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Fwd: Call for Papers: 16th ACM SIGPLAN Erlang Workshop 2017
by Rob Stewart 03 Apr '17

03 Apr '17

Please consider submitting papers to the Erlang Workshop, co-located with ICFP in Oxford, UK, on 8th September. The call for papers is below. ---------- Forwarded message --------- From: Natalia Chechina <Natalia.Chechina(a)glasgow.ac.uk> Date: Mon, 13 Mar 2017 at 13:00 Subject: [Spls] Call for Papers: 16th ACM SIGPLAN Erlang Workshop 2017 To: spls(a)dcs.gla.ac.uk <spls(a)dcs.gla.ac.uk> CALL FOR PAPERS =============== Sixteenth ACM SIGPLAN Erlang Workshop http://erlang.org/workshop/2017/ ----------------------------------------------------------- Oxford, United Kingdom, 8 September 2017 Satellite event of the 22nd ACM SIGPLAN International Conference on Functional Programming (ICFP 2017) 3 - 9 September, 2017 The Erlang Workshop aims to bring together the open source, academic, and industrial communities of Erlang, to discuss technologies and languages related to Erlang. The Erlang model of concurrent programming has been widely emulated, for example by Akka in Scala, and even new programming languages were designed atop of the Erlang VM, such as Elixir. Therefore we would like to broaden the scope of the workshop to include systems like those mentioned above. The workshop will enable participants to familiarize themselves with recent developments on new techniques and tools, novel applications, draw lessons from users' experiences and identify research problems and common areas relevant to the practice of Erlang, Erlang-like languages, functional programming, distribution, concurrency etc. We invite three types of submissions. 1. Technical papers describing language extensions, critical discussions of the status quo, formal semantics of language constructs, program analysis and transformation, virtual machine extensions and compilation techniques, implementations and interfaces of Erlang in/with other languages, and new tools (profilers, tracers, debuggers, testing frameworks, etc.). The maximum length for technical papers is restricted to 12 pages. 2. Practice and application papers describing uses of Erlang in the "real-world", Erlang libraries for specific tasks, experiences from using Erlang in specific application domains, reusable programming idioms and elegant new ways of using Erlang to approach or solve a particular problem. The maximum length for the practice and application papers is restricted to 12 pages. Note that this is a maximum length; we welcome shorter papers also, and the program committee will evaluate all papers on an equal basis independent of their lengths. 3. Poster presentations describing topics related to the workshop goals. Each includes a maximum of 2 pages of the abstract and summary. Presentations in this category will be given an hour of shared simultaneous demonstration time. Workshop Co-Chairs ------------------ Scott Lystig Fritchie, VMware, USA Natalia Chechina, Glasgow University, UK Program Committee ----------------------------- (Note: the Workshop Co-Chairs are also committee members) Clara Benac Earle, Universidad Politecnica de Madrid, Spain Richard Carlsson, Klarna, Sweden Laura M. Castro, University of A Coruna, Spain Viktoria Fördős, Erlang Solutions, Hungary James S. Larson, Google, USA Important Dates ----------------------- Submission deadline: Fri May 26, 2017 Author notification: Fri June 23, 2017 Final submission for the publisher: Sat July 15, 2017 Workshop date: Fri September 8, 2017 Instructions to authors -------------------------------- Papers must be submitted online via EasyChair (via the "Erlang2017" event). The submission page is https://www.easychair.org/conferences/?conf=erlang2017 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines. Each submission must adhere to SIGPLAN's republication policy. Violation risks summary rejection of the offending submission. Accepted papers will be published by the ACM and will appear in the ACM Digital Library. The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Paper submissions will be considered for poster submission in the case they are not accepted as full papers. Venue & Registration Details ------------------------------------------ For registration, please see the ICFP 2017 web site at: http://icfp17.sigplan.org/ Related Links -------------------- ICFP 2017 web site: http://icfp17.sigplan.org/ Past ACM SIGPLAN Erlang workshops: http://www.erlang.org/workshop/ Open Source Erlang: http://www.erlang.org/ EasyChair submission site: https://www.easychair.org/conferences/?conf=erlang2017 Author Information for SIGPLAN Conferences: http://www.sigplan.org/authorInformation.htm Atendee Information for SIGPLAN Events: http://www.sigplan.org/Resources/Policies/CodeOfConduct/ -- You received this message because you are subscribed to the Google Groups "erlang-workshop-sc" group. To unsubscribe from this group and stop receiving emails from it, send an email to erlang-workshop-sc%2Bunsubscribe(a)googlegroups.com<mailto: erlang-workshop-sc%2Bunsubscribe(a)googlegroups.com>. To post to this group, send email to erlang-workshop-sc(a)googlegroups.com <mailto:erlang-workshop-sc@googlegroups.com>. Visit this group at https://groups.google.com/group/erlang-workshop-sc. For more options, visit https://groups.google.com/d/optout. _______________________________________________ SPLS mailing list SPLS(a)mailhost.dcs.gla.ac.uk https://mr1.dcs.gla.ac.uk/mailman/listinfo/spls

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Getting containers updated in Stackage
by David Feuer 03 Apr '17

03 Apr '17

It's come to my attention that Stackage doesn't have the latest and greatest versions of containers. Stackage nightly is on 0.5.7.1, and I just released 0.5.10.2. It'd be nice to get that fixed, but the containers maintainers do not have the time to take responsibility for Stackage curation. Could whoever's responsible please take care of it? Thanks, David

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[Call for Contributions] Haskell Communities and Activities Report, May 2017 edition (32nd edition)
by Mihai Maruseac 03 Apr '17

03 Apr '17

Dear all, We would like to collect contributions for the 32nd edition (10000th edition) of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 2017 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac FAQ: Q: What format should I write in? A: The usual format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2017/hcar.sty that you can use to preview your entry. If you do not know LaTeX or don't want to use it or don't have time to translate your entry into it, then please use plain text, it is better to have an entry in plain-text which we will translate than not have it at all. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then. PNG is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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formal treatment of module dependencies
by Dimitri DeFigueiredo 02 Apr '17

02 Apr '17

It seems to me that specifying and solving module dependencies is a ubiquitous problem in software development. All the languages that I know try to make the problem go away by using version numbers and maximizing backward compatibility. Does anyone know of references that formalize this problem for Haskell? Thanks, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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ANN: Haskell World News
by Gershom B 02 Apr '17

02 Apr '17

Fellow Haskellers, Over the years, a number of newsletters have been produced on news and developments in the Haskell world. There has been the venerable Haskell Weekly News, under multiple editors. More recently, the Haskell Weekly was started. These have been great publications. Nonetheless, in some ways they have been too ambitious, and in other ways, not ambitious enough. Too ambitious in seeking to publish weekly -- a grueling schedule, doomed to sporadic lapses. Insufficiently ambitious in being just roundups of articles by others. The Haskell world is maturing, and it needs a publication that can reflect that, with in-depth reporting, original research, and feature stories. That takes time to produce, and resources. I'm happy to announce the launch of Haskell World News, a subscription-only PDF newsletter with exclusive content that will break new stories and cover the world of Haskell programming in a manner that befits its emerging commercial success. Why subscription-only, you may ask? Because print media, and subscription media, is serious media. Fly-by-night weblogs report false theorems all the time. The wave of the future is not bite-sized tweets and open-letters on medium. It's shoe leather reporting that gets to the real stories. To give just one example of the sorts of things we hope to cover -- everyone agrees Elvis Presley would have been a Functional Programmer. But would he have used Haskell, or OCaml? It takes time and resources to contract a medium to hold a seance and ask his spirit. And those are the resources that Haskell World News will bring to bear. More details will be forthcoming shortly - but for now here's a sneak preview of the cover of the first issue: http://gbaz.github.io/haskell-world-news-1.jpg Credit to James Deikun, Jason Dagit, and Edward Kmett for suggestions as to some of the topics covered. Regards, The Haskell World News Team "Well Typed News Doesn't Break"

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Suggestion on how to solve a type issue with Numeric.AD
by Frederic Cogny 01 Apr '17

