- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Looking for maintainers or comaintainers on my Haskell projects
by sighingnow＠163.com 01 Mar '17

01 Mar '17

Hi! I’m interested in co-maintaining the Hindent project. I use Hindent extensively and I have submitted several patches to Hindent on github during the last year. I would like to contribute to improving Hindent and making it better. Thanks. Tao He

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block comments in cabal files?
by Johannes Waldmann 01 Mar '17

01 Mar '17

I totally could not understand why "stack install" suddenly failed with "ghc: unrecognised flag: -" Then it dawned on me: you cannot use {- .. -} for block comments in cabal files. (the syntax has no block comments at all?) yes, Wadler's law and all, but still. - J.

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Summer of Haskell 2017
by Niki Vazou 01 Mar '17

01 Mar '17

Dear Haskellers, We are sorry to announce that this year Haskell.org <http://haskell.org/> was not accepted for the 2017 Google Summer of Code. But, as last year, we plan to organize our own Summer of Haskell! We ask for your help to plan a successful Summer of Haskell 2017. As last year, one student will be funded directly out of Haskell.org <http://haskell.org/> general funds. The key factor in how many further students we can fund is the generosity and support of donors. Funding a single student plus overhead costs 6000 USD. We are looking for donors willing to cover half, one or more students. If you (or your organization) are interested in becoming a Summer of Haskell donor please reach out to committee(a)haskell.org. We also encourage you to make a smaller donation, as if we receive enough of small donations in total, we will combine them to fund more students. Donation information is at: https://wiki.haskell.org/Donate_to_Haskell.org More information will be forthcoming as we work out the details. Feel free to contact us at (nvazou(a)cs.umd.edu and m(a)jaspervdj.be) if want to help or if you have any ideas, questions, or concerns. Best, Niki Vazou & Jasper Van der Jeugt for the Haskell.org Committee

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[Announce] ZuriHac 2017
by Jasper Van der Jeugt 01 Mar '17

01 Mar '17

Hello all, It is our pleasure to announce ZuriHac 2017, which will take place on 9th to 11th of June, 2017. The Haskell Hackathon is an international, grassroots collaborative coding festival whose goal is to expand the community and to build and improve Haskell libraries, tools, and infrastructure. This is the already the 6th Haskell Hackathon in Zurich! This year, we are privileged to enjoy keynotes from: - Edward Kmett - Julie Moronuki - Neil Mitchell - Stephen Diehl - Simon Thompson This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! This event is open to any experience level, from beginners to gurus. We will have a dedicated beginners' track, and there will be mentors on site whom you can directly approach during the whole event with any Haskell-related question you have. We will be hosting the Hackathon in a new location [1] near the lake which provides space for up to 300 people! Experience from the previous years tells us these places might be gone quickly, so don't wait too long to register. You can find more information and register for the Hackathon here: http://zurihac.info We would also like to thank our sponsors Digital Asset [2], Google Open Source Programs Office [3] and HSR [4] for supporting this great event! Looking forward to see you there, the Zurich HaskellerZ meetup group [1]: https://zurihac.info/#venue [2]: https://digitalasset.com/careers.html [3]: https://developers.google.com/open-source/ [4]: https://www.hsr.ch/

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Re: [Haskell-cafe] Declaring Functors for Type constrained data types
by Jan Bracker 01 Mar '17

