September 2016 - Haskell-Cafe

Re: [Haskell-cafe] [Haskell-community] technical thoughts on stack
by Christopher Allen 15 Sep '16

15 Sep '16

What are you talking about? $ stack build simple-library-0.1.0.0: unregistering (local file changes: src/Lib.hs) simple-library-0.1.0.0: build Preprocessing library simple-library-0.1.0.0... [1 of 1] Compiling Lib ( src/Lib.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/Lib.o ) /home/callen/work/simple-library/src/Lib.hs:7:3: warning: [-Wunused-do-bind] A do-notation statement discarded a result of type ‘[Char]’ Suppress this warning by saying ‘_ <- return "woot"’ Preprocessing executable 'simple-library-exe' for simple-library-0.1.0.0... On Wed, Sep 14, 2016 at 2:10 PM, Patrick Pelletier <ppelleti(a)funwithsoftware.org> wrote: > On 9/14/16 10:47 AM, David Feuer wrote: >> >> >> While we're griping about stack: it seems to place compiler output from >> -ddump-... in mysterious places that are hard to find without Google, and >> (worse) it seems to do something with stack test output that even Google >> can't discover. >> > > Also, compiler warnings get squirreled away in a file that I have to go > hunting for. It would be nice to have an option to print warnings (for > local packages, not dependencies) to stderr. > > --Patrick > > > _______________________________________________ > Haskell-community mailing list > Haskell-community(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-community -- Chris Allen Currently working on http://haskellbook.com

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Re: [Haskell-cafe] technical thoughts on stack
by Olaf Klinke 14 Sep '16

14 Sep '16

I second Richard's observations about project-centered vs. compiler-centered workflows. I used to test all my code in ghci until cabal ended up installing two versions of the same package. Stack helped me to obtain a working Haskell environment again. But: While it is nicer to have the `real' code organized as a proper project, I still haven't figured out how to test individual modules in stack ghci without either loading the entire project or making an executable foo that imports the module and loading the executable with stack ghci project:exe:foo And I can't easily have a traditional GHC install alongside for testing since the haskell platform for the operating system uses a different base package. -- Olaf

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[ANN] Call for Contributions: BOB 2017 - Berlin, Feb 24, 2017
by Stefan Wehr 14 Sep '16

14 Sep '16

BOB Conference 2017 "What happens when we use what's best for a change?" http://bobkonf.de/2017/en/cfp.html Berlin, February 24 Call for Contributions Deadline: October 30, 2016 You are actively engaged in advanced software engineering methods, implement ambitious architectures and are open to cutting-edge innovation? Attend this conference, meet people that share your goals, and get to know the best software tools and technologies available today. We strive to offer a day full of new experiences and impressions that you can use to immediately improve your daily life as a software developer. If you share our vision and want to contribute, submit a proposal for a talk or tutorial! Topics ------ We are looking for talks about best-of-breed software technology, e.g.: - functional programming - persistent data structures and databases - types - formal methods for correctness and robustness - abstractions for concurrency and parallelism - metaprogramming - probabilistic programming - ... everything really that isn’t mainstream, but you think should be. Presenters should provide the audience with information that is practically useful for software developers. This time, we’re especially interested in experience reports. But this could also take other forms, e.g.: - introductory talks on technical background - overviews of a given field - demos and how-tos Requirements ------------ We accept proposals for presentations of 45 minutes (40 minutes talk + 5 minutes questions), as well as 90 minute tutorials for beginners. The language of presentation should be either English or German. Your proposal should include (in your presentation language of choice): - an abstract of max. 1500 characters. - a short bio/cv - contact information (including at least email address) - a list of 3-5 concrete ideas of how your work can be applied in a developer’s daily life -additional material (websites, blogs, slides, videos of past presentations, …) Submit here: https://docs.google.com/forms/d/e/1FAIpQLSfFuyBhBTCOTS0zTXBzY1KVuKpumyIBTuc… Organisation - direct questions to bobkonf at active minus group dot de - proposal deadline: October 30, 2016 - notification: November 15, 2016 - program: December 1, 2016 NOTE: The conference fee will be waived for presenters, but travel expenses will not be covered. Speaker Grants -------------- BOB has Speaker Grants available to support speakers from groups under-represented in technology. We specifically seek women speakers and speakers who are not be able to attend the conference for financial reasons. Details are here: http://bobkonf.de/2017/en/speaker-grants.html Shepherding ----------- The program committee offers shepherding to all speakers. Shepherding provides speakers assistance with preparing their sessions, as well as a review of the talk slides. Program Committee ----------------- (more information here: http://bobkonf.de/2017/programmkomitee.html) - Matthias Fischmann, zerobuzz UG - Matthias Neubauer, SICK AG - Nicole Rauch, Softwareentwicklung und Entwicklungscoaching - Michael Sperber, Active Group - Stefan Wehr, factis research Scientific Advisory Board - Annette Bieniusa, TU Kaiserslautern - Torsten Grust, Uni Tübingen - Peter Thiemann, Uni Freiburg