01 Apr '17

Hello Café *Issue: * I'm having trouble using the AD package presumably due to a wrong use of types (Sorry I know it's a bit vague) Any help or pointer on how to solve this would be greatly appreciated. Thanks in advance *Background: * I've implemented a symbolic expression tree [full code here: https://pastebin.com/FDpSFuRM] -- | Expression Tree data ExprTree a = Const a -- ^ number | ParamNode ParamName -- ^ parameter | VarNode VarName -- ^ variable | UnaryNode MonoOp (ExprTree a) -- ^ operator of arity 1 | BinaryNode DualOp (ExprTree a) (ExprTree a) -- ^ operator of arity 2 | CondNode (Cond a) (ExprTree a) (ExprTree a) -- ^ conditional node deriving (Eq,Show, Generic) An evaluation function on it -- | evaluates an Expression Tree on its Context (Map ParamName a, Map VarName a) evaluate :: (Eq a, Ord a, Floating a) => ExprTree a -> Context a -> a And a few instances on it: instance Num a => Default (ExprTree a) where ... instance Num a => Num (ExprTree a) where ... instance Fractional a => Fractional (ExprTree a) where ... instance Floating a => Floating (ExprTree a) where ... instance (Arbitrary a) => Arbitrary (ExprTree a) where ... This allows me to easily create (using derivation rules) the derivative(s) of such a tree with respect to its variable(s): diff :: (Eq a, Floating a) => ExprTree a -> VarName -> ExprTree a grad :: (Eq a, Floating a) => ExprTree a -> Map VarName (ExprTree a) hessian :: (Eq a, Floating a) => ExprTree a -> Map VarName (Map VarName (ExprTree a)) So far, so good ... Now, to gain assurance in my implementation I wanted to check the derivatives against the Numeric.AD module so I create the two following -- | helper for AD usage exprTreeToListFun :: (RealFloat a) => ExprTree a -- ^ tree -> Map ParamName a -- ^ paramDict -> ([a] -> a) -- fun from var values to their evaluation exprTreeToListFun tree paramDict vals = res where res = evaluate tree (paramDict, varDict) varDict = Map.fromList $ zip (getVarNames tree) vals gradThroughAD :: RealFloat a => ExprTree a -> Context a -> Map VarName a gradThroughAD tree (paramDict, varDict) = res where varNames = Map.keys varDict varVals = Map.elems varDict gradList = AD.grad (exprTreeToListFun tree paramDict) varVals res = Map.fromList $ zip varNames gradList it unfortunately does not type check with message: • Couldn't match type ‘a’ with ‘Numeric.AD.Internal.Reverse.Reverse s a’ ‘a’ is a rigid type variable bound by the type signature for: gradThroughAD :: forall a. RealFloat a => ExprTree a -> Context a -> Map VarName a at src/ SymbolicExpression.hs:452:18 Expected type: [Numeric.AD.Internal.Reverse. Reverse s a] -> Numeric.AD.Internal.Reverse.Reverse s a Actual type: [a] -> a • In the first argument of ‘AD.grad’, namely ‘(exprTreeToListFun tree paramDict)’ In the expression: AD.grad (exprTreeToListFun tree paramDict) varVals In an equation for ‘gradList’: gradList = AD.grad (exprTreeToListFun tree paramDict) varVals • Relevant bindings include gradList :: [a] (bound at src/SymbolicExpression.hs:457:5) varVals :: [a] (bound at src/ SymbolicExpression.hs:456:5) varDict :: Map VarName a (bound at src/SymbolicExpression.hs:453:32) paramDict :: Map ParamName a (bound at src/SymbolicExpression.hs:453:21) tree :: ExprTree a (bound at src/ SymbolicExpression.hs:453:15) gradThroughAD :: ExprTree a -> Context a -> Map VarName a (bound at src/SymbolicExpression.hs:453:1) I was a bit surprised (I guess naively) by this, since to me, 'a' is polymorphic with RealFloat as type constraint and that is "supposed" to work with AD So anyway, I tried to modify as follow: {-# LANGUAGE Rank2Types #-} -- | helper for AD usage exprTreeToListFun :: (RealFloat a) => ExprTree a -- ^ tree -> Map ParamName a -- ^ paramDict -> *(RealFloat b => *[b] -> b) -- fun from var values to their evaluation exprTreeToListFun tree paramDict vals = res where res = *realToFrac* $ evaluate tree (paramDict, varDict) varDict = Map.fromList $ zip (getVarNames tree) *$ map realToFrac *vals This now typechecks and runs but going through AD returns me *all derivatives as zero* as if treating the variables (that passed through *realToFrac*) as constants, so not that much helpful either. Any idea how this can be solved ? Apologies if this is a question that already has an answer somewhere but poking around SO <http://stackoverflow.com/search?q=%5Bhaskell%5D+ad+package> It looks like I'm not the only one having similar issues and unfortunately none of the answers I found really helped me, if anything it confirms the same issue of null derivatives: http://stackoverflow.com/questions/36878083/ad-type-unification-error-with-… Thanks again -- Frederic Cogny +33 7 83 12 61 69

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typeclass for rest resource clients
by Jan von Löwenstein 01 Apr '17

01 Apr '17

Hi, for a project of mine I want to abstract away the rest resources (client side) and provide a stub implementation of the backend that I can use for testing. I thought about a StateT based stub instance, however one of the methods besides the usual crud stuff should be `waitFor` so that I can wait for resources to reach a certain state. Very concretely I want to create kubernetes pods and have my code wait until the pod is `Running`. What would I have to throw into the mix in order to have a stub that creates a pod and only later changes the status to `Running`? Any thoughts, advice or hint? Best Jan

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GHCI says we all drink left-handed
by Andreas Abel 01 Apr '17

01 Apr '17

GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> data Handed = Handed Prelude> let drink beer = beer Prelude> all drink (Left Handed) True -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www.cse.chalmers.se/~abela/

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Re: [Haskell-cafe] [accelerate-haskell] [ANN] Accelerate 1.0
by Trevor McDonell 01 Apr '17

01 Apr '17

Yep, it is just hard coded, but to be fair there have been very few new Accelerate backends. It does pull in extra dependencies though, if you forget to add the appropriate `-f-X` flag. The type class approach you mention sounds like how the LLVM-based packages are architected. -Trev On Fri, 31 Mar 2017 at 20:46 Henning Thielemann < googlegroups(a)henning-thielemann.de> wrote: > > On Fri, 31 Mar 2017, Trevor McDonell wrote: > > > * accelerate-fft: FFI bindings to discrete Fourier transforms (FFTW > and cuFFT) > > (https://hackage.haskell.org/package/accelerate-fft) > > I was curious how you solved the problem of addressing target specific > implementations of functions like the FFT. Looking at the source code it > looks like you have hard-coded a selection of back-ends. I hoped that we > would be able to specify back-end capabilities via a type-class. This way > the user could add more back-ends with their respective FFT > implementations. Maybe that's stuff to be defered to Accelerate-2.0. :-)

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ANN: hledger 1.2
by Simon Michael 01 Apr '17

01 Apr '17

I'm pleased to announce hledger 1.2, on schedule. Thank you to release contributors Peter Simons, Stefano Rodighiero, Moritz Kiefer, Pia Mancini, Bryan Richter, Steven R. Baker, Hans-Peter Deifel, Joshua Chia, Joshua Kehn, Michael Walker, and especially Mykola Orliuk and Justin Le! Notable user-visible changes in this release: new commands list, more powerful balancesheet/incomestatement/cashflow commands, more parseable print output, better --pivot, basic automated postings and periodic transactions support, more and easier addons, bugfixes. Full release notes: http://hledger.org/release-notes#hledger-1.2 The next major release is scheduled for June 30. hledger (http://hledger.org) is a dependable, precise, cross-platform program for tracking money, time, or any other commodity, using double-entry accounting and a simple plain text file format. It is a haskell reimplementation of Ledger, provides command-line, curses and web interfaces, and aims to be a robust, practical tool for daily use. To install this release: Get stack, eg from http://haskell-lang.org/get-started $ stack install --resolver=nightly hledger [hledger-ui] [hledger-web] [hledger-api] $ ~/.local/bin/hledger or see http://hledger.org/download for more installation options. Our IRC channel is #hledger on Freenode, and you can find out more at http://hledger.org. I hope you enjoy hledger and that it helps you achieve your goals. If you have benefitted from hledger, please give back and make it stronger! Donate using one of the funding links on the home page, give feedback, report bugs, send pull requests, write about it, etc. Best! -Simon

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[ANN] Accelerate 1.0
by Trevor McDonell 31 Mar '17