01 Mar '17

Hi, As mentioned before there are several ways of doing this. 1. Constrain the functions working with your types keeping the type itself unconstrained. 2. Write/Use a replacement for the Functor class that allows you to do this. This will usually involve associated type synonyms or functional dependencies. In my own work [1] I use associated type synonyms to do this. But there are other libraries that wrote similar replacements for the Functor class [2,3 and probably some more]. So if you want to use these you fill have to swap out the Functor (Applicative and Monad) hierarchy with another one that supports constraints. So this is definitly possible and no limitation of GHC. The standard library just does not allow it. Categorically a standard functor goes from the category Hask to Hask. A restricted or constrained functor can be seen as a functor that goes from a category C (Hask with the constraints) to Hask together with an injective functor that embeds C in a subcategory of Hask (which allows you to use it in Haskell). Best, Jan [1]: http://hackage.haskell.org/package/supermonad-0.1/docs/Control-Supermonad-C… [2]: https://hackage.haskell.org/package/rmonad-0.8.0.2/docs/Control-RMonad.html… [3]: http://hackage.haskell.org/package/constrained-monads-0.1.0.0/docs/Control-… 2017-02-28 12:00 GMT+00:00 <haskell-cafe-request(a)haskell.org>: > I've done a bit of work on a library that allows you to `fmap` say over > sets, (i.e. fmap :: Ord a, Ord b => (a -> b) -> Set a -> Set b) which is > more powerful than Monofunctor as it doesn't require the types to be the > same. > > And it will involve importing a new version of "Functor" and "fmap", > however it should be largely backwards compatible. > > However, it still needs a little bit of work to be in a state ready to be > uploaded to Hackage (and even then, it probably needs even more work) but > I'll give you a buzz when its up, hopefully in the next week or so. > > On Tue, Feb 28, 2017 at 2:01 PM, Guru Devanla <gurudev.devanla(a)gmail.com> > wrote: > > > Thanks, Will. I think adding the constraint to the functions is a good > > idea. Plus, it makes the intent clearer at the function level. > > > > > > > > On Mon, Feb 27, 2017 at 6:02 PM, Will Yager <will.yager(a)gmail.com> > wrote: > > > >> 1. You could use MonoFunctor (complicated and probably not a good idea > >> here) or just put the Taskable constraint on functions instead of on the > >> Task definition (good and easy). > >> > >> So it would be > >> > >> data Task a = Task a deriving Functor > >> > >> And then put "Taskable a" on functions that require it. > >> > >> 2. You can't do it because it doesn't really make sense. A big part of a > >> functor is that it has to be totally agnostic of what it's parametrized > >> over. Otherwise you could easily violate the functor laws. > >> > >> Good question though, I used to wonder the same thing. > >> > >> cheers, > >> Will > >> > >> On Feb 27, 2017, at 6:48 PM, Guru Devanla <gurudev.devanla(a)gmail.com> > >> wrote: > >> > >> Hello All, > >> > >> I am working on a program that will define a bunch of tasks. Each task > >> will have to implement certain methods as part of a type class. > >> > >> -- task 1 > >> data UpdateAcctsTask = UpdateAccts > >> > >> -- task 2 > >> data EmailConfig = EmaiConfig {someattrs::String} > >> data SendEmailTask = SendEmailsTask EmailConfig > >> > >> -- task 3 > >> data GeneralWriterTask a = GeneralWriterTask a > >> > >> Each of these tasks implement a class, Taskable. The return > >> values are simplified for this example. > >> > >> class Taskable a where > >> process :: a -> Bool > >> can_run :: a -> Bool > >> > >> > >> This works fine. I can expand on these tasks and execute them. > >> > >> Now, I wanted to be able to defined dependencies between these > >> (Taskable's). I decided > >> I could create a data type for this dependency and may be also get a > >> FreeMonad > >> around this structure for further processing using a graph of Tasks. > But, > >> before that I wanted > >> to create an wrapper for these Taskables and create a functor for it as > >> follows > >> > >> The first thing I did was, define a Task, which generalizes over all > >> the above defined (and future Taskables) > >> > >> data Task a where > >> Task :: (Taskable a) => a -> Task a > >> > >> > >> instance Functor Task where > >> fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b > --- > >> THIS DOES NOT WORK > >> fmap f (Task a) = Task $ f a > >> > >> > >> But, I realized that I cannot define an fmap over a type constraint. > >> > >> My questions are: > >> > >> 1. Is there any way to do this. I see there is an answer of SO. I wanted > >> to make sure if there were any improvements to this since that > answer' > >> was posted. > >> http://stackoverflow.com/questions/17157579/functor-instance > >> -for-a-gadt-with-type-constraint > >> > >> 2. Secondly, I would like to know why this is not possible. Is it a > >> current > >> limitation of GHC or if there is some fundamental category theory > >> concepts > >> that dis-allows such declarations that I need to grok! > >> > >> Appreciate any help on this. Thank you! > >> > >> _______________________________________________ > >> Haskell-Cafe mailing list > >> To (un)subscribe, modify options or view archives go to: > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >> Only members subscribed via the mailman list are allowed to post. > >> > >> > > > > _______________________________________________ > > Haskell-Cafe mailing list > > To (un)subscribe, modify options or view archives go to: > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Only members subscribed via the mailman list are allowed to post. > > >

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Call For Presentations: Compose 2017, New York, May 18-19
by Gershom B 28 Feb '17

28 Feb '17

Compose is a conference for typed functional programmers, focused specifically on Haskell, OCaml, F#, SML, and related technologies. It will be held in New York on Thursday and Friday, May 18-19, 2017. Registration will be open shortly. http://www.composeconference.org/2017 To get a sense of Compose, you can check out the great talks from past conferences: https://www.youtube.com/channel/UC0pEknZxL7Q1j0Ok8qImWdQ Below is our call for presentations. We recognize the deadline is tight, so feel free to submit proposals and ideas on the less-polished side. http://www.composeconference.org/2016/cfp/ *** The audience for Compose is Haskell, OCaml, F#, or SML developers who are looking to increase their skills or learn new technologies and libraries. Presentations should be aimed at teaching or introducing new ideas or tools. We are also interested in presentations aiming at taking complex concepts, such as program derivation, and putting them into productive use. However proposals on anything that you suspect our audience may find interesting are welcome. The following are some of the types of talks we would welcome: Library/Tool Talks — Exploring the uses of a powerful toolkit or library, be it for parsing, testing, data access and analysis, or anything else. Production Systems — Experience reports on deploying functional techniques in real systems; insights revealed, mistakes made, lessons learned. Theory made Practical — Just because it’s locked away in papers doesn’t mean it’s hard! Accessible lectures on classic results and why they matter to us today. Such talks can include simply introducing the principles of a field of research so as to help the audience read up on it in the future; from abstract machines to program derivation to branch-and-bound algorithms, the sky’s the limit. We also welcome proposals for more formal tutorials. Tutorials should be aimed at a smaller audience of beginner-to-novice understanding, and ideally include hands-on exercises. The due date for submissions is March 23, 2017. We will send out notice of acceptance by March 30th. We prefer that submissions be via the EasyChair website (https://easychair.org/conferences/?conf=compose2017) Please suggest a title, and describe the topic you intend to speak on. Talks can be either 30 or 45 minutes, please indicate how much time you would prefer to take. Additional information may be included on both your expertise and the interesting elements of your topic, going on what might be included in a public abstract. Furthermore, if your abstract doesn't feel "final"—don't worry! We'll work with you to polish it up. If you want to discuss your proposal(s) before submitting, or to further nail down what you intend to speak on, please feel free to contact us at info(a)composeconference.org. We're happy to work with you, even if you are a new or inexperienced speaker, to help your talk be great. *** Diversity We would like to put an emphasis on soliciting a diverse set of speakers - anything you can do to distribute information about this CFP and encourage submissions from under-represented groups would be greatly appreciated. We welcome your contributions and encourage you to apply! Best, Gershom

