- Haskell-Cafe - Haskell.org

confusion about cabal-install install
by Alexander Solla 11 May '14

11 May '14

I'm trying to install cabal-install. I ran: $ cabal update && cabal install cabal-install After compiling, cabal reported: Installing executable(s) in /home/ajs/.cabal/bin Installed cabal-install-1.20.0.1 But now consider: $ cabal --version cabal-install version 1.16.0.2 using version 1.16.0 of the Cabal library $ which cabal /home/ajs/.cabal/bin/cabal $ /home/ajs/.cabal/bin/cabal --version cabal-install version 1.20.0.1 using version 1.20.0.0 of the Cabal library I am confused. My path is set to point to "$HOME/.cabal/bin/:$PATH"

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Re: [Haskell-cafe] loop problem
by Roelof Wobben 11 May '14

11 May '14

Stuart A. Kurtz schreef op 10-5-2014 18:02: > Dear Roelof, > > >> Thanks , it's working now like this : >> >> loop :: Int -> IO () >> loop n = do >> if n <= 100 >> then do >> putStrLn (show n ); >> putChar ' '; >> putStrLn (show (n * n )) ; >> loop (n + 1); >> else >> return () ; >> >> main :: IO () >> main = loop 1 >> >> But the outcome looks like this: >> >> 1 >> 1 >> 2 >> 4 > You're getting good advice, but this strikes me as extraordinarily unidiomatic Haskell -- it reads like a translated C program. Fixing it doesn't eliminate that sense. > > Why not this? > > main :: IO () > main = putStr . unlines . map (unwords . map show . (\n -> [n,n^2])) $ [1..100] > > Peace, > > Stu > > Like I said to another person . Im now following this course: https://www.fpcomplete.com/school/starting-with-haskell/basics-of-haskell But I get the feeling it's not a good one. Maybe find another tutorial/course where I can learn it the right way and do a lot of exercises so I can see if I understand things right. I learn the best by doing things and not only read about it. Roelof

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ANN: th-alpha
by Julian Arni 11 May '14

11 May '14

Hello Cafe, I just uploaded th-alpha to hackage [1]. This package primarily provides one function, 'areExpAEq', which compares expressions for alpha equivalence (equivalence modulo renaming of bound variables): >> areExpAEq [| \x -> x |] [| \y -> y |] True Pretty unambitious, but I found myself needing this when testing another library that uses Template Haskell, and it might be useful to others in that same situation. Cheers, Julian [1] https://hackage.haskell.org/package/th-alpha-0.1.0.2

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Re: [Haskell-cafe] loop problem
by Roelof Wobben 11 May '14

11 May '14

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Re: [Haskell-cafe] loop problem
by Roelof Wobben 10 May '14

10 May '14

Stuart A. Kurtz schreef op 10-5-2014 18:17: > Dear Roelof, > > I took a quick look at the course -- it wasn't immediately evident to me where you are in it. It looks pretty good, although more examples/exercise are always welcome. > > It occurred to me, though, if you're going to write it so it looks like C, you'd might as well take advantage of Text.Printf: > > import Control.Monad > import Text.Printf > > loop :: Int -> IO () > loop n = when (n <= 100) $ do > printf "%d %d\n" n (n^2) > loop $ n+1 > > main :: IO () > main = loop 1 > > Peace, > > Stu > > Sorry that I did not make that clear. So in your oponion this could be a good way to learn haskell very good. I get the feeling on your remarks that im not really to do things the haskell way and that is what im looking for. I have looked at the book http://learnyouahaskell.com/chapters but I miss there exercises What do you think about this one : http://www.umiacs.umd.edu/~hal/docs/daume02yaht.pdf Roelof

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intelij idea and haskell
by Roelof Wobben 10 May '14

10 May '14

Hello, Does anyone ever tried intelij idea with Haskell. I know there is a plugin. I know Intelij from the fact that I have tried to programm in Clojure and I like the enviroment. Roelof

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loop problem
by Roelof Wobben 10 May '14

10 May '14

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Bug in GHC (probably parser) regarding GADTs
by Brian Nguyen 10 May '14

10 May '14

The following does not work: data Foo a where {Foo :: (->) a (Foo a)} Someone please file a bug for me, I can't on my computer.

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Re: [Haskell-cafe] ANN: hsqml-0.3.0.0 supports Qt 5
by Robin KAY 10 May '14

10 May '14

Dear Andrew, On 09/05/14 16:42, Andrew Voron wrote: [snip] > I've tryed to compile sample project on windows - it compiles good, but > outputs: > Segmentation fault/access violation in generated code > when I'm trying to start executables. As described in the installation section on my web-site [1], this problem is most likely caused by an incompatibility between the MinGW tool-chains shipped with the Haskell Platform and with the Qt binaries. In fact, writing that in the docs was probably too coy, as the latest Haskell Platform and the latest official Qt SDK for Windows will not work together. In the past, this was not the case but the respective copies of MinGW/gcc must now be too far out of step with each other. A relatively simple fix I have found to work is to modify the GHC settings file to point to the version of gcc.exe inside the Qt SDK distribution rather than GHC's. Otherwise, you could build your own Qt binaries using GHC's MinGW but that is rather more involved. [1] http://www.gekkou.co.uk/software/hsqml/#installation Regards, -- Robin KAY

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Haddock issue tracker is now on GitHub.
by Mateusz Kowalczyk 10 May '14

10 May '14

Greetings, The Haddock issue tracker now lives at [1]. The reasoning is that a lot of people have a GitHub account already and signing up just for Trac is tedious. Hopefully this will also allow people to see more easily what's going on. This also removes the maintenance load from Haskell infrastructure: the Trac instance has been bitrotting quite a bit and recently even sending out e-mail about tickets stopped working. Old issues have been migrated over so the ticket numbers still correspond. Please file any new issues or make issue comments on GitHub. If you do not wish to use GitHub (which is fairly understandable) then please send me the report directly and I will create one for you. Thanks. [1]: https://github.com/haskell/haddock/issues?state=open -- Mateusz K.