31 Mar '17

Friends, After much delay, it is my pleasure to announce the release of Accelerate 1.0. Accelerate is an embedded language for computations on dense, multidimensional arrays. Computations are expressed in the form of parameterised collective operations, such as maps, folds, and permutations. These computations are online compiled and can be executed on a range of architectures, such as multicore CPUs and GPUs. This release includes updates to a number core and support packages: * accelerate: Defines the core language (https://hackage.haskell.org/package/accelerate) * accelerate-llvm-native: Accelerate backend to execute computations on multicore CPUs ( https://hackage.haskell.org/package/accelerate-llvm-native) * accelerate-llvm-ptx: Accelerate backend to execute computations on CUDA GPUs (https://hackage.haskell.org/package/accelerate-llvm-ptx) * accelerate-llvm: For developing new backends targeting LLVM (https://hackage.haskell.org/package/accelerate-llvm) * accelerate-io: Conversions between Accelerate arrays and other formats (including vector and repa) (https://hackage.haskell.org/package/accelerate-io) * accelerate-fft: FFI bindings to discrete Fourier transforms (FFTW and cuFFT) (https://hackage.haskell.org/package/accelerate-fft) * accelerate-bignum: Fixed-width large integer arithmetic (> 64-bits) (https://hackage.haskell.org/package/accelerate-bignum) * accelerate-examples: Computational kernels and example applications, as well as regression test suite (https://hackage.haskell.org/package/accelerate-examples) * mwc-random-accelerate: Generate Accelerate arrays filled with pseudorandom numbers (https://hackage.haskell.org/package/mwc-random-accelerate) * gloss-accelerate: Generate gloss pictures from Accelerate (https://hackage.haskell.org/package/gloss-accelerate) * gloss-raster-accelerate: Parallel rendering of raster images and animations (https://hackage.haskell.org/package/gloss-raster-accelerate) * colour-accelerate: Colour representations in Accelerate, including RGB, sRGB, HSV and HSL (https://hackage.haskell.org/package/colour-accelerate ) * lens-accelerate: Lens operators for Accelerate types (https://hackage.haskell.org/package/lens-accelerate) * (soon) linear-accelerate: Linear vector spaces in Accelerate (https://hackage.haskell.org/package/linear-accelerate) (pending https://github.com/ekmett/linear-accelerate/pull/4) The (still under construction) acceleratehs.org website includes some installation instructions and a short tutorial to help you get started. * http://www.acceleratehs.org/get-started.html * http://www.acceleratehs.org/examples/mandelbrot.html If you find a bug or have a question, head over to the GitHub issue tracker or mailing list. * https://github.com/AccelerateHS/accelerate/issues * accelerate-haskell(a)googlegroups.com We welcome new contributors, so please contact us if you would like to get involved!

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Compose Conference Call for Participation [NYC, May 18-19]
by Gershom B 30 Mar '17

30 Mar '17

=============================================== Call for Participation Compose Conference 2017 May 18-19 2017 New York, NY http://www.composeconference.org/2017 =============================================== The practice and craft of functional programming :: Conference Compose is a conference for typed functional programmers, focused specifically on Haskell, OCaml, F#, SML, and related technologies. Typed functional programming has been taken up widely, by industry and hobbyists alike. For many of us it has renewed our belief that code should be beautiful, and that programming can be as enjoyable as it is practical. Compose is about bringing together functional programmers of all levels of skill and experience — from technical leads to novices, and from long-time hackers to students just getting started. It will feature a keynote by Emily Riehl on aspects of category theory and computation, and two days of great talks. * Invited Talks Emily Riehl: TBA * Local Information (venue): http://www.composeconference.org/2017/ * Accepted Talks and Tutorials Barry Burd - Teaching Haskell David Rhodes - Learning F#: Case study with branch and bound Edmund Cape - Multiplying by 1 - an important form of computation and how it reveals distinctions between kleislis Enzo Alda, et al. - Reactive Sheets: an intuitive approach to functional‑reactive computing Hezekiah Carty and Chris Donaher - Distrest - REST access to distributed services Hongbo Zhang - BuckleScript: Making functional programming accessible to JavaScript developers Jennifer Paykin, Kenneth Foner, Antal Spector-Zabusky - `choose` Your Own Derivative Joachim Breitner - Lock-step simulation is child’s play Martín Ceresa, Gustavo Grieco - QuickFuzz Testing for Fun and Profit Michael Chavinda - Android programming in Froid Nikhil Barthwal - Implementing an Event-Driven Microservices Architecture in F#: A case study of Jet.com Nikita Volkov - New Hasql - a simpler, safer and faster Postgres client Sebastien Mondet - Typed-Tagless Final Bioinformatics Stephen Compall - Working with Monads in OCaml Stuart Popejoy - Smart Contracts and Formal Verification with Z3 with Pact Yaron Minsky - Data Driven UIs, Incrementally * Full abstracts: http://www.composeconference.org/2017/program * Registration: http://composeconference.eventbrite.com * Follow @composeconf on twitter for news: https://twitter.com/composeconf * On freenode irc, chat with fellow attendees at #composeconference * Corporate sponsorships are welcome. Current sponsors list forthcoming. * Policies (diversity and anti-harassment): http://www.composeconference.org/conduct * Email us with any questions at info(a)composeconference.org * Please forward this announcement to interested parties and lists.

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Request for white paper
by Michael Litchard 30 Mar '17

30 Mar '17

I'm looking for a freely available version of the below paper. Google Scholar points me to 7 versions, all of which are behind a paywall. Stefan Kahrs, "Red-black trees with types", Journal of functional programming, 11(04), pp 425-432, July 2001

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Call for participation - Programming Languages Mentoring Workshop (PLMW)
by Tobias Grosser 29 Mar '17

29 Mar '17

We invite early graduate students and undergraduates to apply for a scholarship and to participate in the Programming Languages Mentoring Workshop (PLMW) in Barcelona, Spain on Sunday, June 18, 2017. The purpose of this mentoring workshop is to encourage and prepare graduate students and senior undergraduate students to pursue careers in programming language research. The workshop also includes specific mentoring opportunities for students who belong to traditionally underrepresented populations, including women, people of color, students with disabilities, and first-generation students. Throughout the day, we’ll have technical sessions, to discuss cutting-edge research in programming languages; and mentoring sessions, to help you learn about, prepare, and succeed in your research career. You’ll also have the opportunity to talk with top programming language researchers and other students interested in programming language research. This workshop is part of the activities surrounding the Programming Languages Design and Implementation (PLDI) conference and takes place the day before the main conference. One goal of the workshop is to make the PLDI conference more accessible to newcomers. We hope that participants will stay through the entire conference. Other events that follow the workshop include the Programming Languages Summer School and the European Conference on Object-Oriented Programming (ECOOP). Scholarship funds are available for students to attend PLMW@PLDI. These scholarships should cover reasonable expenses (airfare, hotel, and registration fees) for attendance at both the workshop and an associated event (such as the PLDI conference or the summer school), but are limited. We welcome students with alternative sources of funding to attend as well; but everyone who plans to attend needs to apply. Apply for a scholarship and to participate at PLMW@PLDI: https://goo.gl/forms/80aacyFiXNFNnPqA3 The deadline for full consideration of funding is SATURDAY, APRIL 15. Selected participants will be notified by APRIL 29 and will need to pre-register and commit to attending the workshop by MAY 10. Applicants who apply after April 15 but by May 15 may be eligible to receive funding, if funds remain, and will be notified by by May 29. Additionally, if you are currently a student in the United States who cannot (or will not) travel due to concerns about US travel restrictions, we encourage you to apply and let us know. We may be able to arrange another form of mentoring for you. We look forward to seeing you! The PLMW@PLDI Organizers Michael Carbin, MIT Stephen Chong, Harvard University Işıl Dillig, The University of Texas at Austin Ben Wiedermann, Harvey Mudd College

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Queue which tracks actual in-memory element size?
by Bardur Arantsson 29 Mar '17

29 Mar '17

Hi all, So we have the TBQueue[1] which tracks the number of elements, but does anyone know if there's an equivalent queue which the actual combined size of the elements? (I'd be quite happy if it used "gsize" or similar, but I suppose the ideal would be if I could supply a "size estimator" function.) It seems like an obvious generalization of TBQueue[2], but my Google-fu is letting me down at the moment. Regards, --- [1] https://hackage.haskell.org/package/stm-2.4.4.1/docs/Control-Concurrent-STM… [2] TBQueue would be the equivalent of estimating every element to have size 1.

