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cabal or stack command to fetch and add dependency to .cabal file
by xu xiut 07 Sep '15

07 Sep '15

With node.js, you can run `npm install --save package(a)1.01`, and that version of the package will be retrieved and added to your list of dependencies. If you leave off the version number, it will retrieve the latest version from a default repository of packages. Are there any plans to add something this to either cabal or stack?

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ANNOUNCE: graphviz-2999.18.0.0
by Ivan Lazar Miljenovic 06 Sep '15

06 Sep '15

This is my "sorry it took so long" release: I'd like to apologise to all the people that have been nagging^W gently asking me when a GHC 7.10.*-compatible release would be available that it's taken me until now to do this, but I'm finally pleased to be able to say that you can now use my graphviz library to generate/parse Dot code and visualise graphs using the Graphviz suite of tools. As part of this version release, graphviz is now developed using git and can be found on GitHub: https://github.com/ivan-m/graphviz The major version bump is due to an extra attribute being made available, so in most (all?) cases you can safely version bump your existing dependencies. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Problem with lifted-async
by Lana Black 05 Sep '15

05 Sep '15

Hello, I'm writing a thread supervisor that allows implicitly passing some monadic context (e.g. ReaderT) using MonadBaseControl from monad-control. The problem is that I don't know how to tackle this error. GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( ex.hs, interpreted ) ex.hs:20:20: Couldn't match type ‘StM t a0’ with ‘StM t a’ NB: ‘StM’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous Expected type: Async (StM t a0) Actual type: Async (StM t a) Relevant bindings include as :: Async (StM t a) (bound at ex.hs:19:30) t :: Task t a (bound at ex.hs:19:28) td :: TaskDescriptor t (bound at ex.hs:19:10) pollTask :: TaskDescriptor t -> TaskDescriptor t -> t (TaskDescriptor t) (bound at ex.hs:19:1) In the first argument of ‘poll’, namely ‘as’ In a stmt of a 'do' block: st <- poll as Failed, modules loaded: none. This minimal code snippet would be {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE FlexibleContexts #-} import Control.Concurrent.Async.Lifted (Async,async,asyncBound,poll,cancel) import Control.Monad.Trans.Control (MonadBaseControl,StM) data Task m a = Task { taskVal :: m a } data TaskDescriptor m = forall a. TaskDescriptor { task :: Task m a, asyncThread :: Async (StM m a) } runTask :: forall a m. (MonadBaseControl IO m) => Task m a -> m (TaskDescriptor m) runTask = undefined pollTask td(TaskDescriptor t as) = do st <- poll as case st of Nothing -> pure td Just r -> runTask t main = undefined StM is a type from MonadBaseControl typeclass, the definition is class MonadBase b m => MonadBaseControl b m | m -> b where type StM m a :: * liftBaseWith :: (RunInBase m b -> b a) -> m a restoreM :: StM m a -> m a type RunInBase m b = forall a. m a -> b (StM m a) I stripped the comments, the full version is here https://hackage.haskell.org/package/monad-control-1.0.0.4/docs/src/Control-…

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Bullet bindings with Bullet 2.83
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I'm trying to get the bullet binding working with Bullet 2.83.5. I installed libbullet-dev 2.83.5 from the Debian testing archive (I'm running Debian Stretch) and installed the Haskell binding (with cabal install --enable-documentation). Compiling the example results in a bunch of undefined reference errors like this: /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/bullet-0.2.4/libHSbullet-0.2.4.a(Bullet.o):(.data.rel.ro._ZTV20btTetrahedronShapeEx[_ZTV20btTetrahedronShapeEx]+0x60): undefined reference to `btPolyhedralConvexShape::initializePolyhedralFeatures()' I guess this is because, the Debian package doesn't come with a static bullet libary, so I tried to compile it with dynamic libraries (with the -dynamic flag), but it just fails with other undefined references. The installation instructions for the Bullet binding say, it works with Bullet 2.73, so I'm afraid this is some version incompatibility. Does anyone know, how to get the binding working with Bullet 2.83 or what other causes the missing symbols can have? Regards Sven

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Anyone familiar with Hpysics?
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I recently found the physic library/engine Hpysics. It looks really nice and would like to use it for a project, because it's more natural to use in Haskell, than Bullet is. The problem with the library, seems to be, that it doesn't seem, like it retrieved a lot of maintainer attention lately, e.g. after downloading it, I had to fix some type errors in the library (some incompatibility to GL floats), to get it working. The author field in the .cabal file lacks an email, so I can't contact him and ask if he still cares about the package. Given that there doesn't seem to be an active maintainer and I need the package, I would be willing to take over its maintenance. The documentation is not really verbose about most stuff, so there are some things, where I don't know how they are supposed to work. Is there maybe someone, who was involved in writing the library or maintaining it, or someone, who just used it and can help me, with getting it in shape again, or maybe someone, who still feels responsible for it and just didn't update it, because there was no interest? PS: The package can be found on http://code.haskell.org/hpysics/, but not on hackage (another reason, why I believe it's not actively mantained anymore). Regards Sven

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Physics engine for Haskell
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I'm looking for a game physics engine for Haskell. As Bullet is widely used in this field, I went to use the Haskell Bullet binding, but had to find, that it is very inconvenient to use, because of things like making optional parameters necessary in the binding. Is there any higher level binding to Bullet or some other Physics library that is more convenient to use on Haskell? Regards Sven

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GHC 7.8 undefined symbol
by Sven Bartscher 04 Sep '15

04 Sep '15

Greetings, I recently installed a library (namely Hpysics, not on hackage) with shared libraries (they were on by default). When trying to do some stuff with the library (program attached), I get a bunch of undefined symbol errors like this: $ ghc --make Main.hs Linking Main ... /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0x46e): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_constructBoundingSpherezugo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0x4f9): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_zdwlgo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0xaca6): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_constructBoundingSpherezugo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0xad31): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_zdwlgo_info Does anyone know, what this is about? PS: Hpysics can be found at http://code.haskell.org/hpysics/ Regards Sven

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ANN: stack-0.1.4.0 (and 0.1.4.1)
by Emanuel Borsboom 04 Sep '15

04 Sep '15

See README <https://github.com/commercialhaskell/stack#how-to-install> for installation and upgrade instructions. Note: while stack-0.1.4.1 is on Hackage, the only change is to a comment so that stack’s haddocks build. Therefore, no new binaries were generated. Functionally, v0.1.4.0 is identical to v0.1.4.1. Note: we have stopped uploading new versions to the Fedora 20 repository, since that version of Fedora has reached end of life <https://fedoraproject.org/wiki/End_of_life>. If you are using Fedora 20, use stack upgrade or download the general Linux binary <https://github.com/commercialhaskell/stack/wiki/Downloads#linux>. Major changes: - You now have more control over how GHC versions are matched, e.g. “use exactly this version,” “use the specified minor version, but allow patches,” or “use the given minor version or any later minor in the given major release.” The default has switched from allowing newer later minor versions to a specific minor version allowing patches. For more information, see #736 <https://github.com/commercialhaskell/stack/issues/736> and #784 <https://github.com/commercialhaskell/stack/pull/784>. - Support added for compiling with GHCJS - stack can now reuse prebuilt binaries between snapshots. That means that, if you build package foo in LTS-3.1, that binary version can be reused in LTS-3.2, assuming it uses the same dependencies and flags. #878 <https://github.com/commercialhaskell/stack/issues/878> Other enhancements: - Added the --docker-env argument, to set environment variables in Docker container. - Set locale environment variables to UTF-8 encoding for builds to avoid “commitBuffer: invalid argument” errors from GHC #793 <https://github.com/commercialhaskell/stack/issues/793> - Enable translitation for encoding on stdout and stderr #824 <https://github.com/commercialhaskell/stack/issues/824> - By default, stack upgrade automatically installs GHC as necessary #797 <https://github.com/commercialhaskell/stack/issues/797> - Added the ghc-options field to stack.yaml #796 <https://github.com/commercialhaskell/stack/issues/796> - Added the extra-path field to stack.yaml - Code page changes on Windows only apply to the build command (and its synonyms), and can be controlled via a command line flag (still defaults to on) #757 <https://github.com/commercialhaskell/stack/issues/757> - Implicitly add packages to extra-deps when a flag for them is set #807 <https://github.com/commercialhaskell/stack/issues/807> - Use a precompiled Setup.hs for simple build types #801 <https://github.com/commercialhaskell/stack/issues/801> - Set —enable-tests and —enable-benchmarks optimistically #805 <https://github.com/commercialhaskell/stack/issues/805> - --only-configure option added #820 <https://github.com/commercialhaskell/stack/issues/820> - Check for duplicate local package names - Stop nagging people that call stack test #845 <https://github.com/commercialhaskell/stack/issues/845> - --file-watch will ignore files that are in your VCS boring/ignore files #703 <https://github.com/commercialhaskell/stack/issues/703> - Add --numeric-version option Bug fixes: - stack init --solver fails if GHC_PACKAGE_PATH is present #860 <https://github.com/commercialhaskell/stack/issues/860> - stack solver and stack init --solver check for test suite and benchmark dependencies #862 <https://github.com/commercialhaskell/stack/issues/862> - More intelligent logic for setting UTF-8 locale environment variables #856 <https://github.com/commercialhaskell/stack/issues/856> - Create missing directories for stack sdist - Don’t ignore .cabal files with extra periods #895 <https://github.com/commercialhaskell/stack/issues/895> - Deprecate unused --optimizations flag - Truncated output on slow terminals #413 <https://github.com/commercialhaskell/stack/issues/413>

