April 2018 - Haskell-Cafe

GHC Core / STG to supercombinators
by Csaba Hruska 09 Apr '18

09 Apr '18

Hello, I wonder what is the easiest way to compile Haskell to supercombinators (top level functions) using GHC as a library. Is it possible to use GHC simplifier to transform the parsed Haskell source to supercombinators? i.e. to do - eta expansion - closure conversion - lambda lifting Or should it be written from scratch? Is Core or STG suited better for this purpose? Thanks, Csaba

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stack and GHCJS
by Dennis Raddle 08 Apr '18

08 Apr '18

Now I'm trying to use stack to install and run GCHJS. Can someone help me figure out this error? I first created project 'proj01' using the ghcjs template via stack new proj01 ghcjs Then I typed stack build It ran for a while, then gave an error messages that starts "While building custom Setup.hs for package semigroupoids-5.0.0.4 using .... <very long description>, Process exited with code ExitFailure 1" But it didn't halt. Instead it started compiling Main.hs, and ended up saying "at least the following dependencies are missing: tagged >= 0.8.5 && <1 && ==0.8.1" I tried adding "tagged" to the extra-deps in the stack.yaml, but it didn't help. While I'm posting, is there a resource that explains background to this? I don't know much about "packages," "snapshots," "dependencies", means of specifying versions, cabal, or YAML files. Everything I find online seems to assume some background knowledge of these things. For instance the stack documentation talks about snapshots, but I can't find an explanation that I understand for what a "snapshot" actually is. Ditto for all these many concepts that go into configuring and building. D

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PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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Windows: how to perform cleanup actions when the console window closes
by Peter Dedecker 06 Apr '18

06 Apr '18

So I looked at this some more, and determined a few things: - Ctrl-C did not do anything in the original program because the main thread was blocked on a socket accept() call, which seemingly could not be interrupted for an asynchronous exception. - I fixed this by having the main thread initialize everything, then running the main body of code in an async and waiting on that. Apparently this wait is interruptible, so Ctrl-C works, bracket nicely cleans up, and the application exits. Good! - Then I added a console handler to capture 'close' events. Here is the code for the handler: f mainThreadID event = case event of ControlC -> putStrLn "HANDLER" >> throwTo mainThreadID UserInterrupt Break -> putStrLn "HANDLER" >> throwTo mainThreadID UserInterrupt Close -> putStrLn "HANDLER" >> throwTo mainThreadID ThreadKilled >> threadDelay maxBound _ -> pure () Where the 'threadDelay' call serves simply to give the main thread time to exit. This works fine for ControlC and Break, but fails for Close: the program exists immediately, and no cleanup is run. So I still haven't solved how to clean up when the window close button is hit. Any thoughts? Thanks Peter

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ANN: Yampa 0.11
by Ivan Perez 06 Apr '18

06 Apr '18

Dear all, I'm happy to announce version 0.11 of Yampa! Yampa is a mature and robust FRP implementation that has been used to create mobile, desktop, and web games and applications, including games that use devices like wiimotes and kinect, as well as free and commercial mobile games available on iTunes and Google Play. (For reference, see [1-4].) This version includes documentation for all modules and all definitions. We have closed a total of 29 issues. Thanks to @ptvirgo for contributing to this release. A limitation of Haddock prevents us from achieving 100% coverage. Help with that would be appreciated [5, 6]. This version has been tested against all major GHC versions, from 7.6 to 8.4. A minor change in the low-level interface has triggered a major version bump (0.11). I would expect all projects that use Yampa to work by, at most, adapting the version constraints in the cabal files. Everybody is welcome to collaborate. The github repo includes some simple issues especially suitable for beginners, and we are working on the next major version. We are also exploring different research ideas related to FRP, and actively look for research collaborators. Hope you all enjoy it. Happy Haskelling! Ivan [1] https://github.com/ivanperez-keera/Yampa [2] https://github.com/ivanperez-keera/haskanoid [3] https://github.com/ksaveljev/yampa-2048 [4] https://haskell.games [5] https://github.com/haskell/haddock/issues/123 [6] https://github.com/ivanperez-keera/Yampa/issues/72

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Windows: how to perform cleanup actions when the console window closes
by Peter Dedecker 06 Apr '18

