February 2017 - Haskell-Cafe

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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Source code location in IO?
by Tom Ellis 30 Jun '17

30 Jun '17

Is it compatible with the semantics of Haskell to have a function sourceLocation :: IO String which when run returns the source file location at which it is used? For example, suppose Main.hs is module Main where main = do putStrLn =<< sourceLocation putStrLn "Hello" putStrLn =<< sourceLocation It would print the following when run Main.hs:4 Hello Main.hs:6 and module Main where main = do let s = sourceLocation putStrLn =<< s putStrLn "Hello" putStrLn =<< s It would print the following when run Main.hs:4 Hello Main.hs:4 If this is not compatible with the semantics of Haskell, why not? I agree that the two programs must have the same denotation, but is there anything that requires them to have the same output when run? Tom

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Why is Megaparsec treating these two operators differently?
by Jeffrey Brown 14 May '17

14 May '17

I used Text.Megaparsec.Expr to write a minimal (56 lines) 2-operator recursive parser. It's on github[1]. It outputs a binary tree of the following form: data AExpr = Var String | Pair AExpr AExpr deriving (Show) The operator table supplied to makeExprParser defines two operators, # and ##. ## binds after #, but both of them refer to the same function, the Pair constructor of the AExpr data type: aOperators :: [[Operator Parser AExpr]] aOperators = [ [ InfixN # symbol "#" *> pure (Pair) ] , [ InfixN # symbol "##" *> pure (Pair) ] ] The # operator works in isolation: > parseMaybe aExpr "a # b" Just (Pair (Var "a") (Var "b")) Parentheses work with the # operator: > parseMaybe aExpr "(a # b) # (c # d)" Just (Pair (Pair (Var "a") (Var "b")) -- whitespace added by hand (Pair (Var "c") (Var "d"))) And the # and ## operators work together as intended: > parseMaybe aExpr "a # b ## c # d" Just (Pair (Pair (Var "a") (Var "b")) -- whitespace added by hand (Pair (Var "c") (Var "d"))) But the ## operator in isolation does not parse! > parseMaybe aExpr "a ## b" Nothing [1] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/howt… -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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Looking for maintainers or comaintainers on my Haskell projects
by Christopher Done 27 Apr '17