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Templates as typeclasses?
by Mike Meyer 09 May '14

09 May '14

In going over yet again the many failings of template frameworks for building web apps, I recalled some of the features of the best of the lot, and wondered if those features might be useful in a Haskell template framework. Python's Cheetah tool makes templates almost fully functional members of Python's OO system. A template can inherit from a Python class or another template, and a Python class can inherit from a template. Which makes me wonder if something similar might not be useful in a Haskell template framework. Possibly have each template create an appropriate datatype and make it as an instance of the Template typeclass (and possibly others). Is there already a Haskell web framework that does something like this? Would it actually add value to a framework? Thanks, Mike

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[TFPIE2014] final call for participation
by Peter Achten 09 May '14

09 May '14

[FINAL CALL FOR PARTICIPATION] 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/) 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) at Soesterberg, at the “Kontakt der Kontinenten” 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 journal Electronic Proceedings in Theoretical Computer Science (EPTCS). 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). ** Invited Speaker ** TFPIE is pleased to announce that professor Johan Jeuring of Utrecht University and Open University, both in The Netherlands is giving an invited talk entitled: "Automatic tutoring and assessing functional programs". ** 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 ** Program ** The preliminary program of TFPIE 2014 is available at the home site of TFPIE 2014 and the TFPIE wiki (http://wiki.science.ru.nl/tfpie/Main_Page) This year's edition features 14 talks. The program is closed with a plenary discussion. ** Important Dates ** * 1 February 2014: TFPIE submissions open on easychair. * 14 April 2014: Early registration for TFP closes * 21 April 2014: Submission deadline for draft TFPIE papers and abstracts * 27 April 2014: Notification of acceptance for presentation * 15 May 2014: Registration for TFPIE closes - as does late registration for TFP * 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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Templates as typeclasses?
by oleg＠okmij.org 09 May '14

09 May '14

Tobias Dammers wrote: > You can do similar things with Haskell EDSL-style template systems such > as Blaze... > > Still, the big picture with these is that template building blocks are > language elements, and template code and regular code can be mixed > freely. The `h1` function from Blaze is just another Haskell function; > it takes a MarkdownM () value and returns a MarkdownM (), and because > MarkdownM happens to be a Monad, you can use do notation to sequence > HTML elements. MarkdownM () is also an instance of IsString, so if you > enable -XOverloadedStrings, you can use string literals to produce HTML > text nodes. This means that you can write, in plain Haskell: > > h1 $ do > span "Hello, " > span "world!" > > ...and it'll produce <h1><span>Hello, </span><span>world!</span></h1>. In this regard it is worth mentioning HSXML http://okmij.org/ftp/Scheme/xml.html#typed-SXML The document in HSXML is a monoid rather than a monad. It is hard to find a good reason for the document be a monad except for using a do notation, which is not needed in HSXML. The above "Hello, World!" example looks in HSXML as h1 (span "Hello") (span "world!") (Note that we don't need the space after `Hello'.) This produces the desired output (in plain text, HTML or XML). If we don't need span, we can just write h1 "Hello" "world!" The main difference from Blaze shows up if we attempt h1 (p "Hello") "world!" It will be a type error. The error message says No instance for (Build (DC CT_inline d0) (DC CT_block d0) t) arising from a use of `h1' No instance for (Build (DC CT_inline d0) (DC CT_block d0) (DC CT_inline d0)) arising from a use of `p' That is, (p "Hello") produces block-level content and h1 requires its children to be inline-level. HSXML thus implements the block/inline content model of HTML. Arbitrary many content content-level tags may be defined (in fact, HSXML, besides block/inline elements uses document-, attribute-, and table-level elements). It is possible for the same element to be usable in different contexts (e.g., 'title' can be either an attribute or an element used within 'head'). It is rendered differently in different contexts. Although the document itself is just a monoid rather than a monad, rendering a document can be in a monad. For instance, one of the rendering engines works in an IO monad so it can validate local links as the document is rendered. The resulting document does not have dead local links. > For the common case where you have a "master" template and an > incarnation, you could write a master template that takes its inner > blocks as arguments, e.g.: It is very easy to do that in HSXML. Here is an example, the top template for the ChangeLog: toHTML :: MDoc d => CLHead d -> DC CT_root d toHTML (CLHead attrs updates) = document (head (title (cdata (ha_title attrs))) (meta_tag (description (cdata (ha_description attrs)))) author_address (meta_tag (pub_date (ha_DateRevision attrs))) (head_link LR_top (href (ha_top attrs)) (title "All you can find here")) ) (body (h1 "Log of changes on" (aref (ha_top attrs) "this site")) (p nbsp) updates (change_log_prev (ha_history_first attrs)) (change_log_prev (ha_history_last attrs)) ) (One could omit many parentheses using $, but I like parentheses in this context).

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Unicode Haskell source -- Yippie!
by Rustom Mody 09 May '14

09 May '14

I'm mighty pleased to note that the following is valid Haskell code! Do others find this useful/appealing? Any possibilities on making the commented out parts work? [Pragmatics about typing this at the same speed and facility as we do with Ascii is a separate and (IMHO) solvable problem though its not the case at the moment] -------------------- import qualified Data.Set as Set -- Experimenting with Unicode in Haskell source -- Numbers x ≠ y = x /= y x ≤ y = x <= y x ≥ y = x >= y x ÷ y = divMod x y x ⇑ y = x ^ y x × y = x * y -- readability hmmm !!! π = pi -- ⌊ x = floor x -- ⌈ x = ceiling x -- Lists xs ⤚ ys = xs ++ ys -- Bools x ∧ y = x && y x ∨ y = y || y -- ¬x = not x -- Sets x ∈ s = x `Set.member` s -- or keep ∈ for list elem? s ∪ t = s `Set.union` t s ∩ t = s `Set.intersection` t s ⊆ t = s `Set.isSubsetOf` t s ⊂ t = s `Set.isProperSubsetOf` t s ⊈ t = not (s `Set.isSubsetOf` t) -- ∅ = Set.null

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Announcement: First Dutch FinTech Hackaton
by Han Joosten 09 May '14

09 May '14

Hi all, Always wanted to show the industry what Functional Programming can do? Have a look at http://dutchfintechhackathon.nl/ for this is a great opportunity to do so! please spread the word, I think it would be very cool if a FP-team would participate. Cheers! Han Joosten

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ML Family Workshop -- Last call for presentations
by oleg＠okmij.org 09 May '14

