March 2018 - Haskell-Cafe

Fwd: Safe TCP accept loop
by Sumit Raja 30 May '18

30 May '18

Hi, I sent this to beginners but got no responses so I'm hoping someone here can help. I’ve looked at https://wiki.haskell.org/Concurrency_demos/Graceful_exit and used(2) Using throwTo without the use of block and unblock. It runs on 8.2.1 with my limited testing. I can’t find out much about what the > 7.x GHC replacement for block/unblock is other than mask. What does the unblock do in acceptConnections'? Does this method of handling an accept loop still need masking of async exceptions ? If so where does this need to be done? I've got a version with async running for my specific application (https://bitbucket.org/sumitraja/hvrr/src/6927216597a35a9a0d7f2e55cca83fa5b7…) but I don't know if I need to unmask at any point so was hoping for some guidance. Thanks Sumit

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Archival of community.haskell.org
by Mario Rogic 19 May '18

19 May '18

Hi all As announced back in 2015 <http://blog.haskell.org/post/7/the_future_of_community.haskell.org/> we have started archiving community.haskell.org services. We've created https://archives.haskell.org/, and the old projects mailing lists have been the first to be archived. Next on the list is the remainder of projects.haskell.org, and the code.haskell.org site. As always, you can email admin(a)h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

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Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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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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ANN: hledger 1.9
by Simon Michael 31 Mar '18

31 Mar '18

I'm pleased to announce hledger 1.9. The version jump signals some noteworthy changes still landing (normal-positive reports, budget reporting) which are a kind of preview of 2.0. The next major release, on 2018/06/30, will be hledger 2.0. Thank you to release contributors: Eli Flanagan, Peter Simons, Christoph Nicolai, agander, M Parker, Moritz Kiefer, Mykola Orliuk. hledger (http://hledger.org) is a reliable, cross-platform program for tracking money, time or other commodities, using double-entry accounting and simple plain text file formats. It provides command-line, curses and web interfaces, and aims to be a pleasant and practical tool for personal, business or institutional use. What's new ? ------------ Some highlights: * the "financial statement" commands bs/bse/cf/is now show all normal balances as positive numbers, producing more conventional financial reports which are easier to share with others. (experimental) * bal/bs/bse/cf/is can now generate HTML reports * account codes can be used to customize account sorting * numbers in scientific E-notation are supported, eg in journal and CSV data * budget report improvements * general report cleanups More at http://hledger.org/release-notes#hledger-1.9 . Get started ----------- See http://hledger.org/download for all install methods. Often, to get the latest release you will need to build it. Here are some ways to do that: Using the hledger-install script, which requires only bash and will build and install the hledger tools in $HOME/.local/bin/. (Note: building haskell apps can take significant time, memory, and disk space, especially the first time. You can kill and restart the installer without losing progress. If it fails, please help us improve it by reporting the full output): $ curl -O https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… $ less hledger-install.sh # (do security review) $ bash hledger-install.sh # (add -v for more detail; use bash -x to show commands being run) Using hledger-install the more convenient, less secure way: $ curl https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… | bash Using stack: $ stack install hledger-lib-1.9 hledger-1.9 # hledger-ui-1.9 hledger-web-1.9 hledger-api-1.9 Using cabal: $ cabal update $ cabal install hledger-1.9 # hledger-ui-1.9 hledger-web-1.9 hledger-api-1.9 After installation, ensure $HOME/.local/bin is in your $PATH. Now try some commands: $ hledger -h # quick help $ hledger help # list built-in manuals $ hledger add # record some transactions $ hledger # list available commands Now perhaps work through the tutorial at http://hledger.org/step-by-step.html Or review the fine documents, presentations etc. at http://hledger.org and http://plaintextaccounting.org Or say hello and ask questions in the #hledger IRC channel on Freenode: http://irc.hledger.org New users and contributors are always welcome! Give feedback, report bugs, send pull requests, write about it, etc. And if you have been finding hledger useful, consider becoming a sponsor or donor to help to sustain and accelerate our progress (see home page). Our IRC channel is #hledger on Freenode (http://irc.hledger.org)

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Minimizing cascading rebuilds
by ☂Josh Chia (謝任中) 30 Mar '18

30 Mar '18

Hi, In my project, I have multiple packages. One of the packages, packageA, is very fundamental and depended on directly and indirectly by almost all the other packages. It has functions that use some hard-coded data (a ByteString top-level variable) also defined within packageA. This hard-coded data is appended regularly, causing packageA to be rebuilt and thus almost all the other packages to be rebuilt, and building takes a painfully long time. I know I can move this hard-coded data to a file that's read at run-time, but that means one more item to plumb in at run-time (where to find the file), and IO (preventing the functions from being pure), so I would like to keep it hard-coded. Is there an elegant way to prevent or minimize the cascading rebuild of the dependent packages just because the hard-coded data in packageA changed? For analogy, in C or C++, source code gets compiled to .o files, one for each .cpp source file. Multiple .o files get linked into executables. So, unless the interface (.hpp files) also change, an implementation (.cpp file) change does not cause dependents to be recompiled to get new .o files, although dependent executables get relinked. I'm not familiar with the compilation and linking logic in GHC so maybe it has additional complications. BTW, I'm using stack, in case it makes any difference to the nature of the problem. Josh

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Haskell to Ethereum VM ?
by Takenobu Tani 30 Mar '18

30 Mar '18

Hi cafe, Does anyone know about the code generator from Haskell's syntax to Ethereum VM language (bytecode)? That is, what corresponds to Solidity in Haskell. Although Solidity is interesting, it's difficult for me to achieve quality and safety. Does such a project already exist? Regards, Takenobu

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Re: [Haskell-cafe] Accelerating Automatic Differentiation
by Michal J Gajda 29 Mar '18

29 Mar '18

Hi Charlie It certainly looks like exciting project, but the bar is currently placed very high. TensorFlow package not only provides automatic differentiation for whole programs, but also optimizes data processing both on GPU, and reading to achieve large batches. This field has a lot of hot developments, so You would either need to propose something unique to Haskell, or You risk being outclassed by PyTorch and TensorFlow bindings Maybe Dominic suggests something too. Cheers Michal

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ANN: network-2.6.3.5
by evan＠evan-borden.com 28 Mar '18

28 Mar '18

Announcing network-2.6.3.5 * Reverting "Do not closeFd within sendFd" [#271](https://github.com/haskell/network/pull/271) Sorry for any disruption this versioning snafu may have caused. https://hackage.haskell.org/package/network-2.6.3.5

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[Call for Contributions] Haskell Communities and Activities Report, May 2018 edition (34th edition)
by Mihai Maruseac 26 Mar '18

26 Mar '18

Dear all, We would like to collect contributions for the 34th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 2018 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac FAQ: Q: What format should I write in? A: The usual format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2017/hcar.sty that you can use to preview your entry. If you do not know LaTeX or don't want to use it or don't have time to translate your entry into it, then please use plain text, it is better to have an entry in plain-text which we will translate than not have it at all. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then. PNG is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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