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GHCI segfault for program that works fine when compiled
by erik 14 Sep '16

14 Sep '16

Hello, I posted a question on Stackoverflow and reddit's /r/haskell today (see below), the gist of which was that GHCI segfaults when I attempt to create a connection to a Postgresql instance using a function from the postgresql-simple library; however, when I compile and run my code it works fine. I thought it was perhaps odd that GHCI would segfault when the compiled code worked fine (this is on GHC 8.0.1 and GHC 7.10.3), and then I discovered that if I start GHCI in the following way, I can create a connection to my database without any issues: $ stack ghci --library-profiling --ghci-options="-fexternal-interpreter" It seems likely that I have a weird install of Postgresql + postgresql-libpq (albeit probably not too weird for anyone on OSX who uses the EnterpriseDB installer, like I did). As a result, I was unsure if exploring this issue further would be of interest to anyone. Anyway, I am posting here because I was encouraged to do so by a commenter on my Stackoverflow question. In addition, in both places, I received the suggestion to open a GHC ticket, but owing to my limited experience in Haskell, I am hesitant to do so. I do not know how to get a real stack trace for my segfault. Moreover, I would likely require a fair bit of hand-holding to produce any useful data, and I wasn't sure if pursuing this any further would be of value to anyone. More information can be seen here: Stackoverflow question: http://stackoverflow.com/questions/39440416/ghci-segfault-when-creating-a-d… Reddit Thread: https://www.reddit.com/r/haskell/comments/52ht7h/i_have_a_ghci_segfault_for… Should I open a GHC ticket? If so, what are some good strategies for producing useful information for that ticket? Thanks, Erik

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Workshop on Coalgebra, Horn Clause Logic, and Types
by Ekaterina Komendantskaya 13 Sep '16