09 May '14

Important dates Monday May 19 (any time zone): Abstract submission Monday June 30: Author notification Thursday September 4, 2014: ML Family Workshop Higher-order, Typed, Inferred, Strict: ACM SIGPLAN ML Family Workshop Thursday September 4, 2014, Gothenburg, Sweden (immediately following ICFP and preceding OCaml Users and Developers Workshop) Call For Papers http://okmij.org/ftp/ML/ML14.html ML is a very large family of programming languages that includes Standard ML, OCaml, F#, SML#, Manticore, MetaOCaml, JoCaml, Alice ML, Dependent ML, Flow Caml, and many others. All ML languages, beside the great deal of syntax, share several fundamental traits. They are all higher-order, strict, mostly pure, and typed, with algebraic and other data types. Their type systems inherit from Hindley-Milner. The development of these languages has inspired a significant amount of computer science research and influenced a number of programming languages, including Haskell, Scala and Clojure, as well as Rust, ATS and many others. ML workshops have been held in affiliation with ICFP continuously since 2005. This workshop specifically aims to recognize the entire extended ML family and to provide the forum to present and discuss common issues, both practical (compilation techniques, implementations of concurrency and parallelism, programming for the Web) and theoretical (fancy types, module systems, metaprogramming). The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of the members of the ML family. We also encourage presentations from related languages (such as Scala, Rust, Nemerle, ATS, etc.), to exchange experience of further developing ML ideas. The ML family workshop will be held in close coordination with the OCaml Users and Developers Workshop. Format Since 2010, the ML workshop has adopted an informal model. Presentations are selected from submitted abstracts. There are no published proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format encourages the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. The presentations will likely be recorded. Post-conference proceedings The post-proceedings of selected papers from the ML Family and the OCaml Users and Developers workshops will be published in the Electronic Proceedings in Theoretical Computer Science (EPTCS). The Program Committee shall invite interested authors of selected presentations to expand their abstract for inclusion in the proceedings. The submissions are to be reviewed according to the EPTCS standards. Coordination with the OCaml Users and Developers Workshop The OCaml workshop is seen as more practical and is dedicated in significant part to the OCaml community building and the evolution of the OCaml system. In contrast, the ML family workshop is not focused on any language in particular, is more research oriented, and deals with general issues of the ML-style programming and type systems. Yet there is an overlap, which we are keen to explore in various ways. The authors who feel their submission fits both workshops are encouraged to mention it at submission time or contact the Program Chairs. Scope We acknowledge the whole breadth of the ML family and aim to include languages that are closely related (although not by blood), such as Rust, ATS, Scala, Typed Clojure. Those languages have implemented and investigated run-time and type system choices that may be worth considering for OCaml, F# and other ML languages. We also hope that the exposure to the state of the art ML might favorably influence those related languages. Specifically, we seek research presentations on topics including but not limited to * Design: concurrency, distribution and mobility, programming for the web and embedded systems, handling semi-structured data, facilitating interactive programming, higher forms of polymorphism, generic programming, objects * Implementation: compilation techniques, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * Type systems: fancy types, inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * Applications: case studies, experience reports, pearls, etc. * Environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * Education: ML and ML-like languages in college or high-school, in general or computer science curriculum. Four kinds of submissions will be accepted: Informed Positions, Research Presentations, Experience Reports and Demos. * Informed Positions: A justified argument for or against a language feature. The argument must be substantiated, either theoretically (e.g., by a demonstration of (un)soundness, an inference algorithm, a complexity analysis), empirically or by a substantial experience. Personal experience is accepted as justification so long as it is extensive and illustrated with concrete examples. * Research Presentations: Research presentations should describe new ideas, experimental results, or significant advances in ML-related projects. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. These presentations should be structured in a way which can be, at least in part, of interest to (advanced) users. * Experience Reports: Users are invited to submit Experience Reports about their use of ML and related languages. These presentations do not need to contain original research but they should tell an interesting story to researchers or other advanced users, such as an innovative or unexpected use of advanced features or a description of the challenges they are facing or attempting to solve. * Demos: Live demonstrations or short tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. (You will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Submission Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. A submission should have a synopsis (2-3 lines) and a body between 1 and 2 pages, in one- or two-column layout. The synopsis should be suitable for inclusion in the workshop program. Submissions must be uploaded to the workshop submission website before the submission deadline (Monday May 19, 2014). For any question concerning the scope of the workshop or the submission process, please contact the program chair. Program Committee Kenichi Asai Ochanomizu University, Japan Matthew Fluet Rochester Institute of Technology, USA Jacques Garrigue Nagoya University, Japan Dave Herman Mozilla, USA Stefan Holdermans Vector Fabrics, Netherlands Oleg Kiselyov (Chair) Monterey, CA, USA Keiko Nakata Tallinn University of Technology, Estonia Didier Remy INRIA Paris-Rocquencourt, France Zhong Shao Yale University, USA Hongwei Xi Boston University, USA

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reactive-banana GLFW-b event register example
by Vlad Lopatin 09 May '14

09 May '14

Hello List, I am trying to make the following quasi-code example work: registerMouseButton :: IO (Event MouseButton) registerMouseButton = do(addHandler, fire) <- newAddHandler setMouseButtonCallback $ \button _ -> fire button fromAddHandler addHandler According to http://stackoverflow.com/questions/8631816/reactive-banana-how-to-create-an… looks like it should be something straightforward. Can somebody show an example of a functional code that registers a key or a mouse event? Thanks, Vladimir

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Need pointer to explain CPU usage
by Robin Palotai 08 May '14

08 May '14

Hello List, I have a tiny scotty-based application, which when compiled and started in profiling mode, consumes 10x CPU it does in normal mode. Interestingly, if I fire a request on the app, CPU usage drops to minimal after finishing the request for a while. But after a few minutes it starts consuming 10x CPU again. First I would have suspected scotty, however collecting time profiling info with -p, -P or -Pa doesn't reveal much. That still not rules scotty out however. To make matters worse, I inspect strange time profiling tick count I can't explain - the .prof file says "2 ticks @ 1000 us", while the application was clearly running idle for a good few seconds. Any ideas what am I missing? Thanks, Robin

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Haskell Weekly News: Issue 294
by Daniel Santa Cruz 08 May '14

08 May '14

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OCL 2014: First Call for Papers
by Achim D. Brucker 08 May '14

08 May '14

(Apologies for duplicates) CALL FOR PAPERS 14th International Workshop on OCL and Textual Modeling Applications and Case Studies (OCL 2014) Co-located with ACM/IEEE 17th International Conference on Model Driven Engineering Languages and Systems (MODELS 2014) September 28-30 (tbc), 2014, VALENCIA, SPAIN http://www.software.imdea.org/OCL2014/ Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- metamodeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. Venue ===== The workshop will be organized as a part of MODELS 2014 Conference in Valencia, Spain. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, and 2013 in Miami. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Two types of papers will be considered: * short papers (6 pages) and * full papers (10 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://www.easychair.org/conferences/?conf=ocl2014) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Authors of selected papers will be invited to submit an extended version of their workshop paper to a special issue of the Electronic Communications of the EASST (http://journal.ub.tu-berlin.de/eceasst) Important Dates =============== Submission of papers: July 11, 2014 Notification: August 8, 2014 Workshop date: one day during September 28-30, 2014 Organizers ========== Achim D. Brucker, SAP AG, Germany Carolina Dania, IMDEA Software Institute, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Programme Committee (partly confirmation pending) =================== Michael Altenhofen, SAP AG, Germany Thomas Baar, University of Applied Sciences Berlin, Germany Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Fabian Buettner, Ecole des Mines de Nantes, France Achim D. Brucker, SAP AG, Germany Jordi Cabot, INRIA-Ecole des Mines de Nantes, France Yoonsik Cheon, University of Texas, USA Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Madrid, Spain Carolina Dania, IMDEA Software Institute, Madrid, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Atos Research, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Pieter Van Gorp, Eindhoven University of Technology, The Netherlands Heinrich Hussmann, LMU Munchen, Germany Tihamer Levendovszky, Vanderbilt University, USA Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Shane Sendall, Snowie Research SA, Switzerland Michael Wahler, ABB Switzerland Ltd Corporate Research, Switzerland Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ Paris-Sud, France Steffen Zschaler, King’s College, London, UK -- Dr. Achim D. Brucker, SAP AG, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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I would like to add haskell-russian@ mailing list to "User groups" wiki page
by Alexander Alexeev 07 May '14

