- Haskell-Cafe - Haskell.org

pattern match on forall'ed data
by Corentin Dupont 22 Jul '16

22 Jul '16

Hi all, I'm surprised this doesn't work: data SomeData = forall e. (Typeable e, Eq e) => SomeData e (===) :: (Typeable a, Typeable b, Eq a, Eq b) => a -> b -> Bool (===) x y = cast x == Just y test :: SomeData' -> Bool test (SomeData' e) | e === Nothing = True test _ = False It says Could not deduce (Eq a1) arising from a use of ‘===’ How can I achieve something of the same effect? Thanks Corentin

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Re: [Haskell-cafe] Practical use of Stream's monad instance?
by Olaf Klinke 20 Jul '16

20 Jul '16

As someone else on this list whose name I don't recall put it, there is no choice on whether to make Stream a monad or not. It simply _is_ a monad. Anyone with some CS education hearing 'diagonal of inifinite list of infinite lists' should immediately think of Georg Cantor. import Data.Stream import Data.Ratio type Real = Stream Rational -- approximate a real number by an ascending stream -- of lower bounds. supremum :: Stream Real -> Real supremum = join -- If a stream of reals is index-wise ascending, -- the monad instance for Stream computes its supremum. -- Use this e.g. to compute any mathematical quantity -- defined as a supremum. -- Olaf

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Re: [Haskell-cafe] [Haskell] Function to replace given element in list
by amindfv＠gmail.com 20 Jul '16

20 Jul '16

(Moving to haskell-cafe@ -- haskell@ is only for announcements) > El 19 jul 2016, a las 19:44, David Feuer <david.feuer(a)gmail.com> escribió: > > A zipper is a good way to separate the search from the replacement. But the problem at hand does not require such a separation! If you don't need that, a zipper seems like overkill. > > > On Jul 19, 2016 7:39 PM, "Carl Folke Henschen Edman" <carledman(a)gmail.com> wrote: > On Tue, Jul 19, 2016 at 6:13 PM, David Feuer <david.feuer(a)gmail.com> wrote: >> Using a zipper will not get you very far here. The best way would >> likely be to replace the list with a balanced search tree. > > > That depends on the pattern of access and usage. For some a zippered list will outperform a self-balancing tree and vice versa. For others a zippered tree, or something else, will beat either. But when seeing the pattern of changing a single element in the middle of a list, a zippered list is the first improved data structure that comes to mind. > > > Carl Edman > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell

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Local Haskell talk in Leipzig, 27 July 2016 at 3:30pm in HTWK
by Heinrich Apfelmus 19 Jul '16

19 Jul '16

Dear (filter (`livesInOrCloseTo` Leipzig_Germany) haskellers), we would like to invite you to a talk by Stephen Blackheath "They why and how of Functional Reactive Programming with Java/Sodium" on Wednesday, 27 July 2016, at 15:30 Uhr in Room Gu 102, Gutenberg-Bau, HTWK Leipzig Abstract: <https://portal.imn.htwk-leipzig.de/termine/vortrag-the-why-and-how-of-funct… Geographical coordinates: <http://www.openstreetmap.org/?mlat=51.31248&mlon=12.37498#map=18/51.31248/1…> Afterwards (~ 17:00 Uhr) there will be a get-together in the Cafe Suedbrause <http://suedbrause.de>. Stop Listening! Johannes Waldmann and Heinrich Apfelmus

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parsec get line number of lexems
by Olivier Duhart 19 Jul '16

19 Jul '16

hello, I am experimenting with parsec. I am building a very simple language. I have a (almost) wholy funcrionning language : syntactic check, AST building and byte code génération. Now i want to improve error checking (type checking more precisely). this should be done on the AST basis. but in the ast i lose the line number information for precise error message. so my question is : is ther a way to get the line numbers at parsers level ? i would then store it in the ast for semantic check purpose thanks for your answers Olivier

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Call for Participation: ICFP 2016
by Lindsey Kuper 19 Jul '16

