February 2014 - Haskell-Cafe

HaskellWiki account
by Andrey Chudnov 17 Feb '14

17 Feb '14

Hi, Sorry for spamming the list with such trivialities. Long story short, I'd like to get an account for HaskellWiki. Tried e-mailing "nominolo" (at the email service from Google)", but to no avail. Could anyone, please, help me with that? Thanks! /Andrey

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Re: [Haskell-cafe] Yi documentation
by Mateusz Kowalczyk 17 Feb '14

17 Feb '14

On 17/02/14 23:54, David Thomas wrote: > Does this mean generally I can't run an external command with output into a > buffer (or buffer region)? > > > On Mon, Feb 17, 2014 at 2:50 PM, Mateusz Kowalczyk > <fuuzetsu(a)fuuzetsu.co.uk>wrote: > >> On 17/02/14 22:25, David Thomas wrote: >>> Vim doesn't do concurrency either, so that wouldn't be a regression. >>> >>> >> >> Yes, but it does single thread scheduling to compensate (as does emacs) >> while Yi doesn't which means implementing quickfix (and other modes) >> isn't too viable without halting the whole editor. >> >> -- >> Mateusz K. >> > You can, it will just hang the editor while it happens. An easy test is having Yi call something like ‘sleep 5; echo foo’. It should be easy to see why it's a big problem at the moment. Personally I stumbled upon it when trying to implement myself an IRC client inside of Yi but there are many use cases. Anyway, I feel that we're getting a bit off-topic here. If you're interested in helping out with implementing the concurrency stuff, we'd love to have you: pop into #yi, look around the issue tracker and maybe even post to the Yi mailing list. -- Mateusz K.

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ANN: LambdaMC: demo Haskell Android app built with the ghc-android tool.
by Anthony Mulcahy 17 Feb '14

17 Feb '14

Announcing LambdaMC: a demo Haskell Android app to calculate option prices by Monte Carlo simulation built with the ghc-android tool. Based on the example app at https://github.com/neurocyte/android-haskell-activity Functionality is limited, but it may be a useful reference for anybody developing a Haskell Android app: - Android Pause / Resume functionality. - Screen Rotation support. - Background worker thread updates the UI when the calculation is complete. - Interaction between various Android UI elements and Haskell code. Google Play listing: https://play.google.com/store/apps/details?id=org.dragongate_technologies Files LambdaMC.hs : Haskell app code DgAndr.hs : Haskell wrapper for various Android library functions DgMath.hs : Haskell option pricing code LambdaMC.java : Minimal Java code to support the main Activity LicenseActivity.java : Java Activity to display license information (need to move more code to Haskell-land) Prerequisites - An Android cross-compiler built with the ghc-android tool at https://github.com/neurocyte/ghc-android - The foreign-jni package from https://github.com/neurocyte/foreign-jni - The Haskell packages base, deepseq, transformers, mtl, text, time Building ./build_workaround.sh ( to work around the -shared and -r issue described at https://github.com/neurocyte/android-haskell-activity/issues/3 ) Install sudo adb install -r bin/LambdaMC-debug.apk best regards, Anthony Mulcahy

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Re: [Haskell-cafe] Protecting against main thread exit
by Roman Cheplyaka 17 Feb '14

17 Feb '14

* Albert Y. C. Lai <trebla(a)vex.net> [2014-02-17 14:10:35-0500] > On 14-02-15 04:53 PM, Roman Cheplyaka wrote: > >Of course, all guarantees are off when dealing with SIGKILL or power > >reset, but there's one other annoying exception (no pun intended) — when > >the main thread terminates, all other threads are silently shut down, > >without any chance to clean up. > > > >Why is it done that way? Why not kill them with an async exception > >instead? That would be a more uniform behavior. > > > >Are there any good ways to protect again the main thread exit? > > > >(Note that I'm talking about a library, so I can't have direct > >control over what's going on in the main thread.) > > I guess it is a rabbit hole to have GHC RTS courteously send > exceptions to all threads, and still expect certain and timely death. > > Nothing says that a thread voluntarily dies after receiving that > exception, much less when it dies. Sure. But is it a responsibility of the RTS to ensure timely death? My main use case is when the user interrupts the program with Ctrl-C. The main thread can already ignore it, and the RTS is fine with it. I'm just proposing that all threads get a chance to clean up. If timely shutdown is important, there are plenty of external tools with customizable logic that can take care of that. > It seems to me that the cleanest way is still to have the author of > main decide and implement whichever exit strategy is correct for > his/her main and for the library he/she uses. And there are enough > tools to do it tidily, too: main can get a "finally" clause, just > like everybody else. I wouldn't call that tidy. The action for which cleanup is needed can be buried deep in the call hierarchy. For example, let's say 'a' needs to cleanup after doing its job (but no-one needs to wait for a's result). 'b' spawns 'a' in a new thread, 'c' calls 'b', 'd' calls 'c' and so on. We cannot cleanup in the "finally" clause of 'b' even if 'b' is executed in the main thread, because it's okay for 'a' to run longer than 'b'. We cannot simply pass the cleanup action up the chain of calls as a return value (although even that would be rather invasive), because when SIGINT strikes — say, before 'b' returns, the cleanup handler won't be installed yet — it will only be on its way to 'main'. The simplest option seems to be to convert the chain of calls from 'b' up to 'main' to CPS (directly or by using a monad). I hope you agree that it is too much for such a simple thing as (more or less) robust cleanup. Also, this violates DRY: I have to install the handler both for the worker thread itself (in case it gets killed by an async exception), and for the main thread (in case it decides to exit for whatever reason). Moreover, I have to ensure that the cleanup doesn't happen twice because of that. Roman

