- Haskell-Cafe - Haskell.org

Efficiency of passing function arguments
by KS 04 Apr '20

04 Apr '20

Hello everyone, I was coming from a C++ background and I always wondered if there is a distinction between pass-by-value and pass-by-reference in Haskell (or pass-by-thunk? since an argument might be partially evaluated due to laziness in Haskell). Is there a way to guarantee that an function argument is passed in an efficient manner? For example, if we have a data type data Document = Document { title :: Text , author :: Name , content :: Text -- and potentially many more } doc = Document {..} -- omitted and we want to pass doc to a function, we should ideally hope that doc is not getting duplicated when passing it as an argument. Another example might be the Reader monad, where we are passing possibly the same environment to multiple functions. In C++, we could either pass by a pointer or a reference, but I'm not sure what the best practices are in Haskell. The most relevant construct in Haskell seems to be IORef, though I'm not sure if this is the correct way to use it. For exmaple, if I want to pre-load a template document on disk as Text or ByteString into the memory when the program starts, should I put it in a IORef and pass it into (more than one) functions or should I pass it into the function directly? Should I worry about this at all? This question might be related to the implementation of the ghc compiler. It would be better if anyone could shed some lights on the implementation details. Thank you, Kram

5 5

How to safely and fast write repa array to a file?
by cyberfined 04 Apr '20

04 Apr '20

Hello, all. I decide to write parallel ray tracer on haskell with repa. Now, to save repa array to file I use dirty trick casting repa array ptr to bytestring with fromForeignPtr and then writing it to file with hPut. It looks something like that: import qualified Data.Array.Repa as R import qualified Data.Array.Repa.Repr.ForeignPtr as RF import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as BC import qualified Data.ByteString.Internal as BI type Image = Array F DIM2 Pixel writeImage :: FilePath -> Image -> IO () writeImage path img = bracket (openFile path WriteMode) (hClose) $ \hdl -> B.hPut hdl header >> B.hPut hdl body where Z :. h :. w = R.extent img header = BC.pack $ "P6\n" ++ show w ++ ' ':show h ++ "\n255\n" body = BI.fromForeignPtr(castForeignPtr $ RF.toForeignPtr img) 0 (w*h*3) My question is: how to write repa array to file safely and fast?

3 2

ML Family Workshop 2020: Call for presentations
by Leo White 04 Apr '20

04 Apr '20

We are happy to invite submissions to the ML Family Workshop 2020, to be held during the ICFP conference week on Thursday, August 27th. The ML family workshop warmly welcomes submission touching on the programming languages traditionally seen as part of the "ML family" (Standard ML, OCaml, F#, CakeML, SML#, Manticore, MetaOCaml, etc.). The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of the members of the ML family. We also encourage presentations from related languages (such as Haskell, Scala, Rust, Nemerle, Links, Koka, F*, Eff, ATS, etc), to exchange experience of further developing ML ideas. Currently, the workshop is still scheduled to go ahead as planned in Jersey City, however it is likely that the ML workshop will end up being a virtual workshop this year. Either way provisions will be made to allow speakers to present their work remotely. See our detailed CFP online on the ICFP website: https://icfp20.sigplan.org/home/mlfamilyworkshop-2020 Important dates --------------- - Friday 15th May (any time zone): Abstract submission deadline - Friday 26th June: Author notification - Thursday 27th August: ML Family Workshop Program committee ----------------- - Youyou Cong (Tokyo Institute of Technology) - Gowtham Kaki (Purdue University) - Neel Krishnaswami (University of Cambridge) - Daan Leijen (Microsoft Research) - Koko Muroya (Kyoto University) - Atsushi Ohori (Tohoku University) - Jonathan Protzenko (Microsoft Research) - Gabriel Radanne (INRIA) - Claudio Russo (Dfinity) - Leo White (Jane Street) (Chair) - Jeremy Yallop (University of Cambridge) Submission details ------------------ See the online CFP for the details on the expected submission format. Submissions must be uploaded to the workshop submission website https://ml2020.hotcrp.com/ before the submission deadline.

1 0

ANN: stack-2.3.0.1 release candidate
by Emanuel Borsboom 02 Apr '20

02 Apr '20

### First release candidate for v2.3 **Changes since v2.1.3** Release notes: 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 and Cabal 3.0. * Allow relative paths in `--setup-info-yaml` and tool paths [#3394](https://github.com/commercialhaskell/stack/issues/3394) 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)