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[ANN] wiringPi-1.0.1
by Patrick Pelletier 28 Mar '17

28 Mar '17

https://hackage.haskell.org/package/wiringPi https://github.com/ppelleti/hs-wiringPi The most significant addition in this version is the wiringPiISR function, which makes it possible to get a callback when an input pin changes, thus avoiding the need to poll. In a future version, I might add a higher-level facility on top of this, such as the ability to update a TVar when an input changes. (Let me know if you have any thoughts on this.) New in 1.0.1: * Examples for Pi-LITEr and Fish Dish boards. * Added piGpioLayout as a synonym for piBoardRev, to reflect the name change in version 2.36 of the C library. * Addition of wiringPiISR function. (Contributed by Satoshi Ogata.) * Improvements to documentation and examples. About wiringPi: This is a Haskell binding to the wiringPi library, which allows you to interface with the GPIO pins on the Raspberry Pi. Unlike some other solutions for using the Raspberry Pi's GPIO pins, wiringPi provides access to more advanced features, such as enabling the internal pull-up or pull-down resistors. --Patrick

1 0

SublimeHaskell on SublimeText 2
by Scott Michel 28 Mar '17

28 Mar '17

Is there anyone out there who uses the SublimeHaskell plugin for SublimeText 2? Reason for asking: I would sincerely like to remove all vestiges of SublimeText 2 from the plugin. I suspect (know) that it hasn't worked for SublimeText 2 for a long time and the amount of work to backport the current plugin would involve significant effort. For those of you who gave up on SublimeHaskell: Consider giving it another chance. It's more stable now and I've personally been working off issues to make it, in the words of one of the testers, "suck less." (*) It's still not perfect -- known issues are: - The hsdev backend starts up always. I'm refactoring the backend management code, which will take a while, but this will get fixed. It will also enable an enterprising individual to integrate intero and other backends. - Can't connect to a remote hsdev backend. Planned enhancement. - Can't use multiple stack.yamls. Planned enhancement that could get released in the next two weeks. -scooter (*) Paraphrase of Michael Elkin's comment about the Mutt mail client: "All mail users agents suck, Mutt just sucks less."

1 0

Proposal for an extension: in-module namespaces
by Aura Kelloniemi 28 Mar '17

28 Mar '17

Hello I've been thinking about an extension to GHC which would allow creating new module-like namespaces within normal Haskell modules. Because this kind of namespacing system seems quite obvious to me, I ask if it has already been proposed or discussed in detail? The motivation: 1. It would assist in solving the 'pollution of namespace' -problem, of which many Haskellers have complained. 2. It would allow building bundles of related modules which can be exported from one module and then imported into others. This could result in radically fewer import statements. Rant: I'm tired of writing import qualified Data.ByteSstring as B import qualified Data.ByteString.Builder as B import qualified Data.ByteString.Lazy as BL import qualified Data.ByteString.Lazy.Builder as BL import qualified Data.HashMap.Strict as HM import qualified Data.Text as T import qualified Data.Text.Lazy as TL import qualified Data.Text.Lazy.Builder as BL A huge portion of this appears in almos all my Haskell source files. If I wrote my code in Python, I could condense all these imports to: So, you see, it was zero lines of code. I'm also tired of these patterns: (personName p), (companyName c), (objectName o), ... and I hardly prefer (name (p :: Person)), (name (c :: Company)) or (name (o ::Object)) using the DuplicateRecordFields syntax. Proposal: I propose that modules can contain named submodules. They can be exported and imported just like anything else within a module, like this: {-# LANGUAGE Submodules #-} module Library.Things (qualified module Person, qualified module Company, personWorksInCompany ) where -- import ... qualified module Person (Person (..)) where -- This is the module Library.Things.Person. Submodules could possibly -- use some other keywords than 'qualified module'. Maybe just 'module', -- 'submodule' or 'namespace'. data Person = Person { name :: {-# UNPACK #-} !T.Text, id :: {-# UNPACK #-} !Unique, ... } -- Back in Library.Things qualified module Company where data Company = Company { name :: {-# UNPACK #-} !T.Text, employees :: {-# UNPACK #-} !V.Vector Unique, ... } -- back in Library.Things again: personWorksIn :: Person.Person -> Company.Company -> Bool personWorksIn p c = Person.id p `elem` Company.employees c -- ... end of Library.Things And in another file: {-# LANGUAGE Submodules #-} -- The following demonstrates some ways of importing submodules. They are -- not meant to be used like this in real code: import Library.Things import qualified Library.Things as Things -- Now the submodules are also in scope and can be referred to like this: person1 :: Person.Person person2 :: Things.Person.Person company :: Library.Things.Company.Company -- And we also could have written: after importing Library.Things: import Person -- And now data type Person, selector name, etc. are in scope -- If we also imported the Company module: import Things.Company -- selector 'name' would now be ambiguous and need qualification at use site. -- Import lists would also work when bringing submodule contents to current -- namespace. Like this: import Library.Things hiding (module Company) -- or import qualified Library.Things (module Person) -- Which would allow Library.THings.Person.Person to be used requiring use of -- the fully qualified name. -- But the following would not work if Library.Things was not already -- imported: import Library.Things.Person -- because there is no such module, just submodule. -- ... end of other file Some projects probably would like to create a module like this: module Imports (qualified module B, qualified module HM, qualified module T ) where import Data.ByteString as B -- ... and so forth ... Open questions: - Should there be a separate keyword for importing submodules, for example 'use' or 'import namespace'? - Would imports be allowed within submodules? I'd say no for importing other modules, but importing submodules could be ok. - Would it be possible to extend submodules after their initial definition by just writing a new 'qualified module' block? I'd say yes, because submodules are just about namespacing. It is today possible to import many modules qualified and put them in the same namespace, like is often done. - What do you fellow Haskellers think about this all? -- Aura

6 7

Two PhD positions in Ljubljana starting October 2017
by Matija Pretnar 28 Mar '17

28 Mar '17

We are looking for two PhD students at the Faculty of Mathematics and Physics, University of Ljubljana. The programme starts in October 2017 and lasts three years. The positions will be fully funded (subject to approval by the funding agency). The candidates should have a Master's degree in mathematics or computer science. No knowledge of Slovene is required. The first PhD student will be advised by dr. Andrej Bauer. The topic of research is foundations of type theory. The candidate should have interest in mathematical aspects of type theory, and familiarity with proof assistants is desirable. The second PhD student will be advised by dr. Matija Pretnar. The topic of research is the theory of programming languages with a focus on computational effects. The candidate should have interest in both the mathematical foundations and practical implementation of programming languages. Candidates should send their applications as soon as possible, but no later than the end of April, to Andrej Bauer <andrej.bauer(a)fmf.uni-lj.si> or Matija Pretnar <matija.pretnar(a)fmf.uni-lj.si>, depending on their primary interest. Please include a short CV, academic record, and a statement of interest. Andrej Bauer and Matija Pretnar

1 0

Munich Haskell Meeting, 2017-03-29 @ 19:30
by Christian Neukirchen 26 Mar '17

26 Mar '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 29 at Max-Emanuel-Brauerei at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html 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-mar-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0

Void vs ()
by Bardur Arantsson 25 Mar '17

25 Mar '17

Hi all, [Context: I'm working on a CQRS/ES library and in the following, "i" is an identifier for an aggregate root, "e" is an event type, and the "Int" is a sequence number.] So, I have a data type data Foo i e = Foo i e Int where I want to have a corresponding data Batch i e = Batch i [Foo x e Int] (I'll come back to "x". It's just a metasyntactic variable for now. The point of the "Int" is that it's extra information that I really need to keep, but it's not essential to the Foo data structure itself.) with an operation batchFoos :: [Foo i e] -> [Batch i e] batchFoos = ... (basically a standard Data.List.groupBy where I compare for equality on the i's.) I also have an extraction function toList :: Batch i e -> (i, [Foo x e Int]) My question is then: What's the best thing to use for "x", Void or ()? The idea behind "collapsing" the "i" parameter away to an "x" is to signal to users to the API that they can be absolutely certain that when they call "toList", they can be sure that all the Foo's that they get back have the same i value, namely the one which is the first component of the tuple. An alernative I've been considering is actually to create a Foo' type which actually does eliminate the i parameter, but it seems a bit boilerplatey. (It's not a *lot* of code to duplicate, so if it looks like the best option, I'll do that... but it feels odd.) Anyone have any suggestions on what the best way to approach this would be? (Aside: AFAICT there's no reasonable way to implement anything like Foldable or Traversable to avoid the explicit "toList" function, right? AFAICT Foldable/Traversable constrain you to using the *actual* last type parameter, "e", for folding over. Are there any generic alternative type classes that let you "pick your own element type"? I'm guessing Traversal could possibly be that, but if I have to incur a Lens dependency, I'll just live with a "toList" function. This is just about whether it can be done elegantly -- it's not a big deal.) Regards,

4 7

Literate Haskell w/ markdown?
by David Banas 25 Mar '17

25 Mar '17

Hi all, Would anyone care to share their preferred method for writing/processing literate Haskell w/ markdown? Thanks, -db