19 Jul '16

[ Early registration ends 17 August. ] ===================================================================== Call for Participation ICFP 2016 21st ACM SIGPLAN International Conference on Functional Programming and affiliated events September 18 - September 24, 2016 Nara, Japan http://conf.researchr.org/home/icfp-2016 ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: 1 conference, 1 symposium, 10 workshops, tutorials, programming contest results, student research competition, and mentoring workshop * Overview and affiliated events: http://conf.researchr.org/home/icfp-2016 * Program: http://conf.researchr.org/program/icfp-2016/program-icfp-2016 * Accepted Papers: http://conf.researchr.org/track/icfp-2016/icfp-2016-papers#event-overview * Registration is available via: https://regmaster4.com/2016conf/ICFP16/register.php Early registration is due 17 August, 2016. * Programming contest, 5-8 August, 2016: http://2016.icfpcontest.org/ * Student Research Competition (deadline: 3 August, 2016): http://conf.researchr.org/info/icfp-2016/student-research-competition * Follow @icfp_conference on twitter for the latest news: http://twitter.com/icfp_conference There are several events affiliated with ICFP: Sunday, September 18 Workshop on Higher-order Programming with Effects Workshop on Type-Driven Development Scheme and Functional Programming Workshop Programming Languages Mentoring Workshop Monday, September 19 – Wednesday, September 21 ICFP Thursday, September 22 Haskell Symposium – Day 1 ML Family Workshop Workshop on Functional High-Performance Computing Commercial Users of Functional Programming – Day 1 Friday, September 23 Haskell Symposium – Day 2 OCaml Workshop Erlang Workshop Commercial Users of Functional Programming – Day 2 Saturday, September 5 Commercial Users of Functional Programming – Day 3 Haskell Implementors Workshop Functional Art, Music, Modeling and Design Conference Organizers General Co-Chairs: Jacques Garrigue, Nagoya University Gabriele Keller, University of New South Wales Program Chair: Eijiro Sumii, Tohoku University Local Arrangements Co-Chairs: Shinya Katsumata, Kyoto University Susumu Nishimura, Kyoto University Industrial Relations Chair: Rian Trinkle, Obsidian Systems LLC Workshop Co-Chairs: Nicolas Wu, University of Bristol Andres Loeh, Well-Typed LLP Programming Contest Chair: Keisuke Nakano, The University of Electro-Communications Student Research Competition Chair: David Van Horn, University of Maryland, College Park Mentoring Workshop Co-Chairs: Amal Ahmed, Northeastern University Robby Findler, Northwestern University Atsushi Igarashi, Kyoto Universty Publicity Chair: Lindsey Kuper, Intel Labs Video Chair: Iavor Diatchki, Galois Jose Calderon, Galois Student Volunteer Co-Chairs: Yosuke Fukuda, Kyoto University Yuki Nishida, Kyoto University Gabriel Scherer, INRIA Industrial partners: Platinum partners Jane Street Capital Ahrefs Gold partners Mozilla Research Silver partners Ambiata Tsuru Capital Bronze partners Awake Networks Microsoft Research =====================================================================

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ANN: blockhash-0.1.0.0
by KwangYul Seo 19 Jul '16

19 Jul '16

Hi all! I'm pleased to announce the first release of blockhash, a perceptual image hash calculation tool based on algorithm described in Block Mean Value Based Image Perceptual Hashing by Bian Yang, Fan Gu and Xiamu Niu. https://hackage.haskell.org/package/blockhash https://github.com/kseo/blockhash Program: Usage: blockhash [-q|--quick] [-b|--bits ARG] filenames blockhash Available options: -h,--help Show this help text -q,--quick Use quick hashing method -b,--bits ARG Create hash of size N^2 bits. Library: import qualified Codec.Picture as P import Data.Blockhash import qualified Data.Vector.Generic as VG import qualified Data.Vector.Unboxed as V printHash :: FilePath -> IO () printHash :: filename = do res <- P.readImage filename case res of Left err -> putStrLn ("Fail to read: " ++ filename) Right dynamicImage -> do let rgbaImage = P.convertRGBA8 dynamicImage pixels = VG.convert (P.imageData rgbaImage) image = Image { imagePixels = pixels , imageWidth = P.imageWidth rgbaImage , imageHeight = P.imageHeight rgbaImage } hash = blockhash image 16 Precise putStrLn (show hash) For further information on the blockhash algorithm, please visit the web site: http://blockhash.io/ Thanks, Kwang Yul Seo

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Re: [Haskell-cafe] another instance of MonadError
by David Feuer 18 Jul '16

18 Jul '16

You can't do that; sorry. You can always turn a string into an IOException if you like. Or you could write a wrapper around IO. But MonadError has a functional dependency, so you can only have one error type per monad. On Jul 14, 2016 6:19 PM, "Corentin Dupont" <corentin.dupont(a)gmail.com> wrote: Hello, IO is an instance of MonadError IOException... However I also need to make it an instance of MonadError String... Is it possible? I'm trying to instanciate this class: class (Typeable n, Monad n, Applicative n, MonadError String n) => EvMgt n where ... instance EvMgt IO where... Any idea? _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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Linking to a DLL with relative path on Windows using stack build
by Peter Dedecker 18 Jul '16