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Foreign calls and periodic alarm signals
by Phil Ruffwind 04 Sep '15

04 Sep '15

TL;DR: Does 'foreign import safe' silence the periodic alarm signals? I received a report on this rather strange bug in 'directory': https://github.com/haskell/directory/issues/35#issuecomment-136890912 I've concluded based on the dtruss log that it's caused by the timer signal that the GHC runtime emits. Somewhere inside the guts of 'realpath' on Mac OS X, there is a function that does the moral equivalent of: while (statfs64(…) && errno == EINTR); On a slow filesystem like SSHFS, this can cause a permanent hang from the barrage of signals. The reporter found that using 'foreign import safe' mitigates the issue. What I'm curious mainly is that: is something that the GHC runtime guarantees -- is using 'foreign import safe' assured to turn off the periodic signals for that thread? I tried reading this article [1], which seems to be the only documentation I could find about this, and it didn't really go into much depth about them. (I also couldn't find any info about how frequently they occur, on which threads they occur, or which specific signal it uses.) I'm also concerned whether there are other foreign functions out in the wild that could suffer the same bug, but remain hidden because they normally complete before the next alarm signal. [1]: https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Signals

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Private packages on hackage
by Alan & Kim Zimmerman 03 Sep '15

03 Sep '15

Hi all I had a chat with Duncan Coutts of Well-Typed about the possibility of an extension to hackage which would allow private packages to be uploaded and managed by hackage, for a fee. The idea is that it operates in a similar fashion to GitHub, where public packages are free, but there is a monthly fee for hosting private packages. This would give commercial organisations wanting to get started with Haskell a simple way to host their packages, so that members of the organisation could see the private and public packages when logged in, and cabal install would work as expected with the superset. Duncan is worried about the up-front effort required vs an uncertain return. Personally I think that this could be a good way to fund the hackage infrastructure. So the question is, apart from me, would anyone be interested in such a feature? Regards Alan

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platform 2014.2.0.0 for Windows
by Andrew Butterfield 03 Sep '15

03 Sep '15

Where do I download Haskell Platform 2014.2.0.0 for Windows? - the current (new?) Haskell Platform page doesn't link to old versions anymore (I want to build wxHaskell 0.92, but I gather it doesn't build with ghc 7.10 right now) Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland

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How to direct cabal to use an alternative ghc version, ala stack?
by David Banas 03 Sep '15

03 Sep '15

Hi all, How do I direct cabal to use an alternative (i.e. - not my system’s default) version of ghc, which I’ve installed, via stack? I tried this: stack exec cabal install ... but got this: cabal: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with Cabal. Use the flag --package-db to specify a package database (it can be used multiple times). Thanks, -db

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Examples for Control.Lens.Plated?
by Andrey Chudnov 03 Sep '15

03 Sep '15

Hello Cafe, I'm considering porting some of my code that uses uniplate to lens. But Control.Lens.Plated is giving me headaches. So I'm looking for remedy. Has anyone used it and willing to share examples? I hope that seeing some of the combinators in action would help me understand the type signatures and documentation. What I'm particularly interested in is the following: 1) implementation of something like transformBi and rewriteBi using lens 2) usage of *On, *Off and *OnOff combinators (e.g., transformMOn, transformMOff, transformMOnOff). What I want to understand is the difference between the 3 variants. Also, what does it mean to "Transform every element in the tree in a region indicated by a supplied Traversal" and how exactly does the traversal define the region. 3) any recursive tree traversals/transformations using Plated that stop the recursion at a certain point (e.g., upon seeing a certain constructor) I hope this isn't too vague, but, please, let me know if it is. /Andrey

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JOB OPPORTUNITY_ IT Software Developer
by CREATE-NET Jobs 02 Sep '15

02 Sep '15

CREATE-NET is seeking a Software Developer passionate about web applications development and its latest technologies. You will be part of our Innovation IT team based in Trento, Italy and will contribute to the development and testing of our portfolio of specialized software products for supporting the promotion and education of Create-Net innovation result. *Technical skills* ● Working experience with HTML5, CSS, AJAX, JSON APIs, JavaScript ● Working experience with Ruby (on Rails) and PHP plus some knowledge about Java and Node.js ● Knowledge of Mobile application development (Native Android/iOs or PhoneGap/Cordova) and Responsive Web Design ● Knowledge or interest of functional programming languages (Haskell, Common Lisp, others) will be considered a strong plus ● Experience with relational DBMSs (PostgreSQL in particular) ● Experience with distributed version control systems such as Git ● Familiarity working with Linux environments (Redhat, Ubuntu) ● Experience and knowledge of agile methodologies, software design methods are also considered a plus. *Specific Job responsibilities will include:* ● Contribution to the maintenance and evolution of existing tools which support e-learning and virtual incubation (community portal, tasks management app, event mobile app, idea-incubation/learning platform) ● Contribution to the development of new solutions for software integration ● Contribution to technical support. *Organization skills* ● Autonomous thinking and problem solving skills; ● Prioritization, time management skills and capability to adapt to dynamic environments; ● Ability to work hands-on and with little supervision; ● Attitude to collaborative working. Applicants should have Bachelor's degree (or equivalent experience) in computing or other related disciplines and a proven experience in web software development. They should be enthusiastic about web applications technologies, open-minded and willing to share and expand their knowledge and skills on other technologies too. Please include any Github/Bitbucket or otherwise links to open source contributions you have made. *HOW TO APPLY:* Please send your resume/CV in English to careers(a)create-net.org, specifying in the email subject: “IT SOFTWARE DEVELOPER_HC”. Only applications submitted in English will be taken into consideration. *Important:* due to Italian Privacy Protection Law n.196/03 any resume not mentioning explicitly the following wording: “I authorize the use of my personal data in accordance with Italian Privacy Protection Law (30/06/2003, n.196/03)” will be automatically deleted from our database and consequently not taken into consideration. *Languages:* Excellent command of the English language (written and spoken) is essential. knowledge of the Italian language both spoken and written is not required, but can be helpful. *Working Place*: Povo (Trento), Italy

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JOB OPPORTUNITY_ Jr. Functional Software Developer
by CREATE-NET Jobs 02 Sep '15

