February 2017 - Haskell-Cafe

Re: [Haskell-cafe] Declaring Functors for Type constrained data types
by Jan Bracker 28 Feb '17

28 Feb '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 27 Feb '17

27 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 25 Feb '17

25 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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Postdoc at Inria/Irisa on translation validation from LiquidHaskell
by Ranjit Jhala 25 Feb '17

25 Feb '17

Hi all, Inria Project-team TEA is seeking a talented PhD with demonstrated experience in theory and implementation of refinement types in programming and automated verification and proof of programs. The aim of our post-doctoral project is to design a certified code generator from Liquid Haskell to verified, imperative and functional, compiler suites (CompCert, CakeML). We will start by initially focusing on first-order functions in the absence of dynamic memory management. In the spirit of Pnueli's translation validation principle, we will prove code generated from the code generator to obey program properties inferred from source programs, using refinement reflection. We will study extensions to hardware synthesis as well as dynamic memory management, higher-order functions, etc. For context, details, and application, please visit: https://www.inria.fr/ institut/recrutement-metiers/offres/post-doctorat/sejours- post-doctoraux/(view)/details.html?id=PGTFK026203F3VBQB6G68LONZ&LOV5 =4508&nPostingID=11123&nPostingTargetID=17708

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[Haskell-Cafe] Different memory profile in cabal project.
by Thomas Vanhelden 24 Feb '17

24 Feb '17

I have a (dummy) program that reads the contents of a file into a ByteString and then converts the ByteString to a Vector of Floats. To test the laziness of this program, I’ve taken a slice of the the Vector and printed it to the standard output. The program looks like this: import qualified Data.Vector.Unboxed as V import qualified Data.ByteString as BS import Data.Word import System.Environment import GHC.Int main = do [file] <- getArgs samples <- getSamplesFromFile file let slice = V.slice 0 50000 samples print slice getSamplesFromFile = fmap toVector . BS.readFile toVector :: BS.ByteString -> V.Vector Float toVector bs = vgenerate (fromIntegral (BS.length bs `div` 3)) $ \i -> myToFloat [BS.index bs (3*i), BS.index bs (3*i+1), BS.index bs (3*i+2)] where myToFloat :: [Word8] -> Float myToFloat = sum . map fromIntegral vgenerate n f = V.generate n (f . fromIntegral) So I compile this program and generate a memory profile via: ghc Main.hs -O2 -rtsopts -prof ./Main debug48.wav +RTS -hy hp2ps -e8in -c Main.hp ps2pdf Main.ps The file debug48.wav is a 12.9MB file. This is the result: If I now create a cabal project and add the exact same program as the main file by: mkdir testing cd testing cabal init add the program’s code to src/Main.hs add bytestring and vector to the build dependencies (in testing.cabal) add -O2 -rtsopts -prof to the ghc-options (in testing.cabal) cabal install testing debug48.wav +RTS -hy hp2ps -e8in -c testing.hp ps2pdf testing.ps This is the result: How can there be such a big difference in memory usage, just by the code being part of a cabal project?

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Working at Chordify? Working with Music. Coding in Haskell.
by Bas de Haas 24 Feb '17

24 Feb '17

Dear Haskellers, Chordify is hiring! Chordify is a young and fast growing music e-learning platform that helps musicians to play their favorite music. We automatically analyse the chords of a piece of music and display them in an intuitive player. Try it yourself at: https://chordify.net/ . The cool thing is: our backend has been written almost exclusively in Haskell. We hope to broaden our team with a researcher and a developer. We are looking for people who are pro-active, independent, and creative to improve Chordify. * We offer a digital signal processing researcher position in which you can prototype, evaluate, and deploy new algorithms that form the core of Chordify. * We also offer a (front-end) developer position in which you can improve the Chordify user experience, launch new ideas, write Haskell, and have thousands of users use your code every day. If you are interested in working at Chordify, you can find the details here: https://chordify.homerun.co/ All the best, Bas de Haas

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Extensions to Haskell 98 in the GHC Prelude?
by Dylan Thurston 24 Feb '17

24 Feb '17

Pardon me if this is a stupid/frequently asked question... The GHC Prelude has some extensions to the Prelude in Haskell 98 or Haskell 2010. For instance, it has the Applicative type class, whose bindings overlap with some old code of mine. Where are these non-standard extensions documented? The Applicative class is documented at https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.9.0.0/… but it's not picked out as something non-standard. I didn't find it documented where I would have expected in the GHC docs at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#h… nor did I see a way to get a standards-compliant prelude? In this particular case I'll probably rework my code, as Applicative seems like a reasonable core addition. (I am a little annoyed that it uses so many short infix operators; I was using (*>).) I'm just surprised not to see it documented anywhere. Thanks, Dylan Thurston

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