3 2

Code/style review
by Jan von Löwenstein 24 Mar '17

24 Mar '17

Hi, TL;DR I am pretty new to Haskell and have some Haskell code I'd love to get feedback on. ( https://github.com/SAP/bosh-cpi-haskell/tree/a586079dd9383fca030f1536116522… and https://github.com/SAP/bosh-kubernetes-cpi-release/tree/master/src/bosh-kub… ) Longer version: I started looking into Haskell back in November last year. Pretty soon I wanted to try something more "real-world" IO heavy in order to see how that goes. I came up with a side-project at work to implement a Bosh Kubernetes CPI. Bosh (bosh.io) is a tool to install and manage distributed systems on a range of IaaSs. In particular it is used to install Cloud Foundry (a cloud application development platform). I guess I don't have to introduce Kubernetes. The interface between Bosh and its cloud provider interfaces (CPI) is a simple command line application. Bosh provides configuration via file provided as process call argument. It provides the method to be invoked including arguments as JSON via stdin and expects a response via stdout. The CPI can provide a log via stderr. (http://bosh.io/docs/cpi-api-v1.html) The interface to Kubernetes is basically a REST api, i.e. http calls. Currently I use a generated client based on Servant ( https://github.com/soundcloud/haskell-kubernetes) I have put that interface into its own package. I created two typeclasses FileSystem[1] and System[2] to abstract away the system (I guess I should call them MonadFileSystem and MonadSystem to align with how mtl calls the monadic classes). And I created another typeclass MonadCpi[3] to abstract away the concrete implementation towards potentially several infrastructures. The Kubernetes implementation[4] is rather hacked (no tests, low testability, etc.) and I'd like to change that. I thought about a typeclass for REST resources (with methods create, get, list, update, delete, ...) and I begin to have problems integrating that into my current primitives (e.g. I'd like to have a MonadLog instance that uses my System typeclass for printing to stderr). That's when I started to ask myself if I had chosen the right Haskell tools and primitives for what I was doing and thought it was time to get some feedback from more experienced Haskellers aka you. I know this was a pretty long text. Sorry for that. Any feedback, including "you did it completely wrong" is appreciated - as long as it includes "have a look here and here", "look this is what I would do", or similar. Many thanks already for reading. Best Jan [1] https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522… <https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522…> [2] https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522… [3] https://github.com/SAP/bosh-cpi-haskell/blob/a586079dd9383fca030f1536116522… [4] https://github.com/SAP/bosh-kubernetes-cpi-release/blob/master/src/bosh-kub…

1 0

Code Review Request - Unbalanced Parenthesis correction
by Michael Litchard 24 Mar '17

24 Mar '17

I'm prepping for a coding interview, and am examining the task of correcting unbalanced parentheses. The finger tree seems to be the right data structure. As a proof of concept I've used Data.Sequence to test my idea. If this is the right direction to go, I'll write more specialized finger tree code. The code works on the few test cases I have tried. Feedback appreciated. {-# LANGUAGE ViewPatterns #-}module Parenthesis whereimport BasicPrelude hiding (concat,null,empty) import Data.Sequence hiding (length)import Data.Foldable hiding (length,null) balanceParens :: String -> String balanceParens str = go str [] empty where go [] [] (null -> True) = [] go [] [] parens = Data.Foldable.toList parens go ('(':xs) [] (null -> True) = go xs [RP] (singleton '(') go (')':xs) [] (null -> True) = go xs [] (fromList "()") go ('(':xs) debit parens = go xs (RP:debit) (parens |> '(') go (')':xs) [] parens = go xs [] corrected where corrected = ('(' <| parens) |> ')' go (')':xs) (RP:debit) parens = go xs debit (parens |> ')') go (_:xs) debit parens = go xs debit parens go [] (RP:debit) parens = go [] debit (parens |> ')') example: balanceParens "))(""(())()" balanceParens ")))""((()))"

7 13

Why does "enumFromThenTo x x y" depend on "y"?
by Clinton Mead 24 Mar '17

24 Mar '17

I'm implementing Enum for a type I have, and I figured it should replicate what ordinary Enum types, so I did some tests on how Enum behaves. I noticed that: "enumFromThenTo 3 3 5" is not the same as "enumFromThenTo 3 3 1". This seems bizarre to me. Generally increasing the stepsize increases the length of the list. For example, "length (enumFromThenTo 0 10 1000) == 101" "length (enumFromThenTo 0 2 1000) == 501" "length (enumFromThenTo 0 1 1000) == 1001" "length (enumFromThenTo 0 0 1000) == infinite" but "length (enumFromThenTo 0 (-10) (-1000)) == 101" "length (enumFromThenTo 0 (-2) (-1000)) == 501" "length (enumFromThenTo 0 (-1) (-1000)) == 1001" "length (enumFromThenTo 0 0 1000) == 0" It would seem to me that "enumFromThenTo x y z == map negate (enumFromThenTo (-x) (-y) (-z))" should be an invariant that should hold but in this case it clearly doesn't. Is this intentional, and if so, for what reason? Or is this a GHC bug?

2 1

One-Off detection. Question about Space/Time complexity
by Michael Litchard 23 Mar '17

23 Mar '17

Problem spec from CareerCup <https://careercup.com/question?id=5092486548553728>. Given two strings, return boolean True/False if they are only one edit apart. Edit can be insert/delete/update of only one character in the string. Eg. -True xyz,xz xyz,xyk xy,xyz -False xyz,xyz xyz,xzy x,xyz module Strings.OneLetter where` import Preludeimport qualified Data.Text as T` oneLetter :: T.Text -> T.Text -> Bool oneLetter s1 s2 | s1 == s2 = False | max_l > (min_l + 1) = False | max_l == min_l = diff_size == 1 | otherwise = diff_size == 0 where length_s1 = T.length s1 length_s2 = T.length s2 max_l = max length_s1 length_s2 min_l = min length_s1 length_s2 diff_size = length $ filter (\(a,b) -> a /= b) zipped zipped = T.zip s1 s2` So, I used Text instead of String, hoping I could take advantage of fusion. I have the following questions and my initial attempt to answer them. What is the time complexity of oneLetter 0(m+n) where m is the length of s1 and n is the length of s2 what is the space complexity of oneLetter? I'm thinking due to laziness it's O(1), only two Chars are in memory at any one time, or two Ints. But I'm hazy on why. If this is wrong, please articulate why. If I'm right, and my reasoning is wrong or incomplete, please say why. I don't think I can improve the time complexity. Am I right? Can the space complexity be improved? What if I changed from Text to String? I don't think the time complexity changes, but how does this change the space complexity?

4 6

Re: [Haskell-cafe] One-Off detection. Question about Space/Time complexity
by Olaf Klinke 23 Mar '17

23 Mar '17

I believe this is the classic publication on the subject: http://users.monash.edu/~lloyd/tildeStrings/Alignment/92.IPL.html The Haskell implementation is here: http://users.monash.edu/~lloyd/tildeFP/Haskell/1998/Edit01/ In case of a pair of one-off strings, it is O(m) time. Not sure whether the entire strings are kept in memory. My gut feeling is that they must be kept in memory in the worst case (edit distance max(m,n)) but stretches of matched prefixes can be garbage collected. So in your particular case it should indeed be O(1) space. Profiling suggests the algorithm is not O(1) space, but I might have done the input strings wrong. input :: Int -> String input = flip replicate 'a' length . input should be O(1) space, shouldn't it? With ghc 7.8.3 and -O2 it is not. Running the edit distance on the pair (input m, 'b' : input m) and profiling the program suggests the algorithm is O(m) space. Andrew Butterfield's oneOff seems to be linear space, too. -- Olaf $cat test.hs import System.Environment import Data.List (foldl') import EditDistance (d) input :: Int -> String input n = replicate n 'a' main = do args <- getArgs let n = read (head args) let x = input $! (n+1) let y = 'b' : input n print (d x y) $ghc -O2 --make -prof -fprof-auto test.hs [2 of 2] Compiling Main ( test.hs, test.o ) Linking test ... $for n in 1000 10000 100000; do ./test $n +RTS -p && grep 'total alloc' test.prof; done 1 total alloc = 358,128 bytes (excludes profiling overheads) 1 total alloc = 3,022,696 bytes (excludes profiling overheads) 1 total alloc = 29,663,256 bytes (excludes profiling overheads)

1 0

Num nstances for Data.Monoid.{First,Last}
by Oliver Charles 23 Mar '17

23 Mar '17

We already have Num instances on things like Sum and Product, could we have them on First and Last? I mostly want fromInteger on these types. To give you a real world example: keySpeed :: L.Fold KeyboardEventData Double keySpeed = L.foldMap (\KeyboardEventData {keyboardEventKeyMotion} -> case keyboardEventKeyMotion of Pressed -> Last (Just 1) Released -> Last (Just 0)) (fromMaybe 0 . getLast) keySpeed folds (under Control.Foldl) events from a UI system into the number 1 if the key has been pressed and 0 if the key has been released. It's OK in the above form, but if Last had a Num instance, I could just have Pressed -> 1 Released -> 0 Which I find preferable. Any thoughts? - Ollie