02 Sep '15

CREATE-NET is seeking a motivated Junior Functional Software Developer. You will be part our innovation IT team based in Trento, Italy, and will be contributing to the development and testing of our specialized software products which support the workflow related to scientific content organization. *Technical skills* ● Experience with one or more functional programming languages. Haskell and Common Lisp are preferable. ● Experience with web technologies: Javascript, HTML5, CSS, AJAX, JSON APIs, web services, responsive web design. ● Optional knowledge of either Node.js, Ruby on Rails, PHP, Java is considered a plus ● Experience with relational DBMSs (PostgreSQL) ● Experience with distributed version control systems (Git) ● Familiarity in working with GNU/Linux environments (RHEL, Ubuntu, etc.) *Specific job responsibilities will include* ● Contributing to the maintenance and the improvement of our content management platforms and applications; ● Developing new solutions for software integration ● Interacting with the project responsible and the user community to determine requirements and communicating with the support team to handle issues or new features requests. *Organizational skills* ● Autonomous thinking and problem solving skills; ● Prioritization, time management skills and capability to adapt to dynamic environments; ● Ability to work hands-on and with little supervision; ● Ability to timely deliver your work as per an agreed roadmap; ● Attitude to collaborative working; Applicants should have Bachelor’s degree (or equivalent experience) in computing or other related disciplines and a proven experience in software development. They should be enthusiastic about web applications technologies and willing to expand their knowledge and skills on other technologies too. Please include any Github/Bitbucket or otherwise links to open-source contributions you have made. *How to Apply: *Please send your resume/CV in English to careers(a)create-net.org, specifying in the email subject: “FUNCTIONAL SOFTWARE DEVELOPER -HC”. Only applications submitted in English will be taken into consideration. *Important:* Due to Italian Privacy Protection Law n.196/03 any resume not mentioning explicitly the following wording: “I authorize the use of my personal data in accordance with Italian Privacy Protection Law (30/06/2003, n.196/03)” will be automatically deleted from our database and consequently not taken into consideration. *Languages:* Good command of the English language (written and spoken) is essential. Knowledge of the Italian language both spoken and written is not required, but can be helpful. *Working Place*: Povo (Trento), Italy

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Fwd: How to combine simulations
by David Sorokin 02 Sep '15

02 Sep '15

Hi all, I forgot to click on ReplyAll. I thought that my answer could be interested to others as well. Therefore, I resend the letter to Haskell Cafe. Thanks, David ---------- Forwarded message ---------- From: David Sorokin <david.sorokin(a)gmail.com> Date: Wed, Sep 2, 2015 at 9:39 AM Subject: Re: [Haskell-cafe] How to combine simulations To: martin <martin.drautzburg(a)web.de> Hi Martin, Nevertheless, regarding Aivika, did you look at the Signal type (inspired by the IObservable interface of .NET)? This type is quite composable. If I understood you correctly, the Signal is what you call Event. Moreover, if you need a mutable state, then you could use the Ref reference and its signal refChanged. For example, the reference could be used to decide whether to emit a signal based on the intermediate state. Furthermore, if you want to build a computation with linear-looking signal processing then you could use the Process monad (inspired by the Async workflow of F#) and the processAwait function that suspends the current discontinuous process waiting for a signal to come. I hope it helps. In case of need, these ideas can be re-implemented. Thanks, David On Mon, Aug 31, 2015 at 7:48 PM, martin <martin.drautzburg(a)web.de> wrote: > Hello all, > > I've been trying hard to come up with an idea how to build a DES from > smaller parts. So far, I came to the conclusion, > that somewhere there must be an operation which takes an Event and maybe > emits an Event (and appends to a log and > updates some state). Those Events whould come from and go to the > "environment" the simulation runs in. > > My mental model is two billiard tables, which are connected through a hole > in the cushion and which each have a player. > When I look at one such table, it would have to respond to Events from its > player and from the other table and it would > send events to its player ("all balls at rest") and to the other table. > > If I add the other table and the two players then the combined simulation > would not emit any events at all and it would > not respond to any events except maybe as START event. It would only > depend on its initial state. > > But if I add only the player, but not the other table, it would still send > events to the other table and respond to > events from that other table. > > My problem is the type of Events. I could create a type which encompasses > all possible events, but that violates the > idea of composablitly. Somehow I would need to be able to take a system > which accepts "player events" and "other table > events", compose it with an other table and end up with a system which > only accepts "player events" but no more "other > table events" and similarly for the emitted events. And I don't quite know > how to do this. > > Hope this makes some sense. > > Any pointers (which go beyond "aivika has a simulation component") would > also be much appreciated. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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How to combine simulations
by martin 01 Sep '15

01 Sep '15

Hello all, I've been trying hard to come up with an idea how to build a DES from smaller parts. So far, I came to the conclusion, that somewhere there must be an operation which takes an Event and maybe emits an Event (and appends to a log and updates some state). Those Events whould come from and go to the "environment" the simulation runs in. My mental model is two billiard tables, which are connected through a hole in the cushion and which each have a player. When I look at one such table, it would have to respond to Events from its player and from the other table and it would send events to its player ("all balls at rest") and to the other table. If I add the other table and the two players then the combined simulation would not emit any events at all and it would not respond to any events except maybe as START event. It would only depend on its initial state. But if I add only the player, but not the other table, it would still send events to the other table and respond to events from that other table. My problem is the type of Events. I could create a type which encompasses all possible events, but that violates the idea of composablitly. Somehow I would need to be able to take a system which accepts "player events" and "other table events", compose it with an other table and end up with a system which only accepts "player events" but no more "other table events" and similarly for the emitted events. And I don't quite know how to do this. Hope this makes some sense. Any pointers (which go beyond "aivika has a simulation component") would also be much appreciated.

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Backing up ghc version, just for one sandbox?
by David Banas 31 Aug '15

31 Aug '15

Hi all, Is it possible to back up the ghc version, just for a single sandbox? Thanks, -db

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Compiling non-registered packages into a sandbox?
by David Banas 31 Aug '15

31 Aug '15

Hi all, How do I compile a non-Hackage registered package into a local sandbox? Let’s say I have: /proj_dir/ - .cabal-sandbox/ - cabal.sandbox.config - dep_pkg/ - dep_pkg.cabal - Setup.lhs - … I’d like to compile the dep_pkg package into my proj_dir sandbox; can I do this, if dep_pkg is NOT registered with Hackage? If so, how? Thanks, -db

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New Functional Programming Job Opportunities
by Functional Jobs 31 Aug '15

31 Aug '15

Here are some functional programming job opportunities that were posted recently: Full Stack Haskell Software Engineer at Linkqlo Inc http://functionaljobs.com/jobs/8859-full-stack-haskell-software-engineer-at… Cheers, Sean Murphy FunctionalJobs.com

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Thread-safe GHC
by Corentin Dupont 31 Aug '15

31 Aug '15

Hello, I am wondering if GHC itself is thread-safe. I am using Hint, and it reports that GHC is not thread-safe, and that I can't safely run two instances of the interpreter simultaneously. Is that still the case? Thanks! Corentin ---------- Forwarded message ---------- From: Daniel Gorín <jcpetruzza(a)gmail.com> Date: Thu, Aug 27, 2015 at 5:09 PM Subject: Re: Thread-safe Hint To: Corentin Dupont <corentin.dupont(a)gmail.com> Hi Corentin, sorry for the late reply. Until relatively recently, the problem was still on. But I too remember seeing something related to this issue being fixed (iirc, the problem was the runtime linker, which used global state), so perhaps it is already fixed in 7.10. If you can verify this, it shouldn’t be hard to show the error message only on old versions of ghc. I’ll be away for a couple of weeks, but if you want to look into this and send a patch, I’ll merge it when I return. Cheers, Daniel > On 24 Aug 2015, at 10:43 am, Corentin Dupont <corentin.dupont(a)gmail.com> wrote: > > Hello Daniel, > I noticed the following message in Hint: > This version of GHC is not thread-safe,can't safely run two instances of the interpreter simultaneously. > > Is it still the case with recent versions of GHC? > It would be neat to be able to launch several instances of the interpreter. In my game Nomyx I have several "match-up" going on and having one instance of the interpreter would be nicer. Otherwise I am obliged to reset the interpret each time I want to interpret something, which is time consuming (2-3 seconds). > > Thanks, > C

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Re: [Haskell-cafe] Discussion: The CLOEXEC problem
by Donn Cave 31 Aug '15

31 Aug '15

Quoth David Turner <dave.c.turner(a)gmail.com>, > Could you be a bit more specific? Which bits of pre-existing software > didn't have a FD_CLOEXEC bit and would be broken by this proposal? Well, of course to be precise, the bit's always there, it's just normally not set - that's the normal environment that anything written up to now would expect. And of course, anything that depends on a GHC-opened file to stay open over an exec would be broken. I can't enumerate the software that meets that criterion. > Since Python recently decided to go through this exact transition, their > experience should be instructive. Do you know if there was negative fallout > from PEP 0466? I gave up on Python a long time ago and don't follow what goes on. If recently means less than a decade or so, though, it's not much to go on. If the problem addressed by the O_CLOEXEC proposal is obscure, the problems it may create are even more so - I'll certainly concede that - and it could take a lot of experience before those problems would be well known enough to show up if you went looking for them. > When thinking about FDs from outside the Haskell runtime (whether inherited > or simply opened in an external library), can you give an example of a case > where such a FD causes a problem if inherited and yet cannot be set as > FD_CLOEXEC at source? Sorry, I'm confused here. Files opened within GHC and externally have equal potential to work as intended or to cause problems, it seems to me. If we infer from the proposal that GHC-opened files with CLOEXEC unset may cause a problem, then it follows that other files with CLOEXEC unset also may cause the same problem. The proposal addresses only the former, and not the latter, and only for normal files - while the ordinary solution, as implemented in UNIX popen(3), deals with all - pipes, sockets, etc. Donn