27 Apr '17

Hi all, The short version is: I’ve been battling RSI in my fingers for some years. I’m doing various things to mitigate that problem, but I have very limited finger bandwidth these days; enough to work at my usual pace at my job, but not much in the evenings and weekends, and so I can’t manage to do much on my hobby projects. I’m also not as motivated these days to work on my set of open source projects, and am turning my attention to different things. It’s not great, but that’s life. I don’t think that the users of my packages are getting the best maintainership deal. Rather than be “the absentee maintainer”, I’d prefer a straight-forward transition of maintainership or ownership to someone who can put the right energy and time in. In terms of packages, there are really two that have a significant maintenance burden and users aren’t being served very well: - HIndent <https://github.com/chrisdone/hindent> has a significant amount of issues opened for it regularly, and many of them require discussion and debate. If someone would like to become a co-maintainer, let me know. It may (eventually) make sense to move it to a more general GitHub organization like commercialhaskell or haskell. - Intero <https://github.com/commercialhaskell/intero/issues>, which seems to have been a success, has a pretty big maintenance burden on “this doesn’t work” kind of issues which require investigation. There’s some Emacs Lisp work to do on it, and some Haskell work on the intero binary, and a whole lot of platform-specific problems or tooling not working together. On the other hand people really like this project, and there’s a lot of tooling potential. If you want to take xeno <https://github.com/chrisdone/xeno> and make it into a publishable package, please do so. The rest of my projects that are on Stackage are: labels <https://hackage.haskell.org/package/labels>, ace <https://hackage.haskell.org/package/ace>, ical <https://hackage.haskell.org/package/ical>, check-email <https://hackage.haskell.org/package/check-email>, freenect <https://hackage.haskell.org/package/freenect>, frisby <https://hackage.haskell.org/package/frisby>, gd <https://hackage.haskell.org/package/gd>, ini <https://hackage.haskell.org/package/ini>, lucid <https://hackage.haskell.org/package/lucid>, osdkeys <https://hackage.haskell.org/package/osdkeys>, pdfinfo <https://hackage.haskell.org/package/pdfinfo>, present <https://hackage.haskell.org/package/present>, pure-io <https://hackage.haskell.org/package/pure-io>, scrobble <https://hackage.haskell.org/package/scrobble>, shell-conduit <https://hackage.haskell.org/package/shell-conduit>, sourcemap <https://hackage.haskell.org/package/sourcemap>, descriptive <https://hackage.haskell.org/package/descriptive>, wrap <https://hackage.haskell.org/package/wrap>, path <https://hackage.haskell.org/package/path>, weigh <https://hackage.haskell.org/package/weigh>, haskell-docs <https://hackage.haskell.org/package/haskell-docs>, and structured-haskell-mode <https://hackage.haskell.org/package/structured-haskell-mode>. If you’re interested in taking over or co-maintaining any of them, let me know. Some are interesting, others are boring, some are trivial. I have other packages on Hackage, but they’re mostly dead or experiments that don’t need maintenance anyway. I’ve started the process of adding or changing maintainers on my public services: - Haskell News <https://github.com/haskellnews> is now a GitHub organization. Luke Murphy is a co-owner, and has full access to the DigitalOcean account that is running the service. So if you want to work on that project, I’m not in the way. - lpaste <https://github.com/lpaste/lpaste> has been moved to its own DigitalOcean account too. If anyone is interested in taking over the project or co-running it, let me know. - tryhaskell <https://github.com/chrisdone/tryhaskell> doesn’t really require any maintenance, but it’s also on its own DigitalOcean account now too. - IRCBrowse <https://github.com/chrisdone/ircbrowse> is now on its own DigitalOcean account too. It requires maintenance once in a while. If anyone is interested in taking over the project or co-running it, let me know. Cheers!

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Ideas for Haskell Summer of Code
by Niki Vazou 17 Mar '17

17 Mar '17

Hello, As a preparation for the 2017 edition of Haskell Summer of Code, we are crowdsourcing project ideas. Compiler optimizations, new libraries, desired features, or any other ideas you would like to see in the Haskell ecosystem. You can submit tickets to the Haskell Summer of Code Ideas page: https://ghc.haskell.org/trac/summer-of-code/report/1 Alternatively, you can tell us your SoC idea by responding to this thread (and I can generate the ticket). Best, Niki

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reading and writing Data.Ratio.Rational
by Olaf Klinke 07 Mar '17

07 Mar '17

Dear cafe, when processing text files containing numbers of the form "xxxx.yyyy" I used to parse them into Double using that type's Read instance. Obviously, even with arithmetic no more complicated than the field operations the result might have ugly rounding errors like 12.000000000002 due to the fact that numbers like 0.7 are not dyadic rationals. The math in my program is not complicated and the numbers are not large, so I don't care about Rationals having potentially huge memory footprints. So here's my question. A literal like 0.7 has type Fractional a => a, but the Read instance of Rational rejects the string "0.7". Must it be this way? Do I have to go via toRational.(read :: String -> Data.Scientific.Scientific)? Note that the documentation of Data.Scientific explicitly states that using (/) is unsafe, so I'd rather stay with the field Rational. For the output as decimal expansion, there is of course long division as described here [1], but I wonder whether either this exists in some library or there is even a more efficient solution. Any pointers are appreciated. Thanks, Olaf [1] http://stackoverflow.com/questions/30931369/how-to-convert-a-rational-into-…

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Declaring Functors for Type constrained data types
by Guru Devanla 01 Mar '17

