March 2015 - Haskell-Cafe

Existentials for multi-parameter type classes?
by Lyle Kopnicky 20 Mar '15

20 Mar '15

Hi folks, I know how to write existentially quantified data constructors for single-parameter type classes, but I'm having trouble doing it for multi-parameter type classes. Is it not possible, or am I just using the wrong syntax? I have this class: class Collidable a b where collideWith :: a -> b -> String And I tried to define an existentially quantified data constructor thusly: data Collision = forall a. (forall b. Collidable a b => Collision {collider1 :: a, collider2 :: b}) I get a parse error. So then I tried: data Collision = forall a. forall b. Collidable a b => Collision {collider1 :: a, collider2 :: b} I get a different parse error. Is there any way to make this work? Thanks, Lyle

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can't STOP after interrupt winGHCi or ghci
by Kees Bleijenberg 19 Mar '15

19 Mar '15

Now I realize maybe I didn't make my point clear. What I want is not just interrupting the output of the compiler, but also stop the output of the compiler after interrupting. Van: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] Namens Kees Bleijenberg Verzonden: woensdag 18 maart 2015 11:12 Aan: haskell-cafe(a)haskell.org Onderwerp: [Haskell-cafe] can't nterrupt winGHCi or ghci With a haskell program I generated a very big haskell file. Sometimes the generated haskell is not valid. If I load the genearated file in winGHCi a flood of error messages streams over the screen. If I press ctrl-break WinGHCi stops and says interrupted. If I click Ok the program continues printing error messages. Ctrl-c or just c, doesn't help. So I have to wait until WinGHCi has shown all error messages (can take minutes) or close the WinGHCi window. Ghci has (on Windows) the same behaviour. Is there a special key combination to stop Ghc or Ghci or is this a bug? Kees

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Incremental sequence sorting
by David Feuer 18 Mar '15

18 Mar '15

I had an idea, and it's kind of complicated, so I was wondering if I could get a sanity check from someone before putting *too* much time into it. Specifically, I was thinking about a sort of incremental sorting of sequences (in Data.Sequence). The idea is very closely related to the incremental sorting described by Navarro and Paredes ( http://www.dcc.uchile.cl/~gnavarro/ps/algor09.pdf ). The basic concept: sequences are represented as Hinze-Paterson 2-3 finger trees. They are fairly lazy, so building them from the top down tends to be a good thing. Suppose I have a sequence to be sorted. To start building the result sequence, I need to know which elements go in the first digit and which go in the last digit. To do this, I need two order statistics, starting with one separating the first digit from the rest, then one separating the last digit from the rest. Once I have separated these, I can produce the top of the sequence. The digit separation is the same all down the spine. Within digits, 2-3 trees split similarly. I can get the order statistics using median-of-medians. As Navarro and Paredes (and presumably others) describe, median-of-medians partially sorts the sequence in the process of finding the desired order statistic. Navarro and Paredes use a stack of indices into an array to take advantage of this, but I need things rather more incremental. What I was thinking is that I could use a finger tree of sequences (a sequence of sequences making it easy to find where a particular total length is reached) to separate the different segments from each other. Each time I need to split on a particular order statistic, I can isolate the necessary segment, use median-of-medians (splitting it into pieces), and then append the pieces onto either side as appropriate. Does anyone have a sense of 1. Whether this is sane and/or 2. What the chances are that its performance will be competitive with either of the current Data.Sequence sorting functions and/or 3. How I should actually implement median-of-medians for this particular application (as there seem to be very few Haskell implementations of the algorithm, and I've found no optimized ones)? Thanks, David Feuer

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Proposed significant breaking changes to Win32
by Yitzchak Gale 18 Mar '15