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Idiomatic way to parse bytecodes
by Gautier DI FOLCO 31 Aug '15

31 Aug '15

Hello all, I have some bytecodes to parse and I look for the most idiomatic way to do it. It's mostly JVM Bytecode, long story short, I have some embedded JVM Bytecodes: * one part is specification-defined, from 0x00 to 0xB8, differ from the regular JVM Bytecode * and the other part is proprietary and changes often I have two main goals: * print the signification of each bytecodes (a more explicit one than the hexadecimal value of the bytecode) * Do some analysis on it, spot patterns, apply markov model on it and so on. The issue with JVM Bytecode in general is that each bytecode hasn't been made equal. Some of them requires parameters, some of them modify the length of the next bytecode, etc. I'm not looking for a way to parse bytecode quickly, or for a library doing it for me. I look for a simple/composable/idiomatic way to do it. So if you have any thoughts, papers, articles or even code snippets (part of a library of not) that is done in the Haskell-way, please, share it :) Thanks in advance for your help.

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Haskell Engineer Needed for Machine Learning Startup
by Charles Weitzer 30 Aug '15

30 Aug '15

Voleon Capital Management LP is a startup quantitative hedge fund located in Berkeley, California. We would like to hire a senior software engineer with strong functional programming experience as soon as possible. Voleon's founders previously worked together at one of the most successful quantitative hedge funds in the world. Our CEO has a PhD in Computer Science from Stanford and has been CEO and founder of a successful Internet infrastructure startup. Our Chief Investment Officer has a PhD in Statistics from Berkeley. Voleon's team includes PhD's from leading departments in statistics, computer science, and mathematics. We have made several unpublished advances in the field of machine learning and in other areas as well. Here is our formal job description: ********************************************************** * Senior Software Engineer * Technology-driven investment firm employing cutting-edge statistical machine learning techniques seeks an exceptionally capable software engineer. You will architect and implement new production trading systems, machine learning infrastructure, data integration pipelines, and large-scale storage systems. The firm researches and deploys systematic trading strategies designed to generate attractive returns without being dependent on the performance of the overall market. Join a team of under 30 people that includes a Berkeley statistics professor as well as over ten PhD's from Berkeley, Chicago, CMU, MIT, Princeton, Stanford, and UCLA, led by the founder and CEO of a successful Internet infrastructure technology firm. The firm's offices are walking distance from BART and the UC Berkeley campus in downtown Berkeley, California. We have a casual and collegial office environment, catered lunches, and competitive benefits packages. We seek candidates with a proven track record of writing correct, well-designed software, solving hard problems, and delivering complex projects on time. You should preferably have experience designing and implementing fault-tolerant distributed systems. Experience with building large-scale data infrastructure, stream processing systems, or latency-sensitive programs is a bonus. We are growing rapidly. Willingness to take initiative and a gritty determination to productize are essential. Required experience: - strong experience developing in a functional programming environment (Haskell, Erlang, others). - developing with C/C++/Python/Go in a Linux environment with a focus on performance, concurrency, and correctness. - working in TCP/IP networking, multi-threading, and server development. - working with common Internet protocols (IP, TCP/UDP, SSL/TLS, HTTP, SNMP, etc.). - architecting and designing highly available systems. - architecting and designing large-scale data management infrastructure. - working in large codebases and building modular, manageable code. Preferred experience.: - debugging and performance profiling, including the use of tools such as strace, valgrind, gdb, tcpdump, etc. - working with build and test automation tools. - working with well-defined change management processes. - diagnosing RDBMS performance problems, exploiting indexing, using EXPLAIN PLAN, optimizing at the code layer, etc. - working with messaging queues (RabbitMQ, Redis, etc.) as well as distributed caching systems. Interest in financial applications is essential, but experience in finance is not a primary factor in our hiring. Benefits and compensation are highly competitive. ********************************************************** The above job description is just a starting point in terms of possible duties and seniority. We can be very flexible for the right person. If you are interested or if you know of anyone who might be interested, let us know the best way to get in touch and we can discuss details. Thank you, Charles Weitzer Senior Recruiter Voleon Capital Management LP Charles(a)Voleon.com<mailto:Charles@Voleon.com> Office: 510.704.9870 x 7012 Mobile: (510) 558-9182 www.Voleon.com<http://www.voleon.com/>

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Re: [Haskell-cafe] Why Haskell is beautiful to the novice
by Olaf Klinke 30 Aug '15

30 Aug '15

Dear cafe inhabitants, first of all thanks for all the elaborate answers. I did not imagine to trigger such an avalanche of opinions. Let me try to steer away from the Python/Haskell comparison (as comparison is what I wanted to avoid) and collect some points I found important in the past discussion. 1. There is a difference between teaching how the machines we call computers work and how computers are used to solve problems. What is computer science? The science about computers or science that can be executed on a computer? Both subjects are beautiful, in different ways. Haskell, I used to think, was designed explicitly to abstract away from how its programs are actually run on the hardware, to the point where one can not even tell (and care) what order certain computations are executed in, or whether they are executed at all. Therefore Haskell is certainly not the best language to teach about the mechanics of computing. Similarly, there are practical problems that may be more difficult to express in a functional language, because the problem may be already of a sequential, mutable, effect-full nature (e.g. writing an adventure game). 2. For teaching purposes, the remark that kids want something they can show off to others should not not be under-estimated. In order to quickly obtain showable results, one might choose a toy language (DSL?) or program robots. For this reason I considered HTML as a first introduction to syntax, even if it is not a programming language as such. But there exist syntax checkers and one can create and view HTML on every modern device. By the way, is there a live CD (not DVD) image that contains the Haskell platform and some other languages? 3. One skill/revelation that I want to install in the students of both math and CS, and which I believe Haskell is more suitable for than imperative languages, is the fact that in mathematics and computer science there are no hidden layers/aspects, no mysteries. Everything is composed of smaller, simpler parts. And if one wishes, one can unwrap one layer and look inside. How deeply one wishes to look inside is a question of audience, then. The layering might consist of recursion, classes, objects. It does not matter. The important point is that the path to a solution must descend the layers to the simple parts. This is in contrast with arts and humanities, where one can study the same piece of art, the same novel, several times in a lifetime and each time discover new things, and probably still miss others which require expert education. Mergesort, once grasped, is there in all its completeness, simplicity and beauty, there is no further interpretation. As others have remarked in this discussion, the `no mysteries' only applies to the denotational semantics, but not the operational semantics in case of Haskell. For assembler it might very well be the other way around. Thanks, Olaf

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Haskell related talks at Curry On!
by Jurriaan Hage 30 Aug '15

30 Aug '15

Dear Haskell-minded, Curry On! in Prague was co-located with Ecoop and had some Haskell-oriented talks. In case you were not there, and had not noticed: video’s of the talks are available on YouTube. Of particular note are Julian Arny’s talk on Servant, url at https://www.youtube.com/watch?v=snOBI8PcbMQ Then there is the entertaining discussion by Thomas Arts of Quviq on awakening industry to the use of QuickCheck (it’s Erlang, I know, but coming from Haskell): https://www.youtube.com/watch?v=Ar0FPW2GSgo and finally my own on domain-specific type error diagnosis in Haskell: https://www.youtube.com/watch?v=bPrM1gONdII And when you’ve seen these, YouTube is bound to suggest many others. enjoy, Jurriaan Hage PS. Curry On! was in fact a great event with many interesting talks. It is going on repeat it seems, so catch it if you can.