18 Jul '16

Hello all, I am writing a Haskell program that controls a scientific instrument (a very sensitive camera). The instrument vendor makes a Windows DLL available that exposes a C interface. I have already set up the FFI bindings. I am building the application using Stack on Windows 8. But I am not sure how I can get “stack build” to find the DLL at link time. My Google searches have not cleared things up. My questions: 1. What global directory should I place the DLL in so “stack build” will pick it up? I tried placing it in C:\Windows\SysWOW64 but that doesn’t work. I realize I could add an "extra-lib-dirs” argument in the .cabal file but I would like to know what the default linker search paths are. But in fact I prefer to not install the DLL in a hardcoded, global location. Since the instrument is attached to a specific measurement computer, there is no point in having it installed on the developer PCs. I would prefer to have it all self-contained into a single project. In my C++ development I solve this by placing the DLLs (well, import libraries, really) in a project-specific “lib” directory next to the source code, and under version control. For example, in Visual Studio my linker path is simply "$(SolutionDir)\..\lib”, which works nicely. 2. But: why can’t I use a project-relative "extra-lib-dirs" argument in the .cabal file? What I would really like is to be able to say "extra-lib-dirs={$projectroot}/lib” in the cabal file. And then, after compilation, I can just copy the binary to the measurement computer (controlling the instrument), copy the DLL into the .exe folder, and have it all work. Except that it doesn’t work. A relative path in the .cabal file results in "lib is a relative path which makes no sense (as there is nothing for it to be relative to)." I don’t understand why it makes no sense in this case (where I am linking to a DLL). Seems to me like this does make sense? Any suggestions on how to go about this? I have stumbled across this: http://stackoverflow.com/questions/24444675/use-relative-paths-for-extra-li… but that seems awfully hackish and I haven’t been able to get it to work. With thanks Peter

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Announcing MuniHac 2016
by Michael Adler 18 Jul '16

18 Jul '16

Hi fellow Haskellers! Together with Alexander Lehmann from TNG Technology Consulting GmbH and Andres Löh from Well-Typed LLP, I am organizing a new Haskell Hackathon that will take place in Munich, from Friday September 2 - Sunday September 4. TNG is graciously offering to host the Hackathon at their premises. This Hackathon is in the tradition of other Haskell Hackathons such as ZuriHac, HacBerlin, UHac and others. We have capacity for 80-100 Haskellers to collaborate on any project they like. Hacking on Haskell projects will be the main focus of the event, but we will also have a couple of talks by renowned Haskellers. More details and a link to the registration platform can be found on www.munihac.de Hope to see you in Munich! Best regards, Michael -- Michael Adler TNG Technology Consulting michael.adler(a)tngtech.com TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring Geschäftsführer: Henrik Klagges, Christoph Stock, Dr. Robert Dahlke Amtsgericht München, HRB 135082

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OCL 2016: ** Deadline Extension ** Submit Your Paper Until July 24, 2016
by Achim D. Brucker 17 Jul '16

17 Jul '16

(Apologies for duplicates) If you are working on the foundations, methods, or tools for OCL or textual modelling, you should now finalise your submission for the OCL workshop! *** The submission deadline has been extended to July 24th, 2016! *** CALL FOR PAPERS 16th International Workshop on OCL and Textual Modeling Co-located with ACM/IEEE 19th International Conference on Model Driven Engineering Languages and Systems (MODELS 2016) October 2, 2016, Saint-Malo, France http://oclworkshop.github.io 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 to 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 or formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- meta-modeling 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 - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - 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 tools that support - in a very broad sense - textual modeling languages (if you have implemented OCL.js to run OCL in a web browser, this is the right workshop to present your work) as well as textual model transformations. Venue ===== The workshop will be organized as a part of MODELS 2016 Conference in Saint-Malo, France. 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, 2013 in Miami, 2014 in Valencia, Spain, and 2015 in Ottawa, Canada. 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 contributions (between 6 and 8 pages) describing new ideas, innovative tools or position papers. * full papers (between 12 and 16 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://easychair.org/conferences/?conf=ocl16) 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 post-conference edition of CEUR (http://www.ceur-ws.org) Important Dates =============== Submission of papers: July 24, 2016 Notification: August 14, 2016 Workshop date: October 2, 2016 Organizers ========== Achim D. Brucker, The University of Sheffield, UK Jordi Cabot, ICREA - Open University of Catalonia, Spain Adolfo Sánchez-Barbudo Herrera, University of York, UK Programme Committee (TBC) ========================= Thomas Baar, University of Applied Sciences Berlin, Germany Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Domenico Bianculli, University of Luxembourg Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Indra Sistemas S.A., Spain Geri Georg, Colorado State University, USA Martin Gogolla, University of Bremen, Germany Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Massimo Tisi, Mines de Nantes, France Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ. Paris-Sud - LRI, France Steffen Zschaler, King's College, UK -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.uk/ | https://logicalhacking.com/blog

