February 2015 - Haskell-Cafe

Code review. API Guidance.
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 14 Feb '15

14 Feb '15

Hello everyone. I have an application (a calculator repl [1]) and I'm looking to convert it into an package that provides both library and executable. I'm looking for suggestions for what might be a good API for such a library. I'm not looking for an extended code review, just the placement of functions in modules, and which functions to export. The current source tree looks like this. src/ $ tree . . ├── Calculator │ ├── Color.hs -- Colored output │ ├── Evaluator │ │ ├── Base.hs -- Common evaluation functions │ │ ├── Cmd.hs -- Evaluate special commands │ │ ├── Expr.hs -- Expression evaluation │ │ ├── Func.hs -- Convert specification for a function into a mathematical function │ │ └── Statement.hs -- Evaluate either expression or command │ ├── Help.hs -- Help message │ ├── Parser │ │ ├── Base.hs -- Common parsers, number, id etc. │ │ ├── Cmd.hs │ │ ├── Expr.hs │ │ └── Statement.hs │ └── Prim -- Primitives │ ├── Bindings.hs -- Variable and Function Bindings │ ├── Cmd.hs │ ├── Definitions.hs -- Basic definitions, pre-defined variables and functions etc. │ ├── Expr.hs │ ├── Function.hs -- Tried to formalize the notion of a function of multiple arguments without currying. │ └── Statement.hs ├── Main.hs └── Model └── Arithmetic.hs -- A calculator model built using Text.ParserCombinators.Parsec.Expr (for model testing) I'm trying to have a library because I want to use a lot of the functionality provided by calculator in plot-lab [2], which is a plotting application. I want to have a stable API, because I don't want to irresponsibly keep changing it according to my personal needs. [1] : https://github.com/sumitsahrawat/calculator [2] : https://github.com/sumitsahrawat/plot-lab -- Regards Sumit Sahrawat

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ANN: ZuriHac 2015
by Gleb Peregud 14 Feb '15

14 Feb '15

We am glad to announce ZuriHac 2015, a annual three-day Haskell hackathon in Zurich. This year it's sponsored by Google. When: May 29th - May 31st Where: Google office, Zurich, Switzerland ZuriHac is an international Haskell hackathon: a grassroots, collaborative coding festival with a simple focus, to build and improve Haskell libraries, tools, and infrastructure. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools, or even start new ones! I am happy to announce that Edward Kmett will give a talk. We are still in progress of nailing down the details, they will be available at the wiki page: http://www.haskell.org/haskellwiki/ZuriHac2015 Registration is not yet open, we will send an additional announcement email when it's open. We look forward to seeing you there! -- The organisers of ZuriHac 2015 Gleb Peregud Simon Meier Johan Tibell Alexander Bernauer

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Changes to Haskell Weekly News: In with the Old, In with the New
by Kim-Ee Yeoh 14 Feb '15

14 Feb '15

Dear Reader, A few days ago Daniel Santa Cruz stepped down from the helm of Haskell Weekly News. With others that read HWN regularly, I thank him for his long service and wish him and his family godspeed. Did you know John Goerzen started HWN in August 2, 2005? Now a week from today, February 21, 2015, Simon and Simon shall close your window of opportunity to opine on drastic changes to the prelude. By sheer coincidence, the very first issue of HWN links to a thread on Updating the Haskell Standard. John himself started the thread. Today the talk is about the Foldable and Traversable type classes. Back then it was about Overlapping and Undecidable instances, the FFI, and a Hierarchical namespace. But these are all features we take for granted today. What sweeping changes took hold over the last ten years. There was no hackage. Only in September 2005 did Isaac Jones and David Himmelstrup announce beta-testing of a cabal prototype. Imagine life without hackage. Today there are 7664 packages all ready to go. So it is with this ladened awareness that I have volunteered and been accepted to the open vacancy at HWN. I seek to alleviate a sorely felt void in the Haskell diaspora by elucidating the newsworthy. The first decision I made might catch you by surprise, so let me explain. You see, Semen Trygubenko has also stepped up to contribute to HWN by continuing the leaderboard format pioneered by Daniel. I proposed and he agreed to alternate weeks between HWN Leaderboard and HWN Light. I'm excited by this dual format. Lower frequency means longer wavelength, which also means looking at events from a wider angle. At the same time the dual format affords continuity from past editions that a readership has grown accustomed to. Even though some readers will be mortified by the loss in uniformity, I trust you won't be among them. I take full responsibility for any untowardness caused by this to-ing and fro-ing, including innocent kittens mysteriously vanishing into the night. The Light edition will come out this Tuesday; the Leaderboard edition next. Thank you for reading Haskell Weekly News. >From the desk of Poobah of HWN, Kim-Ee Yeoh