1 1

[ANN] acme-dont-1.2 - a "don't" construct
by Jack Kelly 02 Apr '20

02 Apr '20

Hello all, I have taken over the maintainership of acme-dont, and am pleased to announce a new acme-dont release on Hackage[1]. acme-dont provides a `don't` function, which does not execute its argument. For example, the following code will not print anything: import Acme.Dont (don't) quiet :: IO () quiet = don't $ putStrLn "You'll never see this on stdout" New in this release: * `don't` has been generalised from `Monad` to `Applicative`. If you have been waiting since GHC 8.0.1's release in 2016[2] to use `don't` with `-XApplicativeDo`, wait no longer. * A test suite, to ensure that `don't` does not do anything. Previous versions could have been defined as `don't = void` for all we knew. Also, the acme-dont community now has a new home on sourcehut. On there, you can find the following invaluable resources: * A source repository[3], to track further developement and new ways of not doing things, with a commit history reconstructed from the release tarballs uploaded to Hackage. * A mailing list[4] for the acme-dont community to discuss the best ways of not doing things. * A bug tracker[5] to handle reports, in case someone observes acme-dont doing something. * Continuous integration[6], to ensure that acme-dont continues to not do things across multiple versions of GHC. I would like to thank Gracjan Polak for uploading the initial releases of `acme-dont`, and for maintaining the package these past eleven years. I do not intend to betray the responsibility he has assigned me in handing over this critical package. Best, -- Jack [1]: https://hackage.haskell.org/package/acme-dont-1.2 [2]: https://www.haskell.org/ghc/download_ghc_8_0_1 [3]: https://git.sr.ht/~jack/acme-dont [4]: https://lists.sr.ht/~jack/acme-dont-discuss [5]: https://todo.sr.ht/~jack/acme-dont [6]: https://builds.sr.ht/~jack/acme-dont

5 6

Using State monad with lens
by Michael Hull 02 Apr '20

02 Apr '20

Hi, I use the State monad in a lot of places, but when I use it with the lens library, I find myself writing code like: myFunc :: StateT MyState ([ResultType]) myFunc = do st <- get case st ^. some_property of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye" I want to do something based on a 'state-variable', but I have to write 'get' -- is there a pattern that allows me to avoid the intermediate 'st' variable? I want to write something like: myFunc = do case (get ^. some_property) of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye" but that won't compile. Best wishes, Mike

6 6

Re: [Haskell-cafe] [ANN] acme-dont-1.2 - a "don't" construct
by Jack Kelly 01 Apr '20

01 Apr '20

Dear Henk-Jan, You can view the preprint yourself with the following commands: $ true > dont.txt $ less dont.txt You'll then start seeing it everywhere you look, and I'm not sure that wider distribution is possible. -- Jack "Henk-Jan van Tuyl" <b.vantuyl1(a)upcmail.nl> writes: > Thanks, > > The `don't` function is a big step forward in computering; it will > surely prevent many bugs from occurring, with little effort to > software developers. > > Is it published in a scientific magazine yet? > > Regards, > Henk-Jan van Tuyl > > > On Wed, 01 Apr 2020 10:38:08 +0200, Jack Kelly <jack(a)jackkelly.name> wrote: > >> Hello all, >> >> I have taken over the maintainership of acme-dont, and am pleased to >> announce a new acme-dont release on Hackage[1]. >> >> >> acme-dont provides a `don't` function, which does not execute its >> argument. For example, the following code will not print anything: >> >> import Acme.Dont (don't) >> >> quiet :: IO () >> quiet = don't $ putStrLn "You'll never see this on stdout" >> >> >> New in this release: >> >> * `don't` has been generalised from `Monad` to `Applicative`. If you >> have been waiting since GHC 8.0.1's release in 2016[2] to use >> `don't` with >> `-XApplicativeDo`, wait no longer. >> >> * A test suite, to ensure that `don't` does not do anything. Previous >> versions could have been defined as `don't = void` for all we knew. >> >> >> Also, the acme-dont community now has a new home on sourcehut. On there, >> you can find the following invaluable resources: >> >> * A source repository[3], to track further developement and new ways of >> not doing things, with a commit history reconstructed from the release >> tarballs uploaded to Hackage. >> >> * A mailing list[4] for the acme-dont community to discuss the best ways >> of not doing things. >> >> * A bug tracker[5] to handle reports, in case someone observes acme-dont >> doing something. >> >> * Continuous integration[6], to ensure that acme-dont continues to not >> do things across multiple versions of GHC. >> >> >> I would like to thank Gracjan Polak for uploading the initial releases >> of `acme-dont`, and for maintaining the package these past eleven >> years. I do not intend to betray the responsibility he has assigned me >> in handing over this critical package. >> >> Best, >> >> -- Jack >> >> [1]: https://hackage.haskell.org/package/acme-dont-1.2 >> [2]: https://www.haskell.org/ghc/download_ghc_8_0_1 >> [3]: https://git.sr.ht/~jack/acme-dont >> [4]: https://lists.sr.ht/~jack/acme-dont-discuss >> [5]: https://todo.sr.ht/~jack/acme-dont >> [6]: https://builds.sr.ht/~jack/acme-dont >> _______________________________________________ >> 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.