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Passing a cabal flag to stack
by Rustom Mody 16 Jul '16

16 Jul '16

I’ve been trying to compile hoodle following the instructions in the readme: https://github.com/wavewave/hoodle Went further than year or two earlier when attempt got mired in cabal-hell This time stack build went through (after a couple of deb packages needing installation) Now I get this error on trying to run GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported Asking on the hoodle mailing list I was told that one has to build poppler with option -fgtk3 Where/How to give that? Looking at https://github.com/commercialhaskell/stack/issues/191 I tried the following: 1. Delete every file/directory under .stack-work that has the word poppler 2. stack build poppler --flag gtk:gtk3 3. stack build Seems to have built Ok I get the same warning that stack seems to be generally giving — dozens of them — viz ============ No packages found in snapshot which provide a "gtk2hsC2hs" executable, which is a build-tool dependency of "glib" Missing build-tools may be caused by dependencies of the build-tool being overridden by extra-deps. This should be fixed soon - see this issue https://github.com/commercialhaskell/stack/issues/595 ============= But thereafter stack exec hoodle results in the same error. So whats the official way of putting the -fgtk3 flag into stack? Any other suggestions? stack first tried from deb; then redone from stack website $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 ubuntu 16.4 (if that matters) Thanks

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another instance of MonadError
by Corentin Dupont 14 Jul '16

14 Jul '16

Hello, IO is an instance of MonadError IOException... However I also need to make it an instance of MonadError String... Is it possible? I'm trying to instanciate this class: class (Typeable n, Monad n, Applicative n, MonadError String n) => EvMgt n where ... instance EvMgt IO where... Any idea?

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network package and SIGVTALRM
by Ruben Astudillo 14 Jul '16