1 0

GHC 8.0.2 and extra dynamic libraries
by Bryan Gardiner 23 Mar '17

23 Mar '17

Hi all, I'm not sure if this is the right place to ask this, but I'd like to see if there were any changes between GHC 8.0.1 and 8.0.2 in how linking works on Linux. I inject an extra library directory (extraLibDirs, for building qtah) via preBuild and preTest Cabal hooks[1], and while GHC 8.0.1 (or is this Cabal?) seems to pass this as both "-L$dir" and "-optl-Wl,-rpath,$dir" to the ghc link command, GHC 8.0.2 (Cabal?) appears to only pass the former, causing the built library to be unable to find its dependency in that directory. Is this a known change? Do I need to pass -optl-Wl,... myself now? I'm running Gentoo with the Haskell overlay, Cabal-1.24.2.0, and cabal-install-1.24.0.2 in both cases. Happy to provide more info if necessary. Thanks for any help you can provide, Bryan [1] https://gitlab.com/khumba/qtah/blob/master/qtah/Setup.hs

1 0

Fwd: Semigroup and Monoid instances for ReaderT
by David Menendez 22 Mar '17

22 Mar '17

On Tue, Mar 21, 2017 at 3:53 AM, Louis Pan <louis(a)pan.me> wrote: > Hi all, > > In my Glazier GUI library, I had to use newtype wrappers to create > Semigroup and Monoid instances for ReaderT. > > Is there a reason why ReaderT doesn't have an instance of Semigroup and > Monoid? > There are at least three reasonable instances of Semigroup and Monoid for ReaderT. instance (Applicative m, Monoid a) => Monoid (ReaderT r m a) where mempty = pure mempty mappend a b = mappend <$> a <*> b instance (Alternative m) => Monoid (ReaderT r m a) where mempty = empty mappend = (<|>) instance (Monoid (m a)) => Monoid (ReaderT r m a) where mempty = ReaderT $ mempty mappend a b = ReaderT $ mappend a b In the absence of a principled reason to prefer one over the others and a general consensus, I think it’s better not to choose. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

2 1

Semigroup and Monoid instances for ReaderT
by Louis Pan 22 Mar '17

22 Mar '17

Hi all, In my Glazier GUI library, I had to use newtype wrappers to create Semigroup and Monoid instances for ReaderT. Is there a reason why ReaderT doesn't have an instance of Semigroup and Monoid? The reader ((->) a) is a Monoid and a Semigroup. https://hackage.haskell.org/package/base-4.9.1.0/docs/src/ GHC.Base.html#line-268 https://hackage.haskell.org/package/base-4.9.1.0/docs/src/ Data.Semigroup.html#line-150 Could the following be added to the transformers package? Or is it not lawful? instance (Applicative m, Semigroup a) => Semigroup (ReaderT r m a) where f <> g = (<>) <$> f <*> g {-# INLINABLE (<>) #-} instance (Applicative m, Monoid a) => Monoid (ReaderT r m a) where mempty = pure mempty {-# INLINABLE mempty #-} f `mappend` g = mappend <$> f <*> g {-# INLINABLE mappend #-} Does it make sense to extend the monoid instance to all the other transformers? Eg. instance (Monad m, Semigroup a) => Semigroup (StateT s m a) where f <> g = (<>) <$> f <*> g {-# INLINABLE (<>) #-} instance (Monad m, Monoid a) => Monoid (StateT s m a) where mempty = pure mempty {-# INLINABLE mempty #-} f `mappend` g = mappend <$> f <*> g {-# INLINABLE mappend #-} instance (Monad m, Monoid w, Semigroup a) => Semigroup (WriterT w m a) where f <> g = (<>) <$> f <*> g {-# INLINABLE (<>) #-} instance (Monad m, Monoid w, Monoid a) => Monoid (WriterT w m a) where mempty = pure mempty {-# INLINABLE mempty #-} f `mappend` g = mappend <$> f <*> g {-# INLINABLE mappend #-} and also for MaybeT, IdentityT, ExceptT, etc Regards, Louis

2 2

Full-time (remote acceptable) -- Haskell engineering at Vacation Labs, India
by Saurabh Nanda 22 Mar '17

22 Mar '17

Cross-posted from Reddit: https://www.reddit.com/r/haskell/comments/60tkh7/fulltime_remote_acceptable… We are a startup based out of Goa (India), building travel-related technology products. Our flagship SaaS product is already in the market and has paying customers. We're expanding our product portfolio, and building new products in Haskell (the current product is written in Rails + AngularJS). If you're interested in using Haskell (and related FP languages, like Elm & Purescript) to build web/mobile products, do apply via http://www.vacationlabs.com/we-are-hiring/software-engineer-haskell/ PS: Our current stack is Rails, AngularJS v1, Postgres, Redis, Bootstrap+LessCSS. The stack for future product-dev is Haskell (on the server) with Typescript/Elm/Purescript (on the UI). -- Saurabh.

1 0

Semigroup and Monoid instances for ReaderT
by Olaf Klinke 21 Mar '17

21 Mar '17

Maybe I missed something, but aren't the transformers a red herring here? instance (Semigroup a, Applicative f) => Semigroup (f a) where (<>) = liftA2 (<>) My first guess was that you could add this instance to your code without the need for changes to the transformers package. The problem with the semigroup instance above is that it conflicts with semigroup instances for f. For example, there is a semigroup instance for [a] when a is not a semigroup. For the declaration above, ghci 7.8.3 gave me: Overlapping instance declarations: instance (Semigroup a, Applicative f) => Semigroup (f a) -- Defined at <interactive>:4:10 instance (Semigroup a, Semigroup b) => Semigroup (a, b) -- Defined in ‘Data.Semigroup’ Hence I guess you must use a newtype wrapper to declare when you want the semigroup instance of a and when you want the instance of f. Olaf

1 0

Registration Open: Compose 2017, New York, May 18-19
by Gershom B 20 Mar '17

20 Mar '17

http://www.composeconference.org/2017 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. Early Bird registration is now open at eventbrite: https://www.eventbrite.com/e/cmpse-conference-2017-tickets-32106159387 Additionally, we're happy to announce our keynote speaker as Emily Riehl, who is a professor of mathematics at Johns Hopkins University, and the author of the book Category Theory in Context (as well as much else). To get a sense of Compose, you can check out the great talks from past conferences: https://www.youtube.com/channel/UC0pEknZxL7Q1j0Ok8qImWdQ Finally, as a reminder, the Call for Presentations closes on March 23, so hurry up and get those talk proposals submitted! http://www.composeconference.org/2017/cfp/ Cheers, Gershom

1 0

Has anyone used Graql in Haskell?
by Jeffrey Brown 20 Mar '17

20 Mar '17

Graql[1] is a (and stands for) graph query language. It is unusually expressive, allowing relationships to be members of other relationships[2] -- effectively, allowing one edge to be an endpoint of another edge. Someone named Felix posted a Haskell library for generating Graql queries[3]. Does anyone have a Haskell program that not only generates but uses such a query? [1] https://hackage.haskell.org/package/graql [2] https://discuss.grakn.ai/t/nested-relationships/ [3] https://discuss.grakn.ai/t/graql-haskell-how-to-run-the-test-program/ -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

4 5

Best practices for linting wrt performance
by Adam Bergmark 19 Mar '17

19 Mar '17

I'd like to make aeson hlint clean. aeson contains a lot of performance optimizations which made me wonder; Which linting refactorings should you think twice about when it comes to performance? The best way to find an answer to this for aeson is of course to run the benchmarks, but are there any rules-of-thumb that you take into account when linting? Here's the default hlint output for the current aeson master: https://gist.github.com/bergmark/5dd63c24fe6bdcf67eae0e88495a24e7 Cheers, Adam

3 6

Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 19 Mar '17

19 Mar '17

Just adding to the programming contest anecdotes. Here is what happens if you issue an applied math assignment without specifying the programming language to be used. The first student solves it in bash, to demonstrate how inefficient the solution can be. The second student writes a Turing machine simulator in Maple (I can't recall whether it was the brainf*ck dialect), then programs that Turing machine. The third student writes the solution in postscript and makes the printer solve the differential equation. All were languages available in the setting. I have no idea whether any of the above submissions were inspected by the tutor, but received a mark. Olaf

