April 2016 - Haskell-Cafe

Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 152, Issue 27
by Julian Leviston 23 Apr '16

23 Apr '16

There were quite a few responses to you so far, but no one seemed to mention that what you actually asked for are called anamorphisms. https://en.wikipedia.org/wiki/Anamorphism <https://en.wikipedia.org/wiki/Anamorphism> They’re not a *class* per se (as in… they’re not a typeclass), but rather they’re a kind of higher-order function which we call unfold (the kind-of “opposite" action to a fold). Here’s a nice little thing I found which is quite helpful as an explanation: https://gist.github.com/CMCDragonkai/b586ee9ecce297ceeeb3 and Data.List has unfoldr: http://hackage.haskell.org/package/base-4.8.2.0/docs/Data-List.html#v:unfol… <http://hackage.haskell.org/package/base-4.8.2.0/docs/Data-List.html#v:unfol…> If you google around for either "anamorphism Haskell” or “unfold Haskell” you’ll find some nice explanatory resources. Hope that helps! Julian http://www.happylearnjavascripttutorial.com/ <http://www.happylearnjavascripttutorial.com/> - begin programming with JavaScript today using our awesome new method! http://www.happylearnhaskelltutorial.com/ <http://www.happylearnhaskelltutorial.com/> - begin programming with Haskell today using our awesome new method! > On 23 Apr 2016, at 10:00 PM, haskell-cafe-request(a)haskell.org wrote: > > Hi. I'm more in the shallow end of the Haskell pool right now, so > forgive me if this is really basic... In Haskell, what is the class for > things that can provide an infinite sequence? I mean, things that > produce a value as well as function for producing the next value? > Clearly RandomGen does this, but what is the next level of abstraction > above RandomGen (i.e., RandomGen minus the randomness aspect).

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wxhaskell: Play sound not working on OSX?
by Tilmann 23 Apr '16

23 Apr '16

Is it possible that play . sound might not work on OSX or I'm I doing something wrong? This advice from around 2006 is all I could find: "Did you build wxWidgets with configuring --enable-sound build option?" Is this still valid? If so, how do I do that? Tilmann |import Graphics.UI.WX main :: IO () main = start $ do f <- frame [] p <- panel f [] set f [layout := minsize(Size 100 100) $ widget p] play (sound "alarm.wav") | Using OSX 10.9.5, GHC 7.10.2, WX 0.92.2.0 http://stackoverflow.com/questions/36809989/play-sound-with-wxhaskell-not-w…

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[ANN] Aivika: A parallel distributed discrete event simulation library
by David Sorokin 23 Apr '16

23 Apr '16

Hi Cafe! I’m glad to announce the first add-on module [3] of Aivika simulation library that allows creating and running parallel distributed discrete event simulations. It is destined for clusters and supercomputers, although it works on a stand-alone notebook too. My module implements an optimistic strategy that implies transparent rollbacks. It is mainly inspired by ideas of the Time Warp algorithm, which first publication is dated early 80s. Please note that this is the first version. It is mainly experimental, but it already shows good results. For example, when using the reproducible random number generators on different nodes, I always get the same simulation results. Furthermore, for testing purposes, I rewrote one of my examples with discrete processes and the shared resource so that it uses too much messages just to see how it works. The add-on module allows doing everything that the generalized version [2] of Aivika can do in a part related to the discrete event simulation. Moreover, the parallel nodes can communicate to each other sending and receiving asynchronous messages, but these messages have the send and receive times. If the node receives the past message, a transparent rollback occurs so that the node can process the received message. When rolling the computation back, the node sends so called anti-messages to cancel the invalidated messages that came from the node. It allows us to have a predictable distributed simulation satisfying the time constraints and causality. There are different customization parameters that allow tuning the library. For example, by default the library waits 5 seconds to consider the local time of the node defined. Therefore, when running test models, you can see a small delay, which is necessary for finishing the synchronization between the nodes and so called time server. Unfortunately, there is a lack of documentation. But the main idea is as follows. The generalized version [2] is almost the same as the core library [1] which already has a documentation. The represented add-on [3] just allows using the generalized version for creating and running parallel distributed discrete event simulations within the specified computation. Best regards, David Sorokin, Yoshkar-Ola, Russia [1] http://hackage.haskell.org/package/aivika [2] http://hackage.haskell.org/package/aivika-transformers [3] http://hackage.haskell.org/package/aivika-distributed

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Generators? Iterators?
by Christopher Howard 23 Apr '16

23 Apr '16

Hi. I'm more in the shallow end of the Haskell pool right now, so forgive me if this is really basic... In Haskell, what is the class for things that can provide an infinite sequence? I mean, things that produce a value as well as function for producing the next value? Clearly RandomGen does this, but what is the next level of abstraction above RandomGen (i.e., RandomGen minus the randomness aspect). -- http://justonemoremathproblem.com To protect my privacy, please use PGP encryption. It's free and easy to use! My public key ID is 0x340EA95A (pgp.mit.edu)

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Documenting extensions
by Doug McIlroy 23 Apr '16