14 Jul '16

Hi all I am doing a DCC subsystem on a irc client. After all the handshakes are done I just connect to the server and start `recv`. The code I use for this is: getPackets :: MVar Int -> FilePath -- ^ Name media -> Int -- ^ File size -> AddrInfo -> ExceptT DCCError IO () getPackets mvar name totalSize addr = do receivedSize <- lift $ bracket acquire release receive let delta = (totalSize - receivedSize) if delta > 0 then throwE (NotFullRecv delta) else return () where bufferSize = 16384 acquire :: IO (IO.Handle,Socket) acquire = (,) <$> (IO.openFile name IO.WriteMode) <*> newSocket addr release :: (IO.Handle,Socket) -> IO () release (hdl, sock) = IO.hClose hdl >> close sock receive :: (IO.Handle,Socket) -> IO Int receive (hdl, sock) = flip execStateT 0 . fix $ \loop -> do mediaData <- lift (B.recv sock bufferSize) unless (B.null mediaData) $ do S.modify' (+ (B.length mediaData)) currentSize <- S.get lift $ B.hPut hdl mediaData >> B.send sock (int2BS currentSize) >> swapMVar mvar currentSize loop Part of the protocol is that on each `recv` I send the current received size on network byte order. Hence the B.send line on receive I use this function: -- | given a number forms a bytestring with each digit on a separated -- Word8 in network byte-order int2BS :: Int -> B.ByteString int2BS i | w <- (fromIntegral i :: Word32) = B.pack [ (fromIntegral (shiftR w 24) :: Word8) , (fromIntegral (shiftR w 16) :: Word8) , (fromIntegral (shiftR w 8) :: Word8) , (fromIntegral w :: Word8)] Everything works correctly until around 1/2 of a test transfer (ie in a file of 340M it gets 170). That first half is gotten in the right order (I tested with a video and it was playable until the middle). On tinier files the bug doesn't happen, the file is received completly. I did a little bit of `strace` and `tcpdump` and I got this -- strace -e trace=network -p $client (..) 30439 recvfrom(13, "\312\255\201\337\376\355\253\r\177\276\204X]8\6\221\301#\361<>\273+\355\5\343B \333\366\351W"..., 16384, 0, NULL, NULL) = 1380 30439 sendto(13, "\n\273\31l", 4, 0, NULL, 0) = 4 30439 recvfrom(13, 0x20023f010, 16384, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) 30439 recvfrom(13, "\222llq_H\23\17\275\f}\367\"P4\23\207\312$w\371J\354aW2\243R\32\v\n\251"..., 16384, 0, NULL, NULL) = 1380 30439 sendto(13, "\n\273\36\320", 4, 0, NULL, 0) = -1 EAGAIN (Resource temporarily unavailable) 30438 --- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_value={int=0, ptr=0}} --- 30438 --- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_value={int=0, ptr=0}} --- (..) -- tcpdump 05:20:22.788273 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 48004680, win 489, options [nop,nop,TS val 627805332 ecr 675358947,nop,nop,sack 1 {48006060:48066780}], length 0 05:20:22.975627 IP 198.255.92.74.36103 > tapioca.36346: Flags [.], seq 48004680:48006060, ack 82629, win 0, options [nop,nop,TS val 675359033 ecr 627805248], length 1380 05:20:23.014991 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 48066780, win 4, options [nop,nop,TS val 627805559 ecr 675359033], length 0 05:20:23.768012 IP 198.255.92.74.36103 > tapioca.36346: Flags [P.], seq 48066780:48067292, ack 82629, win 0, options [nop,nop,TS val 675359232 ecr 627805559], length 512 05:20:23.768143 IP tapioca.36346 > 198.255.92.74.36103: Flags [.], ack 48067292, win 0, options [nop,nop,TS val 627806312 ecr 675359232], length 0 05:20:24.523397 IP 198.255.92.74.36103 > tapioca.36346: Flags [.], ack 82629, win 0, options [nop,nop,TS val 675359421 ecr 627806312], length 0 What bothers me is that SIGVTALRM on the strace output. I am not the greatest unix hacker but that signal is related to settimer and I haven't explicitly set that up. So I am scratching me head a little. Maybe somebody has experienced something related with the network package? Do you notice something on the logs? thanks in advance. -- -- Ruben Astudillo

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How to support multiple string types in Haskell?
by KwangYul Seo 14 Jul '16

14 Jul '16

Hi all, There are multiple string types in Haskell – String, lazy/strict ByteString, lazy/strict Text to name a few. So to make a string handling function maximally reusable, it needs to support multiple string types. One approach used by TagSoup library is to make a type class StringLike which represents the polymorphic string type and uses it where a String type is normally needed. For example, parseTags :: StringLike str => str -> [Tag str] Here parseTags takes a StringLike type instead of a fixed string type. Users of TagSoup can pick any of String, lazy/strict ByteString, lazy/strict Text because they are all instances of StringLike type class. It seems StringLike type class is quite generic but it is used only in the TagSoup package. This makes me wonder what is the idiomatic way to support multiple string types in Haskell. What other approaches do we have? Thanks, Kwang Yul Seo

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Memory Management and Lists
by Christopher Howard 14 Jul '16

14 Jul '16

Hi, short version of the question: when elements are 'drop'ped from a list, when is the memory for those elements released? And is there some way to control that? Longer version: I have this situation where I've got a State monad carrying a Data.Matrix, and I... 1) generate in infinite list of monadic operations with (repeat mf) where mf is a monadic function. 2) 'sequence' (from Control.Monad) to get a monad containing the infinite list of all states (i.e., a list of Matrices). 3) evalState(T) to extract the list 4) extract the nth state with (head (drop (n-1))) I like this approach. However, there seems to be a memory management issue: a Matrix itself should only be, I'm guessing, somewhere around 100KB. But instead I'm maxing out the 3GB of RAM on my old T60 laptop. Maybe I'm generating list elements (Matrices) a lot faster than memory management is releasing them...? -- http://qlfiles.net To protect my privacy, please use PGP encryption. It's free and easy to use! My public key ID is 0x340EA95A (pgp.mit.edu)

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efficient operations on immutable structures
by Christopher Howard 14 Jul '16