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Discussion: The CLOEXEC problem
by Niklas Hambüchen 30 Aug '15

30 Aug '15

Hello Cafe, I would like to point out a problem common to all programming languages, and that Haskell hasn't addressed yet while other languages have. It is about what happens to file descriptors when the `exec()` syscall is used (whenever you `readProcess`, `createProcess`, `system`, use any form of `popen()`, Shake's `cmd` etc.). (A Markdown-rendered version containing most of this email can be found at https://github.com/ndmitchell/shake/issues/253.) Take the following function f :: IO () f = do inSomeTemporaryDirectory $ do BS.writeFile "mybinary" binaryContents setPermissions "mybinary" (setOwnerExecutable True emptyPermissions) _ <- readProcess "./mybinary" [] "" return () If this is happening in parallel, e.g. using, forkIO f >> forkIO f >> forkIO f >> threadDelay 5000000` then on Linux the `readProcess` might often fail wit the error message mybinary: Text file busy This error means "Cannot execute the program 'mybinary' because it is open for writing by some process". How can this happen, given that we're writing all `mybinary` files in completely separate temporary directories, and given that `BS.writeFile` guarantees to close the file handle / file descriptor (`Fd`) before it returns? The answer is that by default, child processes on Unix (`fork()+exec()`) inherit all open file descriptors of the parent process. An ordering that leads to the problematic case could be: * Thread 1 writes its file completely (opens and closes an Fd 1) * Thread 2 starts writing its file (Fd 2 open for writing) * Thread 1 executes "myBinary" (which calls `fork()` and `exec()`). Fd 2 is inherited by the child process * Thread 2 finishes writing (closes its Fd 2) * Thread 2 executes "myBinary", which fails with `Text file busy` because an Fd is still open to it in the child of Process 1 The scope of this program is quite general unfortunately: It will happen for any program that uses parallel threads, and that runs two or more external processes at some time. It cannot be fixed by the part that starts the external process (e.g. you can't write a reliable `readProcess` function that doesn't have this problem, since the problem is rooted in the Fds, and there is no version of `exec()` that doesn't inherit parent Fds). This problem is a general problem in C on Unix, and was discovered quite late. Naive solutions to this use `fcntl`, e.g. `fcntl(fd, F_SETFD, FD_CLOEXEC)`: http://stackoverflow.com/questions/6125068/what-does-the-fd-cloexec-fcntl-f… which is the equivalent of Haskell's `setFdOption` to set the `CLOEXEC` flag to all Fds before `exec()`ing. Fds with this flag are not inherited by `exec()`ed child processes. However, these solutions are racy in multi-threaded programs (such as typical Haskell programs), where an `exec()` made by some thread can fall just in between the `int fd = open(...); exec(...)` of some other thread. For this reason, the `O_CLOEXEC` flag was added in Linux 2.6.23, see e.g. `man 2 open` http://man7.org/linux/man-pages/man2/open.2.html to the `open()` syscall to atomically open a file and set the Fd to CLOEXEC in a single step. This flag is not the default in Haskell - but maybe it should be. Other languages set it by default, for example Python. See PEP-433: https://www.python.org/dev/peps/pep-0433/ and the newer PEP-446: https://www.python.org/dev/peps/pep-0446/ for a very good description of the situation. Python >= 3.2 closes open Fds *after* the `exec()` when performed with its `subprocess` module. Python 3.4 uses O_CLOEXEC by default on all Fds opened by Python. It is also noted that "The programming languages Go, Perl and Ruby make newly created file descriptors non-inheritable by default: since Go 1.0 (2009), Perl 1.0 (1987) and Ruby 2.0 (2013)": https://www.python.org/dev/peps/pep-0446/#related-work A work-around for Haskell is to use `O_CLOEXEC` explicitly, as in this example module `System/Posix/IO/ExecSafe.hsc`: https://gist.github.com/nh2/4932ecf5ca919659ae51 Then we can implement a safe version of `BS.writeFile`: https://gist.github.com/nh2/4932ecf5ca919659ae51 Using this form of `writeFileExecSafe` helps in cases when your program is very small, when you can change all the code and you don't use any libraries that open files. However, this is a very rare case, and not a real solution. All multi-threaded Haskell programs that write and execute files will inherently trigger the `Text file busy` problem. We need to discuss what to do about this. Let us run this discussion on haskell-cafe and move to the libraries@ mailing list once we've got some ideas and opinions. My personal stance is that we should follow Python's example, and all functions in our standard libraries should open files with the O_CLOEXEC flag set. Niklas

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Final CFP: FLOPS 2016, International Symposium on Functional and Logic Programming
by Oleg 30 Aug '15

30 Aug '15

NEW: revised submission deadlines (Sep 21 for abstracts, Sep 25 for papers) FLOPS 2016: March 3-6, 2016, Kochi, Japan Final Call For Papers http://www.info.kochi-tech.ac.jp/FLOPS2016/ Writing down detailed computational steps is not the only way of programming. The alternative, being used increasingly in practice, is to start by writing down the desired properties of the result. The computational steps are then (semi-)automatically derived from these higher-level specifications. Examples of this declarative style include functional and logic programming, program transformation and re-writing, and extracting programs from proofs of their correctness. FLOPS aims to bring together practitioners, researchers and implementors of the declarative programming, to discuss mutually interesting results and common problems: theoretical advances, their implementations in language systems and tools, and applications of these systems in practice. The scope includes all aspects of the design, semantics, theory, applications, implementations, and teaching of declarative programming. FLOPS specifically aims to promote cross-fertilization between theory and practice and among different styles of declarative programming. Scope FLOPS solicits original papers in all areas of the declarative programming: * functional, logic, functional-logic programming, re-writing systems, formal methods and model checking, program transformations and program refinements, developing programs with the help of theorem provers or SAT/SMT solvers; * foundations, language design, implementation issues (compilation techniques, memory management, run-time systems), applications and case studies. FLOPS promotes cross-fertilization among different styles of declarative programming. Therefore, submissions must be written to be understandable by the wide audience of declarative programmers and researchers. Submission of system descriptions and declarative pearls are especially encouraged. Submissions should fall into one of the following categories: * Regular research papers: they should describe new results and will be judged on originality, correctness, and significance. * System descriptions: they should contain a link to a working system and will be judged on originality, usefulness, and design. * Declarative pearls: new and excellent declarative programs or theories with illustrative applications. System descriptions and declarative pearls must be explicitly marked as such in the title. Submissions must be unpublished and not submitted for publication elsewhere. Work that already appeared in unpublished or informally published workshops proceedings may be submitted. See also ACM SIGPLAN Republication Policy. The proceedings will be published by Springer International Publishing in the Lecture Notes in Computer Science (LNCS) series, as a printed volume as well as online in the digital library SpringerLink. Post-proceedings: The authors of 4-7 best papers will be invited to submit the extended version of their FLOPS paper to a special issue of the journal Science of Computer Programming (SCP). Important dates Monday, September 21, 2015 (any time zone): Abstract Submission Friday, September 25, 2015 (any time zone): Submission deadline (FIRM) Monday, November 16, 2015: Author notification March 3-6, 2016: FLOPS Symposium March 7-9, 2016: PPL Workshop Invited Talks - Kazunori UEDA (Waseda University) The exciting time and hard-won lessons of the Fifth Generation Computer Project - Atze Dijkstra (Utrecht University) UHC: Coping with Compiler Complexity Submission Submissions must be written in English and can be up to 15 pages long including references, though pearls are typically shorter. The formatting has to conform to Springer's guidelines. Regular research papers should be supported by proofs and/or experimental results. In case of lack of space, this supporting information should be made accessible otherwise (e.g., a link to a Web page, or an appendix). Papers should be submitted electronically at https://easychair.org/conferences/?conf=flops2016 Program Committee Andreas Abel Gothenburg University, Sweden Lindsay Errington USA Makoto Hamana Gunma University, Japan Michael Hanus CAU Kiel, Germany Jacob Howe City University London, UK Makoto Kanazawa National Institute of Informatics, Japan Andy King University of Kent, UK (PC Co-Chair) Oleg Kiselyov Tohoku University, Japan (PC Co-Chair) Hsiang-Shang Ko National Institute of Informatics, Japan Julia Lawall Inria-Whisper, France Andres Loeh Well-Typed LLP, UK Anil Madhavapeddy Cambridge University, UK Jeff Polakow USA Marc Pouzet Ecole normale superieure, France Vitor Santos Costa Universidade do Porto, Portugal Tom Schrijvers KU Leuven, Belgium Zoltan Somogyi Australia Alwen Tiu Nanyang Technological University, Singapore Sam Tobin-Hochstadt Indiana University, USA Hongwei Xi Boston University, USA Neng-Fa Zhou CUNY Brooklyn College and Graduate Center, USA Organizers Andy King University of Kent, UK (PC Co-Chair) Oleg Kiselyov Tohoku University, Japan (PC Co-Chair) Yukiyoshi Kameyama University of Tsukuba, Japan (General Chair) Kiminori Matsuzaki Kochi University of Technology, Japan (Local Chair) flops2016 at logic.cs.tsukuba.ac dot jp

