December 2017 - 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] brittany-0.9.0.0 (on both hackage&stackage)
by lennart spitzner 25 Mar '18

25 Mar '18

- Implement `IndentPolicyLeft`: A layouting setting that disables "hanging indentation". This indentation mode may become default in the future. Only configurable via config-file for the time being. - (!) commandline interface is reworked slightly to support multiple inputs/outputs (`brittany --write-mode=inplace *.hs`). This removes the --input/--output flags. - Ensure compatibility with stackage - Change default global config path (old configs are still respected) - Improve support for per-project config - Fix several bugs and Improve language extension support full changelog at https://hackage.haskell.org/package/brittany-0.9.0.0/changelog brittany was recently added to stackage, and should hopefully be included in the next LTS release. -- lennart

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[ANNOUNCE] GHC 8.4.1-alpha1 available
by Ben Gamari 21 Jan '18

21 Jan '18

The GHC development team is pleased to announce the first alpha release of the 8.4.1 release. The usual release artifacts are available from https://downloads.haskell.org/~ghc/8.4.1-alpha1 Note that this release drops compatibility with GCC 4.6 and earlier. While we generally try to place as few constraints on system toolchain as possible, this release depends upon the __atomic__ builtins provided by GCC 4.7 and later (see #14244). === Notes on release scheduling === The 8.4.1 release marks the first release where GHC will be adhering to its new, higher-cadence release schedule [1]. Under this new scheme, major releases will be made in 6-month intervals with interstitial minor releases as necessary. In order to minimize the likelihood of schedule slippage and to ensure adequate testing, each major release will be preceeded by a number of regular alpha releases. We will begin issuing these releases roughly three months before the final date of the major release and will issue roughly one every two weeks during this period. This high release cadence will allow us to quickly get fixes in to users hands and allow better feedback on the status of the release. GHC 8.4 is slated to be released in mid-February but, due to technical constraints, we are starting the alpha-release cycle a bit later than planned under the above schedule. For this reason, it would be greatly appreciated if users could put this alpha through its paces to make up for lost time. As always, do let us know if you encounter any trouble in the course of testing. Thanks for your help! Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/blog/2017-release-schedule

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Shadowing record field names
by ☂Josh Chia (謝任中) 01 Jan '18

01 Jan '18

Quite often, I need to use record types like this: data Whole1 = Whole1 { part :: Part, ... } data Whole2 = Whole2 { part :: Part, ... } Where Whole1 & Whole2 are types of things that have a Part and some other things. E.g. a Book has an Author, a Title, etc and so does an Article. The problem is that I'm not actually allowed to use the same name (author/part) in two different record types. Some people use lens to solve this. You can have a lens called 'author' for dealing with the Author in both Book and Article (e.g. using makeClassy). That's fine, but there's yet another problem. Let's say I have a function that takes an Author and a [Library] and returns all the Libraries that have Books or Articles matching the Author. So: findAuthorLibraries :: Author -> [Library] -> [Library] findAuthorLibraries author libraries = ... But I already have a lens called 'author' and ghc will complain about shadowing. So, to avoid shadowing, should I use 'theAuthor' instead of 'author' for the function argument? Or, should I name the lens 'authorLens', 'authorL' or 'lAuthor' instead of 'author'? Prefixing with 'the' is quite unreadable because whether or not an argument has that prefix depends on whether there's a lens with a conflicting name so it adds noise to the code. Adding a 'Lens' prefix to the 'author' lens also seems quite an overbearing eyesore because for consistency I would have to use the prefix for all my field-accessing lenses. Maybe I should use Lens.Control.TH.makeClassy and then define: findAuthorLibraries :: HasAuthor a => a -> [Library] -> [Library] findAuthorLibraries hasAuthor libraries = ... But that may be making my function more complicated and general than I want, affecting readability, simplicity, compilation time and maybe even performance. In summary, I find that there are ways around the problem but they really affect readability. I could also disable the warning about shadowing but that seems pretty dangerous. It may be OK to disable the warning for the specific cases where a function argument shadows something from the topmost scope, but GHC does not allow such selective disabling of that warning. In a code base that deals mainly with concrete business logic, this problem probably crops up more than in a code base that deals mainly with more abstract things. What do people do to address this problem? Any recommendations or best practices? Josh

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ANN: hledger 1.5
by Simon Michael 31 Dec '17