14 Jul '16

Hi again all. From some online research, I understand that operations on complex immutable structures (for example, a "setter" function a -> (Int, Int) -> Matrix -> Matrix which alters one element in the Matrix) is not necessarily inefficient in Haskell, because the (compiler? runtime?) has some means of sharing unchanged values between the input and output structure. What I am not clear on, however, is how this works, and how you ensure that this happens. Could someone perhaps elaborate, or point me to a really good read on the subject? -- http://qlfiles.net To protect my privacy, please use PGP encryption. It's free and easy to use! My public key ID is 0x340EA95A (pgp.mit.edu)

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Is the #functional programming paradigm antithetical to efficient strings? #Haskell
by KC 14 Jul '16

14 Jul '16

Is the #functional programming paradigm antithetical to efficient strings? #Haskell -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Use GHC API in standalone executable?
by Mitchell Rosen 13 Jul '16

13 Jul '16

Hi, I'd like to compile an executable that internally uses the GHC API, but otherwise does not require users to have GHC installed on their machine. Is this possible? As far as I can tell, lots of functions in the GHC API read package databases per $GHC_PACKAGE_PATH and things of that nature. Thanks, Mitchell

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Can't log into to GHC trac
by Mitchell Rosen 13 Jul '16

13 Jul '16

Hi, embarrassingly I've forgotten my password (and username) to the GHC trac, and there's no automatic way to reset any of it: - When I try to register an account it says my email address is already in use - To reset my password I need to give both my username and password, but I don't know my username Who can I contact to help resolve the issue?

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ANN: tagsoup-megaparsec-0.1.0.0
by KwangYul Seo 13 Jul '16

13 Jul '16

Hello all, I'm pleased to announce the first release of tagsoup-megaparsec, a Tag token parser library. This library helps you build a megaparsec parser using TagSoup's Tags as tokens. https://hackage.haskell.org/package/tagsoup-megaparsec https://github.com/kseo/tagsoup-megaparsec Regards, Kwang Yul Seo

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Cannot install any packages after upgrading to GHC 8.0.1
by Magicloud Magiclouds 12 Jul '16

12 Jul '16

Hi, I update my GHC to 8.0.1 and failed to install any packages since then. If I use cabal, I got 'installed package info from too old version of Cabal (key field does not match id field)' for every package. If I use Setup.hs, I got 'Configuring Cabal-1.24.0.0... Setup.hs: ghc-pkg dump failed'. What should I do? Googling did not get me an answer.

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Re: [Haskell-cafe] Is the #functional programming paradigm antithetical to efficient strings? #Haskell
by Alex Belanger 11 Jul '16

11 Jul '16

That's not just about Haskell, it's C.S. in general. You're always going to make space and time tradeoffs. Funny how the same constraints appear in physics; I wonder if it is trying to tell us something... - nitrix On Jul 11, 2016 8:54 AM, "David Fox" <dsf(a)seereason.com> wrote: On Sun, Jul 10, 2016 at 11:44 AM, KC <kc1956(a)gmail.com> wrote: > Is the #functional programming paradigm antithetical to efficient strings? > #Haskell > I can certainly see how it might seem so. Our main string representation uses about 10 times more memory than seems necessary, and is relatively fast, though probably not compared to C. Recently I spent some time looking into how to reduce this memory overhead, and I modified the pretty library (https://github.com/ddssff/pretty/tree/textdetails) so it could use any type that was an instance of ListLike and StringLike ( https://github.com/JohnLato/listlike) Then I tried the UnitLargeDoc test with several different data types. This just concatenates a list of ten million "Hello" strings. Using String this happens in about 5 seconds. Using strict or lazy Text it immediately blew the stack. When I increased the stack size to over 1GB it took minutes to complete. When I used the Data.Text.Lazy.Builder type instead it took about 30 seconds to complete. Results with utf8 encoded ByteStrings were siimilar. When I mentioned some of this, I was told "different data structures for different purposes", which sort of makes sense, but honestly if Haskell is a language where you have to sit down and choose a representation every time you want to build some text, I get a bit discouraged. So is functional programming antithetical to efficient strings? In theory maybe not, but I would love to see some hard evidence. </rant> _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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ANN: enchant-0.1.0.0
by KwangYul Seo 11 Jul '16

11 Jul '16

Hello, I'm happy to announce the first release of enchant, binding for the Enchant library. https://hackage.haskell.org/package/enchant-0.1.0.0 https://github.com/kseo/enchant What is Enchant? (from http://www.abisource.com/projects/enchant/) On the surface, Enchant appears to be a generic spell checking library. You > can request dictionaries from it, ask if a word is correctly spelled, get > corrections for a misspelled word, etc... > > Beneath the surface, Enchant is a whole lot more - and less - than that. > You'll see that Enchant isn't really a spell checking library at all. > > "What's that?" you ask. Well, Enchant doesn't try to do any of the work > itself. It's lazy, and requires backends to do most of its dirty work. > Looking closer, you'll see the Enchant is more-or-less a fancy wrapper > around the dlopen() system call. Enchant steps in to provide uniformity and > conformity on top of these libraries, and implement certain features that > may be lacking in any individual provider library. Everything should "just > work" for any and every definition of "just working." Thanks, Kwang Yul Seo

