- Haskell-Cafe - Haskell.org

BlockedIndefinitelyOnMVar
by Massimo Zaniboni 16 Jul '18

16 Jul '18

Hi, I have a code like this ``` producer :: Int -> MVar Int -> IO () producer c mvar = case c > 10 of True -> throwIO HighNumberException False -> do putMVar mvar (c + 1) producer (c + 1) mvar consumer :: MVar Int -> IO () consumer mvar = do _ <- takeMVar mvar consumer mvar coordinator :: MVar Int -> IO () coordinator mvar = do withAsync (producer 0 mvar) $ \p -> withAsync (consumer mvar) $ \c -> do wait c wait p main :: IO () main = do mvar <- newEmptyMVar withAsync (coordinator mvar) (wait) putStrLn "done" ``` The producer send the informative exception HighNumberException, but the coordinator job receives a generic BlockedIndefinitelyOnMVar, because the producer stop working. How can I discard the generic exception, and re-throw/catch only HighNumberException? In case the complete source code is on https://github.com/massimo-zaniboni/blocked-on-mvar Thanks in any case, Massimo

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Package takeover: bencoding
by Sergey Vinokurov 16 Jul '18

16 Jul '18

I'd like to take over maintainership of the bencoding package [1]. The package was not updated to work with ghc 8.0 and will likely require more fixes to work with ghc 8.4 and some work to make a Stackage release. There's a PR open for GHC 8.0, but it's still open for a year and a half [2]. I've tried to ask the author in the PR thread to make me a maintainer in January but it still hasn't happened. Cheers, Sergey [1]: http://hackage.haskell.org/package/bencoding [2]: https://github.com/pxqr/bencoding/pull/7

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[ANNOUNCE] GHC 8.6.1-alpha2 available
by Ben Gamari 16 Jul '18

16 Jul '18

The GHC development team is pleased to announce the second alpha release leading up to GHC 8.6.1. The usual release artifacts are available from https://downloads.haskell.org/~ghc/8.6.1-alpha2 This alpha fixes many of the bugs reported in the first alpha and brings many of the core libraries to their (hopefully) final release versions. Unfortunately, the documentation troubles affecting the first alpha haven't quite been resolved in this release. Consequently you will again find there is no documentation uploaded. However, let us know if anything else is amiss. Thanks for your help! Cheers, - Ben

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Re: [Haskell-cafe] What is your favourite Haskell "aha" moment?
by Vilem-Benjamin Liepelt 15 Jul '18

15 Jul '18

Dear MarLinn, > Of course you need all that data because there's a ghc in that data, but > it should say so on the tin. So please never send a script with this > line to a "newbie" without big warnings so they can make an informed > decision. That is a very good point. I just happen to use some particular LTS because I have it cached, so it works instantly for all my scripts. Thank you for reminding me that I shouldn't carelessly just pass this on for others to use without mentioning the caveats you pointed to. > Also, while I do like the scripting possibilities, interact is possibly > the worst way to introduce a newcomer to it because you can't really > test it in ghci. Your concern about testability in GHCi is valid but minor; you can just make the string transformation function a top-level declaration like so: ``` main = interact sumCLI sumCLI :: String -> String sumCLI = show @Double . sum . mapMaybe readMaybe . words ``` IMO `interact` is the easiest way for newcomers to write a script that they can actually use, hence why I opted for this instead of `print . sumCLi =<< getContents`. Lastly, SPJ asked for our "aha" moments—this was mine. He always does a good job at serving information in a way that is digestible for the audience, so you need not worry that any lambs will be led astray. Best, Vilem > On 2018-07-14, at 13:00, <haskell-cafe-request(a)haskell.org> <haskell-cafe-request(a)haskell.org> wrote: > > [...] > > Message: 4 > Date: Sat, 14 Jul 2018 13:33:31 +0200 > From: MarLinn <monkleyon(a)gmail.com> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] What is your favourite Haskell "aha" > moment? > Message-ID: <c7a00886-f32b-c4ae-617d-450394278b58(a)gmail.com> > Content-Type: text/plain; charset="utf-8"; Format="flowed" > > Warning: I'll be overly harsh in this mail because I assume some people > that are new to Haskell might browse this thread now and in the future. > I don't mean my comments to be considered personal. On the contrary, I > think some of us can learn for their own teaching endeavours. > >> #!/usr/bin/env stack >> -- stack script --resolver lts-10.1 --install-ghc > > Imagine a "newbie" running this script just to see what it does. After > 15 minutes they'll be disappointed why your small demo took so long to > build. Only /then/ do they realize that, what is it today, about half a > gigabyte maybe, of their monthly "high speed" bandwidth and of their > precious space on the cheap but decade-old hard drive is gone. > > Of course you need all that data because there's a ghc in that data, but > it should say so on the tin. So please never send a script with this > line to a "newbie" without big warnings so they can make an informed > decision. Maybe they would want to install ghc at their uni to save on > that precious bandwidth at home or something similar. > > In fact don't even send scripts with this line to seasoned haskellers. > If your code is compatible with the ghc 8.2.1 on my system, why should > your script assume I want stack to install 8.2.2 right now? > > Of course this behaviour is ingrained in the DNA of stack – after all it > values convenience for the professional user more than convenience for > the unprivileged newcomer – but AFAIU it /can/ be persuaded to be more > inclusive. So especially here, we should maybe try to do that. > >> interact >> ( show @Double >> . sum >> . mapMaybe readMaybe >> . words >> ) > > Also, while I do like the scripting possibilities, interact is possibly > the worst way to introduce a newcomer to it because you can't really > test it in ghci. Apart from all the other shortcomings it has. It is > nice in that it hides IO, but… > > Again, not meant as an attack, but since we're talking about giving > newcomers a good experience, I thought these two points important. > > Cheers, > MarLinn >