18 Mar '15

There are some discussions going on in this GitHub issue: https://github.com/haskell/win32/issues/24 with proposals for extensive breaking changes to the Win32 library. Well over 100 functions would be involved. These changes would finally fix many long outstanding defects in the API, some of which make it difficult to use important Win32 functions in the way originally intended. It is estimated that about 70 packages on Hackage depend on Win32 and could potentially be affected. If you might be affected, have an opinion, or just are interested, please drop by the GitHub issue. There is also a thread on the libraries mailing list. Thanks, Yitz

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can't nterrupt winGHCi or ghci
by Kees Bleijenberg 18 Mar '15

18 Mar '15

With a haskell program I generated a very big haskell file. Sometimes the generated haskell is not valid. If I load the genearated file in winGHCi a flood of error messages streams over the screen. If I press ctrl-break WinGHCi stops and says interrupted. If I click Ok the program continues printing error messages. Ctrl-c or just c, doesn't help. So I have to wait until WinGHCi has shown all error messages (can take minutes) or close the WinGHCi window. Ghci has (on Windows) the same behaviour. Is there a special key combination to stop Ghc or Ghci or is this a bug? Kees

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[TFP'15] final call for papers - deadline extended march 31 -
by Peter Achten 18 Mar '15

18 Mar '15

----------------------------- L A S T C A L L F O R P A P E R S ----------------------------- ======== TFP 2015 =========== 16th Symposium on Trends in Functional Programming June 3-5, 2015 Inria Sophia Antipolis, France http://tfp2015.inria.fr/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. The selected revised papers will be published as a Springer Lecture Notes in Computer Science (www.springer.com/lncs) volume. TFP 2015 will be the main event of a pair of functional programming events. TFP 2015 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on June 2nd. The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * and in Soesterberg (The Netherlands) in 2014. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == INVITED SPEAKERS == TFP is pleased to announce talks by the following two invited speakers: * Laurence Rideau is a researcher at INRIA and is interested in the semantics of programming languages , the formal methods, and the verification tools for programs and mathematical proofs. She participated in the beginnings of the Compcert project (certified compiler), and is part of the Component Mathematical team in the MSR-INRIA joint laboratory, who performed the formalization of the Feit-Thompson theorem successfully. Thirty years ago, computers barged in mathematics with the famous proof of the Four Color Theorem. Initially limited to simple calculation, their role is now expanding to the reasoning whose complexity is beyond the capabilities of most humans, as the proof of the classification of finite simple groups. We present our large collaborative adventure around the formalization of the Feit-Thompson theorem (http://en.wikipedia.org/wiki/Feit%E2%80%93Thompson_theorem) that is a first step to the classification of finite groups and that uses a palette of methods and techniques that range from formal logic to software (and mathematics) engineering. * Anil Madhavapeddy == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2015 program chair, Manuel Serrano. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == SPONSORS == TFP is financially supported by Erlang Solutions. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp2015 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == IMPORTANT DATES == Submission of draft papers: March 31, 2015 Notification: April 7, 2015 Registration: May 4, 2015 TFP Symposium: June 3-5, 2015 Student papers feedback: June 9, 2015 Submission for formal review: July 1, 2015 Notification of acceptance: September 8, 2015 Camera ready paper: October 8, 2015 == PROGRAM COMMITTEE == Janis Voigtländer University of Bonn, DE Scott Owens University of Kent, UK Neil Sculthorpe Swansea University, UK Colin Runciman University of York, UK Manuel Serrano Inria (PC chair), FR Rinus Plasmeijer University of Nijmegen, NL Tomas Petricek University of Cambridge, UK Marco T. Morazan Seton Hall University, USA Wolfgang De Meuter Vrije Universiteit Brussel, BE Michel Mauny Ensta ParisTech, FR Sam Lindley The University of Edinburgh, UK Daan Leijen Microsoft, USA Jurriaan Hage Utrecht University, NL Andy Gill University of Kansas, USA Thomas Gazagnaire University of Cambrige, UK Lars-Ake Fredlund Universidad Politécnica de Madrid, ES Jean-Christophe Filliatre Université Paris Sud Orsay, FR Marc Feeley Université de Montréal, CA Olaf Chitil University of Kent, UK Edwin Brady University of St Andrews, UK