07 May '14

Hello. Could someone add https://groups.google.com/forum/#!aboutgroup/haskell-russian ... to this wiki page http://www.haskell.org/haskellwiki/User_groups#Russia ... please? Or I could do it by myself if someone would create me an account at Haskell Wiki. Username I would like to have - eax (like @ Hackege). -- Best regards, Alexander Alexeev http://eax.me/

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Splicious is now accepting applications for alpha user trials
by Greg Meredith 07 May '14

07 May '14

Dear Haskellians and CurriedOnes, Splicious is now accepting applications for alpha user trials<http://www.splicious.net/> . To learn more about splicious, apart from that it is written in a higher-kinded functional language and makes extremely cool use of delimited continuations, please check out these links. - splicious and distributed identity<https://www.youtube.com/watch?v=Tz2pmVGgrG4> - splicious, BitCoin, content creation, curation, and sharing<https://www.youtube.com/watch?v=f700b6IHZcc> - Vlad Patryshev's review of splicious<https://www.youtube.com/watch?v=nRmu9-AL2OM> - The specialK white paper<https://docs.google.com/document/d/1aM5OIJWOyW89rHdUg6d9-YVbItdtxxiosP_fXZQ…> - A link to our crowd funding site<http://www.rockethub.com/projects/42500-splicious> Love to all, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Cabal exit codes when work to be done
by Andrew Cowie 06 May '14

06 May '14

Is there a way to run e.g. `cabal install --only-dependencies` and have it return a non-zero exit code if, for instance, there IS something to do? I tried `--dry-run` but the exit code is still 0 if something is missing. [or have it return non-zero if there isn't work to do, or...] AfC Sydney

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Questions about Functor, Applicative, Monad
by Corentin Dupont 06 May '14

06 May '14

Hi everybody! Two questions in one: - When you define and instance of a Monad, why don't you get the Applicative and Functor instances for free? I seem that you can mechanically write them. - Do you have examples of things that are Functors but not Applicative? Applicative but not Monads? Tchuss Corentin

3 2

extensible records - Vinyl - alternatives?
by Peter Althainz 05 May '14

05 May '14

Dear All, I'm searching a handsome library for handling large amounts of configuration data in a flexible manner. So to speak I want to use some kind of inheritance of data fields. I think this is where the extensible records discussion is a proper description of my use case. I've found 2 libraries making this work, so far: HList Vinyl I currently prefer Vinyl due to the complex types in HList, lens compatibiliy, named labels. (I love the HList and OOHaskel papers, but for practical use, I currently prefer Vinyl). What do you think, what is the best way nowadays to use extensible records? Is Vinyl a good choice from your point of view? regards Peter

7 13

ANN: hsqml-0.3.0.0 supports Qt 5
by Robin KAY 05 May '14

05 May '14

Dear All, I would like to announce version 0.3.0.0 of the HsQML graphics library, a Haskell binding to the Qt Quick framework. The library allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell and vice versa. Documentation including a brief tutorial is available on my web-site [1]. HsQML requires an installation of Qt 5 (including Qt Quick) present on your path. This is the first release to support Qt 5 and it has been a much requested feature. Several other new features are also introduced in this release, including support for marshalling lists and signalling property notifications. For more information, please see the release announcement [2]. The release is available for download from Hackage [3] along with a demo application implementing Nine Men's Morris [4] and some other sample programs [5]. Linux, Windows, and Mac OS platforms should all be supported. [1] http://www.gekkou.co.uk/software/hsqml [2] http://blog.gekkou.co.uk/2014/05/hsqml-0300-released-now-with-qt-5.html [3] http://hackage.haskell.org/package/hsqml-0.3.0.0 [4] http://hackage.haskell.org/package/hsqml-demo-morris-0.3.0.0 [5] http://hackage.haskell.org/package/hsqml-demo-samples-0.3.0.0 Regards, -- Robin KAY

1 0

Quick question on Lens
by Corentin Dupont 05 May '14

05 May '14

Hello! I have a record like this: data A = { _bs :: [B]} funcA :: State A () funcA = bs ??? funcB funcB :: State B () funcB = ... What lens operator should I use instead of ???, to update all my Bs? Cheers, Corentin

3 2

Experimenting a new plotting library
by Kai Zhang 05 May '14

05 May '14

Hi cafe, I'm experimenting a new plotting library which makes use of the API of diagrams. I decided to start this work, because: 1. There is few plotting library written in pure Haskell 2. Haskell-chart, one of the most featured library, is hard to extend and there is no straightforward way to compose or modify the plots generated by this library. I had some ideas for the new library. You can get some feeling from here: https://github.com/kaizhang/haskell-plot I'm still relatively new to haskell and don't have much experience in making plots either. Hopefully I can get some suggestions from you guys and improve my programming skills through this process.

2 3

[ANN] codex 0.0.2 - generate tags file from dependencies
by Alois Cochard 04 May '14

04 May '14

Hi, Just to let you know that I released a new version of 'codex' (a tool which allow to generate a tags[1] file for a given cabal project using the sources of all the dependencies of that project). The main new feature is the support of "workspace resolution", which basically mean that if you have the following projects: ~/projects/foo ~/projects/bar Where 'foo' have a dependency on 'bar'... when you run `codex update` in 'foo' directory it will generate tags using the sources available in the sibling directory 'bar' instead of taking it from hackage (as long as versions match). It's specially useful when working on multiple modules, kudos to Edward Kmett for the original idea. [1]Those tags file basically contain references to functions/types definition in source code and allow "jump to definition" like functionality in text editors. source: http://github.com/aloiscochard/codex hackage: http://hackage.haskell.org/package/codex -- *Alois Cochard* http://twitter.com/aloiscochard http://github.com/aloiscochard

1 0

cabal repl fails with GHC 7.8.2
by Omari Norman 04 May '14

04 May '14

I have a package for which "cabal repl" works fine with cabal version 1.20 and GHC 7.6.3. The package has a lot of FFI bindings and uses hsc2hs, so getting a REPL without "cabal repl" is a pain. The package also comes with the C sources that get linked in. But with GHC 7.8.2, "cabal repl" fails with: Loading object (static) dist/build/decnumber/src/decDouble.o ... /usr/bin/ld: dist/build/decnumber/src/decDouble.o: relocation R_X86_64_32S against `DPD2BIN' can not be used when making a shared object; recompile with -fPIC Any idea on how to fix this? "cabal build" and "cabal install" work fine and I can even build and execute a test binary against the library. It seems the problem is just with "cabal repl". Thanks. Omari