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ECOOP/PLDI 2017 in Barcelona / Call for Student Participation
by Tobias Grosser 27 Feb '17

27 Feb '17

ECOOP/PLDI 2017 in Barcelona / Call for Student Participation http://conf.researchr.org/home/pldi-ecoop-2017 Interested in hearing the latest about Scala, Perl, Clojure, Rust, Swift, JavaScript, and many many other languages and systems? Curious about new ideas such as gradual types, JIT compilers for dynamic languages, differential privacy or deep learning? Always wanted to talk about computer science face to face with senior researchers and practitioners? Let’s come to share, challenge and complement your views! Join us in Barcelona from Sunday 18 to Friday 23 June 2017 The ECOOP/PLDI conferences offers support to students and professionals interested in attending. To apply: application form # Bachelor, Master, and early Doctoral Students The ECOOP Summer School provides an easy introduction to the world of research in programming languages. No background is required other than an interest in languages technologies. Info: http://2017.ecoop.org/track/ecoop-2017-Summer-School # Doctoral Students The Doctoral Symposium is designed to provide a forum for PhD students at any stage in their research to present their topic and get detailed feedback and advice. The main objectives of this event are: to allow PhD students to practise writing clearly and to communicate and present their research effectively to receive constructive feedback from other researchers and peers to offer opportunities to form research collaborations to contribute to the conference goals through interaction with other researchers at the main conference Info: http://2017.ecoop.org/track/ecoop-2017-Doctoral-Symposium # All Students Register to be Student Volunteers, help with the organization of the conference, get a free registration and a ticket for one of the banquets. This year PLDI and ECOOP are co-located. Student Volunteers can attend events from both conferences and might be working for either. Preference can be stated in the application form. Info: http://2017.ecoop.org/track/ecoop-2017-Student-Volunteers#About

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Re: [Haskell-cafe] What does "1 = 2" mean in Haskell?
by Harendra Kumar 27 Feb '17

27 Feb '17

CCing the list. I guess you intended to cc but forgot. On 24 February 2017 at 09:27, <ok(a)cs.otago.ac.nz> wrote: > In Erlang, the equivalent of a let fails. > 1> 1=2. > ** exception error: no match of right hand side value 2 > > In SML, the equivalent of a let fails. > - val 1 = 1; > - val 1 = 2; > > uncaught exception Bind [nonexhaustive binding failure] > raised at: stdIn:2.5-2.10 > > The problem is not that let 1 = 2 ... is *legal* but that > - the compiler is *silent* about it > - the runtime is *silent* about it. > Compiling the little program > > main = let 1 = 2 in print "hi" > > I expected that the compiler would be silent but that > there would be some sort of "matched failed" error at > run time. Silly me. > > > The thing is, it is not just bindings that bind no variables > that act as if they were not there. > > main = let [x] = [1,2] in print "hi" > > also compiles silently and runs without error. Change it to > > main = let [x] = [1,2] in print ("hi" ++ show x) > > and you get a runtime error > > <object>: <source>: Irrefutable pattern failed for pattern [x]. > > I wish the compiler would report an error something like > > "<location>: possibly failing match deleted > because it binds no live variables" > > > >

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Munich Haskell Meeting, 2017-02-28 @ 19:30
by Christian Neukirchen 26 Feb '17

26 Feb '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, Feburary 28 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html The location is carnival-free. If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-feb-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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What does "1 = 2" mean in Haskell?
by Harendra Kumar 26 Feb '17

26 Feb '17

Kids have this amazing ability to break any toy in minutes. I gave my seven year old daughter ghci to play with and in a little while she said it is broken: >> let 1 = 2 >> 1 1 >> Earlier, I had explained to her about symbols and assigning values to symbols, and I said numbers are not symbols. But when she came up with this I could not explain what's going on. How can "1 = 2" be a valid equation? Am I missing something fundamental here, or it is just broken? -harendra

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