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ANN: Implementations of TaPL
by Kyle Marek-Spartz 17 Feb '14

17 Feb '14

Hello, I’ve been working through Types and Programming Languages, implementing each language with a figure in the book. So far I’ve done Boolean, Arith, TypedBoolean, and TypedArith. Before I get too far, I’d like some feedback on my approach, project structure, and how idiomatic the code is. Here’s the GitHub repo: https://github.com/zeckalpha/TaPL Thanks in advance! I’m aware of the OCaML implementation, and a few previous Haskell implementations, but learning by doing is important, plus there were things I didn’t like about the other implementations. -- Kyle Marek-Spartz

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Problems using tasty-golden
by Jan Stolarek 17 Feb '14

17 Feb '14

Hi *, I've been trying to write a testsuite using tasty-golden and I've run into two technical problems. I'll start with some background. I'm working on singletons library that provides Template Haskell functionality. I would like to tests whether some particular TH splices compile at all and if they compile whether they produce expected output. My plan is to attempt compilation of a file with GHC. If compilation fails then the test fails. If it succeedes then output is compared with a golden file. I am directly calling goldenTest function from Test.Tasty.Golden.Internal. I wrote this function to produce the test output: compileWithGHC :: IO (Either BSL.ByteString BSL.ByteString) It runs GHC with some specified options and returns Left stderr (if compilation fails) or Right stdout (if it succeedes). This function is passed as third parameter to goldenTest (using liftIO). Then I have a comparing function: cmp :: Either BSL.ByteString BSL.ByteString -> Either BSL.ByteString BSL.ByteString -> IO (Maybe String) If it gets two Rights then it compares them. If the second param is left then the function returns Just stderr. I've run into following problems: 1. When I try to run my test I get: message threw an exception: MY_GOLDEN_FILE: hGetBufSome: illegal operation (handle is closed) I suspect that lazy IO is getting in the way but I have no idea why exactly things go wrong. 2. I had to hardcode the path to my golden file. Otherwise the file could not be found. I suspect the reason is that test are run somewhere in dist/ subdirectory whereas my golden files are located alongside my test files. What can I do to have the golden files located without giving specific subdirectories in the source code? A bonus question. I redefined vgReadFile from Test.Tasty.Golden.Internal to return Either: vgReadFileE :: FilePath -> ValueGetter r (Either BSL.ByteString BSL.ByteString) vgReadFileE path = (fmap Right . liftIO . BSL.hGetContents =<<) $ ValueGetter $ ContT $ \k -> bracket (openBinaryFile path ReadMode) hClose k The only modification is the "fmap Right .". How can I return Left if opening the file fails? Janek

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[Announcement] New release of SBV: SMT Based Verification, v3.0
by Levent Erkok 16 Feb '14

16 Feb '14

Home page: http://leventerkok.github.io/sbv/ ChangeLog: http://github.com/LeventErkok/sbv/blob/master/CHANGES.md I'm happy to announce a new release of SBV, v3.0. The most important addition is support for reasoning about IEEE-74 floating point numbers, i.e., programs using the Float and Double types. See the change-log for full details. Reasoning with floating-point is a tricky task, and SMT solvers are slowly starting to provide support for true machine arithmetic as prescribed by the IEEE-754 standard. In particular, the semantics of NaN and Infinity are both properly captured, along with proper rounding. Also supported is fused-multiply-add, and square-root functions at the logic level. Currently, only Z3 (the developer version release) has official support for this logic, and the implementation in SBV builds on top of Z3's implementation. It's likely that both Z3's and SBV might have some rough edges, so any bug reports are most welcome. There's also a new example file demonstrating the floating-point arithmetic: Formally establishing that floating point addition is not associative (finding a concrete counter-example), multiplicative inverses do not necessarily exist, etc. See: http://hackage.haskell.org/package/sbv-3.0/docs/Data-SBV-Examples-Misc-Floa… Happy theorem proving! -Levent.