5 14

Fwd: SBLP 2014 new deadline
by Simon Thompson 04 May '14

04 May '14

CALL FOR PAPERS - NEW DEADLINE: May 23rd 18th Brazilian Symposium on Programming Languages (SBLP 2014) A member of CBSoft joint conference http://www.ic.ufal.br/evento/cbsoft2014/ to be held in Maceio, Brazil. SBLP happens on Thursday, Oct 2nd, and Friday, Oct 3rd. CBSoft dates are September 28, 2014 to October 3, 2014 This CFP is also available at the webpage of the event: http://www.ic.ufal.br/evento/cbsoft2014/en/call-for-papers-sblp.html IMPORTANT DATES Paper abstract submission: May 23rd, 2014 Full paper submission: May 23rd, 2014 Notification of acceptance: June 27th, 2014 Final papers due: July 11th, 2014 INTRODUCTION The 18th Brazilian Symposium on Programming Languages, SBLP 2014, will be held in Maceio, a beautiful city in the Northeastern part of Brazil. The conference will happen on October 2nd, and October 3rd, 2014. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. SBLP 2014 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. Papers should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. Accepted papers written in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer. For this reason, all papers must be prepared using the LNCS template, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. We encourage the submission of short papers reporting partial results of on-going master dissertations or doctoral theses. Submissions should be done through the SBLP 2014 page at EasyChair, which is available at http://www.easychair.org/conferences/?conf=sblp2014. As in previous editions, a journal's special issue, with selected papers from accepted contributions, is anticipated. Selected papers from the 2003 to the 2008 editions of SBLP were published in special issues of the Journal of Universal Computer Science, by Springer. The post-proceedings of SBLP from 2009 to 2012, also with selected papers from the conference, have been published as special issues of Science of Computer Programming, by Elsevier. KEYNOTE SPEAKERS Louis-Noel Pouchet, University of California Los Angeles Fabrice Rastello, INRIA PROGRAMME CHAIR Fernando Magno Quintao Pereira, UFMG PROGRAMME COMMITTEE Alberto Pardo, Universidad de la Republica Alex Garcia, IME Alvaro Moreira, Federal University of Rio Grande do Sul Andre Rauber Du Bois, Federal University of Pelotas Carlos Camar√£o, Federal University of Minas Gerais Christiano Braga, Fluminense Federal University Fabio Mascarenhas, Federal University of Rio de Janeiro Fabrice Rastello, INRIA Fernando Pereira, Federal University of Minas Gerais Fernando Castor, Federal University of Pernambuco Francisco Carvalho-Junior, Federal University of Ceara Hans-Wolfgang Loidl, Heriot-Watt University Jeremy Singer University of Glasgow Jo√£o Saraiva, University of Minho Joao F. Ferreira, Teesside University Louis-Noel Pouchet, University of California, Los Angeles Lucilia Figueiredo, Federal University of Ouro Preto Luis Barbosa, University of Minho Manuel A. Martins, University of Aveiro Marcello Bonsangue, Leiden University Marcelo Maia, Federal University of Uberlandia Marcelo D'Amorim, Federal University of Pernambuco Mariza Bigonha, Federal University of Minas Gerais Martin Musicante, Federal University of Rio Grande do Norte Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Rafael Lins, Federal University of Pernambuco Renato Cerqueira, PUC-Rio Roberto Bigonha, Federal University of Minas Gerais Roberto Ierusalimschy PUC-Rio Rodrigo Geraldo, Federal University of Ouro Preto Sandro Rigo, State University of Campinas Sergio Medeiros, Federal University of Rio Grande do Norte Simon Thompson, University of Kent Varmo Vene, University of Tartu Zongyan Qiu Peking University Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

1 0

Re: [Haskell-cafe] Haskell Programming Font with Ligatures
by Garrett Mitchener 04 May '14

04 May '14

I just tried this in gedit and it looks *really* nice! Thanks for putting this together. Doesn't work in emacs though :-(

4 4

Parsec lookAhead with choice (<|>) doesn't work as expected?
by Baojun Wang 03 May '14

03 May '14

Hi List, There is my first email on haskell-cafe, I must admit haskell is a amazing language even though I found it hard to program. I'm trying to use Parsec to parse a text file something like: format 1) HEADER BODY format 2) BODY BODY = many DATA_FIELDS I must distinguish the format is either 1) or 2), because I need process them differently. So I'm trying to use lookAhead & <|>, something like: format1Header = do HEADER_parser1 HEADER_parser2 return .. format2Header = do -- whatever return .. headerVersion = do version <- (lookAhead format1Header) <|> (lookAhead format2Header) return version Howerver if format1Header parser failed headerVersion just return failure without trying format2Header parser. Full source code is attached, you can load the source then just run ``main'' I searched seems similar issue had been reported: https://groups.google.com/forum/#!topic/fa.haskell/NGjQqR05-TA Is there anything I'm fundamentally wrong with lookAhead and <|> in Parsec? How can I get headerVersion works as expected? Thank you Baojun

3 4

Prevent replicateM_ from repeating expensive (pure) computations
by suhorng Y 01 May '14

01 May '14

Hi all, I've been writing loops in a monad using `replicateM_`. However, recently I discovered that it caused my program become extremely slow. For example, > module Main where > > import Control.Monad (replicateM_) > > fib :: Int -> Int > fib 0 = 0 > fib 1 = 1 > fib n = fib (n-1) + fib (n-2) > > main = do > n <- readLn > let fn = fib n > replicateM_ 20 (print fn) The program ran fine if no optimization is turned on. However, when I used `-O` or `-O2` with GHC (7.4/7.6), the program became extremely slow, and if I replaced `replicateM_` by `mapM_`, the problem disappeared. I suspected that GHC inlined `fn`, causing `replicateM_` to recalculate the value in every loop. Though the problem could be solved by either using `mapM_`, lifting `print fn` to a global definition or manually demanding it be evaluated strictly, I would expected `fn` not to be computed multiple times. Any suggestions around this? Thanks! Sincerely, suhorng

2 1

forLoop + strict State monad is much faster than foldl'
by Niklas Hambüchen 01 May '14

01 May '14

This is just a short notice that using foldl' (+) 0 [0..100000::Int] is over 10 times slower than using flip execState 0 $ forLoop (0 :: Int) (< n) (+1) $ \i -> do x <- get put $! x + i with `loopFor` as on https://github.com/nh2/loop. Even using an IORef is twice as fast as the pure foldl' (but still 5 times slower than strict State). The benchmark is at http://htmlpreview.github.io/?https://github.com/nh2/loop/blob/master/resul…. (All benchmarks are linked from https://github.com/nh2/loop.) You can see there that the problem is gone when using Vector.foldl', but only for Int - for Word32 it persists. It seems that manual looping is beneficial even when dealing with prime examples of pure code that GHC ought to optimize well. Niklas