2 1

Loop with the StateT monad
by p75213＠gmail.com 19 Mar '17

19 Mar '17

This is an exercise to learn the StateT monad. The program implements the game Morra. The two players are the computer and a person. The state accumulates the score of the computer and player. The program works for one iteration of function morra only. However I am at a loss how to loop it. I have tried a few things but nothing seems to work. I have narrowed the problem down to the "p <- liftIO getChar" statement. The second time through the loop it isn't executed. I have bracketed the statement with "liftIO $ putStrLn "before" and liftIO $ putStrLn "after". This is the program: module Morra where import Control.Monad.Trans.State.Lazy import Control.Monad.IO.Class import Data.Char (isDigit, digitToInt) import System.Random (randomRIO) import Control.Monad (when) morra :: StateT (Int, Int) IO () morra = do liftIO $ putStrLn "before" p <- liftIO getChar liftIO $ putStrLn "after" when (isDigit p) $ do let p' = digitToInt p c <- liftIO $ randomRIO (1, 2) liftIO $ putStrLn ("P: " ++ [p]) liftIO $ putStrLn ("C: " ++ show c) (pt, ct) <- get if even (c + p') then do liftIO $ putStrLn "Computer Wins" put (pt, ct + 1) else do liftIO $ putStrLn "Player Wins" put (pt + 1, ct) morra main :: IO () main = do putStrLn "-- p is Player" putStrLn "-- c is Computer" putStrLn "-- Player is odds, Computer is evens." (personS,compS) <- execStateT morra (0,0) putStrLn ("Person Score: " ++ show personS) putStrLn ("Computer Score: " ++ show compS) if personS > compS then putStrLn "Winner is Person" else putStrLn "Winner is Computer" and this is the output: *Morra> main -- p is Player -- c is Computer -- Player is odds, Computer is evens. before 1 after P: 1 C: 2 Player Wins before after Person Score: 1 Computer Score: 0 Winner is Person *Morra>

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Re: [Haskell-cafe] benchmarking a function that returns an unlifted type
by David Feuer 18 Mar '17

18 Mar '17

The critical feature of whnf (as I understand it) is that GHC does not use the static argument transformation. The function passed to "go" therefore doesn't inline into it. So you'll have to write your version in some fashion that similarly prevents undesirable inlining. Could you just copy the code and specialize it to the desired type? On Mar 18, 2017 1:32 PM, "Richard Eisenberg" <rae(a)cs.brynmawr.edu> wrote: Apologies for a newbie question, but I am seeking advice on how to benchmark a pure function that returns an unlifted type. I've heard tell that `criterion` is good for benchmarking, and so I'm using that library (but have no particular allegiance to it, if something else is better). Specifically, I'm comparing a function over unlifted types with the same function written over lifted types, trying to observe how much we pay for the boxing/unboxing. To benchmark the lifted version, I just use criterion's whnf function. That seems to be working splendidly. Of course, whnf doesn't work over unlifted types, so I found its source code and inlined it. This compiles. But it doesn't work! No matter what I do, the unlifted version measures in the nanoseconds. I'm guessing that GHC "cleverly" is avoiding recomputation. How can I stop this? Here is my code: > main :: IO () > main = do > initializeTime > let lifted = whnf fastSumPrimes 100000 > > unlifted = Benchmarkable go > where > go n > | n <= 0 = return () > | otherwise = let x = fastSumPrimes# 100000# in go (n-1) > > benchmark lifted > benchmark unlifted I've tried increasing the argument in the unlifted case, to no avail. I've also tried using `case` instead of `let`. -ddump-simpl suggests that fastSumPrimes# is really being called, but I'm dubious. Can anyone offer advice? Thanks! Richard _______________________________________________ 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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benchmarking a function that returns an unlifted type
by Richard Eisenberg 18 Mar '17

18 Mar '17

Apologies for a newbie question, but I am seeking advice on how to benchmark a pure function that returns an unlifted type. I've heard tell that `criterion` is good for benchmarking, and so I'm using that library (but have no particular allegiance to it, if something else is better). Specifically, I'm comparing a function over unlifted types with the same function written over lifted types, trying to observe how much we pay for the boxing/unboxing. To benchmark the lifted version, I just use criterion's whnf function. That seems to be working splendidly. Of course, whnf doesn't work over unlifted types, so I found its source code and inlined it. This compiles. But it doesn't work! No matter what I do, the unlifted version measures in the nanoseconds. I'm guessing that GHC "cleverly" is avoiding recomputation. How can I stop this? Here is my code: > main :: IO () > main = do > initializeTime > let lifted = whnf fastSumPrimes 100000 > > unlifted = Benchmarkable go > where > go n > | n <= 0 = return () > | otherwise = let x = fastSumPrimes# 100000# in go (n-1) > > benchmark lifted > benchmark unlifted I've tried increasing the argument in the unlifted case, to no avail. I've also tried using `case` instead of `let`. -ddump-simpl suggests that fastSumPrimes# is really being called, but I'm dubious. Can anyone offer advice? Thanks! Richard

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Tools to assist with development cycle of hackage packages
by Clinton Mead 17 Mar '17

17 Mar '17

I've only just started uploading packages to hackage (my package listing is here <https://hackage.haskell.org/user/clinton>) but currently there's a lot of repetitive activities for creating and updating packages. For example, here's the steps in creating a package: 1. Initialise a repository on github 2. Initialise git repository locally 3. Set github repository as remote 4. Add a LICENSE file 5. Add a standard ".gitignore file" 6. Create a cabal file with the appropriate files that hackage requires including: 1. Git repository source 2. Issues page 3. Licence 4. Licence file 7. Run multi-ghc-travis <https://github.com/hvr/multi-ghc-travis> to create a ".travis.yml" file 8. Make an initial commit and push 9. Refresh travis-ci.org's repository list so it detects the new repository There's also the stack stuff, but the GUI I'm using, IntelliJ with a Haskell plugin, handles most of that. It also creates a cabal file, but it's missing a number of key fields as mentioned above. When I actually want to upload the package I go though these steps: 1. Push to github 2. Wait for Travis-CI to compile the package (this is a test to ensure it builds in a clean remote environment). 3. Run "cabal sdist 2>&1" 4. Parse the output of sdist to see where the dist file is. 5. Run "hup <https://hackage.haskell.org/package/hup> packup fileFromSDist" to upload the package, putting in my hackage user/pass 6. Run "hup docboth", to both build and upload the documentation. 7. Tag the commit as a release Currently, I've got two scripts with help with a lot of this, but it's a bit adhoc, and it's not fully automated (for example, I still have to manually ensure all the correct fields are in the cabal file, usually by copy/pasting from another package and modifying). Are there any tools that I haven't found that make this process a bit more painless? I'm a bit new to this area, and I've only started using git recently as a prelude to uploading my Haskell packages, so admittedly I may have missed something obvious or perhaps I'm just doing it all wrong. But if other people do find it painful like me, perhaps I'll put some effort into rewriting my perl scripts into nice haskell packages and executables for others to use.

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Package takeover: dbus
by Andrey Sverdlichenko 17 Mar '17

17 Mar '17

I'd like to take over maintenance of the package dbus. It depends on several other packaged with upper bounds too restrictive for the latest versions. I tried to contact owner a few times weeks ago, but to no avail. My hackage username is blaze.

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Ideas for Haskell Summer of Code
by Niki Vazou 17 Mar '17

17 Mar '17

Hello, As a preparation for the 2017 edition of Haskell Summer of Code, we are crowdsourcing project ideas. Compiler optimizations, new libraries, desired features, or any other ideas you would like to see in the Haskell ecosystem. You can submit tickets to the Haskell Summer of Code Ideas page: https://ghc.haskell.org/trac/summer-of-code/report/1 Alternatively, you can tell us your SoC idea by responding to this thread (and I can generate the ticket). Best, Niki

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How can I use “cabal repl” without base
by Adam Steen 17 Mar '17

17 Mar '17

Hi How can I use cabal repl (ghci) without base? The following contrived example shows the error -- minimal.hsmodule Minimal where myId :: a -> a myId x = x and -- minimal.cabal name: minimal version: 0.0.1 build-type: Simple cabal-version: >= 1.10 library exposed-modules: Minimal --build-depends: base default-language: Haskell2010 default-extensions: NoImplicitPrelude cabal build succeeds $ cabal build Building minimal-0.0.1... Preprocessing library minimal-0.0.1...[1 of 1] Compiling Minimal ( Minimal.hs, dist\build\Minimal.o ) but cabal repl fails $ cabal repl Preprocessing library minimal-0.0.1... GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help <interactive>:1:6: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:30: error: Not in scope: `System.IO.stdin' No module named `System.IO' is imported. <interactive>:1:46: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. <interactive>:1:70: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:94: error: Not in scope: `System.IO.stdout' No module named `System.IO' is imported. <interactive>:1:111: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. <interactive>:1:135: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:159: error: Not in scope: `System.IO.stderr' No module named `System.IO' is imported. <interactive>:1:176: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. if i restore "build-depends: base" if works as expected. $ cabal repl Preprocessing library minimal-0.0.1... GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help[1 of 1] Compiling Minimal ( Minimal.hs, interpreted ) Ok, modules loaded: Minimal.*Minimal> i know just keeping "build-depends: base" will make it work, but thats not what i am asking. Note: this also applies for stack Cheers Adam ps this is cross posted from stackoverflow <http://stackoverflow.com/q/42823547/415681>