1 0

mmap Failure in: 24:Counters should be zero
by YueCompl 01 Apr '20

01 Apr '20

Hi mmap maintainers, Today I found the last test case out of 25 failed like: ```bash $ cabal v2-run -f mmaptest mmaptest Resolving dependencies... Build profile: -w ghc-8.6.5 -O1 In order, the following will be built (use -v for more details): - mmap-0.5.9 (exe:mmaptest) (configuration changed) Configuring mmap-0.5.9... Preprocessing executable 'mmaptest' for mmap-0.5.9.. Building executable 'mmaptest' for mmap-0.5.9.. <no location info>: warning: [-Wmissing-home-modules] These modules are needed for compilation but not listed in your .cabal file's other-modules: System.IO.MMap <no location info>: warning: [-Wmissing-home-modules] These modules are needed for compilation but not listed in your .cabal file's other-modules: System.IO.MMap ### Failure in: 24:Counters should be zero tests/mmaptest.hs:256 expected: 0 but got: 4 Cases: 25 Tried: 25 Errors: 0 Failures: 1 ``` And the test code is: ```haskell test_counters_zero = do System.Mem.performGC threadDelay 1000 counters <- c_system_io_counters return (counters @?= 0) ``` I see it is to make sure no resource held open after 1 millisecond after garbage collection is triggered. I tried extending the threadDelay to 10 second and still got 4 thus failing the test, maybe newer machinery of GC with GHC 8.6 should be leveraged to prevent such leakage? I CC haskell-cafe list to seek for broader community help, maybe some one can confirm that I can just live with this specific failure without worries? Thanks & best regards, Compl

2 1

How to make a Hashable instance for (Finite n)?
by David Banas 31 Mar '20

31 Mar '20

What is the most expedient way to get around this error? Could not deduce (Hashable (Finite n)) Thanks, -db

1 0

Research Assistant/Associate Position (Postdoc) at Imperial College London
by David Castro 30 Mar '20

30 Mar '20

Hello, We are looking for candidates for this position at Imperial College London. The official text of the advertisement follows: Department of Computing, Imperial College London Research Assistant/Associate Position (Full Time) 35,477 GBP to 47,579 GBP per annum Reference: ENG01270 Fixed-term: 2 years (with a possible 24 month extension) Starting date: as soon as possible (no later than 1st October 2020) Closing Date: 26th April 2020 The Research Assistant will work under the EPSRC Established Career Fellowship Project, POST: Protocols, Observabilities and Session Types. Imperial College London provides a flexible arrangement to be able to start and work remotely until we re-open the campus (this depends on your nationality and current living address). Please contact with Nobuko Yoshida (n.yoshida(a)imperial.ac.uk) Imperial College London if you would like to apply to the position to have informal discussions. The application deadline is also extendable. ------------------------------------------------------------ The project has particular emphasis on putting theory into practice, by embedding session types in a range of programming languages and applying them to case studies; or developing the links between session types and other areas of theoretical computer science. The research programme includes collaboration with several companies and organisations. Candidates for the post-doc position will need to have expertise in either: 1. programming language design and implementation; or 2. formal semantics, type theory and concurrency theory Different positions will be suitable for different points on the theory/practice spectrum. We are especially interested in candidates with a combination of theoretical and practical skills. For more details, see http://mrg.doc.ic.ac.uk. The focus of Imperial College London Group is theories and applications of (Multiparty) Session Types (JACM,POPL'08) which include: -- Go (POPL'19,ICSE'18,POPL'17,CC'16), Scala (PLDI'19,ECOOP'17,ECOOP'16), F# (CC'18), Erlang (CC'17), Haskell (CC'20,POPL'16), OCaml (SOC), Java (FASE'16,FASE'17), MPI-C (FPL'16,OOPSLA'15,CC'15) and Python (FOAC,LMCS,FMSD); -- session types theories (ESOP'20,POPL'19,ESOP'19,ESOP'16,CONCUR'15), automata theories (CAV'19,CONCUR'19,FoSSaCs'17,TACAS'16,CONCUR'15,POPL'15), game semantics (POPL'19,FoSSaCs'19), Implicit Complexity (LICS'18) and linear logic (ESOP'18,FoSSaCs'18,CONCUR'15); or -- mechanisation of session types meta-theory (Coq, Isabelle, Agda, etc) (TACAS 20) -- other applications such as blockchains (FSE'19) and robotics (ECOOP'19) The contact person is Professor Nobuko Yoshida, Imperial College London (n.yoshida(a)imperial.ac.uk) The candidate is welcome to contact her. Details: https://www.imperial.ac.uk/jobs/description/ENG01270/research-assistant-res…

1 0