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ANN: qtah-0.1.0
by Bryan Gardiner 11 Jul '16

11 Jul '16

Hello all, I'm happy to announce the first Hackage-ready release of Qtah, Qt bindings for Haskell. I announced Qtah earlier this year, but a lot of work was needed to get it to the point where you can just "cabal install" it. That work has now been done, so I invite you to install qtah-qt5 or qtah-examples[1] (currently just the Qt notepad example), and check out: http://khumba.net/projects/qtah https://gitlab.com/khumba/qtah As a small demonstration, I have a good portion of Goatee ported (board rendering clearly missing): http://khumba.net/tmp/20160710-goatee-qt.png However, there is still lots of the Qt API left to cover. Thanks to ezyang for help working with some Cabal internals. Cheers, Bryan [1] cabal install --enable-executable-dynamic qtah-examples

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Practical use of Stream's monad instance?
by Petr Pudlák 10 Jul '16

10 Jul '16

Hi, we've been debating the usefulness of Monad instances for Stream [2] or homogeneous tuples (fixed-length vectors). The Applicative implementation for them is simply zipping, very useful indeed. The "join" of their Monad instances takes the diagonal of a 2-dimensional plane (finite for tuples, infinite for Stream). (See also [1].) The question is, are such monad instances used in the wild? Are they actually useful or interesting for anything? [1] https://mail.haskell.org/pipermail/haskell-cafe/2009-April/thread.html#59079 [2] https://hackage.haskell.org/package/Stream-0.4.7.2/docs/Data-Stream.html Thanks Petr

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What's wrong in my Data.Generic.Zipper code?
by martin 10 Jul '16

10 Jul '16

Hello all, I do not understand why this simple piece of code returns Nothing instead of [1,2,3]. Can anybody help? {-# LANGUAGE BangPatterns,NoMonomorphismRestriction, DeriveDataTypeable #-} import Data.Typeable import Data.Data import Data.Generics.Zipper ex_list1 :: [Int] ex_list1 = [1,2,3] t1 = toZipper ex_list1 -- *Main> getHole t1 -- Nothing

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Re: [Haskell-cafe] Commutative monads vs Applicative functors
by Olaf Klinke 10 Jul '16

10 Jul '16

Anders Kock has a nice paper on commutative monads [1]. He relates commutativity to the Fubini theorem, because that is what commutativity is for the monad of meaures: You can change the order in integration by parts. He works in a symmetric monoidal closed category, so Hask works, too. It is the sort of category theory paper where you can go along the paper and implement most of it in Haskell. In particular, he mentions that MonadPlus has an alternative set of laws: A monad m is a MonadPlus when there are isomorphisms iso0 :: () -> m Void iso2 :: (m a,m b) -> m (Either a b) Indeed, let mzero = liftM absurd (iso0 ()) mplus x y = liftM codiagonal (iso2 (x,y)) where codiagonal (Left x) = x codiagonal (Right y) = y Conversely, let iso0 () = mzero iso2 (x,y) = (liftM Left x) `mplus` (liftM Right y) Then Kock shows that the MonadPlus operations are automatically Eilenberg-Moore algebra homomorphisms, which is the abstract way of saying that addition distributes over multiplication. On a related note, I wonder whether the following is true: Let m be a monad and let r be an object which is an m-algebra. Think of m as the free semigroup monad. Now consider the continuation monad type T x = (x -> r) -> r Observe that (x -> r) inherits the m-algebra structure of r. Let T' x be the subset of maps in T x which are m-linear. Is T' a monad, too? Can this me made concrete in a Haskell function? Regards, Olaf [1] @article{kock12, author="Kock, Anders", title="Commutative monads as a theory of distributions", journal="Theory and Applications of Categories", volume=26, number=4, pages="97-131", year=2012, url="http://www.tac.mta.ca/tac/volumes/26/4/26-04.pdf" }

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Links on https://hackage.haskell.org/package/Cabal-1.24.0.0/docs/Language-Haskell-Extension.html are broken. How can I help?
by Michael Litchard 10 Jul '16

