >>> formatting, attributes, validations, modifiers and callbacks, that >>> are composable, so the pieces are reusable and return type safe >>> responses to the calling process. Even the links are part of widgets >>> and return back type safe inputs at compile time to the calling server >>> process. Tho glue these components, ordinary applicative combinators >>> and other extra combinators are used. >>> >>> - The widgets and the communication don´t make assumptions about the >>> architecture, so it can be adapted to non-web environments. This >>> versions has interface for WAI-warp, Hack, Text.XHtml (xhtml) , and >>> Haskell Server Pages. >>> >>> -The widget rendering can be converted to ByteStrings automatically >>> with special combinators. A mix of widgets with different formats can >>> be combined in the same source file. For example Text.Html and HSP >>> (Haskell server pages) >>> >>> -These widgets can be cached, to avoid widget rendering on every interaction. >>> >>> -To handle the back button, and because the processes are stateful, >>> they can run backwards until the response match. This is transparent >>> for the programmer, thanks to the embedded FlowM monad. >>> >>> -All the code is in pure Haskell. No deployment, special scripts, >>> formats etc are necessary. >>> >>> -Besides automatic state persistence, TCache provides transactions and >>> user data persistence, that can be configured for SQL databases. >>> Default persistence in files permit very rapid prototyping. Just code >>> and run it with runghc. >>> >>> -Has AJAX support >>> >>> All of this sounds very complicated, but really it is simple!. Most of >>> these things are transparent. The resulting code is quite readable and >>> has very little plumbing! >>> >>> There is a non trivial example that some of these functionalities >>> embedded here that you can run: >>> >>> http://hackage.haskell.org/packages/archive/MFlow/0.1.5.3/doc/html/MFlow-Fo… >>> >>> Take a look and tell me your opinion. I hope that you find it as >>> exciting as me. >>> >>> I´m looking for people to collaborate in the development of MFlow. >>> >>> Although still it is experimental, it is being used in at least one >>> future commercial project. So I have te commitment to continue its >>> development. There are many examples in the documentation and in the >>> package. >>> >>> Alberto >>> >>> _______________________________________________ >>> Haskell mailing list >>> Haskell(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell

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Getting started with Shake
by Richard Wallace 24 Sep '12

24 Sep '12

Hey all, I'm going to be working on a webapp in Haskell in the upcoming months and am thinking Shake would be a good fit - I'll need to do js, css, and probably some graphics processing as part of the build and would like to use Shake to automate deployment. I'm not entirely sure how to get started with Shake. I've used to make, jam, maven, sbt and other build tools in the past. All of these keep a build configuration in the project and use that to build the project. In sbt, which is most like Shake, the build configuration is written in Scala and compiled by sbt when you run the build. The result of the compilation is saved and reused unless it detects that the build source files changed. With shake I'm not sure exactly how to get started. Should I have a separate project where I create the build system for the webapp? Or can I setup something similar to sbt? Also, how do I handle dependencies with shake? cabal will pull in packages from hackage and do the needful, is there anything in shake to do the same? If not, how is it normally done? Thanks, Rich

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ANN: New wiki page and mailing list for combinatorics and Haskell
by Brent Yorgey 24 Sep '12

24 Sep '12

Quite a while ago there was a mailing list thread where the idea was brought up of creating a mailing list for those interested in combinatorics in the setting of Haskell. Well, I've finally gotten around to it. If you're interested in (probably extremely low-traffic) discussion and announcements relating to doing combinatorics in Haskell (computing combinatorial/ number-theoretic functions; exhaustive or random generation of combinatorial objects; type algebra; etc.), please join! http://projects.haskell.org/cgi-bin/mailman/listinfo/combinatorics There is also a wiki page for collecting information here: http://www.haskell.org/haskellwiki/Combinatorics -Brent

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Organizaing tests in Haskell
by Jan Stolarek 24 Sep '12

24 Sep '12

Dear list, during many years of Java programming I've been faithful to TDD methology. Recently I've been trying to figure out how to do tests in Haskell. Thanks to RWH and help from great folks at #haskell I've managed to get on my feet. There is however one issue I wasn't able to solve. In Java it is very easy to separate test code from the actual source. A Java project simply contains two folders: src and tests. These two are treated as the source directories. Both these directories have the same subdirectory structure. For example I have a source file src/myPackage/mySubpackage/MyClass.java and test for it are kept in file tests/myPackage/mySubpackage/MyClassTest.java. These two files are considered by Java to be in the same package. Here's the main trick: fileds and methods that are marked as protected in Java are accessible to other classes in the same package. This allows to test internal methods of a class by marking them as protected instead of private. This breaks encapsulation but only within a package, which is acceptable. Now I'd like to achieve something similar in Haskell. I'm using cabal's support for testing. I created separate src and tests directories, both with the same subdirectory structure. I keep tests for each module in a separate file (e.g. I have src/Math/MyModule.hs and tests/Math/MyModuleTest.hs) and I have one file that assembles all the tests into a single test suite (I use test-framework for that). The only problem is that in order to test some function from a module I have to expose that function, which pretty much forces me to give up on encapsulation. Is there any better solution to organize tests in Haskell? Should I just give up on module encapsulation, or should I only test functions exposed by the module and don't worry about internal functions? Perhaps I should use some different approach? Jan