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[ANNOUNCE] GHC 8.4.3 released
by Ben Gamari 14 Jul '18

14 Jul '18

Hello everyone, The GHC team is pleased to announce the availability of GHC 8.4.3. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.4.3 This release includes a few bug fixes including: * A code generation bug resulting in crashing of some programs using UnboxedSums has been fixed (#15038). * #14381, where Cabal and GHC would disagree about abi-depends, resulting in build failures, has been worked around. Note that the work-around patch has already been shipped by several distributions in previous releases, so this change may not be visible to you. * By popular demand, GHC now logs a message when it reads a package environment file, hopefully eliminating some of the confusion wrought by this feature. * GHC now emits assembler agreeable to newer versions of Gnu binutils, fixing #15068. * SmallArray#s can now be compacted into a compact region Thanks to everyone who has contributed to developing, documenting, and testing this release! As always, let us know if you encounter trouble. How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standardized lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs: GHC home page https://www.haskell.org/ghc/ GHC developers' home page https://ghc.haskell.org/trac/ghc/ Haskell home page https://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: https://ghc.haskell.org/trac/ghc/wiki/TeamGHC Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: https://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site: https://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at https://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Many GHC developers hang out on #haskell on IRC: https://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: https://www.haskell.org/ghc/reportabug

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Package takeover: bencoding
by Sergey Vinokurov 14 Jul '18

14 Jul '18

I'd like to take over maintainership of the bencoding package [1]. The package was not updated to work with ghc 8.0 and will likely require more fixes to work with ghc 8.4 and some work to make a Stackage release. There's a PR open for GHC 8.0, but it's still open for a year and a half [2]. I've tried to ask the author in the PR thread to make me a maintainer in January but it still hasn't happened. Cheers, Sergey [1]: http://hackage.haskell.org/package/bencoding [2]: https://github.com/pxqr/bencoding/pull/7

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Request for Nominations to the GHC Steering Committee
by Joachim Breitner 14 Jul '18