13 Sep '16

Call for Papers, Presentations and Participation Workshop on Coalgebra, Horn Clause Logic Programming and Types 28-29 November 2016, Edinburgh, UK https://ff32.host.cs.st-andrews.ac.uk/lpti16/ =================================================== The workshop marks the end of the EPSRC Grant Coalgebraic Logic Programming for Type Inference, by K. Komendantskaya and J. Power and will consist of two parts: Part 1 - Semantics: Lawvere theories and Coalgebra in Logic and Functional Programming Part 2 - Programming languages: Horn Clause Logic for Type Inference in Functional Languages and Beyond We invite all colleagues working in related areas to present and share their results. We envisage a friendly meeting with many stimulating discussions, and therefore welcome presentations of already published research as well as novel results. Authors of original contributions will be invited to submit their papers to EPTCS post-proceedings. We especially encourage early career researchers to present and participate. ======================= Venue ----- The workshop will be held at the International Center for Mathematical Sciences, in Edinburgh city center, just 2 minutes walk from the Informatics Forum. Invited speakers and tutorials ------------------------------ * Relational Specification of type systems using Logic Programming, Ki Yung Ahn, Nanyang Techological University, Singapore * Logic programming for type inference in object-oriented languages, Davide Ancona, University of Genoa, Italy * Logic programming: laxness and saturation, John Power, University of Bath, UK * Classes for the masses, Claudio Russo, Microsoft Research Cambridge, UK * Comodels and interaction, Tarmo Ustalu, Tallinn University of Technology, Estonia =============================== Proceedings publication ----------------------- Presentations: We invite submission of 2-page extended abstracts via Easychair, by the 15th October 2016. These will be subject to light review process. Preliminary proceedings will be made available at the conference in electronic form. Post-proceedings: Authors presenting original work will be invited to submit full papers to the post-proceedings of the workshop. The post-proceedings volume will be published in Electronic Proceedings in Theoretical Computer Science and peer-reviewed according to EPTCS standards by the PC members. ================================ Important dates --------------- Extended Abstract Submission: 15 October, 2016 Author notification: 25 October, 2016 Workshop: 28–29 November, 2016 EPTCS post-proceedings invitations: 15 December, 2016 EPTCS final version submission: 30 January, 2017 ======================== Programme committee ------------------- Ki Yung Ahn, Nanyang Techological University, Singapore Davide Ancona, University of Genoa, Italy Filippo Bonchi, CNRS, ENS de Lyon, France Iavor Diatchki, Galois, Inc, USA Peng Fu, Heriot-Watt University, Edinburgh, UK Neil Ghani, University of Strathclyde, UK Patricia Johann, Appalachian State University, USA Ekaterina Komendantskaya, Heriot-Watt University, Edinburgh, UK Clemens Kupke, University of Strathclyde, UK J. Garrett Morris, University of Edinburgh, UK Fredrik Nordvall Forsberg, University of Strathclyde, UK John Power, University of Bath, UK Claudio Russo, Microsoft Research Cambridge, UK Martin Schmidt, DHBW Stuttgart and University of Osnabrueck, Germany Stephan Schulz, DHBW Stuttgart, Germany Aaron Stump, The University of Iowa, USA Niki Vazou, University of California, San Diego, USA Joe Wells, Heriot-Watt University, Edinburgh, UK Fabio Zanassi, Radboud University of Nijmegen, Netherlands Workschop chairs -------- Ekaterina Komendantskaya, Heriot-Watt University, UK John Power, University of Bath, UK Publicity chair --------------- František Farka, University of Dundee, UK and University of St Andrews, UK

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Re: [Haskell-cafe] Haskell eXchange and related events -- Call for Participation
by Bence Kodaj 13 Sep '16

13 Sep '16

Hi all, If you haven't registered yet for the Haskell eXchange, please get in touch - I bought an early bird ticket but I cannot attend after all. I'd be happy to sell my ticket to anybody for 80 GBP (originally it was 95 GBP + VAT). Cheers, Bence On Friday, 9 September 2016 17:32:24 UTC+2, Andres Löh wrote: > > Hi everyone. > > As Tom Ellis has correctly pointed out in another thread, I should > have announced the Haskell eXchange on this list. Let me do that now: > > ===================================================================== > Haskell eXchange 2016 > London, 6-7 October 2016 > ===================================================================== > > The Haskell eXchange is a general Haskell conference aimed at Haskell > enthusiasts of all skill levels. The Haskell eXchange is organized > annually, and 2016 is its fifth year. For the second year in a row, the > venue will be Skills Matter’s CodeNode, where we have space for three > parallel tracks. New this year: a large number of beginner-focused > talks. At all times, at least one track will be available with a talk > aimed at (relative) newcomers to Haskell. Of course, there are also > plenty of talks on more advanced topics. > > Keynote speakers: > > Don Stewart, Conor McBride, Graham Hutton, Simon Peyton Jones > > Talks and workshops (some changes may still occur) by: > > Julian Arni, Christiaan Baaij, Thomas van Binsbergen, Mathieu Boespflug, > Lars Brünjes, Ben Clifford, Philip Cunningham, Tom Ellis, Matthias > Fischmann, Oleg Grenrus, Nickolay Kudasov, Robert Henderson, Csaba > Hruska, Robin Kay, Csongor Kiss, Steven Kutsch, Kris Jenkins, David > Luposchainsky, Neil Mitchell, Andrey Mokhov, Romeo Moura, Jann Müller, > Boldizsár Németh, Andor Penzes, Ivan Perez, Michał Płachta, Nicolas > Pouillard, Arian van Putten, Andrzej Rybczak, Jeremy Singer, Nicolas Wu > > Homepage and registration: > > https://skillsmatter.com/conferences/7276-haskell-exchange-2016 > > If you're interested in the Haskell eXchange, you might also be > interested in the Haskell eXchange Haskell Hackathon and/or in the > Haskell courses (2-day introductory, 1-day on types, 2-day on > performance) offered by Well-Typed on the surrounding days. More info > here: http://www.well-typed.com/blog/125/ > > Cheers, > Andres > > -- > Andres Löh, Haskell Consultant > Well-Typed LLP, http://www.well-typed.com > > Registered in England & Wales, OC335890 > 250 Ice Wharf, 17 New Wharf Road, London N1 9RF, England >

