February 2014 - Haskell-Cafe

Unmaintained packages and hackage upload rights
by Roman Cheplyaka 09 Feb '14

09 Feb '14

In the recent past I took over two unmaintained packages: bert and ansi-terminal. I don't mind spending a bit of time to keep our ecosystem from bitrotting. However, both times I had to go through an irritating procedure of contacting hackage admins, asking them to grant me upload rights, explaining why the maintainers can't do that themselves and why I think the packages are abandoned. Instead of a feeling that I'm doing something good and useful, I have a feeling that I'm bothering people with my own problems. It also adds unnecessary latency to my work. So from now on I'll simply fork the packages I need to fix. Others are of course welcome to use my forks. (This email was prompted by regex-tdfa which doesn't build on GHC 7.8, and whose maintainer hasn't responded. My fork is at http://hackage.haskell.org/package/regex-tdfa-rc .) Roman

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Puppet module for installing cabal packages?
by Andrew Pennebaker 09 Feb '14

09 Feb '14

I'm using Vagrant <http://www.vagrantup.com/> and Puppet<http://puppetlabs.com/puppet/what-is-puppet> to automate setting up development, testing, and production environments. For now, I'm executing shell commands to install my favorite Cabal packages (`cabal update`, `cabal install <package>`), but I'd prefer to use something a bit more high level. Would anyone be interested in creating a Puppet module for Cabal? This could encourage more use of awesome packages like Darcs and ShellCheck by non-Haskellers. -- Cheers, Andrew Pennebaker www.yellosoft.us

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[ANN] deka - decimal arithmetic
by Omari Norman 08 Feb '14

08 Feb '14

Announcing deka, a library for decimal arithmetic. Why decimal arithmetic is important is discussed at length at http://speleotrove.com/decimal/ but for a short explanation, go into ghci and type: print $ 0.1 + 0.1 + 0.1 This is 0.3, right? But on my machine I get 0.3 plus a small fraction, which is simply not acceptable for applications where it is essential to get exact results, such as financial applications. deka is a binding to the decNumber C library, a thoroughly tested and fast implementation of decimal arithmetic. deka is fully tested and I encourage you to run the tests using the usual "cabal test" method. deka is available at http://hackage.haskell.org/package/deka and is on Github at https://github.com/massysett/deka Because deka is a binding to the decNumber C library, first you will need to install decNumber. An easy to install version with the GNU autotools is at: https://github.com/massysett/decnumber/releases decNumber's website is at http://speleotrove.com/decimal/decnumber.html A literate Haskell file illustrating the use of deka is available at https://github.com/massysett/deka/blob/master/lib/Data/Deka/Docs/Examples.l…

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An arrow transformer
by Roman Cheplyaka 08 Feb '14

08 Feb '14

I wrote (and found useful) the following arrow transformer: data Partial ar a b = Partial (ErrorArrow () ar a b) | Total (ar a b) toPartial :: ArrowChoice ar => Partial ar a b -> ErrorArrow () ar a b toPartial (Partial f) = f toPartial (Total sf) = liftError sf instance ArrowChoice ar => Category (Partial ar) where id = Total id Total f . Total g = Total $ f . g f . g = Partial $ toPartial f . toPartial g instance ArrowChoice ar => Arrow (Partial ar) where arr = Total . arr first (Total f) = Total $ first f first (Partial f) = Partial $ first f It allows to work with partial functions/arrows and at the same time tracks which of them are actually total. I wonder if it is already defined somewhere or is perhaps an instance of a more general construction. Roman

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ANN: Wheb -- A WAI framework
by Kyle Hanson 08 Feb '14

08 Feb '14

Hello all, I was rather frustrated with the use of Template Haskell as the main entry point for the big framework projects (Yesod, Snap, etc.) While these frameworks offer template-free options, they seem like second class citizens. So I started work on Wheb with the goal that out of the box I could start a project quickly, without Template Haskell and without the dreaded long list of Language Pragma's at the top. I was inspired by the simplicity of the Scotty project which showed how easy a Haskell server could be to write. I have included a couple of plugin examples for handling Auth and Sessions. Wheb is database and template agnostic, but I plan to write some plugins soon to make it easier to use the major libraries with Wheb. I just started work on it last weekend but wanted to share my progress. Take a look if it interests you! http://hackage.haskell.org/package/Wheb-0.0.1.1 https://github.com/hansonkd/Wheb-Framework Thanks, Kyle Hanson