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Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 17 Mar '17

17 Mar '17

Johannes, Thanks for writing this. Actually, your negative abstract turns out to be for a quite positive article. List are like iterators, that's the essence, and as long as you are iterating, it's fine to use []. The prominence of singly linked lists in Haskell has tought me to write my data processing programs in a streaming style. To think about what data must be held in memory (use Maps or whatever for this) and read the other data in as a list. I'd be interested whether there is a way to check which of my lists in the source code the compiler managed to "deforest" away. Which intermediate files should I look at? What are the tools to inspect? Olaf

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amortized time and space for set operations
by Michael Litchard 17 Mar '17

17 Mar '17

What's the best way to calculate the amortized time and space complexity for the operations in "Implementing Sets Efficiently in a Functional Language". http://groups.csail.mit.edu/mac/users/adams/BB/92-10.ps

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Package Management with Stack?
by Jun Inoue 16 Mar '17

16 Mar '17

I've gotten fed up with Cabal+Hackage build errors and I want a more stable way to manage Haskell packages. I'm NOT trying to build a Haskell project of my own, but I just want a system-wide (or at least user-wide) installation of GHC "with batteries", along with some Haskell programs that other people have written. I need xmonad installed somewhere on my $PATH and I need lens available with :m+ when I fire up ghci. I do NOT want a bunch of sandboxes scattered throughout my file system, some of which have these things installed while others don't. I was hoping Stack+Stackage would fit that bill, but going through the instructions on https://haskell-lang.org/get-started, it seems that stack is meant really for setting up sandboxes for particular projects. When I tried using it as a system-wide (or rather, user-wide) package manager, it fails. For instance, as suggested on that page, I created /tmp/HelloWorld.hs and type $ stack HelloWorld.hs which seemed to successfully download the lts-8.4 build plan and ghc-nopie-8.0.2. So far so good. But then when I typed $ stack install xmonad-contrib it said Error: While constructing the build plan, the following exceptions were encountered: In the dependencies for xmonad-contrib-0.13: X11-xft must match >=0.2, but the stack configuration has no specified version (latest applicable is 0.3.1) Recommended action: try adding the following to your extra-deps in /home/jun/.stack/global-project/stack.yaml: - X11-xft-0.3.1 You may also want to try the 'stack solver' command Plan construction failed. I've tried going down the "stack solver" rabbit hole, to no avail; it just tells me I need to register a directory containing a .cabal file. IIUC, that's a directory containing some project I'm developing? But I have no such project! (At the moment.) Am I fundamentally misunderstanding how stack is supposed to be used? Is stack usable as a package manager in the way cabal sort of was, and in the way apt-get or emerge can be used? If not, what do people recommend I use for that purpose? Nix? Or, perhaps, do the errors above indicate misconfiguration? I don't want to install haskell-platform because it's too outdated. I need at least GHC >= 8. I'm on the latest stable release of Ubuntu if that makes a difference. -- Jun Inoue

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Parsing a bunch of strings without backtracking
by Javran Cheng 16 Mar '17

16 Mar '17

Hi all, I come across an idea about parsing a bunch of strings, just want to share this idea and looking for some comments and maybe pointing me to some related articles. The idea is when you have: 1 <$ string "aabb" <|> 2 <$ string "aacc" you can rewrite it as: string "aa" >> (1 <$ string "bb" <|> 2 <$ string "cc") so that you don't have to deal with the common prefix "aa" twice. Then I further extend this idea: if we have a set of strings (s1, s2, ...), whose element is not a prefix of any other set member, why don't we turn things like (v1 <$ string s1 <|> v2 <$ string s2 <|> ...) into a "tree-structure" so we don't need to deal with any common prefix more than once? So this motivates my little toy implementation at https://gist.github.com/Javran/1cbfe9897d9a5c8fae5d20a33fd83813 basically given a list [(s1,v1),(s2,v2)...], we want to generate a parser equivalent to (v1 <$ string s1 <|> v2 <$ string s2 <|> ...) but perhaps with less backtracking. My code does this by turning the set of strings into a tree structure (see function "compact") and using this "Compact" representation as a driver to generate the parser. I used ReadP to have a concrete parser for testing, but this should work with any parsing library supporting MonadPlus. I think they are bunch of things that can be improved: - for now every intermediate node of "Compact" contains just a char, so this will end up generating a parser that has branch `char 'f' >> char 'o' >> 'o'` in it instead of the slightly more efficient version `string "foo"` - we could further attach a weight to each string, this allows rearranging (<|>) chains so that a more frequent string parser appears earlier. - if the input [(s1,v1),(s2,v2)...] are all known constants at compile time, probably TemplateHaskell can be used for generating the parser at compile time (as a Haskell expression). I'm not sure how difficult this would be though, as I have very limited understanding of TH. but before I explore further, I want to collect some comments as I said in the beginning. Best, -- Javran (Fang) Cheng

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Re: [Haskell-cafe] Lazy vs Strict ByteStrings
by Dimitri DeFigueiredo 16 Mar '17

16 Mar '17

I think I need to be more specific here. My gripe with ByteString is that there are two distinct ByteString types disguised as one. There are “disguised” simply by having the same name. This leads me to think the use of ByteString will be simpler than it usually turns out to be. I fear Backpack may soon make this worse. Obviously, this is not exclusive to ByteString. Using the same name for strict and lazy versions is all over the place. Here is another example: https://hackage.haskell.org/package/unordered-containers/docs/Data-HashMap-… https://hackage.haskell.org/package/unordered-containers/docs/Data-HashMap-… But the problem is specially insidious with BytStrings because of their usage patterns and also because the process of finding out which version is which through GHC error messages is very inefficient. Here is a very ugly example where I believe I was misled by both ByteString types having the same name. I needed to make a “random looking but secretly not random” number and package it up in a 128-bit UUID. In other words, I wanted to mark the random numbers I create, but in a way that nobody else knows they are mine. The `hash` function from the cryptonite library accepts ByteString and its output can be converted to ByteString. The UUID type also has a function `fromByteString`. What could be easier? Here’s what I ended up with: import System.Random import Data.Maybe import Data.Word import Data.UUID import Crypto.Hash import Crypto.Hash.Algorithms import Data.ByteArray import Data.ByteString.Builder import qualified Data.ByteString as BS import Data.ByteString.Lazy as BSL -- Ugly! Step-by-step markAsSelf :: Word64 -> Word64 -> UUID markAsSelf selfDetectKey rand = let selfKeyAsLazyBS = toLazyByteString (word64BE selfDetectKey) randAsLazyBS = toLazyByteString (word64BE rand) hashInput = BSL.concat [randAsLazyBS, selfKeyAsLazyBS] digest = hash (toStrict hashInput) :: Digest SHA256 hashBitsStrict = BS.take 8 (convert digest) halfAndHalf = BSL.concat [randAsLazyBS, fromStrict hashBitsStrict] in fromJust (fromByteString halfAndHalf) secret :: Word64 secret = 12345 main = do r <- randomIO print (markAsSelf secret r) --------- end --------- in Data.ByteString.Builder toLazyByteString :: Builder -> ByteString -- exists toStrictByteString :: Builder -> ByteString -- was not found Similarly, in Data.UUID fromByteString :: ByteString -> Maybe UUID — only exists for lazy I think this lack of a common interface makes the code above a type conversion disaster and the process of debugging it painfully inefficient. Is there a better approach to go about doing this? Cheers, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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[ANN] Haskell Paris meetup on 2017-03-27
by lucas di cioccio 15 Mar '17

15 Mar '17

Dear cafe readers, The next Haskell Paris Meetup will take place on 2017-03-27 at Murex near Trocadéro in Paris. There will be two entry-level/intermediary-level talks (in French) as well as some time for socializing. You'll find more details about the event as well as a registration form at https://www.meetup.com/haskell-paris/events/238394643/ . Best, -- Lucas DiCioccio

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