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Facing issue: "No instance for (Fractional a0) arising from a use of ‘it’ The type variable ‘a0’ is ambiguous"
by Balraj Singh 13 Sep '16

13 Sep '16

Hi, I am writing a sample code below and getting an error. Please help to resolve it: Code: import Data.List findLoot val [] = 0.0 findLoot val ((x:y:[]):xs) | val == 0 = 0.0 | val < y = ((fromIntegral val) * (fromIntegral x)) / fromIntegral y | val > y = ((div val y)*x) + (findLoot (mod val y) xs) Error: No instance for (Fractional a0) arising from a use of ‘it’ The type variable ‘a0’ is ambiguous Note: there are several potential instances: instance HasResolution a => Fractional (Fixed a) -- Defined in ‘Data.Fixed’ instance Integral a => Fractional (Ratio a) -- Defined in ‘GHC.Real’ instance Fractional Double -- Defined in ‘GHC.Float’ ...plus one other In the first argument of ‘print’, namely ‘it’ In a stmt of an interactive GHCi command: print it Sample Input and Output: Input 1:- val = 50 ((x:y:[]):xs) = [[120, 30], [100,50],[60, 20]] Output 1:- 180.0000 Input 2:- val = 10((x:y:[]):xs) = [[500,30]] Output 2:- 166.6667

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Re: [Haskell-cafe] xls: A library for parsing MS excel spreadsheets
by David Feuer 12 Sep '16

12 Sep '16

Also important is the extent to which the maintainers are willing and able to change the library to respond to changes in users' needs and expectations. Parsec, for example, seems to have been largely supplanted by Megaparsec because it failed to evolve. And some libraries need to track developments in implementation techniques, either replacing old techniques or adding new ones alongside. On Sep 12, 2016 2:53 PM, "Joachim Durchholz" <jo(a)durchholz.org> wrote: The best signal is a compact description of what the library does, and, more importantly, what it does not do. Stability isn't a particularly interesting metric anyway. You want metrics like use case coverage, interface complexity, performance predictability, and you also want the update intensity over time (the shape of the curve correlates with kinds of team activity - you need to validate anything you infer from the curve, but it tells you what things to validate). Am 12.09.2016 um 11:49 schrieb Theodore Lief Gannon: > ...and, "it will probably be replaced by something meaningful in the > future." But it hasn't, nor has it been removed, and it's a better signal > than no signal at all. > > On Sun, Sep 11, 2016 at 4:01 AM, Tom Ellis < > tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk> wrote: > > On Sun, Sep 11, 2016 at 03:48:38AM -0700, Theodore Lief Gannon wrote: >> >>> In fact, there's a "Stability" field where you can indicate that it's >>> officially experimental. >>> >> >> Six years ago Simon Marlow pronounced that the "Stability" field was >> "mostly >> defunct now, and shouldn't be used" >> >> https://stackoverflow.com/questions/3841218/conventions- >> for-stability-field-of-cabal-packages/3847493#3847493 >> >> Tom >> > _______________________________________________ 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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xls: A library for parsing MS excel spreadsheets
by Harendra Kumar 12 Sep '16

12 Sep '16

Hi, Some time back I wrote a small Haskell library to parse the old MS excel format (.xls). There were libraries to parse ".xlsx" but there was none for ".xls". I uploaded it on github (https://github.com/harendra-kumar/xls) so that others looking for something similar can find and use it. -harendra

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[ANN] hs-di; dependency injection library for Haskell; deterministic even for fairly arbitrary and complex IO code
by Milán Nagy 10 Sep '16

10 Sep '16

Hi, Are there people here who are interested in the techniques of unit testing, mocking, dependency injection? Reproducible, deterministic tests, even for fairly arbitrary and complex IO code? There is a recent project of mine that strives to do all of the above: https://github.com/Wizek/hs-di I'd be curious to hear what some of you may think about it. I'm also looking to find at least one other person who can get excited about the potential of these techniques; someone who could be interested in discussing certain details of this project and related questions.

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