01 Mar '17

Hello All, I am working on a program that will define a bunch of tasks. Each task will have to implement certain methods as part of a type class. -- task 1 data UpdateAcctsTask = UpdateAccts -- task 2 data EmailConfig = EmaiConfig {someattrs::String} data SendEmailTask = SendEmailsTask EmailConfig -- task 3 data GeneralWriterTask a = GeneralWriterTask a Each of these tasks implement a class, Taskable. The return values are simplified for this example. class Taskable a where process :: a -> Bool can_run :: a -> Bool This works fine. I can expand on these tasks and execute them. Now, I wanted to be able to defined dependencies between these (Taskable's). I decided I could create a data type for this dependency and may be also get a FreeMonad around this structure for further processing using a graph of Tasks. But, before that I wanted to create an wrapper for these Taskables and create a functor for it as follows The first thing I did was, define a Task, which generalizes over all the above defined (and future Taskables) data Task a where Task :: (Taskable a) => a -> Task a instance Functor Task where fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b --- THIS DOES NOT WORK fmap f (Task a) = Task $ f a But, I realized that I cannot define an fmap over a type constraint. My questions are: 1. Is there any way to do this. I see there is an answer of SO. I wanted to make sure if there were any improvements to this since that answer' was posted. http://stackoverflow.com/questions/17157579/functor-instance-for-a-gadt-wit… 2. Secondly, I would like to know why this is not possible. Is it a current limitation of GHC or if there is some fundamental category theory concepts that dis-allows such declarations that I need to grok! Appreciate any help on this. Thank you!

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Summer of Haskell 2017
by Niki Vazou 28 Feb '17

28 Feb '17

Dear Haskellers, We are sorry to announce that this year Haskell.org <http://haskell.org/> was not accepted for the 2017 Google Summer of Code. But, as last year, we plan to organize our own Summer of Haskell! We ask for your help to plan a successful Summer of Haskell 2017. As last year, one student will be funded directly out of Haskell.org <http://haskell.org/> general funds. The key factor in how many further students we can fund is the generosity and support of donors. Funding a single student plus overhead costs 6000 USD. We are looking for donors willing to cover half, one or more students. If you (or your organization) are interested in becoming a Summer of Haskell donor please reach out to committee(a)haskell.org. We also encourage you to make a smaller donation, as if we receive enough of small donations in total, we will combine them to fund more students. Donation information is at: https://wiki.haskell.org/Donate_to_Haskell.org More information will be forthcoming as we work out the details. Feel free to contact us at (nvazou(a)cs.umd.edu and m(a)jaspervdj.be) if want to help or if you have any ideas, questions, or concerns. Best, Niki Vazou & Jasper Van der Jeugt for the Haskell.org Committee

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[Announce] ZuriHac 2017
by Jasper Van der Jeugt 28 Feb '17

28 Feb '17

Hello all, It is our pleasure to announce ZuriHac 2017, which will take place on 9th to 11th of June, 2017. The Haskell Hackathon is an international, grassroots collaborative coding festival whose goal is to expand the community and to build and improve Haskell libraries, tools, and infrastructure. This is the already the 6th Haskell Hackathon in Zurich! This year, we are privileged to enjoy keynotes from: - Edward Kmett - Julie Moronuki - Neil Mitchell - Stephen Diehl - Simon Thompson This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! This event is open to any experience level, from beginners to gurus. We will have a dedicated beginners' track, and there will be mentors on site whom you can directly approach during the whole event with any Haskell-related question you have. We will be hosting the Hackathon in a new location [1] near the lake which provides space for up to 300 people! Experience from the previous years tells us these places might be gone quickly, so don't wait too long to register. You can find more information and register for the Hackathon here: http://zurihac.info We would also like to thank our sponsors Digital Asset [2], Google Open Source Programs Office [3] and HSR [4] for supporting this great event! Looking forward to see you there, the Zurich HaskellerZ meetup group [1]: https://zurihac.info/#venue [2]: https://digitalasset.com/careers.html [3]: https://developers.google.com/open-source/ [4]: https://www.hsr.ch/

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