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Darcs on Windows 7
by Vasili I. Galchin 24 Sep '12

24 Sep '12

Hello Haskellers, I installed darcs on a Windows 7 machine. A darcs folder was created under "Program Files(x86)" folder. However, when I pull up "Program..." on the left side, darcs not there for me to run it. Why? Is darcs run only from the CLI? Regard, Vasili

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Platform Versioning Policy: upper bounds are not our friends
by Bryan O'Sullivan 23 Sep '12

23 Sep '12

Hi, folks - I'm sure we are all familiar with the phrase "cabal dependency hell" at this point, as the number of projects on Hackage that are intended to hack around the problem slowly grows. I am currently undergoing a fresh visit to that unhappy realm, as I try to rebuild some of my packages to see if they work with the GHC 7.6 release candidate. A substantial number of the difficulties I am encountering are related to packages specifying upper bounds on their dependencies. This is a recurrent problem, and its source lies in the recommendations of the PVP itself (problematic phrase highlighted in bold): When publishing a Cabal package, you should ensure that your dependencies > in the build-depends field are accurate. This means specifying not only > lower bounds, *but also upper bounds* on every dependency. I understand that the intention behind requiring tight upper bounds was good, but in practice this has worked out terribly, leading to depsolver failures that prevent a package from being installed, when everything goes smoothly with the upper bounds relaxed. The default response has been for a flurry of small updates to packages in which the upper bounds are loosened, thus guaranteeing that the problem will recur in a year or less. This is neither sensible, fun, nor sustainable. In practice, when an author bumps a version of a depended-upon package, the changes are almost always either benign, or will lead to compilation failure in the depending-upon package. A benign change will obviously have no visible effect, while a compilation failure is actually *better* than a depsolver failure, because it's more informative. This leaves the nasty-but-in-my-experience-rare case of runtime failures caused by semantic changes. In these instances, a downstream package should *reactively* add an upper bound once a problem is discovered. I propose that the sense of the recommendation around upper bounds in the PVP be reversed: upper bounds should be specified *only when there is a known problem with a new version* of a depended-upon package.

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[Snap] Argument Substitution in Heist Templates with Splices
by Sebastian Fischer 23 Sep '12

23 Sep '12

Hello, the following program demonstrates that arguments in Heist templates are sometimes not substituted in presence of splices: {-# LANGUAGE OverloadedStrings #-} import Blaze.ByteString.Builder (toByteString) import qualified Data.ByteString.Char8 as BS import Data.Functor ((<$>)) import Data.Maybe (fromJust) import Text.Templating.Heist -- just return input node unchanged testSplice :: Splice IO testSplice = (:[]) <$> getParamNode main = do writeFile "test.tpl" "<arg /><test attr='${arg}'><arg /></test>" state <- either error id <$> loadTemplates "." defaultHeistState (builder,_) <- fromJust <$> renderWithArgs [("arg","42")] state "test" BS.putStrLn $ toByteString builder -- 42<test attr='42'>42</test> let state' = bindSplices [("test",testSplice)] state (builder',_) <- fromJust <$> renderWithArgs [("arg","42")] state' "test" BS.putStrLn $ toByteString builder' -- 42<test attr='42'><arg></arg></test> Without using splices, all occurrences of 'arg' in the template are substituted. When using a splice, 'arg' is not substituted underneath the input node of the splice. It is substituted in an attribute of the input node. Is this intentional? How can I ensure substitution also underneath the input node? Best, Sebastian

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Fwd: ANNOUNCE: hsqml-0.1.1 (and hsqml-morris-0.1.0)
by Matthew West 22 Sep '12

22 Sep '12

Hello, In case anyone is wondering this does not compile on vanilla Mac OS 10.6 because XCode 3.2 doesn't support C++11. I believe that XCode 4.2 does support it however I've not yet paid Apple the $100 they want before I can download it for 10.6. Macports has more recent versions of GCC however this option turned out to be fraught with difficulties[1][2]. Has anybody got HsQML working with XCode 4.2? Matt [1] https://trac.macports.org/wiki/UsingTheRightCompiler [2] http://www.haskell.org/haskellwiki/Mac_OS_X_Strike_Force On 11 Sep 2012, at 08:47, Robin KAY wrote: > Dear All, > > I would like to announce a new package called 'hsqml' [1]. > > HsQML provides a Haskell binding to the Qt Quick framework. It allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell. > > HsQML requires an installation of Qt 4.7 (including QtDeclarative) or later on your path. It has been tested with GHC 7.4 on both Linux and Windows. > > There is also a simple example program available using HsQML on Hackage called 'hsqml-morris'. It implements a human v computer game of Nine Men's Morris with game logic and AI written in Haskell and the presentation layer in QML. > > The library is currently limited by a lack of support for Qt signals, so calls can only be made from JavaScript to Haskell and not the other way. I plan to add support for these in a future release. Migration over to Qt 5.x is also likely once it's released. > > [1] http://hackage.haskell.org/package/hsqml-0.1.1 > [2] http://hackage.haskell.org/package/hsqml-morris-0.1.0 > > Regards, > > -- > Robin KAY

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Haskell Wiki News
by Colin Adams 22 Sep '12

22 Sep '12

This doesn't seem to be up-to-date. It announces GHC 7.4 released (but 7.6.1 was released a couple of weeks ago).

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