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Haskell Weekly News
by Kim-Ee Yeoh 18 Mar '15

18 Mar '15

*Top picks:* - Simon Marlow, who built much of GHC's runtime, now works at Facebook. He contributes to Haxl, a tool to "automatically batch and overlap requests for data from multiple data sources in a rule engine for identifying malicious content." In a talk last year at FP Days <http://fpdays.net/2014/sessions/index.php?session=21>, he explains how you too can use Haxl in your projects. The video was just made available at <http://www.infoq.com/presentations/remote-access-haxl> InfoQ <http://www.infoq.com/presentations/remote-access-haxl>. Hat-tip to redditor Pikachut <http://www.reddit.com/r/haskell/comments/2z049t/remote_access_made_easy_and…> for links to slides <https://github.com/simonmar/fpdays14/blob/master/Haxl%20-%20FP%20Days%20201…> and code <https://github.com/simonmar/fpdays14>. - GHC builds can be frightfully slow. Karel Gardas <http://comments.gmane.org/gmane.comp.lang.haskell.ghc.devel/8233> wonders how he can speedup builds on his 32-threaded SPARC T2000. Folks point him toward Andrey Mokhov's ongoing investigation <https://github.com/snowleopard/shaking-up-ghc> of Neil Mitchell's shake <http://community.haskell.org/~ndm/shake/> to replace GHC's current use of GNU make. SPJ hopes that it "will make a big difference." - Back in September last year, Ahmad Fatoum <https://github.com/haskell/win32/issues/24> asks whether we can fix the Win32 FFI so that it supports optional params? Some Win32 functions -- e.g. see FindWindow <https://msdn.microsoft.com/en-us/library/windows/desktop/ms633499%28v=vs.85…> -- accept NULL pointers as invocation of default behavior, currently impossible from Haskell. One option is to make copies of over 100 functions to prevent existing client code from breaking. A more radical one is to extend the existing FFI in-place and alter the type signatures by Maybe-fying the params that are actually optional. The vocal opinion is in favor of the latter, deeper fix. Yitzchak Gale <http://haskell.1045720.n5.nabble.com/Proposed-significant-breaking-changes-…> recently brought up the issue on haskell-libraries. - Maurizio Vitale <http://haskell.1045720.n5.nabble.com/extracting-a-single-declaration-using-…> asks for a way to extract verbatim a single definition from Haskell source. Michael Sloan shows a way to do it <https://gist.github.com/mgsloan/ac77dd33326322fc6ccd> using haskell-src-exts <https://hackage.haskell.org/package/haskell-src-exts>. - Conor McBride <http://www.reddit.com/r/haskell/comments/2y25ly/language_deriveapplicative/…> discusses Naperian / Representable functors as a subclass of Applicative. Elsewhere in the thread, Ed Kmett remarks that "The Log of a coinductive container being an inductive type is a new result for me." - A subreddit called haskelltil <http://www.reddit.com/r/haskelltil/> (TIL = Today I Learned) for little discoveries was announced on haskell reddit <https://pay.reddit.com/r/haskell/comments/2zcnsl/haskelltil_a_subreddit_for…> . *Repo of the week: *Calvin Cheng <http://calvinx.com/about/>, co-founder and CTO at AlgoAccess, releases code <https://github.com/calvinchengx/learnhaskell> and slides <http://www.slideshare.net/calvinchengx/functional-programming-part01> on learning Haskell for the OO programmer. *In your neighborhood:* - Kat Chuang organized an NYC-based study group <http://www.meetup.com/NY-Haskell/events/220997346/> to dissect Nishan Shukla's Haskell Data Analysis Cookbook <http://haskelldata.com/>. "This is your AHA! <https://github.com/ny-haskell/Action-Haskell-Analysis> Haskell Moment." - Redditor netroby <http://www.reddit.com/r/haskell/comments/2y4e0r/haskell_chinese_user_group_…> launched a QQ instant messaging group <http://en.wikipedia.org/wiki/Tencent_QQ>, code number 424801832, to talk Haskell. *Quote of the week:* Dimitri DeFigueiredo <https://mail.haskell.org/pipermail/beginners/2015-March/014900.html> says about the haskell-beginners mailing list, "This is the friendliest mailing list I have ever subscribed to. The people in this list are so nice that even if Haskell were not so wonderfully elegant, I would like to learn it just to be able to chat and work with them." *Acknowledgments* Thanks to Henk-Jan van Tuyl for help with the Quotes. Thanks to Gershom Bazerman, Roman Cheplyaka, and Artyom for offering assistance on web hosting HWN. *Letter to the Editor:* Can I just say that I'm very much in favour of the new format. There's interesting content that can't just be gained from occasionally checking reddit, there's an actual comment on each one rather than just the headline, and most importantly the quote is still there ;) Sorry if this is adding to a deluge of email that you're now getting because of this. Yours, Anon *Response:* Dear Anon, Glad you're enjoying HWN as much as I do creating and publishing it. I love the quotes too, and I depend on my readers to email me what they'd like to share with Team Haskell. (Sneak peak at the top-secret contingency plan: If I don't have quotes for the week, I ransack the archives. Shhh!! Don't tell anyone.) Yours is the only thank you I received. Appreciation like yours is what keeps HWN going, so let me thank you in return for sharing the love, for I remain Yours editorially, Kim-Ee Yeoh p.s. Stay tuned for an upcoming editorial on what the future holds for HWN. -- Kim-Ee