8 20

ANN: hledger-0.23, hledger-web-0.23
by Simon Michael 01 May '14

01 May '14

I'm pleased to announce hledger and hledger-web 0.23! This release includes command-line fixes and polish, a new accounts command, and a number of changes to the balance command relating to --depth, --flat, and multicolumn mode, which I find has made it much more useful. Release notes: http://hledger.org/release-notes#hledger-023-201451 . Contributors to this release: Peter Simons and Marko Kocić. hledger (http://hledger.org) is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. It can also read CSV or timelog files. It provides useful reports, and can also help you record new transactions interactively. Add-on commands include hledger-web (a web interface), hledger-irr (for calculating internal rate of return) and hledger-interest (for generating interest transactions). hledger is inspired by and largely compatible with Ledger. Installation: cabal update; cabal install hledger [hledger-web] (see http://hledger.org/installing for help) or sponsor a binary at http://hledger.org/download Best! -Simon

1 0

Re: [Haskell-cafe] Stupid newbie question of the day: why is newMVar in the IO monad
by Ryan Yates 01 May '14

01 May '14

There is extra work done to make this safe. See the following: https://github.com/ghc/ghc/blob/af6746fb6b5adb5ba5be6e0f647c4ebe767ce084/rt… https://ghc.haskell.org/trac/ghc/ticket/5558 Ryan On Thu, May 1, 2014 at 8:25 AM, Mikulas Patocka < mikulas(a)artax.karlin.mff.cuni.cz> wrote: > > Also note linked from the global variables page on the wiki is: > > http://www.haskell.org/haskellwiki/Top_level_mutable_state > > > > Important for the `unsafePerformIO` option is the NOINLINE pragma to > ensure > > that only one global variable exists. In the STM case it is also > important > > to use `newTVarIO` rather then `unsafePerformIO $ atomically newTVar` > which > > does not work. > > > > Ryan > > Hi > > I think it isn't correct even with NOINLINE. > > > Read the paper "Haskell on a Shared-Memory Multiprocessor" > http://research.microsoft.com/pubs/67424/2005-haskell.pdf > > Among other things, the paper presents these ideas: > > (section 3.1) When multiple threads start to evaluate the same thunk > concurrently, we could use interlocked compare-and-swap instruction to > make sure that only one thread starts to evaluate it. However, the > compare-and-swap instruction is slow and executing it at every thunk > evaluation decreases performance. So, the designers avoid the > compare-and-swap instruction (section 3.2) and consequently there is a > race condition - there is a small window where two or more threads may > evaluate the same thunk concurrently. > > Normally, this race condition doesn't matter, because Haskell is purely > functional language, so when multiple threads evaluate the same thunk they > end up with the same result - but in this particular example, where one is > using unsafePerformIO to construct a unique variable, it can hurt, because > you end up executing "unsafePerformIO $ newMVar 0" concurrently (section > 3.5 of the paper). > > > So - code like this isn't correct, the numbers returned by "uniq" may not > be unique if we hit the race condition when evaluating "global": > > global :: MVar Integer > global = unsafePerformIO $ newMVar 0 > {-# NOINLINE global #-} > uniq = modifyMVar global (\i -> return (i+1,i)) > > > Has anything changed since that time, so that this construct is safe? > > Mikulas >

1 0

Dealing with GHC 7.8
by Ivan Lazar Miljenovic 01 May '14

01 May '14

I've been trying to update the various Haskell packages for Exherbo, but for some packages I keep getting errors like this (taken from vector-th-unbox, which is ultimately needed by criterion): runhaskell Setup copy --destdir=/var/tmp/paludis/build/dev-haskell-vector-th-unbox-0.2.0.2/image/ Installing library in /var/tmp/paludis/build/dev-haskell-vector-th-unbox-0.2.0.2/image/usr/lib64/x86_64-linux-ghc-7.8.2/vector-th-unbox-0.2.0.2 Setup: Error: Could not find module: Data.Vector.Unboxed.Deriving with any suffix: ["dyn_hi"] in the search path: ["dist/build"] When trying to find solutions for this, every result happens to be for Windows... which this isn't. Has some way of dealing with building packages using just runhaskell Setup changed? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

