- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Decorating exceptions with backtrace information
by Michal J Gajda 09 May '20

09 May '20

Ben, I agree with you that is a great idea! I can add a few more real world examples: * we get exception from a foreign library that we bound, * we get an exception from a platform (I believe Windows supports throwing exceptions to programs), * user presses CTRL-C and we want to know where our program hanged. * we get infamous <<loop>>, because in theory nobody wants non-terminating programs, but in practice everybody gets them sometimes. I also use `ExceptT`, `EitherT` for processing large sets of data, because that allows me to contain the errors efficiently. However from time to time, I get an error to blow up **and I cannot even locate which library was guilty**. It would be nice to extract them automatically and put them into error database before they are prioritized. -- Cheers Michał

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network's example echo server seems leaking socket on error binding
by Compl Yue 09 May '20

09 May '20

Hi folks, I just filed an issue https://github.com/haskell/network/issues/447 <https://github.com/haskell/network/issues/447> with the network package, I'd like to post here too in seeking your wisedom at the same time. In top-level doc, the minimal example echo server uses Control.Exception.bracket like this: E.bracket (open addr) close loop where resolve = do let hints = defaultHints { addrFlags = [AI_PASSIVE] , addrSocketType = Stream } head <$> getAddrInfo (Just hints) mhost (Just port) open addr = do sock <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr) setSocketOption sock ReuseAddr 1 withFdSocket sock setCloseOnExecIfNeeded bind sock $ addrAddress addr listen sock 1024 return sock loop sock = forever $ do (conn, _peer) <- accept sock void $ forkFinally (server conn) (const $ gracefulClose conn 5000) I happened to copy the configuration with another machine's IP address to run on my dev machine, then of course it failed binding to the IP, but in this case I suspect sock above is leaked without close, as open failed at all, so sock is not given to bracket for it to do the cleanup. Is my suspicion correct ? Or I missed something that the example actually won't leak on binding errors ? Best regards, Compl

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Windows Haskell CI
by Ben Franksen 06 May '20

06 May '20

Like ghc itself, we (the Darcs team) also have trouble providing adequate support for Windows. I personally haven't (seriously) used Windows for decades. So I thought we could at least do some automated building and testing for Windows on some CI system, perhaps even automatically build Windows binaries. Is there a free CI service that you could recommend that ideally supports at least Windows, MacOS, and Linux and that has good support for Haskell programs, i.e. some recent ghc and cabal pre-installed? If not, what could be an alternative solution? Any kind of comments or hints are appreciated. Cheers Ben

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FMBC 2020: 2nd Workshop on Formal Methods for Blockchains (3rd CfP, Deadline Extension)
by Bruno Bernardo 06 May '20