14 Jul '18

Dear Haskell community, the GHC Steering committee is seeking nominations for a new member. The committee scrutinizes, nitpicks, improves, weights and eventually accepts or rejects proposals that extend or change the language supported by GHC and other (public-facing) aspects of GHC. Our processes are described at https://github.com/ghc-proposals/ghc-proposals which is also the GitHub repository where proposals are proposed. We are looking for a member who has the ability * to understand such language extension proposals, * to find holes and missing corner cases in the specifications, * foresee the interaction with other language features and specifications, * comment constructively and improve the proposals, * judge the cost/benefit ratio and * finally come to a justifiable conclusion. We look for committee members who have some of these properties: * have substantial experience in writing Haskell applications or libraries, which they can use to inform judgements about the utility or otherwise of proposed features, * have made active contributions to the Haskell community, for some time, * have expertise in language design and implementation, in either Haskell or related languages, which they can share with us. The committee’s work requires a small, but non-trivial amount of time, especially when you are assigned a proposal for shepherding. We estimate the workload to be around 2 hours per week, and our process works best if members usually respond to technical emails within 1-2 weeks (within days is even better). Please keep that in mind if your email inbox is already overflowing. The GHC developers themselves are already well represented already. We seek Haskell _users_ more than GHC hackers. There is no shortage of people who are very eager to get fancy new features into the language, both in the committee and the wider community. But each new feature imposes a cost, to implement, to learn, (particularly) through its unexpected interaction with other features. We need to strike a balance, one that encourages innovation (as GHC always has) while still making Haskell attractive for real-world production use and for teaching. We therefore explicitly invite “conservative” members of the community to join the committee. To make a nomination, please send an email to me (as the committee secretary) at mail(a)joachim-breitner.de until July 23th. I will distribute the nominations among the committee, and we will keep the nominations and our deliberations private. We explicitly encourage self-nominations. You can nominate others, but please obtain their explicit consent to do so. (We don’t want to choose someone who turns out to be unable to serve.) On behalf of the committee, Joachim Breitner -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

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Re: [Haskell-cafe] What is your favourite Haskell "aha" moment?
by Vilem-Benjamin Liepelt 14 Jul '18

14 Jul '18

Dear Simon, I was always impressed by Haskell, but my "aha" moment was when I realised how great it is as a scripting language. How cool is the fact that you can glue together a bunch of small programs and make a bigger program out of it without toil? Haskell helps you get mundane tasks done while being a pleasure to write (in relative terms)! There are much better examples online—I love Gabriel Gonzalez' turtle library—but I find the following compelling enough, and anybody can understand more or less what is going on with some explanation: #!/usr/bin/env stack -- stack script --resolver lts-10.1 --install-ghc {-# LANGUAGE TypeApplications #-} import Data.Maybe (mapMaybe) import Text.Read (readMaybe) main :: IO () main = interact ( show @Double . sum . mapMaybe readMaybe . words ) -- Adapted from a script by Taylor Fausak: https://gist.github.com/tfausak/fcd11dfcec616622033dfb64eb2378e1 Good luck with the presentation! Best, Vilem

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Taking over unicode-show
by Yuji Yamamoto 14 Jul '18

14 Jul '18

Hello Haskellers, I'm trying to take over the unicode-show <http://hackage.haskell.org/package/unicode-show> package, whose author Takayuki Muranushi died about a year ago [1]. We Japanese (and perhaps non-alphabet writers) are often annoyed by the result of the show function, which converts various characters into non-human-readable strings. The unicode-show package solves the problem, and I'm going to introduce the package in an educational content I'm creating to help Japanese Haskell newbies. Thanks to the simple structure, there is nothing to maintain (unfortunately except hard one). It works even on GHC 8.4. [1]: (In Japanese) http://www.asj.or.jp/tennet/archives/msg07622.html -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep Google+: https://plus.google.com/u/0/+YujiYamamoto_igrep

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Lifetime control of webserver
by ☂Josh Chia (謝任中) 13 Jul '18

13 Jul '18

Hi, My application is not a webserver but needs to run a webserver for part but not all of its lifetime. Particularly, there's a part that does some complex computation and I want a webserver to expose its internal state/progress through HTTP. It seems that the typical and expected way to use a webserver is to designate the entire app itself as a webserver, so that functions like the following are basically infinite loops that serve HTTP: Web.Scotty.scotty: http://hackage.haskell.org/package/scotty-0.11.2/docs/Web-Scotty.html#v:sco… Network.Wai.Handler.Warp.run: http://hackage.haskell.org/package/warp-3.2.22/docs/Network-Wai-Handler-War… Can I run a webserver with explicit control of its lifetime? Particularly, I want to be able to kill it without killing the app or explicitly serve one request at a time. I'm not restricted to scotty or warp. Josh

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