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STM and unsafePerformIO/bracket
by Rob Leslie 07 Feb '14

07 Feb '14

I’ve run into a difficulty I’d appreciate some advice to solve. I am trying to make a call to C (through the FFI) within an STM transaction to compute what is effectively a pure result. By itself this doesn’t seem to be a problem, except that the C function depends on some global state in a way that compromises thread safety, and so I would like to serialize the process of setting up that state and calling the FFI. I have tried wrapping my call up with: > unsafePerformIO $ bracket (takeMVar lock) (putMVar lock) $ \_ -> do > setupFFIGlobalState > callFFI This seems to work as intended, with one significant exception. If the surrounding STM transaction is aborted (i.e. because of a TVar conflict detected by another thread) while inside the bracket, then apparently the second argument to ‘bracket’ may never be evaluated, leaving my ‘lock’ mutex forever empty, and eventually causing deadlock. Here is a working example of the problem: http://lpaste.net/99399 When I run that, I get: taking lock... lock acquired taking lock... stm-test: thread blocked indefinitely in an MVar operation Is this expected behavior? I realize performing any IO within an STM transaction is inherently unsafe, but I am a little surprised that ‘bracket’ fails here. Is there a better way to do what I’m trying to accomplish? How can I provide a pure interface to my foreign function that will work within an STM transaction? Thanks, -- Rob Leslie rob(a)mars.org

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Filing bugs for haskell packages
by Szymon Pyżalski 07 Feb '14

07 Feb '14

Hello! I am new to haskell and I found a bug in package http://hackage.haskell.org/package/regex-compat-0.92/docs/Text-Regex.html On the hackage page it says that the maintainer e-mail of this package is: libraries(a)haskell.org. So i sent a bug report there, but bounced back to me because of quota. So apparently nobody reads these e-mails. How can I file a bug report for this package? I don't feel good enough in haskell to try and fix it myself. Greetings Szymon Pyżalski

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2nd Call for Papers - TFPIE 2014
by Peter Achten 06 Feb '14

06 Feb '14

All, Please find below the call for papers for the 3rd International Workshop on Trends In Functional Programming in Education, TFPIE 2014. Apologies in advance for multiple copies you may receive. Best regards, James Caldwell Call for Papers ___________________________________________________________________________________ *3rd International Workshop on Trends in Functional Programming in Education (TFPIE 2014)* May 25, 2014 Utrecht University Soesterberg, The Netherlands (http://www.cs.uwyo.edu/~jlc/tfpie14/ <http://www.cs.uwyo.edu/%7Ejlc/tfpie14/>) The 3rd International Workshop on Trends in Functional Programming in Education, TFPIE 2014, will be co-located with the Symposium on Trends in Functional Programming (TFP 2014) <http://www.cs.uu.nl/wiki/TFP2014/> at Soesterberg, at the "Kontakt der Kontinenten" <http://www.kontaktderkontinenten.nl/conferentiehotel/home.aspx?lang=en-US> hotel in the Netherlands on Sunday, May 25th. TFP will follow from May 26-28. The goal of TFPIE is to gather researchers, teachers and professionals that use, or are interested in the use of, functional programming in education. TFPIE aims to be a venue where novel ideas, classroom-tested ideas and work-in-progress on the use of functional programming in education are discussed. The one-day workshop will foster a spirit of open discussion by having a review process for publication after the workshop. The program chair of TFPIE 2014 will screen submissions to ensure that all presentations are within scope and are of interest to participants. Potential presenters are invited to submit an extended abstract (4-6 pages) or a draft paper (up to 16 pages) in EPTCS style. The authors of accepted presentations will have their preprints and their slides made available on the workshop's website/wiki. Visitors to the TFPIE 2014 website/wiki will be able to add comments. This includes presenters who may respond to comments and questions as well as provide pointers to improvements and follow-up work. After the workshop, presenters will be invited to submit (a revised version of) their article for review. The PC will select the best articles for publication in the journalElectronic Proceedings in Theoretical Computer Science (EPTCS) <http://published.eptcs.org/>. Articles not selected for presentation and extended abstracts will not be formally reviewed by the PC. TFPIE workshops have previously been held in St Andrews, Scotland (2012) and in Provo Utah, USA (2013). *Program Committee* James Caldwell, (Program Chair) University of Wyoming Peter Achten, Radboud University, Nijmgen Edwin Brady, University of St Andrews, St Andrews Jurriaan Hage, Universiteit Utrecht Philip Holzenspies, University of Twente Daniel R. Licata, Wesleyan University Marco T Morazan, Seton Hall University Christian Skalka, University of Vermont David Van Horn, Northeastern University *Submission Guidelines* There will be two types of presentations at TFPIE 2014. Regular papers and "best lecture" presentations. The best lecture talks are intended to allow for presentations or short lectures of purely pedagogical material. Papers and abstracts can be submitted via easychair at the following link: https://www.easychair.org/conferences/?conf=tfpie2014 *Papers* TFPIE 2014 welcomes submissions describing techniques used in the classroom, tools used in and/or developed for the classroom and any creative use of functional programming (FP) to aid education in or outside Computer Science. Topics of interest include, but are not limited to: * FP and beginning CS students * FP and Computational Thinking * FP and Artificial Intelligence * FP in Robotics * FP and Music * Advanced FP for undergraduates * FP in graduate education * Engaging students in research using FP * FP in Programming Languages * FP in the high school curriculum * FP as a stepping stone to other CS topics * FP and Philosophy * * *Best Lectures* In addition to papers, this year we are requesting "best lecture" presentations. What's your best lecture topic in an FP related course? Do you have a fun way to present FP concepts to novices or perhaps an especially interesting presentation of a difficult topic? In either case, please consider sharing it. Best lecture topics will be selected for presentation based on a short abstract describing the lecture and its interest to TFPIE attendees. *Important Dates* * 1 February 2014: TFPIE submissions open on easychair. * 7 April 2014: TFP and TFPIE registration opens * 21 April 2014: Submission deadline for draft TFPIE papers and abstracts * 27 April 2014: Notification of acceptance for presentation * 25 May 2014: Presentations in Soesterberg, Netherlands * 29 June 2014: Full papers for EPTCS proceedings due. * 16 August 2014: Notification of acceptance for proceedings * 8 September 2014: Camera ready copy due for EPTCS Submission of an abstract implies no obligation to submit a full paper; abstracts with no corresponding full versions by the full paper deadline will be considered as withdrawn. At least one author from each accepted presentation must attend the workshop.