06 May '20

[ Please distribute, apologies for multiple postings. ] ======================================================================== 2nd Workshop on Formal Methods for Blockchains (FMBC) 2020 - Third Call https://fmbc.gitlab.io/2020 July 19, 2020 Co-located with the 32nd International Conference on Computer-Aided Verification (CAV 2020) http://i-cav.org/2020/ *Due do the COVID-19 pandemic, the workshop is likely to be held online.* ------------------------------------------------------------- IMPORTANT DATES -------------------------------- Abstract submission: May 12, 2020 (extended) Full paper submission: May 19, 2020 (extended) Notification: June 23, 2020 (extended) Camera-ready: July 14, 2020 (extended) Conference: July 19, 2020 Deadlines are Anywhere on Earth: https://en.wikipedia.org/wiki/Anywhere_on_Earth -------------------------------- -------------------------------- TOPICS OF INTEREST -------------------------------- Blockchains are decentralized transactional ledgers that rely on cryptographic hash functions for guaranteeing the integrity of the stored data. Participants on the network reach agreement on what valid transactions are through consensus algorithms. Blockchains may also provide support for Smart Contracts. Smart Contracts are scripts of an ad-hoc programming language that are stored in the Blockchain and that run on the network. They can interact with the ledger’s data and update its state. These scripts can express the logic of possibly complex contracts between users of the Blockchain. Thus, Smart Contracts can facilitate the economic activity of Blockchain participants. With the emergence and increasing popularity of cryptocurrencies such as Bitcoin and Ethereum, it is now of utmost importance to have strong guarantees of the behavior of Blockchain software. These guarantees can be brought by using Formal Methods. Indeed, Blockchain software encompasses many topics of computer science where using Formal Methods techniques and tools are relevant: consensus algorithms to ensure the liveness and the security of the data on the chain, programming languages specifically designed to write Smart Contracts, cryptographic protocols, such as zero-knowledge proofs, used to ensure privacy, etc. This workshop is a forum to identify theoretical and practical approaches of formal methods for Blockchain technology. Topics include, but are not limited to: * Formal models of Blockchain applications or concepts * Formal methods for consensus protocols * Formal methods for Blockchain-specific cryptographic primitives or protocols * Design and implementation of Smart Contract languages * Verification of Smart Contracts -------------------------------- -------------------------------- SUBMISSION -------------------------------- Submit original manuscripts (not published or considered elsewhere) with a maximum of twelve pages (full papers), six pages (short papers), and two pages (extended abstract) describing new and emerging ideas or summarizing existing work). Each paper should include a title and the name and affiliation of each author. Authors of selected extended-abstracts are invited to give a short lightning talk. At least one author of an accepted paper is expected to present the paper at the workshop as a registered participant. Submission link: https://easychair.org/conferences/?conf=fmbc2020 The authors are encouraged to use LaTeX and the EasyChair style files: https://easychair.org/publications/for_authors -------------------------------- -------------------------------- PROCEEDINGS -------------------------------- All submissions will be peer-reviewed by at least three members of the program committee for quality and relevance. Accepted regular papers (full and short papers) will be included in the workshop proceedings, published as a volume of the OpenAccess Series in Informatics (OASIcs) by Dagstuhl. -------------------------------- -------------------------------- INVITED SPEAKER -------------------------------- Grigore Rosu, Professor at University of Illinois at Urbana-Champaign, USA and Founder of Runtime Verification http://fsl.cs.illinois.edu/index.php/Grigore_Rosu https://runtimeverification.com/ -------------------------------- -------------------------------- PROGRAM COMMITTEE -------------------------------- PC CO-CHAIRS * Bruno Bernardo (Nomadic Labs, France) (bruno(a)nomadic-labs.com) * Diego Marmsoler (University of Exeter, UK) (D.Marmsoler(a)exeter.ac.uk) PC MEMBERS * Wolfgang Ahrendt (Chalmers University of Technology, Sweden) * Lacramioara Astefanoei (Nomadic Labs, France) * Massimo Bartoletti (University of Cagliari, Italy) * Bernhard Beckert (Karlsruhe Institute of Technology, Germany) * Achim Brucker (University of Exeter, UK) * Silvia Crafa (Universita di Padova, Italy) * Zaynah Dargaye (Nomadic Labs, France) * Jérémie Decouchant (University of Luxembourg, Luxembourg) * Ansgar Fehnker (University of Twente, Netherlands) * Georges Gonthier (Inria, France) * Florian Kammueller (Middlesex University London, UK) * Maurice Herlihy (Brown University, USA) * Igor Konnov (Informal, Austria) * Andreas Lochbihler (Digital Asset, Switzerland) * Anastasia Mavridou (NASA Ames, USA) * Simão Melo de Sousa (Universidade da Beira Interior, Portugal) * Andrew Miller (University of Illinois at Urbana-Champaign, USA) * Karl Palmskog (KTH, Sweden) * Vincent Rahli (University of Birmingham, UK) * Andreas Rossberg (Dfinity Foundation, Germany) * Claudio Russo (Dfinity Foundation, USA) * César Sanchez (Imdea, Spain) * Clara Schneidewind (TU Wien, Austria) * Ilya Sergey (Yale-NUS College/NUS, Singapore) * Bas Spitters (Aarhus University/Concordium, Denmark) * Mark Staples (CSIRO Data61, Australia) * Meng Sun (Peking University, China) * Simon Thompson (University of Kent, UK) * Philip Wadler (University of Edinburgh / IOHK, UK)

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 04 May '20

04 May '20

============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 31st May 2020 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, twice per year the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 31st May 2020: o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 350 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting; if your original abstract exceeds the word limit, please submit an abridged version within the limit) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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cost of Text.Parsec.Expr
by Johannes Waldmann 02 May '20

02 May '20

Dear Cafe, I use `buildExpressionParser` https://hackage.haskell.org/package/parsec-3.1.14.0/docs/Text-Parsec-Expr.h… with a table similar to https://hackage.haskell.org/package/haskell-src-exts-1.23.0/docs/Language-H… and I find it's somewhat expensive. I think the algorithm makes me pay for each operator that is in the table even when just a few of them are actually in the input. (I removed from the table those that don't appear in my test cases, and it got faster.) Certainly this is known? What could be done about it? What does Megaparsec do? It seems that Text.Megaparsec.Expr vanished from version 6 to version 7. A classical shift/reduce algorithm would help (?) but not with the given type of table, which does not allow to parse an operator just once, and then look up its precedence. - J.W.

