September 2015 - Haskell-Cafe

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

5 5

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

7 10

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

2 1

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

5 8

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

2 2

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

1 0

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

1 0

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 >

1 0

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.

2 1