2 3

Haskell Weekly News: Issue 293
by Daniel Santa Cruz 01 May '14

01 May '14

Welcome to issue 293 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from April 20 to 26, 2014 Quotes of the Week * monochrom: linux's ulimit for main memory will not help because linux kernel's allocator is a lazy yes-man (think irrefutable pattern) * edwardk: lens is the only library I've ever written that generates hate mail and invites to give talks in almost equal measure * shachaf: You don't need lens to produce or consume lenses. You just need it to complain about them. * glguy_: Reddit has shown you don't need to have used the lens package to complain about it, either Top Reddit Stories * What I Wish I Knew When Learning Haskell 2.0 Domain: dev.stephendiehl.com, Score: 191, Comments: 37 On Reddit: [1] http://goo.gl/PkvrnZ Original: [2] http://goo.gl/ljYe4w * Announcing cabal 1.20 Domain: blog.johantibell.com, Score: 141, Comments: 38 On Reddit: [3] http://goo.gl/H16RoZ Original: [4] http://goo.gl/2r3vBH * Lens is unidiomatic Haskell Domain: ro-che.info, Score: 124, Comments: 231 On Reddit: [5] http://goo.gl/Hm7Zbl Original: [6] http://goo.gl/HrI0L6 * wreq: a capable new HTTP client library Domain: serpentine.com, Score: 121, Comments: 128 On Reddit: [7] http://goo.gl/kUZp4s Original: [8] http://goo.gl/y8QFpR * Applicative became a superclass of Monad Domain: ghc.haskell.org, Score: 94, Comments: 46 On Reddit: [9] http://goo.gl/0OIiPz Original: [10] http://goo.gl/DKCUsD * Open Sourced! Cryptol: A Domain Specific Language for Specifying Cryptographic Algorithms Domain: cryptol.net, Score: 91, Comments: 7 On Reddit: [11] http://goo.gl/32oaZd Original: [12] http://goo.gl/30Y9g0 * Model-view-controller, Haskell-style Domain: haskellforall.com, Score: 81, Comments: 55 On Reddit: [13] http://goo.gl/yJH5Ck Original: [14] http://goo.gl/pjqFPN * Lenses don't compose backwards. Domain: self.haskell, Score: 73, Comments: 35 On Reddit: [15] http://goo.gl/p9e9v8 Original: [16] http://goo.gl/p9e9v8 * 14 Haskell Projects accepted for Google Summer of Code Domain: google-melange.com, Score: 59, Comments: 28 On Reddit: [17] http://goo.gl/bw3jrU Original: [18] http://goo.gl/O91dDJ * New issue of The Monad Reader (#23) Domain: themonadreader.wordpress.com, Score: 48, Comments: 11 On Reddit: [19] http://goo.gl/PgFaAg Original: [20] http://goo.gl/M62yM8 * Lens from the ground up Domain: mth.io, Score: 44, Comments: 25 On Reddit: [21] http://goo.gl/7hb2dZ Original: [22] http://goo.gl/W2C4x2 * Lenses You Can Make at Home Domain: duplode.github.io, Score: 40, Comments: 17 On Reddit: [23] http://goo.gl/5Fx3re Original: [24] http://goo.gl/qs2nbt Top StackOverflow Questions * How do experienced Haskell developers approach laziness at *design* time? votes: 26, answers: 1 Read on SO: [25] http://goo.gl/obf2MG * Lax monoidal functors with a different monoidal structure votes: 14, answers: 3 Read on SO: [26] http://goo.gl/gR954y * Maximizing Haskell loop performance with GHC votes: 13, answers: 2 Read on SO: [27] http://goo.gl/TNaeB7 * What does it mean that the semantics (of Haskell) are affected by the inferred types (of return type polymorphism)? votes: 12, answers: 2 Read on SO: [28] http://goo.gl/e702Su * Write a parallel array Haskell expression once, run on CPUs & GPUs with repa and accelerate votes: 10, answers: 2 Read on SO: [29] http://goo.gl/gvSnV8 * Best way to save 10 GB vector to disk in Haskell votes: 9, answers: 3 Read on SO: [30] http://goo.gl/6SjJeG * What is the purpose of the `Typeable (* -> Constraint) Monoid` instance? votes: 9, answers: 1 Read on SO: [31] http://goo.gl/QeLSHm Until next time, [32]+Daniel Santa Cruz References 1. http://dev.stephendiehl.com/hask 2. http://www.reddit.com/r/haskell/comments/23srcm/what_i_wish_i_knew_when_lea… 3. http://blog.johantibell.com/2014/04/announcing-cabal-120.html 4. http://www.reddit.com/r/haskell/comments/23i6ih/announcing_cabal_120/ 5. http://ro-che.info/articles/2014-04-24-lens-unidiomatic.html 6. http://www.reddit.com/r/haskell/comments/23uzpg/lens_is_unidiomatic_haskell/ 7. http://www.serpentine.com/wreq/ 8. http://www.reddit.com/r/haskell/comments/23q8kc/wreq_a_capable_new_http_cli… 9. https://ghc.haskell.org/trac/ghc/browser/ghc/libraries/base/GHC/Base.lhs?re… 10. http://www.reddit.com/r/haskell/comments/23ob5a/applicative_became_a_superc… 11. http://cryptol.net/index.html 12. http://www.reddit.com/r/haskell/comments/23w3su/open_sourced_cryptol_a_doma… 13. http://www.haskellforall.com/2014/04/model-view-controller-haskell-style.ht… 14. http://www.reddit.com/r/haskell/comments/2407hs/modelviewcontroller_haskell… 15. http://www.reddit.com/r/haskell/comments/23x3f3/lenses_dont_compose_backwar… 16. http://www.reddit.com/r/haskell/comments/23x3f3/lenses_dont_compose_backwar… 17. http://www.google-melange.com/gsoc/projects/list/google/gsoc2014 18. http://www.reddit.com/r/haskell/comments/23mfwj/14_haskell_projects_accepte… 19. http://themonadreader.wordpress.com/2014/04/23/issue-23/ 20. http://www.reddit.com/r/haskell/comments/23u25r/new_issue_of_the_monad_read… 21. http://mth.io/talks/lens-from-the-ground-up/ 22. http://www.reddit.com/r/haskell/comments/23z5xb/lens_from_the_ground_up/ 23. http://duplode.github.io/posts/lenses-you-can-make-at-home.html 24. http://www.reddit.com/r/haskell/comments/241aec/lenses_you_can_make_at_home/ 25. http://stackoverflow.com/questions/23280936/how-do-experienced-haskell-deve… 26. http://stackoverflow.com/questions/23316255/lax-monoidal-functors-with-a-di… 27. http://stackoverflow.com/questions/23315001/maximizing-haskell-loop-perform… 28. http://stackoverflow.com/questions/23282921/what-does-it-mean-that-the-sema… 29. http://stackoverflow.com/questions/23202145/write-a-parallel-array-haskell-… 30. http://stackoverflow.com/questions/23247289/best-way-to-save-10-gb-vector-t… 31. http://stackoverflow.com/questions/23277838/what-is-the-purpose-of-the-type… 32. https://plus.google.com/105107667630152149014/about

1 0

ANN: reinterpret-cast - byte-casting word-size numbers
by Niklas Hambüchen 30 Apr '14

30 Apr '14

Announcing the first version of the reinterpret-cast package: Hackage: http://hackage.haskell.org/package/reinterpret-cast Github: https://github.com/nh2/reinterpret-cast Takes the bit representation of a number and uses it for a different numeric type. This is reinterpret_cast from C++ and float f = 1.23; int i = * (int *) &f; from C. I started writing this because the PointCloudLibrary project (PCL) makes RGB colours available as an int-packed format cast to a float value - that's silly but oh the legacy. I was surprised that fast reinterpret casts are not readily available in base or at least GHC, but there are improvements going on (see links in the README). Meanwhile this package is for dealing with it. The way implemented here is the fastest possible way known to me as of now. In particular, it is faster than what data-binary-ieee754 does at the moment. Contributions are very welcome.

1 0

Escaping phantom type
by Miguel Mitrofanov 30 Apr '14

30 Apr '14

Hi cafe! Assume I have some function foo :: A x -> B x Now, the type B is somewhat complicated. It consists of several parts. So, if we call foo twice, like let b1 = foo a b2 = foo a then it's possible that we'd use some parts of b1 and b2 together, mistakenly assuming they are from the same B, not from different ones. I want to exclude this possibility. So I introduce a phantom type, and the signature becomes this: foo :: A x -> exists y. B x y (of course that's not a legitimate Haskell, in reality I'll use some GADT) Now b1 and b2 would have different types, and we can't mix them up by mistake. So far so good. But let's assume that we want to use "foo" recursively. Without the safety precautions it would be easy: generateA :: B x -> OtherArgs -> A x ... b = foo (generateA b otherArgs) Unfortunately, we can't do the same thing with this safety net in place. If we do generateA :: B x y -> OtherArgs -> A x where x doesn't depend on y, then generateA would loose the phantom type, so let b = foo (generateA b otherArgs) in b and let b1 = foo (generateA b1 otherArgs) b = foo (generateA b1 otherArgs) -- sic! in b would be indistinguishable. And that is definitely not what we want. In reality, we want x to depend on y. Unfortunately, there are lots of possibilities regarding the way x would depend on y. I know for sure that there would be many different "generateA" functions, and I don't want to encode them all in types. I've come up with one solution: foo :: (forall y. exists x. ((B x y -> A x), (B x y -> z))) -> z This would work, as in the end I would be interested not in B, but in something that would have nothing to do with this phantom types. But that requires explicit CPS transformation, which makes the code difficult to read. Is there some way to simplify things? To make them easier to use?

1 0

How to create a background program that monitors input (linux)
by EatsKittens 30 Apr '14

30 Apr '14

Essentially, I want to create a program that is simply ran in the background, not in a terminal, that monitors keypresses and mouse presses and takes certain actions on specific keypresses. Where to start?