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Rationale for two separate map functions in ClassyPrelude
by Clinton Mead 14 Feb '15

14 Feb '15

ClassyPrelude has two map functions, namely: 1. "map" 2. "omap" "map" works on any Functor. However, things like "Text" are not functors as they aren't generic containers. As can be seen in the following code: module Main where import Prelude () import ClassyPrelude import qualified Data.Text as T import Data.Char as C main = do let l = [1,2,3] :: [Int] let t = (T.pack "Hello") let m = Just 5 print $ map (*2) l print $ map (*2) m print $ omap C.toUpper t return () Notice one has to use "omap" to deal with the Text. The thing is, I found it trivially easy to get "map" to work for both calls. Here's the code: {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} module Main where import Prelude hiding (map) import qualified Data.Text as T import Data.Char as C import Control.Monad (Functor) class CanMap a b where type Element a :: * type Container a b :: * map :: (Element a -> b) -> a -> Container a b instance (Functor f) => CanMap (f a) b where type Element (f a) = a type Container (f a) b = f b map = fmap instance CanMap T.Text Char where type Element T.Text = Char type Container T.Text Char = T.Text map = T.map main = do let l = [1,2,3] :: [Int] let m = Just 5 let t = (T.pack "Hello") print $ map (*2) l print $ map (*2) m print $ map C.toUpper t return () All that's required is to add instances to CanMap for any monomorphic containers. ClassyPrelude already does this anyway with "omap" in the Data.MonoTraversable module. I suspect however there's a good reason I'm missing about why there should be two separate map functions to deal with these alternate situations, but I'm wondering what that is.

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Why doesn't TH expose its parser?
by Christopher Done 13 Feb '15

13 Feb '15

What's the reason that there isn't a parseExp :: String -> Q Exp, parseDecl, etc.? It's a pretty common use case to write some syntactic extension that merely extends regular Haskell. We end up with the haskell-src-meta package which doesn't support full GHC Haskell and doesn't know how to parse infix applications. It doesn't give a strong story for quasi-quotes to be useful for extensions to Haskell's syntax itself, as opposed to just mini-DSLs. Here's a case in point: idiom brackets. You'd like to implement them like this: [i|foo bar mu|] → foo <$> bar <*> mu You want it to be full Haskell inside, so foo could easilly be a case or a let, etc., so a trivial way to do that would be to just parse the string "foo bar mu" with parseExp and then do the above simple transformation. Job done. In practice, we can't do this. Our best option is HSE, but it's not ideal. Another way to do this is: $(idiom [|foo bar mu|]) And because [|…|] produces a Q Exp, you can just go ahead and do the rewrite on the resulting Exp. Yay, we now have full GHC Haskell inside the idiom bracket, but we've lost the syntactic convenience. Given that [|…|] already does the String → Q Exp step, why not allow us to re-use this functionality in quasi quotes? Ciao!

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Complex Event Processing in Haskell
by Tom Ellis 13 Feb '15

13 Feb '15

I'm looking for examples of production-capable Complex Event Processing systems in Haskell along the lines of Esper: http://esper.codehaus.org/ I know of Nettle (and McNettle and Procera) from Yale, and it claims to have excellent performance. http://haskell.cs.yale.edu/nettle/ Is there anything else? Is there anything in another functional language I should look at? I know FRP systems in general deal with processing events, but I am looking specifically for things similar to Esper. Tom

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ICFP 2015: Final Call for Papers
by David Van Horn 13 Feb '15