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Question, re: type class syntax.
by David Banas 30 Aug '15

30 Aug '15

Hi all, I can’t figure out why this is invalid: class (Show a, Num a) => LogTree2 a where evaluator :: (LogTree2 a) t => t -> SizedVector a -> SizedVector a error| ‘LogTree2’ is applied to too many type arguments In the type ‘(LogTree2 a) t => t -> SizedVector a -> SizedVector a’ In the class declaration for ‘LogTree2’ Can anyone help me understand? Thanks, -db

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General Functor and Traversable
by M Farkas-Dyck 29 Aug '15

29 Aug '15

Consider this: class (Category c, Category d) => Functor c d f where map :: c a b -> d (f a) (f b) Clearly mapM of Traversable is simply map :: Kleisli m a b -> Kleisli m (f a) (f b). But what is traverse? can it be so defined? I am thinking no, as Kleisli composition is not defined in general, but I know very little category theory so I may easily be wrong.

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Re: [Haskell-cafe] List of nitpicks
by Johannes Waldmann 28 Aug '15

28 Aug '15

Re: http://repetae.net/recent/out/classalias.html Prelude> :set -XConstraintKinds Prelude> class C a; class D a; type E a = (C a, D a); instance E a => C (Maybe a) https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/constraint-… - J.W.

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[ANN] dejafu-0.1.0.0: Overloadable primitives for testable, potentially non-deterministic, concurrency.
by Michael Walker 28 Aug '15

28 Aug '15

Hi cafe, For the past few months I've been working on a library for concurrency testing in Haskell, which I'll be giving a talk on at the Haskell Symposium next week. Now it's on Hackage! --- Déjà Fu is a library for developing and testing concurrent Haskell programs, it provides a typeclass-abstraction over GHC’s regular concurrency API, allowing the concrete implementation to be swapped out. Why do we need this? Well, concurrency is really hard to get right. Empirical studies have found that many real-world concurrency bugs can be exhibited with small test cases using as few as two threads: so it’s not just big concurrent programs that are hard, small ones are too. We as programmers just don’t seem to have a very good intuition for traditional threads-and-shared-memory-style concurrency. The typical approach to testing concurrent programs is to just run them lots of times, but that doesn’t provide any hard coverage guarantees, and then we need to wonder: how many runs do we need? Fortunately, there has been a lot of research into testing concurrency in the past several years. Systematic concurrency testing is an approach where the source of nondeterminism, the actual scheduler, is swapped out for one under the control of the testing framework. This allows possible schedules to be systematically explored, giving real coverage guarantees for our tests. This is a library implementing systematic concurrency testing. It provides two typeclasses, MonadConc to abstract over much of Control.Concurrent and related modules, and MonadSTM, to similarly abstract over Control.Monad.STM. ## How to use it: If you’re not making use of any IO in your code other than for concurrency, the transition to using MonadConc and MonadSTM will probably just be a textual substitution: - IO is replaced with MonadConc m => m - STM with MonadSTM m => m - *IORef with *CRef - *MVar with *CVar - *TVar with *CTVar - Most other things have the same name, and so can be replaced by just swapping imports around. If you are using other IO, you will need a gentle sprinkling of MonadIO and liftIO in your code as well. ## Is this really just a drop-in replacement for IO/STM? That’s the idea, yes. More specifically, the IO instance of MonadConc and the STM instance of MonadSTM just use the regular IO and STM functions, and so should have no noticeable change in behaviour, except for CRef, the IORef equivalent, where modifyCRef behaves like atomicModifyIORef, not modifyIORef. There are some other differences which can lead to incorrect results when testing, but which should not affect code when used in an IO or STM context. Specifically: Control.Monad.Conc.Class.getNumCapabilities can lie to encourage more concurrency when testing; and Control.Exception.catch can catch exceptions from pure code, but Control.Monad.Conc.Class.catch can’t (except for the IO instance). --- There is a blog post with references to prior posts and related papers here: http://www.barrucadu.co.uk/posts/2015-08-27-announce-dejafu.html -- Michael Walker (http://www.barrucadu.co.uk)

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Re: [Haskell-cafe] Why Haskell is beautiful to the novice
by Nicola Gigante 28 Aug '15

28 Aug '15

> Il giorno 28/ago/2015, alle ore 15:37, Torsten Otto <torsten.otto(a)hamburg.de> ha scritto: > > What an interesting topic to discuss! > >> Am 28.08.2015 um 12:12 schrieb Nicola Gigante <nicola.gigante(a)gmail.com>: >> >> Using Haskell (specifically some kind of concepts, e.g. equational reasoning) >> will also make easier for students to see the connection between math >> and programming, making them more motivated to learn math as well. > > Can you elaborate on that? What would you do in class in terms of „equational reasoning“? > It depends on the age of the students and the math they already know. I don’t have specific examples of in-class exercises or things like that. However, the point is that writing Haskell code you can often come up with shorter or more elegant solutions by manipulating terms exactly as done in elementary algebra. Starting from a 10lines function and coming up with a simple and readable oneliner is fun and rewarding, and shows one of the possible connections to math, especially if you can make them reason about types, in which case the task becomes very similar to theorem proving than to algebraic manipulations alone. Another possible connection arise when you teach things such as the Monoid typeclass. Even if students have not been exposed to abstract mathematics, they can still appreciate the usefulness of abstracting similar operations (addition in this case) to being able to uniformly reason about apparently different concepts. Abstraction is the core of mathematics, but is also a fundamental concept in programming, of course, so you’re implicitly teaching two things at once. > Regards, > Torsten Greetings, Nicola

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LaTeX question: interpreter examples
by Christopher Howard 28 Aug '15

28 Aug '15

Hi. I'm doing a lot of Literate Haskell lately, with proofs and commentary in each module. I'm not very familiar with LaTeX, but I'm learning. (I'm using lhs2tex and texi2pdf.) However, in my commentary, I occasionally have blocks of example interpreter interaction, and I'm not sure what to do with it. Is there a certain command enclosure that would be appropriate? Ideally, it would treat the text like monospace, and also preserve line breaks. Example text would be: *GCD> extEuclidean 546 2022 (6,100,-27) *GCD> 546 * 100 + 2022 * (-27) == 6 True But this does not come out looking similar in the final output. -- http://justonemoremathproblem.wordpress.com/

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an idea for modifiyng data/newtype syntax: use `::=` instead of `=`
by Alexey Muranov 28 Aug '15

28 Aug '15

Hello, i would like to suggest an idea for modifying the basic data/newtype syntax in Haskell: replace the equality sign `=` with `::=`. When i started learning Haskell, the most confusing part of the syntax for me was the equality sign in `data` definition. I could not even guess what the `data` definition meant without reading a chapter or two about types in Haskell, and i think it was partially due to the equality sign. I still find this notation inconsistent with other uses of the equality sign in Haskell and in general. For example, in type Name = String data Date = Date Int Int Int data Anniversary = Birthday Name Date | Wedding Name Name Date the second line is particularly disorienting IMO because on two sides of the equality, `Date` denotes different things. As far as i understand, in all contexts except those of `data` and `newtype` definitions, the equality sign in Haskell denotes the actual equality for all purposes: if a line foo x y = bar y x is present in a program, `foo a b` and `bar b a` can be used more or less interchangeably elsewhere in the program. Similarly, if the line type Name = String is present, `Name` can be used as `String`. Clearly, the equality in data Date = Date Int Int Int does not have such property. I think that if `::=` was used instead of `=` in `data` and `newtype` definitions, this would suggest to a newcomer that the syntax of the two sides might be different, and would helpfully remind of the Backus–Naur Form for syntax rules. I think that a newcomer to Haskell, like myself, would have had a better chance of guessing the meaning of type Name = String data Date ::= Date Int Int Int data Anniversary ::= Birthday Name Date | Wedding Name Name Date IMO this would make the program easier to read in general and the difference between `type` and `newtype` more clear. Maybe the can even make the use of keywords redundant, by allowing to write simply Name = String Date ::= Date Int Int Int Anniversary ::= Birthday Name Date | Wedding Name Name Date What do you think? Alexey.