23 Apr '16

I'd like to believe the Haskell report is a bible. I learned H98 using the Prelude as a source of examples and Hugs as a testbed. That was a comfortable and rewarding combination. Later I discovered that by default GHC rejected almost all my report-conforming (H98 and H2010) code. Ironically, its website at the time asserted, "Haskell is a standard language." In fact, far from reflecting biblical authority, the de facto Haskell discussed in this cafe comes in some 2^99 flavors -- enough to use a different language for every run of the compiler for the life of the universe. Witness: ghc --supported-languages | grep -v '^No[A-Z]' | wc -l (99 is the answer for GHC 7.8.4. In fact not all combinations of -X options are legal, but you get the point.) Unfortunately there is no coherent discussion of all those Haskells. The extensions section of the GHC User Guide gives no formal syntax, often teaches semantics only by example, and is replete with sales pitches, partial analogies, inconsistent terminology, and (helpful) appeals to outside sources. Definitely not a bible. I understand that many GHC extensions are probationary. Nevertheless, in the interest of facilitating their full and correct use, I would plead that a report-grade description accompany each language feature incorporated in official releases. This seems like a small amount of effort beyond the hard work of conception and implementation. Are there cogent reasons for doing otherwise? Doug

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Haskell in Leipzig 2016: Call for Papers
by Joachim Breitner 22 Apr '16

22 Apr '16

Haskell in Leipzig September 14-15, 2016 HTKW Leipzig, Germany http://hal2016.haskell.org/ == About HaL == The workshop series “Haskell in Leipzig”, now in its 11th year, brings together Haskell developers, Haskell researchers, Haskell enthusiasts and Haskell beginners to listen to talks, take part in tutorials, and join in interesting conversations. Everything related to Haskell is on topic, whether it is about current research, practical applications, interesting ideas off the beaten track, education, or art, and topics may extend to functional programming in general and its connections to other programming paradigms as well. This year, HaL is colocated with two related conferences, * the Workshop on Functional and (Constraint) Logic Programming (WFLP) and * the Workshop on (Constraint) Logic Programming (WLP), to form the Leipzig Week of Declarative Programming (L-DEC): http://nfa.imn.htwk-leipzig.de/LDEC2016/ In order to accommodate and welcome a more international audience, this year’s HaL will be held in English. == Submissions == Contributions can take the form of * talks (about 30 minutes), * tutorials (about 90 minutes), * demonstrations, artistic performances, or other extraordinary things. Please submit an abstract that describes the content and form of your presentation, the intended audience, and required previous knowledge. We recommend a length of 2 pages, so that the PC and the audience get a good idea of your submission, but this is not a hard requirement. You can submit your abstract, as an PDF document, at https://easychair.org/conferences/?conf=hal2016 until Friday, July 1, 2016. You will be notified by July 15, 2016. == Program committee == * Andreas Abel, Chalmers and Gothenburg University, Sweden * Heinrich Apfelmus, Leipzig, Germany * Joachim Breitner, Karlsruhe Institute of Technology, Germany (Chair) * Matthias Fischmann, Zerobuzz, Germany * Petra Hofstedt, BTU Cottbus-Senftenberg, Germany * Wolfgang Jeltsch, Institute of Cybernetics at Tallinn University of Technology, Estonia * Andres Löh, Well-Typed LLP, Germany * Alejandro Serrano Mena, Universiteit Utrecht, Netherlands * Neil Mitchell, Standard Chartered Bank, UK * Katie Ots, Facebook, UK * Peter Stadler, University of Leipzig, Germany * Henning Thielemann, Freelancer, Germany * Niki Vazou, University of California, San Diego, USA If you have any questions, please don’t hesitate to contact Joachim Breitner <breitner(a)kit.edu>. -- Dipl.-Math. Dipl.-Inform. Joachim Breitner Wissenschaftlicher Mitarbeiter http://pp.ipd.kit.edu/~breitner

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I have a question about Haskell
by Eunsu Kim 22 Apr '16

22 Apr '16

Hi i have a problem in my code! here is my code: here is error message: in very bottom function (polyEvaluate), why is not working “result <- map (\(a,b) -> a+b) powers” ??? in Prelude, it is working Thanks!

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I've wanted a Haskell shirt for awhile
by Stegosaurus Armatus 21 Apr '16

21 Apr '16

Does anyone know where I can get one? Failing that, I made one to celebrate today. https://teespring.com/haskell-stay-lifted Disclaimer: I don't intend to be spammy, but I totally understand and respect if some people don't like this. I was just happy and wanted to share. Please don't hate.

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Generics changes in GHC 8 break containers
by David Feuer 21 Apr '16

21 Apr '16

containers does not currently build with GHC 8 because of changes to Generics. I only have a shallow understanding of GHC.Generics. Could someone give me a hand figuring out what I need to change? Thanks, David Feuer

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ANN: New Haskell.org committee members
by John Wiegley 21 Apr '16

21 Apr '16

The Haskell.org committee has selected its new member following the March self-nomination period. The new (returning) member is: * Gershom Bazerman Thank you to everyone who submitted a self-nomination. Note that if you have self-nominated in the past, but not been picked, please self-nominate again in the future. -- John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2

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