13 Feb '15

===================================================================== 20th ACM SIGPLAN International Conference on Functional Programming ICFP 2015 Vancouver, Canada, August 31 - September 2, 2015 http://www.icfpconference.org/icfp2015 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Friday, February 27 2015, 23:59 UTC-11 https://icfp15.hotcrp.com/ Author response: Tuesday, April 21, 2015 through Thursday, 23 April, 2015 Notification: Friday, May 1, 2015 Final copy due: Friday, June 12, 2015 Scope ~~~~~ ICFP 2015 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * Language Design: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; type systems; interoperability; domain-specific languages; and relations to imperative, object-oriented, or logic programming. * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling. * Foundations: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types. * Analysis and Transformation: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation. * Applications: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia and 3D graphics programming; scripting; system administration; security. * Education: teaching introductory programming; parallel programming; mathematical proof; algebra. * Functional Pearls: elegant, instructive, and fun essays on functional programming. * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working. If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By Friday, 27 February 2015, 23:59 UTC-11, submit a full paper of at most 12 pages (6 pages for an Experience Report) in standard ACM conference format, including bibliography, figures, and appendices. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.sigplan.org/Resources/Policies/Republication * Authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions will have a choice of one of three ways to manage their publication rights. These choices are described at http://authors.acm.org/main.html Presentations will be videotaped and released online if the presenter consents. The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available at http://www.sigplan.org/Resources/Author/ Submission: Submissions will be accepted on the web using a link that will be posted at https://icfp15.hotcrp.com/ Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 72-hour period, starting at 0:00 UTC on Tuesday, 21 April 2015, to read reviews and respond to them. ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking the definitive version of ACM article should reduce user confusion over article versioning. After your article has been published and assigned to your ACM Author Profile page, please visit http://www.acm.org/publications/acm-author-izer-service to learn how to create your links for free downloads from the ACM DL. Publication date: The official publication date of accepted papers is the date the proceedings are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. General Chair: Kathleen Fisher Tufts University (USA) Program Chair: John Reppy University of Chicago (USA) Program Committee: Amal Ahmed Northeastern University (USA) Jean-Philippe Bernardy Chalmers University of Technology (Sweden) Matthias Blume Google (USA) William Byrd University of Utah (USA) Andy Gill University of Kansas (USA) Neal Glew Google (USA) Fritz Henglein University of Copenhagen (Denmark) Gabriele Keller University of New South Wales and NICTA (Australia) Andrew Kennedy Microsoft Research Cambridge (UK) Neelakantan Krishnaswami Birmingham University (UK) Daan Leijen Microsoft Research Redmond (USA) Keiko Nakata FireEye Dresden (Germany) Mike Rainey INRIA Rocquencourt (France) Andreas Rossberg Google (Germany) Manuel Serrano INRIA Sophia Antipolis (France) Simon Thompson University of Kent (UK) David Van Horn University of Maryland (USA) Stephanie Weirich University of Pennsylvania (USA)

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[TFP 2015] 2nd call for papers
by Peter Achten 13 Feb '15

13 Feb '15

----------------------------- S E C O N D 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 SPEAKER == TFP is pleased to announce a talk by the following invited speaker: * 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. == 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 <to be announced> == 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 17, 2015 Notification: March 24, 2015 Registration: April 7, 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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example
by Закиров Марат 13 Feb '15

13 Feb '15

Hi all, Algorithms on graphs in my opinion is something bad for parallel execution (especially such of them which have O(n) comlexity). So if I haskell give me an advantage in some graph application it will be great benchmark. Recently I tried to think about well-known by compiler people - so called "topologic-sort" or reverse post order numeration, but I gave up... There are just nothing to execute in parallel. Currently I am searching for graph (compiler) algorithm example written in haskell with persistent data structures which can be executed in parallel. And in the same time this example written in imperative style must not have good parallel execution form. Summarizing: I want to find example of data_persistence&functional style killer graph (compiler) application. Do you know such an example? -- *Regards, Marat.* *С уважением Марат.*

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A Question about the IO type detection
by Zongzhe Yuan 13 Feb '15

13 Feb '15

Hi i meet a problem when i was using readLn to read a int from user my function is like that do (some thing else that is working) n <- readLn (some thing else) f n x1 x2 x3 …… f has the type of Int -> something else because the type in this function is fixed, if the type of n is not Int, it will have exception However i want to detect whether the user input is valid or not and i have this function import Data.Typeable isInteger :: Typeable a => a -> Bool isInteger n = (typeOf n) == (typeOf (1::Int)) and add to do (some thing else that is working) n <- readLn if isInteger n then (some thing else) f n x1 x2 x3 …… else (something else) but it doesn’t work because of the reason “type has been inferred and fixed” How could i solve this problem? Zongzhe Yuan This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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