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ANN: wxHaskell 0.92
by Henk-Jan van Tuyl 27 Aug '15

27 Aug '15

L.S., I am happy to announce a new version of wxHaskell (0.92). What is it? ----------- wxHaskell[1] is a portable and native GUI library for Haskell. The goal of the project is to provide an industrial strength GUI library for Haskell, but without the burden of developing (and maintaining) one ourselves. wxHaskell is therefore built on top of wxWidgets – a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X. Furthermore, it is a mature library (in development since 1992) that supports a wide range of widgets with the native look-and-feel. What's new? ----------- - wxc/setup.hs now stops searching for wxWidgets when a compatible version is found (this solves bug ticket 96) - Support for simple Wizards added - Calendar support added - GCC > 4.5 can now be used on Windows, which is a big improvement, as wxWidgets and wxHaskell must use the exact same GCC, to prevent compatibility problems - wxAui is added to wxc, wxAuiNotebook events are added to wxcore and wx - Missing GLAttributes added - Packet version limits adapted to the newest Haskell Platform - Bitness check on Windows no longer uses an external executable - wxHaskell can now be installed with MSYS2 - wxc/Setup.hs is modified to also link to the wx OpenGL libraries for wxGLCanvas - The "warning: Adding duplicate image handler for '... file'" messages are removed - Created a new class Updating with corresponding event "update". Provided instances for TextCtrl and ComboBox: update gets called when the text changes. - Support for wxSplashScreen in wxc and wxcore is added - Many warnings are solved Links ----- See the homepage of wxHaskell for more information: https://wiki.haskell.org/WxHaskell The packages are: - wxc https://hackage.haskell.org/package/wxc - wxdirect https://hackage.haskell.org/package/wxdirect - wxcore https://hackage.haskell.org/package/wxcore - wx https://hackage.haskell.org/package/wx Regards, Henk-Jan van Tuyl [0] https://www.wxwidgets.org [1] https://wiki.haskell.org/WxHaskell -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Stack transitive dependencies
by Corentin Dupont 27 Aug '15

27 Aug '15

Hello, I have a package depending on another: Foo/stack.yaml: flags: {} packages: - '.' - location: ../Bar extra-dep: true extra-deps: - Bar-0.7.6 resolver: lts-2.19 When I compile (stack build), I obtain: -- While attempting to add dependency, Could not find package DebugTraceHelpers in known packages -- Failure when adding dependencies: DebugTraceHelpers: needed (==0.12.*), but not present in build plan, latest is 0.12 needed for package: Bar-0.7.6 I find it strange that it asks for DebugTraceHelpers because it's not a direct dependency of my package, but a dependency of a dependency. It is correctly mentioned in Bar/stack.yml: flags: {} packages: - '.' extra-deps: - DebugTraceHelpers-0.12 resolver: lts-2.19

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Haskell hacking internships at Los Alamos National Laboratory (Spring 2016, undergraduate)
by Kei Davis 27 Aug '15

27 Aug '15

Hello, We have an ongoing project developing an auto-parallelizing pure functional language implementation based loosely on Haskell/STG. If you are a United States citizen or permanent resident alien studying computer science or mathematics at the undergraduate level with strong interests in Haskell programming, compiler/runtime development, and pursuing a spring semester (2016) internship at a national laboratory this could be for you. We don't expect applicants to necessarily already be highly accomplished Haskell programmers--such an internship is expected to be a combination of (further) developing your programming/Haskell skills and putting them to good use. If you're already a strong C hacker we could use that too. The application process for spring semester internships is open here http://science.energy.gov/wdts/suli/. Note the deadline of Oct. 9. Email me if interested, and feel free to pass this along. -- Kei Davis Applied Computer Science Group CCS-7, Mail Stop B287 Los Alamos National Laboratory Los Alamos, NM 87545, U.S.A.

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Template Haskell and packages using Obj-C FFI
by Hiromi ISHII 27 Aug '15

27 Aug '15

Hi cafe, I'm currently writing objc specific marshaller for inline-c, and faced with strange behaviour of Template Haskell and FFI. It turns out that it's irrelevant to inline-c and matter of GHC. Here is minimal example package for failure: https://github.com/konn/objcth This tiny package is built up with two parts: 1. A wrapper library to call NSLog function with Haskell's String value. This uses FFI interface, 2. An executable which calls the above library to say hello. If 2. does not contain any Template Haskell macros, everything works fine. But if any TH macro presents, even only top level splice `$(return [])`, then compiler halts with following error: ``` <command line>: can't load .so/.DLL for: /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib (dlopen(/Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib, 5): Symbol not found: _OBJC_CLASS_$_NSString Referenced from: /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib Expected in: flat namespace in /Users/myname/Documents/Programming/Haskell/lab/ghcbugs/objcth/.stack-work/dist/x86_64-osx/Cabal-1.22.4.0/build/libHSobjcth-0.1.0.0-IBzJnZWD04lEmS0Ot4HRVO-ghc7.10.2.dylib) ``` I used stack to build automatically, but cabal-install produces almost the same result (modulo paths to dist files). TH uses GHCi internally, so perhaps it's related to GHCi? If I replace the call for NSLog and [NSSring stringWith...] with puts c-function, everything gets fine. Is there any workaround, or it's the bug of GHC? Here is my environment: * OS X Yosemite 10.10.5 * Haskell Platform 7.10.2a * llvm-3.5 (installed via brew install llvm35) $ tail -n3 /Library/Frameworks/GHC.framework/Versions/7.10.2-x86_64/usr/lib/ghc-7.10.2/settings , ("LLVM llc command","llc-3.5") , ("LLVM opt command","opt-3.5") ] -- Hiromi ISHII konn.jinro(a)gmail.com

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Re: [Haskell-cafe] List of nitpicks
by Johannes Waldmann 27 Aug '15

27 Aug '15

>> quickCheck $ ( (\ xs -> xs == reverse xs ) :: Eq a => [a] -> Bool) > This can be solved, by the way: > https://ro-che.info/articles/2013-02-19-smallcheck#required-type-annotations Very good indeed. But (and here's another nit to pick - about ghc) - your > smallCheck 5 $ \x y -> x == (y :: Integer) would look much clearer that way: > smallCheck 5 $ \ (x::Integer) (y :: Integer) -> x == y but ghc complains about missing ScopedTypeVariables and that error message is fully incomprehensible (e.g., to beginning students) as there is no type variable anywhere. There was a PatternSignatures option but it seems gone (deprecated). I believe that (PatternSignatures and) ScopedTypeVariables should be on by default. It is currently off only to help those that are too lazy to write "forall t . " in (inner) declarations? Leaving out quantifiers should never be encouraged. And putting type declarations next to the name declaration (that is, on the "x" under the lambda) should be the normal thing to do. Everything else feels like a work-around, and it is a distraction when teaching. Putting language options at the top of the source file, or in .ghci, is also a distraction. - J.W.