31 Dec '17

Happy New Year all! I'm very pleased to announce hledger 1.5. 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. A big thank you to the release contributors: Dmitry Astapov, Mykola Orliuk, Eli Flanagan, Elijah Caine, Sam Jeeves, Matthias Kauer, Hans-Peter Deifel, Mick Dekkers, Nadrieril, Alvaro Fernando García. In 2017, four quarter-end releases were shipped on schedule, and our feature set, marketing reach and contributor activity continue to grow. The next release is scheduled for 2018/03/31. 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) What's new ? ------------ Some highlights from the release notes at http://hledger.org/release-notes#hledger-1.5 : * Ledger-style automated posting rules to modify transactions, enabled with --auto flag * Ledger-style periodic transaction rules to generate forecast transactions (--forecast) and budget goals * a new budget report (balance --budget) comparing amounts with per-period and per-account budget goals * more expressive period expressions * space can be used as digit group separator character in numbers * commodity or default commodity directives give more control over display precision and decimal point/digit group separator Get started ----------- See http://hledger.org/download for all install methods. Windows users can download nightly binaries. On other platforms, you may need to build the latest release yourself. The easiest and most reliable way is to run the hledger install script. This requires only bash and will install the hledger tools in $HOME/.local/bin/. (It uses an installed cabal or stack if possible, otherwise installs stack and GHC in $HOME/.local/bin and $HOME/.stack/). Here's the responsible way to run it: $ 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) or the more convenient, less secure way: $ curl https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… | bash or, to install individual tools: $ stack install hledger-1.5 # hledger-ui-1.5 hledger-web-1.5 hledger-api-1.5 etc. or: $ cabal update $ cabal install hledger-1.5 # hledger-ui-1.5 hledger-web-1.5 hledger-api-1.5 etc. 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. 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 Wishing you a highly prosperous and serene 2018, -Simon Happy New Year all! I'm very pleased to announce hledger 1.5. 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. A big thank you to the release contributors: Dmitry Astapov, Mykola Orliuk, Eli Flanagan, Elijah Caine, Sam Jeeves, Matthias Kauer, Hans-Peter Deifel, Mick Dekkers, Nadrieril, Alvaro Fernando García. In 2017, four quarter-end releases were shipped on schedule, and our feature set, marketing reach and contributor activity continue to grow. The next release is scheduled for 2018/03/31. 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) What's new ? ------------ Some highlights from the release notes at http://hledger.org/release-notes#hledger-1.5 : * Ledger-style automated posting rules to modify transactions, enabled with --auto flag * Ledger-style periodic transaction rules to generate forecast transactions (--forecast) and budget goals * a new budget report (balance --budget) comparing amounts with per-period and per-account budget goals * more expressive period expressions * space can be used as digit group separator character in numbers * commodity or default commodity directives give more control over display precision and decimal point/digit group separator Get started ----------- See http://hledger.org/download for all install methods. Windows users can download nightly binaries. On other platforms, you may need to build the latest release yourself. The easiest and most reliable way is to run the hledger install script. This requires only bash and will install the hledger tools in $HOME/.local/bin/. (It uses an installed cabal or stack if possible, otherwise installs stack and GHC in $HOME/.local/bin and $HOME/.stack/). Here's the responsible way to run it: $ 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) or the more convenient, less secure way: $ curl https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… | bash or, to install individual tools: $ stack install hledger-1.5 # hledger-ui-1.5 hledger-web-1.5 hledger-api-1.5 etc. or: $ cabal update $ cabal install hledger-1.5 # hledger-ui-1.5 hledger-web-1.5 hledger-api-1.5 etc. 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. 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 Wishing you a highly prosperous and serene 2018, -Simon

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singToList
by Dmitry Olshansky 31 Dec '17

31 Dec '17

Hello cafe, Does a function like `singToList` exist somewhere: singToList :: Sing (xs :: [k]) -> [SomeSing k] singToList = \case SNil -> [] SCons x xs -> SomeSing x : singToList xs ?

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Re: [Haskell-cafe] [ANNOUNCE] GHC 8.4.1-alpha1 available
by Emanuel Borsboom 31 Dec '17

31 Dec '17

Hi Ben, Are there any plans to start releasing Linux bindists linked with libncurses.so.6? A number of Linux distributions have upgraded from 5, including Fedora, Arch and Gentoo, and the currently available official bindists don't work out of the box on those anymore. -- Manny

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