April 2020 - Haskell-Cafe

[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

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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

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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.

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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

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