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Elementary math question: primality test for huge numbers
by Christopher Howard 26 Aug '15

26 Aug '15

Hi. I was working through a math chapter explaining the RSA algorithm, and I was implementing it in Haskell for learning sake. However, the chapter does not explain how you efficiently pick the two prime numbers. I scanned through the WP page on primality tests but it was somewhat overwhelming. What would be the standard primality test for this sort of application, and is it already built in to Base somewhere? -- Biblical creationism: http://tinyurl.com/qfyeg4a Free Bible software: http://xiphos.org Software freedom: http://tinyurl.com/qjnpnsm Free computer operating system: http://tinyurl.com/7wczchu Alternative to MS Office: http://tinyurl.com/aw9p6d4

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'Scientific' loses significant digits
by Alex 26 Aug '15

26 Aug '15

Hi: Does anyone know why is this true? ((fromJust . readMaybe) "1.00000000" :: Scientific) == ((fromJust . readMaybe) "1" :: Scientific) -- Alex

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Munich Haskell Meeting, 2015-08-27 @ 19:30 Augustiner-Keller Biergarten
by Christian Neukirchen 25 Aug '15

25 Aug '15

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 27 at **Augustiner-Keller Biergarten** (Arnulfstr. 52) in the self-service area (rougly here: https://goo.gl/maps/QmnMc) at 19h30. For details see: http://www.haskell-munich.de/dates Please note the changed location this time! The weather will be nice. If you plan to join, please add yourself to this dudle. It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-aug-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Compilation time
by Corentin Dupont 25 Aug '15

25 Aug '15

Hi guys, I have an application with around 7K loc in 4 packages. It takes 1min30 to compile with stack. Is it a normal compilation time? My computer is fairly recent with Intel i7... I run some music in background :). Is there some statistics about compilation time somewhere? Thanks. Corentin

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[ANN] frpnow-0.12
by Atze van der Ploeg 25 Aug '15

25 Aug '15

Dear Cafe, We have released the (nearly) first version of FRPNow, the functional reactive programming library based on the ICFP 2015 paper "Principled Practical FRP: Forget the Past, Change the Future, FRPNow!" ( https://www.reddit.com/r/haskell/comments/3ai7hl/principled_practical_frp_f… ) The main package: http://hackage.haskell.org/package/frpnow Examples: https://github.com/atzeus/FRPNow/tree/master/Examples Gloss interoperability: http://hackage.haskell.org/package/frpnow-gloss GTK interoperability: http://hackage.haskell.org/package/frpnow-gtk (hackage doesn't like the newer GTK docs, so you can read the docs at http://www.cse.chalmers.se/~atze/frpnow-gtk/ ) Cheers, Atze

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New Functional Programming Job Opportunities
by Functional Jobs 24 Aug '15

24 Aug '15

Here are some functional programming job opportunities that were posted recently: Haskell Engineer at Wagon http://functionaljobs.com/jobs/8857-haskell-engineer-at-wagon Cheers, Sean Murphy FunctionalJobs.com

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Alternative to newtypes or orphan instances
by Clinton Mead 24 Aug '15

24 Aug '15

Lets say there's a type `T` I've imported from a package I don't control. I can easily write new functions to work on `T`. For example, I could write: f :: T -> T -> T f x y = ... I've extended T's functionality. No need for newtypes. This makes sense. Whilst a newtype in the following case: newtype Time = Time Int because in this case `Time` is conceptually different to `Int`. For example, you shouldn't multiply two `Time`s So in summary, we've just extended T's functionality appropriately, without introducing a newtype. But this could be dangerous. Lets say another module writer writes a similar function "f". Then if someone imports their module and ours, there will be a clash. Fortunately the compiler tells about this when it happens, and we can then use qualified imports to workaround the clash. This I'll get back to later. Now lets say I want to extend T's functionality some more, but this time by making it an instance of class C. Class C already exists in a package I don't control. Now it would be unreasonable for every class writer to write instances for every possible data type that is appropriate for their class. As you can see, it's an O(n^2) problem, so there will undoubtably be situations where one needs to write instances for data types and classes in separate packages. One recommended approach here is to newtype T, like the following: newtype MyT = MyT T however I find this incredibly ugly. Conceptually, there is no "newtype", and now there's different types of Ts bashing around which really are the same thing. Not only that, they're incompatible, for no good reason. Instead of these different Ts being different types because they're conceptually different things that shouldn't be mixed, the type of these objects depends solely on what operations I want to apply to them. Imagine a rule where: f :: T -> T -> T was only legal in the module T was defined. If you wanted to define T anywhere else, you'd have to do: f :: MyT -> MyT -> MyT That's how messy the newtype solution is, it wouldn't be accepted for ordinary functions, and I don't think it's reasonable to apply that messiness to class functions. A second approach is an orphan instance. The recommendation here is to put the orphan instances in their own module, so the user can choose to import them. This may works ok if your user is writing an executable. But what if your user is writing a library themselves. But once, you, or your user, directly uses one of the instances, they need to import it, and they pollute the global instance namespace for anyone that uses their package. This is okay if you're the only person doing it, but I think it's reasonable to say that if your practices would be bad if someone else was doing them also, then they're a bad practice. Indeed, if someone else writes their own instances and they clash with yours, people simply can't use your package and their package in the same program. This is bad. In the simple "function" case. We deal with the clash of two functions named "f" using qualified imports. We can't do that instances, nor can we even hide one of the instances. So to recap here we've tried: (1) Copying the data type. We've decided this has lots of problems. (2) Just making an orphan instance. This has serious problems too. I want to suggest a third option: (3) Copying the class. What would this involve: a) Making a new class (say C1), with the same class methods as the existing one (say C). b) Making an instance of the class like so: instance (C x) => C1 x where f = ModuleC.f ... which forwards all the calls to C. c) Add your new instances to C1 (these will overlap with the above but they will be more specific so with overlapping instances this is allowed). Now if your users want to use your instances, they import C1. They can still use the data type T as usual on the old instances, or on any existing functions defined on T directly. The big question is, what if someone else does this, and creates C2? Then they'll be a clash. But in this case, classes, unlike instances, can be explicitly imported and qualified. Your packages won't be impossible to use together, just the user will have to import one of them qualified. That's acceptable, we understand we have to do this sometimes with clashing function names too. So I guess my questions are: What do you think of my analysis? Are there parts that are incorrect? What do you think of my solution? Is there reasons why it is not as good as the "newtype" or "orphan instances" approach? Is there a way to make my solution easier to implement, i.e. less typing?

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getNumCapabilities always returns 1 with GHC 7.10.2 on OS X
by Geraldus 24 Aug '15

24 Aug '15

I have OS X Yosemite 10.10.4. There are two bundles of GHC (from ghcformacosx.github.io): 7.10.1 and 7.10.2. The latter always shows me one core when running `getNumCapabilities` action: *GHC 7.10.2* > stack ghci +RTS -N8 Using resolver: lts-3.0 from global config file: /Users/arthurfayzrakhmanov/.stack/global/stack.yaml Configuring GHCi with the following packages: GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent Prelude Control.Concurrent> getNumCapabilities 1 *GHC 7.10.1* > /Applications/ghc-7.10.1.app/Contents/bin/ghci +RTS -N8 GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent Prelude Control.Concurrent> getNumCapabilities 8 It looks like a 7.10.2 bug, can some one confirm that?

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ANN: MicrosoftTranslator-0.1.0.1
by Ernesto Rodriguez 23 Aug '15

23 Aug '15

Dear Cafe, I needed to translate text in my Haskell code and Microsoft Translator[2] seems to be the easiest and cheapest (free 2M chars/month) way to do it. I wrapped my code in this package[1]. I tried to make it easy to use. If you plan to use Microsoft Translator in Haskell but don't want to use my package, I still recommend you look at my source code since the documentation in Microsoft's page appears to be incorrect (I had to see the source code of the Python package to get it to work). In any case, it is very easy to use: > translate "clientId" "clientSecret" "hello" English German Right "hallo" It is in an early stage and I haven't added most of the languages supported by Microsoft Translator. Pull requests are welcome :) Best Regards, Ernesto Rodriguez [1] http://hackage.haskell.org/package/MicrosoftTranslator-0.1.0.1 [2] https://www.microsoft.com/en-us/translator/default.aspx -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

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Re: [Haskell-cafe] octree implementation - followup to "How to model outer space for a MUD"
by Michal J Gajda 23 Aug '15

23 Aug '15

Hi, And if you search for Octree on Hackage, you would have also found my Octree implementation. It does not do re-balancing, but in this type of application you talk about it may not be needed. http://hackage.haskell.org/package/Octree Funny part, is that implementation may be easier to read than algo description I read first time: http://hackage.haskell.org/package/Octree-0.5.4.2/docs/src/Data-Octree-Inte… . It worked perfectly for my use cases (checking for collisions of 1000-100000 objects), but (just in case) the initial creation from list structure make the octree balanced. Note that balanced octree is supposed to be much faster than KD-tree, and may be also faster than R-tree in memory, since it is very shallow structure - log_8 between 10k and 1m changes only from below 5 to below 7! I was also very happy with its cache behaviour, although in theory R-tree could be better. I am also extremely interested in making sure that Haskell has a good choice of multidimensional metric data structures, so I am closely following maintenance of kd-tree, Ashley´s, and Gloss´ quadtrees. And if you really want to have re-balancing, please contact me on my email. -- Best regards Michal

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