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ICFP 2020 will be held ONLINE Aug 23-28
by Sam Tobin-Hochstadt 01 May '20

01 May '20

The ICFP 2020 organizers would like to announce that the conference and co-located events, originally scheduled for August 23-28, in Jersey City, New Jersey, will now be held online during the same dates. Further information for presenters, authors, attendees, sponsors, and the ICFP community will be provided as it becomes available. The ICFP Organizing Committee

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[Haskell-Cafe] How to deal with looped references inside instances?
by Daneel Yaitskov 01 May '20

01 May '20

{- Hi, I am trying to translate a Java sample with looped references below to Haskell. I've found 2 solutions, but both of them look ugly, because they require a helper class. So I am looking for an advice for best practices on working with looped references in polymorphic types. interface Engine { String showEng(); } class Car<E extends Engine> { int id; E eng; String show() { return "Car with " + eng.showEng(); } int getId() { return id; } } class Steam implements Engine { Car<Steam> car; Stream(Car<Steam> car) { this.car = car; } String showEng() { return "id " + car.getId(); } } Car<Steam> car = new Car<>(); car.id = 42; car.eng = new Stream(car); car.show(); -} {-# LANGUAGE UndecidableInstances #-} -- Fix2 shortcoming class Car c where getId :: c -> Int class Engine e where showEng :: e -> String data SteamEng c = SteamEng c instance (Car c) => Engine (SteamEng c) where showEng (SteamEng c) = "id " ++ show (getId c) {- First solution is via ShowEng class. I don't know how to tell type checker that the type which is got after application of type function is instantiating Engine class?? instance (Engine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ showEng eng -} data Car1 eng = Car1 Int (eng (Car1 eng)) class ShowEng f where showEng2 :: (Car a) => f a -> String instance ShowEng SteamEng where showEng2 = showEng {- A side note. Intuitive alternative for ShowEng which is not working! showEng2 = showEng means showEng2 gets Engine, but class HaveEngine f where cast :: (Car a) => f a -> f a instance HaveEngine SteamEng where cast = id ... instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng (cast eng)) nor instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng eng) -} instance Car (Car1 e) where getId (Car1 id _) = id instance (ShowEng eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng2 eng) c1 = Car1 42 (SteamEng c1) {- Second solution is removing argument from engine type parameter. -} data Car0 eng = Car0 Int eng instance Car (Car0 e) where getId (Car0 id _) = id instance (Engine eng) => Show (Car0 eng) where show (Car0 _ eng) = "Car0 with " ++ showEng eng {- in this case I have to avoid infinite type and introducing an extra wrapper and instantiating business logic classes for him. λ c0 = Car0 42 (SteamEng c0) <interactive>:30:6: error: • Occurs check: cannot construct the infinite type: car ~ Car0 (SteamEng car) • In the expression: Car0 (SteamEng c0) In an equation for ‘c0’: c0 = Car0 42 (SteamEng c0) • Relevant bindings include c0 :: car (bound at <interactive>:30:1) -} newtype Fix2 f g = Fix2 (f (g (Fix2 f g))) instance (Show (f (g (Fix2 f g)))) => Show (Fix2 f g) where show (Fix2 a) = show a {- λ c0 = Car0 42 (SteamEng (Fix2 c0)) <interactive>:62:1: error: • No instance for (Car (Fix2 Car0 SteamEng)) arising from a use of ‘print’ • In a stmt of an interactive GHCi command: print it -} instance (Car (f (g (Fix2 f g)))) => Car (Fix2 f g) where getId (Fix2 a) = getId a c0 = Car0 42 (SteamEng (Fix2 c0)) {- Thanks, Daniil -}

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A quick poll about GHC's -with-rtsopts flag
by Merijn Verstraaten 30 Apr '20

30 Apr '20

Who here is: 1) Aware that multiple invocations of -with-rtsopts clobber each other, and 2) Actually relies on this behaviour? This quick and informal poll in relation to this GHC issue: https://gitlab.haskell.org/ghc/ghc/issues/18117 Cheers, Merijn

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ANN: stack-2.3.1
by Emanuel Borsboom 30 Apr '20

