- Haskell-Cafe - Haskell.org

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

7 10

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

1 0

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 ?

2 2

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

2 1

Question about ST arrays
by Jean-Marc Alliot 29 Dec '17

29 Dec '17

Hi, This is my first post to this list so I apologize in advance if I don't use it properly, or if my question is too simple or inapropriate. I come from the Caml world and I am quite new to Haskell (but not to functional programming). I am currently trying to get the hang of Haskell arrays. I have gone through regular arrays, IO Arrays and I am now working with ST Arrays. This is the problem I am currently stuck with. I write the following code: arr = newArray (-1, 1) 0 :: ST s (STArray s Int Int) get :: Int -> Int get i = runST (arr >>= (\b -> readArray b i)) Here everything is perfectly OK. Now I want a more general version that could deal with any array like arr. So I write: get2 :: ST s (STArray s Int Int) -> Int -> Int get2 tab i = runST (tab >>= (\b -> readArray b i)) And the compiler is clearly very upset by my code: Couldn't match type ‘s’ with ‘s1’ ‘s’ is a rigid type variable bound by the type signature for: get2 :: forall s. ST s (STArray s Int Int) -> Int -> Int at testst.hs:17:9 ‘s1’ is a rigid type variable bound by a type expected by the context: forall s1. ST s1 Int at testst.hs:18:14 Expected type: ST s1 Int Actual type: ST s Int I am pretty sure that the compiler is right and I am wrong, but I don't get why... Anyone could help? Thanks

5 8

[Call for Project Ideas] Haskell.org Google Summer of Code 2018
by Jasper Van der Jeugt 25 Dec '17

25 Dec '17

Google Summer of Code will take place again in 2018 [1]. Last year, Haskell.org was not selected, and we decided to run our own program [2], which ended very successfully [3]. This year, we would like to apply to Google Summer of Code again, since their sponsorship is very significant. The main feedback we received from Google last year was that we didn't really have a great homepage for Summer of Code with ideas for students (things were very rushed and we ended up submitting a link to an outdated bug tracker -- not ideal!). We already started fixing that last year by building a nicer webpage to host ideas [4]. We would now like to call on the community to submit ideas for the students. If you are the maintainer or the user of a Haskell project, and you have an improvement in mind which a student could work on during the summer, please submit an idea here: https://summer.haskell.org/ideas.html Or contact Niki Vazou (nvazou [AT] cs.umd.edu) or myself (m [AT] jaspervdj.be) directly. For context, Google Summer of Code is a program where Google sponsors students to work on open-source projects during the summer. Haskell.org has taken part in this program from 2006 until 2015. Many important improvements to the ecosystem have been the direct or indirect result of Google Summer of Code projects, and it has also connected new people with the existing community. Projects should benefit as many people as possible -- e.g. an improvement to GHC will benefit more people than an update to a specific library or tool, but both are definitely valid. New libraries and applications written in Haskell, rather than improvements to existing ones, are also accepted. Projects should be concrete and small enough in scope such that they can be finished by a student in three months. Best, Niki Vazou & Jasper Van der Jeugt for the Haskell.org Committee [1]: https://opensource.googleblog.com/2017/09/announcing-google-summer-of-code-… [2]: https://summer.haskell.org/news/2017-02-28-2017-announce.html [3]: https://summer.haskell.org/news/2017-09-15-final-results.html [4]: https://summer.haskell.org/ideas.html

1 0

Trouble building happy/alex with Stack lts-9.20
by Eric McCorkle 25 Dec '17

25 Dec '17

Hi folks, I'm trying to update my language project to happy >= 9.16, so I can use typeclass-based parsers. Unfortunately, I'm running into build errors after switching to lts-9.20. The log file is attached. I see similar problems when trying to build alex (the lexer generator). It seems to be claiming that there is no cabal file, which is odd, as there clearly is one (in the source at least). Has anyone else seen this problem, and if so, what is the workaround?

3 4

Supporting Haskell.org
by Tikhon Jelvis 25 Dec '17

25 Dec '17

A significant part of the Haskell community infrastructure—including the haskell.org website, Hackage, Hoogle and the build infrastructure for GHC—runs on donations from the Haskell community administered by haskell.org . You can donate right now using several methods including PayPal and check: https://wiki.haskell.org/Donate_to_Haskell.org Haskell.org can also accept donations through employers via Benevity using the unique ID 475236502. As Haskell.org is a 501(c)(3) non-profit, donations are tax-deductible. This year marked the second Haskell Summer of Code program which provided grants for 15 students working on open source Haskell projects over the summer. The projects improved Haskell tooling and libraries as well as supporting Haskell education through improvements to code.world. A detailed retrospective can be found on summer.haskell.org: https://summer.haskell.org/news/2017-09-15-final-results.html With look forward to continuing this work with your support in 2018 as well as exploring new projects to improve the Haskell community infrastructure. Best wishes, Tikhon Jelvis on behalf of the haskell.org committee.

2 1

ANN: progress-meter 1.0.0
by esz＠posteo.de 25 Dec '17

25 Dec '17

Hi everybody, I have just released a complete API rewrite of the `progress-meter` package: * <https://hackage.haskell.org/package/progress-meter> * <https://github.com/esoeylemez/progress-meter> With this package you can easily add live diagnostics to any long-running application from a simple progress bar to full task monitoring. The `System.Progress` module contains a tutorial. Features: * update progress from multiple threads, * non-progress diagnostics that just scroll by (e.g. logging), * breaking the state apart for individual threads, so they can update their relevant portion of the progress bar without interfering with each other ("zooming"), * smart throttling (no updates drawn when nothing is happening, and only ever drawn at a maximum rate). Here is a very simple example: import Control.Concurrent import System.Progress main = withProgress_ "" id $ \pm -> do -- Set the progress bar: setMeter pm "Working..." -- Do some "work": threadDelay 3000000 -- Print something while temporarily hiding the progress bar: putMsgLn pm "Some extra diagnostics that scrolls by" setMeter pm "Almost done..." threadDelay 1000000 putMsgLn pm "Done." The first argument of 'withProgress_' is the initial state and the second is a renderer for the current state: render :: Int -> String render p = "Progress: " ++ show p ++ "%" withProgress_ 0 render $ \pm -> do threadDelay 1000000 setMeter pm 10 threadDelay 8900000 setMeter pm 99 -- The famous last percent: threadDelay 10000000 putMsgLn pm "Done." Progress-meter will do The Right Thing when you update the state or produce log messages from multiple threads. Happy holidays ertes

1 0

Hackage / Stackage Idea
by alex＠centromere.net 23 Dec '17

23 Dec '17

Hi all, I use Haskell in production, and when choosing a library I wonder, "Is this package production-ready?". In addition to evaluating the quality of the code, I consider various factors such as the most recent upload date, number of downloads, the age of the last commit, the author's reputation, the number of packages which depend on that package, and the number of issues open and closed. I think it would be interesting to add a feature to Hackage and/or Stackage which allows companies to advertise that they use a package in production. If they elect to be anonymous, only the count would increase. I believe that this data would not only help inform my library choices, but also help communicate Haskell's merits to industry. What do you all think? Alex

2 1