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extracting a single declaration using haskell-src-ext
by Maurizio Vitale 17 Mar '15

17 Mar '15

G'day! Is there an easy way to extract a single declaration from an Haskell source file and print it with the original formatting? [I could live with pretty printing, but the default prettyPrint gives me a very strange formatting where something like: main = do putStrLn "Hello" becomes: main = do putStrLn "Hello"] The closest I've got is to parse the file with comments and then get inside Module ...decls and do printExact with an empty comment list for the decl I'm interested in. This gives me the original formatting and empty lines where comments were that I can then remove. But if there was anything already out there that just take a String for the toplevel I'm interested in and convert that only, I'd love to know. Thanks, Maurizio

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Full page Haskell 2010 report request
by Christopher Done 17 Mar '15

17 Mar '15

Hi, has anyone managed to produce a single-page HTML export of the Haskell 2010 report? Here's one I made by munging together the multi-page one: https://www.haskell.org/report/mono/2010#x8-440003.12 (this link is currently unpublished on the home page). But it's incomplete due to link references and things. I'd like a proper output but in the interest of my mental health prefer not to touch LaTeX except by proxy through other poor souls. Anyone managed to do this? A full, linkable page (don't worry about the section/para links, I added them with a few lines of JS, I can do it again) is really conducive to discussion of Haskell's finer points. Ciao!

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Whole program analysis as a GHC plugin
by David Darais 17 Mar '15

17 Mar '15

Hello cafe, I'm currently implementing a whole-program analysis for Haskell programs using the GHC api. I have a GHC plugin written and I'm successfully slurping in programs and processing them with my analysis tools. I'm getting stuck when my tool encounters an identifier that lives in another module. I'd like to unfold the identifier and retrieve its source code, allowing me to do whole-program analysis. Assuming I have the source code of the whole program, and that I get to (re)compile everything with my plugin enabled, I'm wondering: 1) Does the GHC api already support a nice way of doing this? 2) I see that there is support for unfolding identifiers if they've been specially marked in .hi files. Is there a way to mark everything to support unfolding, and would this give me the information I'm looking for? 3) Does anybody have experience with implementing a whole-program analysis for GHC as a plugin, and if so, how did you go about it? Many Thanks, David

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