30 Apr '20

See https://haskellstack.org/ for installation and upgrade instructions. Release notes: - We have reduced the number of platforms that we support with binary releases. The reason behind this is that we've been slowed down in our release process until now with issues trying to build binaries for less common platforms. In order to make sure we can address issues more quickly (like supporting new GHC versions), we're limiting support from the Stack team to: - Linux 64-bit (static) - macOS - Windows 64-bit If others want to provide additional binaries, we will definitely be happy for the support. But since our CI system is currently able to produce these three bindists only, that's what we will be providing with the next release. - Since we no longer have dynamically linked Linux binaries, we are removing removing the -static suffix from the static Linux binaries. If you have scripts to download the latest stable Linux binary, update them to use linux-x86_64 instead of linux-x86_64-static (if you are already using the former, nothing needs to change). For this release, both are supported, but the next release will no longer have the -static variant. - We are also deprecating the download links at https://stackage.org/stack. See this page for the current installation instructions: https://docs.haskellstack.org/en/stable/install_and_upgrade/. - These are the canonical locations to download the latest stable binaries from, and will continue to be supported going forward: - Linux 64-bit (static): https://get.haskellstack.org/stable/linux-x86_64.tar.gz - macOS: https://get.haskellstack.org/stable/osx-x86_64.tar.gz - Windows 64-bit: https://get.haskellstack.org/stable/windows-x86_64.zip As always, binaries for specific versions are available from the Github releases: https://github.com/commercialhaskell/stack/releases. *Changes since v2.1.3.1* Major changes: - setup-info-locations yaml configuration now allows overwriting the default locations of stack-setup-2.yaml. #5031 <https://github.com/commercialhaskell/stack/pull/5031> #2983 <https://github.com/commercialhaskell/stack/issues/2983> #2913 <https://github.com/commercialhaskell/stack/issues/2913> - The setup-info configuration key now allows overwriting parts of the default setup-info - The --setup-info-yaml command line flag now may be used in all stack commands such as stack build, and not only in stack setup - The --setup-info-yaml may specify multiple locations for stack-setup.yaml files. - The stack upload can read first reads environment Variable $HACKAGE_USERNAME and $HACKAGE_PASSWORD if they are missing only then asks for username or password - Fully remove GHCJS support. - Remove the freeze command. It has been replaced by lock files. Behavior changes: - Remove the deprecated --stack-setup-yaml command line argument in favor of --setup-info-yaml #2647 <https://github.com/commercialhaskell/stack/issues/2647> - We now recommend checking in generated cabal files for repos. When generating lock files for extra-deps that only include package.yaml files, a deprecation warning will be generated. Also, those packages will no longer be included in the generated lock files. See #5210 <https://github.com/commercialhaskell/stack/issues/5210>. Other enhancements: - Add build-output-timestamps flag in yaml. Setting it to true prefixes each build log output line with a timestamp. - Show warning about local-programs-path with spaces on windows when running scripts. See #5013 <https://github.com/commercialhaskell/stack/pull/5013> - Add ls dependencies json which will print dependencies as JSON. ls dependencies --tree is now ls dependencies tree. See #4424 <https://github.com/commercialhaskell/stack/pull/4424> - Remove warning for using Stack with GHC 8.8-8.10, and Cabal 3.0-3.2. - Allow relative paths in --setup-info-yaml and tool paths #3394 <https://github.com/commercialhaskell/stack/issues/3394> - Added the --only-locals flag. See #5272 <https://github.com/commercialhaskell/stack/issues/5272> Bug fixes: - Upgrade pantry: module mapping insertions into the database are now atomic. Previously, if you SIGTERMed at the wrong time while running a script, you could end up with an inconsistent database state. - --resolver global doesn't retrieve snapshots list from the internet beause doesn't need it. See #5103 <https://github.com/commercialhaskell/stack/issues/5103> - Fix using relative links in haddocks output. See #4971 <https://github.com/commercialhaskell/stack/issues/4971>. - Do not include generated cabal file information in lock files. See #5045 <https://github.com/commercialhaskell/stack/issues/5045>. - Use proper Hoogle executable path when installed automatically. See #4905 <https://github.com/commercialhaskell/stack/issues/4905> - Fix GHC version for batched package unregistration. See #4951 <https://github.com/commercialhaskell/stack/issues/4951> - Use Hoogle from the snapshot used and not the latest version. See #4905 <https://github.com/commercialhaskell/stack/issues/4905> - Resolve "'stty' is not recognized". See #4901 <https://github.com/commercialhaskell/stack/issues/4901> - Fix missing reconfigure check causing errors when a package gets used in multiple projects. See #5147 <https://github.com/commercialhaskell/stack/issues/5147> *Thanks to all our contributors for this release:* - Adam McCullough - Akshay Mankar - Brian Wignall - Cheah Jer Fei - Chris Done - David Baynard - davit nalchevanidze - Dino Morelli - Emanuel Borsboom - Felix Yan - flip111 - Flowers Safety - Fred Miller - Fyrbll - gdziadkiewicz - Good boy - Hypnoes.Liu - Iisus - Iustin Pop - ivanbakel - Javier Neira - Jeff Happily - jeffhappily - jneira - Justin Le - Kirill Zaborsky - Leif Metcalf - Leonhard Markert - Lokesh Mohanty - Matt Audesse - Michael Snoyman - Mihai Maruseac - NightRa - Phil Ruffwind - qinzhenlong - Robert Hensing - Sasha Bogicevic - Sibi - Sibi Prabakaran - Simon Michael - Steve Mao - Vlad Ki - Xia Li-yao

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