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problem with happy and type identity
by Richard Cobbe 06 Feb '14

06 Feb '14

I'm working on a small project that involves an Alex scanner and a Happy parser, and I'm getting an error from the type-checker that I do not understand. Can anyone help shed some light on what's going on? I'm running Haskell Platform 2013.2.0.0, on MacOS 10.8.5 with XCode 4.6.3. I've reduced the problem to a very small example, which I've attached as a tar file. It's a cabal package, and it contains a library with some test cases. If I run "cabal configure && cabal build" then the library builds with no problems whatsoever. But if I run cabal clean && cabal configure --enable-tests && cabal build then I get the following error message: RunTests.hs:16:27: Couldn't match expected type `sample-0.4:Ast.Entry' with actual type `Entry' In the return type of a call of `Entry' In the second argument of `(~?=)', namely `Entry "mumble"' In the expression: parse "entry" ~?= Entry "mumble" And this doesn't make any sense to me, because the two types are supposed to be the same. If I take Happy and Alex out of the picture by replacing the Parser module with a hand-written parser, the error goes away. This is hardly feasible in the actual project, of course :-) so I'm hoping someone can shed some light on what I'm doing wrong here. Thanks! Richard (I apologize for the attachment, by the way, but since filesystem layout is potentially important here, and since it's only slightly larger than 1K, I figured it was the best way. If there's a better way to do this in the future (which doesn't assume I can throw it on the web somewhere), then I'd love to hear about it.)

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Generalized null / zero
by Alvaro J. Genial 06 Feb '14

06 Feb '14

1. Is there a more general version of `null`? (e.g. for a Monad, Functor, Applicative, Traversable or the like.) The closest I can come up with is, in decreasing clunkiness: zero :: (MonadPlus m, Eq (m a)) => m a -> Bool zero = m == mzero zero :: (Alternative f, Eq (f a)) => f a -> Bool zero = m == empty zero :: (Monoid m, Eq m) => m -> Bool zero = m == mempty Though requiring Eq seems ugly and unnecessary, in theory. 2. In that vein, is there an existing function for "a value or a default if it's zero"? E.g.: orElse :: (Monoid m) => m -> m -> m a `orElse` b = if zero a then b else a Thank you, Alvaro http://alva.ro

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