4 5

Why is this code slow?
by Jakub Arnold 29 Apr '14

29 Apr '14

Hey guys, in my stumbling over Reddit I happened to find the following comment http://www.reddit.com/r/programming/comments/248abo/range_comprehensions_c/…, where the poster shows a simple algorithm which is really slow when implemented in Haskell, compared to a C++ counterpart which does it the same way. I’ve tried to figure this out, but my profiling skills don’t seem to be good enough for this. I’m not sure if this is somehow a limitation of list comprehensions, or if something else is wrong. -- Jakub

2 1

ANNOUNCE: haskintex-0.5.0.0. Evaluating Haskell inside LaTeX.
by Daniel Díaz Casanueva 29 Apr '14

29 Apr '14

Dear cafe readers. I have released today a new version (0.5.0.0) of the haskintex program, which is written in Haskell, for Haskell people, and by Daniel Díaz (that's me!). Haskintex is a program that processes files which contains LaTeX and (maybe!) some Haskell expressions on it. Exactly as you are expecting, it will evaluate those expressions and replace them by their results. Neat, but there is more. If the Haskell expression is of type LaTeX (yes, there is a type for that, look in [1]), then it will be replaced by the LaTeX expression that value represents. This allows magic tricks like plotting a Haskell function in a LaTeX file in just a few seconds! (see [2]). So, what is new in this version? Haskintex can now memorize results of evaluations, to avoid repeating computations. The values are shared in different runs of the program, so you can save lot of time. This feature was proposed by a kind haskintex user in the #hatex IRC channel under the alias of colmcbeardman (I hope I have spelled it correctly). Find more about haskintex in: http://daniel-diaz.github.io/projects/haskintex Happy hacking, Daniel Díaz. -- References [1] http://hackage.haskell.org/package/HaTeX-3.10.0.0/docs/Text-LaTeX-Base-Synt… [2] http://hackage.haskell.org/package/HaTeX-3.10.0.0/docs/Text-LaTeX-Packages-…

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Parse error when #ifdef pragma enabled
by Rob Stewart 29 Apr '14

29 Apr '14

Hi, I'm missing something obvious. I'd like to compile the following code. --8<---------------cut here---------------start------------->8--- {-# LANGUAGE CPP #-} module CPP where #ifdef CUDA import qualified Data.Array.Accelerate.CUDA as CUDA #endif f = "lolcats" --8<---------------cut here---------------end--------------->8--- Without the CUDA pragma, it's all good: $ ghc --make CPP.hs [1 of 1] Compiling CPP ( CPP.hs, CPP.o ) With the pragma thought, I get a compilation error: $ ghc --make CPP.hs -DCUDA [1 of 1] Compiling CPP ( CPP.hs, CPP.o ) CPP.hs:6:39: parse error on input `.' Where's my mistake? Thanks! -- Rob

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ANNOUNCE (2nd call): Applied Functional Programming (AFP) Summerschool 7-18 July 2014, Utrecht, Netherlands
by Atze Dijkstra 28 Apr '14

28 Apr '14

Dear all, we bring the following announcement to your attention a 2nd time as the registration deadline (may 5) approaches. thank you, regards, Atze Dijkstra =========== AFP Summerschool 2014 =========== Applied Functional Programming (AFP) Summerschool July 7-18, 2014 Utrecht University, Department of Information and Computing Sciences Utrecht, The Netherlands Summerschool & registration website: http://www.utrechtsummerschool.nl/courses/science/applied-functional-progra… AFP website with edition 2013 info : http://www.cs.uu.nl/wiki/USCS contact : Uscs-afp(a)lists.science.uu.nl *** The 2014 edition of the Applied Functional Programming (AFP) Summerschool in Utrecht, Netherlands will be held from 7-18 July 2014. The summerschool teaches Haskell on both beginners and advanced levels via lectures and lab exercises. More info can be found via the references above, included here is an excerpt from the summerschool website: ``Typed functional programming in Haskell allows for the development of compact programs in minimal time and with maximal guarantees about robustness and correctness. The course introduces Haskell as well as its theoretical underpinnings such as typed lambda calculus, and Damas-Milner type inference. There is ample opportunity to put this all in practice during lab sessions. Typed functional programming languages allow for the development of robust, concise programs in a short amount of time. The key advantages are higher-order functions as an abstraction mechanism, and an advanced type system for safety and reusability. This course introduces Haskell, a state-of-the-art functional programming language, together with some of its theoretical background, such as typed lambda calculi, referential transparency, Damas-Milner type inference, type level programming, and functional design patterns. We will combine this with applications of functional programming, concentrating on topics such as language processing, building graphical user interfaces, networking, databases, and programming for the web. The goal of the course is not just to teach the programming language and underlying theory, but also to learn about the Haskell community and to get hands-on experience by doing lab exercises or a Haskell project of your own.'' *** regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)uu.nl ............... / |___\

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Why this doesn't work?
by Kai Zhang 28 Apr '14

28 Apr '14

Hi Cafe, I don't understand why this doesn't work: import qualified Data.Vector.Generic as G import Control.Monad.ST test :: G.Vector v a => v a -> v a test x = runST . (>>= G.freeze) . G.thaw $ x But if I replace "." with "$", it can compile. test x = runST $ (>>= G.freeze) . G.thaw $ x I originally though f $ g x can always be written as f . g $ x

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New Functional Programming Job Opportunities
by Functional Jobs 28 Apr '14

28 Apr '14

Here are some functional programming job opportunities that were posted recently: functional software developer at OpinionLab http://functionaljobs.com/jobs/8706-functional-software-developer-at-opinio… Cheers, Sean Murphy FunctionalJobs.com

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BayHac 2014 (May 16-18) Reminder
by Jonathan Fischoff 27 Apr '14

27 Apr '14

This is your three week reminder for the up coming Bay Area Haskell Hackathon, held Friday May 16 to Sunday May 18 at the Hacker Dojo in Mountain View CA. Here is the signup sheet: https://docs.google.com/forms/d/16QEHqAioGQeHHOlnMTEmjdgtO4YNN2_Qc-rbgLOFat… BayHac is time for Haskellers of all skill levels to collaborate on projects, disseminate knowledge and socialize. This year, we responded to requests to have more teaching, and there will be 15 classes in addition to experience reports, demos, and lightning talks. More information can be found on the wiki: http://www.haskell.org/haskellwiki/BayHac2014 Happy Hacking, Jonathan

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Baltimore Haskell Users Group
by Michael Orlitzky 27 Apr '14

27 Apr '14

For those who don't reddit: http://www.reddit.com/r/haskell/comments/23zevt/baltimore_haskell_users_gro… https://groups.google.com/forum/#!forum/bmore-hug And if like me you have no idea how to Google Groups, just send an email to bmore-hug+subscribe(a)googlegroups.com to subscribe to the list.

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