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create a record with all record accessors
by Henning Thielemann 16 Feb '14

16 Feb '14

I have records of type (f a) where all fields are of type "a". I want to create records that contain all field getters as elements. I can do this generically: getters :: (Traversable f, Applicative f) => f (f a -> a) getters = fmap (\n x -> Fold.toList x !! n) $ MS.evalState (Trav.sequenceA (pure (MS.state $ \n -> (n, succ n)))) 0 Applicative is only needed for "pure". That is, "Pointed f" would be enough. However, (!!) is not total and not very efficient. Is there already a type class in a package with "getters" as method or something I can build an efficient "getters" from? I suspect the answer lies in one of the "lens" modules. :-)

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Announce: Software Engineering for Systems Developed in Functional Languages
by Simon Thompson 16 Feb '14

16 Feb '14

The 32nd CREST Open Workshop: Software Engineering for Systems Developed in Functional Languages Date: 24th - 25th March 2014 Venue: University College London, UK Through the kind support of the EPSRC in the form of a platform grant, we have funding to support the workshops, so registration is free (but there is a charge for registering and then *not* attending, since places are limited). Registration includes lunch on both days and light refreshments. Overview: Functional programming has become a mature practical tool for software development. Large complex functional programs require many of the software engineering techniques developed for other languages and paradigms, such as testing, slicing and refactoring. Some of these techniques, originally designed for imperative languages, such as amorphous slicing and refactoring may prove highly amendable to functional styles of programming. This workshop will bring together software engineers and functional programmers to concentrate on aspects of software engineering that can be extended, adapted and augmented for application to functional programs. Confirmed Speakers David Binkley, Computer Science Department, Loyola University Maryland, USA Sebastian Danicic, Department of Computing - Goldsmiths College London, UK Kevin Hammond, School of Computer Science, at the University of St. Andrews, UK Torben Hoffmann, Erlang Solutions, Denmark Yue Jia, CREST Centre, SSE Group, Department of Computer Science, UCL, UK Raghavan Komondoor, Indian Institute of Science, India Alan Mycroft, Computer Laboratory, University of Cambridge, UK Jerry Swan, Computing Science and Mathematics, University of Stirling, UK Don Syme, Microsoft Research, Cambridge, UK Simon Thompson, School of Computing, University of Kent, UK Dimitrios Vytiniotis, MSRC PPT group, Microsoft Research, Cambridge, UK More details and registration at: http://crest.cs.ucl.ac.uk/cow/32/ Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Wheb 0.1 released -- Graceful shutdown, Mongo plugin, blaze example and more.
by Kyle Hanson 16 Feb '14

16 Feb '14

Hello all, This is a small release, but I thought I would tell you about it. I have added some new stuff to my Haskell server framework, Wheb. For those of you (most) who don't know about Wheb, here is Wheb's description from github: > Wheb's a framework for building robust, high-concurrency web applications > simply and effectively. Its primary goal is to extend the functionality of > the base WAI library as well as provide an easy entry point into Haskell > web servers. The only prerequisite is "Learn you a Haskell" or another > introductory Haskell course. I fleshed out some more examples, rewrote some of the documentation and made some feature improvements to make it easier to build plugins. The github repo's readme now contains a small tutorial. Probably the biggest change this release is the addCleanupHook which you can use to hook in and run IO actions after server shutdown to cleanup any open resources. Wheb handles the SIGINT signal and waits for all requests to complete before shutting down. Here is the complete changelog: - Ability to read settings from a file - Add graceful shutdown and cleanup hooks. - Ability to monitor number of active connections. - Remove Data.Default - Add builder helper function - URL encoding You can find Wheb on hackage or github: - http://hackage.haskell.org/package/Wheb - https://github.com/hansonkd/Wheb-Framework In addition to these changes, I have released Wheb-Mongo which provides Auth, Sessions and document manipulation/querying for mongoDB. - http://hackage.haskell.org/package/wheb-mongo If you have any recommendations please feel free to let me know! -- Kyle Hanson

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