06 Apr '18

Hi Joachim, > You need to intercept the various events that may cause a console > windows to be closed; this is done via the Windows API call > SetConsoleCtrlHandler . > https://stackoverflow.com/a/30843219/6944068 gives an overview very informative, thanks! > I don't know how to register a Haskell function as a Windows callback, > that will have to be answered by somebody else. With your tips, I came across https://hackage.haskell.org/package/base-4.11.0.0/docs/GHC-ConsoleHandler.h…. (The documentation doesn't work, but you can look at the source to see the gist of it). I think this may do what you describe. The documentation for this function says that > When the event is received, if the > 'Handler' value is @Catch f@, then a new thread will be spawned by > the system to execute @f e@, where @e@ is the 'ConsoleEvent' that > was received. I wonder if I can add a handler function that detects Control-C and Close events, and responds by throwing a userInterrupt exception to the main thread, and then sleeping forever. The main thread then sees an asynchronous exception, the bracket cleanup action fires (which is my goal here), main thread shuts down and the program stops. Will try to experiment with that later. Thanks again Peter

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Windows: how to perform cleanup actions when the console window closes
by Peter Dedecker 06 Apr '18

06 Apr '18

Hello all, I have a console (command-line) Haskell application that communicates with some hardware equipment. The equipment needs to be initialized when my program starts, and needs to be cleanly shut down when the program ends. This is the basic structure of the program: main = readAvailableEquipment >>= \descs -> withEquipment descs $ \availableEquipment -> <<< more initialization then an infinite loop as a sockets-based server, communicating with the hardware via availableEquipment >>> withEquipment is defined as withEquipment descs action = bracket (initializeEquipment descs) closeEquipment action Basically I want closeEquipment to be executed when the program shuts down. The program is stopped by the user closing the console window (hitting the upper right 'X' in the Windows title bar). I would expect that the 'closeEquipment' action is performed since it is in the cleanup part of the 'bracket', but my testing suggests that these actions never execute. Incidentally, stopping the program via Ctrl-C doesn't work. I have tested this with GHC 8.2.2 installed via Stack (LTS 11.3). This is on Windows 10. Summary question and TLDR: how do I reliably perform cleanup actions in a Windows console program that is closed by the user by clicking the 'X' symbol in the window title bar? Thanks! Peter

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Re: [Haskell-cafe] Proposal: Professionalizing GHC Development
by Doug McIlroy 05 Apr '18

05 Apr '18

Gershom, An emergent team-building solution (with or without an enabling blockchain protection ring) is all very well, but is it monadic? Doug

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DataHaskell User Survey
by Marco Zocca 05 Apr '18

05 Apr '18

Dear all, the community around datahaskell.org has been slowly but steadily growing since its inception in 2016, and with it the diversity of its members. Its mission is to provide a home for data science and numerical computation students and practitioners who would like to use Haskell in their work and explorations. In order to best represent and serve the community, we'd like to know it in the first place, which is why we have prepared this little questionnaire (10 questions, which should take around 5 minutes to complete): https://www.surveymonkey.com/r/3FBBJWR No personal data are required and the results will only be published in aggregate form at a later stage. The raw data will only be stored and used for producing the aggregate report, after which it will be deleted. Thank you in advance for your time; Best, Marco Zocca github.com/ocramz hackage.haskell.org/user/ocramz

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[Haskell, FP] Anduril Industries is Hiring
by Travis Whitaker 05 Apr '18

05 Apr '18

Anduril Industries (https://www.anduril.com) is hiring. TL;DR: Come write Haskell, Rust, and Nix (and some C++ when necessary) to make autonomous robots and drones go! We're a team of software and hardware engineers from various backgrounds (game development, computer graphics, financial technology, government intelligence, biotechnology) working to improve the state of defense technology. Our strategy involves focusing on product development instead of traditional governmental processes. By funding product development ourselves instead of relying on government funds, we're able to create more focused products faster and with significantly fewer resources. By leveraging hardware and techniques that have only recently become feasible to deploy at scale (e.g. GPGPU computing), we can significantly advance the state of the defense technology market. We're searching for generally competent, mathematically inclined software engineers, and we're especially interested in those with experience in computer vision (first principles techniques and machine learning), sensor fusion, detection and tracking, and statistical parameter estimation. Our team is increasingly applying functional programming and related technologies; we run Haskell and Rust code in production and use Nix to achieve reproducible build environments and keep deployment, CI, and cross-compilation sane. If you like functional programming, interfacing with hardware, and solving problems in detection, tracking, and autonomous vehicle control (land and air), drop me a line at travis(a)anduril.com

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