10 Jul '16

Links to documentation are broken. I would be happy to fix. But how? https://hackage.haskell.org/package/Cabal-1.24.0.0/docs/Language-Haskell-Ex…

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Re: [Haskell-cafe] Strange error with cabal install on windows:
by Anton Kholomiov 10 Jul '16

10 Jul '16

For those who wants to help. Pavel discovered the cause of the problem for me. I've got too many modules in my package and it causes the error. The length of the command line strings is limited for windows so if you have too many modules the final build command can not list them all. I need to regroup my modules I guess.. It's solved! 8 июля 2016 г., 20:24 пользователь Anton Kholomiov < anton.kholomiov(a)gmail.com> написал: > Ноо тогда он их не включит в пакет. Не сгенерит объектные файлы для этих > модулей. Я думаю, что мне надо перегруппировать модули. Чтобы их стало > поменьше. Но не слишком сильно, чтобы ghc смог скомпилировать файлы. Если > файл слишком большой он будет крайне долго компилировать. > > 8 июля 2016 г., 13:07 пользователь Pavel Troev <pavel.troev(a)gmail.com> > написал: > > ну да.. у меня давно зуб на виндовую консоль по поводу utf-8 >> >> похоже надо подправить .cabal файл - >> убрать все модули из секции Library / Other-Modules: >> >> >> >> C:\sap\sharc_timbre\sharc-timbre-0.1>cabal install >> Warning: The package list for 'hackage.haskell.org' is 81.2 days old. >> Run 'cabal update' to get the latest list of available packages. >> Resolving dependencies... >> Configuring sharc-timbre-0.1... >> Building sharc-timbre-0.1... >> Preprocessing library sharc-timbre-0.1... >> In-place registering sharc-timbre-0.1... >> Creating package registration file: >> C:\Users\root\AppData\Local\Temp\pkgConf-sharc-timbre-048275436.1 >> Installing library in >> >> C:\Users\root\AppData\Roaming\cabal\x86_64-windows-ghc-7.10.3\sharc-timbre-0.1-Ig1DfnlqQG6G3PN7S26idD >> Registering sharc-timbre-0.1... >> Installed sharc-timbre-0.1 >> >> >> [image: Встроенное изображение 1] >> >> >> 8 июля 2016 г., 12:02 пользователь Anton Kholomiov < >> anton.kholomiov(a)gmail.com> написал: >> >> Похоже слишком много модулей, я этот пакет кодогенерил из БД, стоит >>> пересмотреть структуру пакета, >>> спасибо, теперь я знаю причину.. я что-то слышал о том, что в windows >>> есть лимит на длину строк в командной строке, и мой пакет упёрся в это >>> ограничение >>> >>> 8 июля 2016 г., 12:51 пользователь Pavel Troev <pavel.troev(a)gmail.com> >>> написал: >>> >>>> cabal get sharc-timbre >>>> cabal cd sharc-timbre-0.1 >>>> cabal configure >>>> cabal build -v >>>> видим строку ghc --make .... >>>> удаляем где-то половину нот до "Sharc.Instruments.ViolinMuted.Note15" >>>> начал компилировать, похоже дело именно в длине команды >>>> [ 618 of 1379] Compiling Sharc.Instruments.Cell >>>> arc\Instruments\CelloPizzicato\Note25.o ) >>>> [ 619 of 1379] Compiling Sharc.Instruments.Cell >>>> arc\Instruments\CelloPizzicato\Note26.o ) >>>> ... >>>> >>>> >>>> 8 июля 2016 г., 11:40 пользователь Anton Kholomiov < >>>> anton.kholomiov(a)gmail.com> написал: >>>> >>>>> я пока ещё не изучал вопрос, но вроде работает и там очень много >>>>> обновлений, но судя по тому что они писали в HCAR могут быть сюрпризы, они >>>>> планируют следующие релизы посвятить стабилизации ghc. >>>>> >>>>> >>>>> 2016-07-08 12:27 GMT+04:00 Pavel Troev <pavel.troev(a)gmail.com>: >>>>> >>>>>> а стоит на ghc 8 переходить? >>>>>> >>>>>> 2016-07-08 11:24 GMT+03:00 Anton Kholomiov <anton.kholomiov(a)gmail.com >>>>>> >: >>>>>> >>>>>>> Спасибо, то есть это от версии может зависеть.. >>>>>>> >>>>>>> У меня ghc-8.0.1. со стэком у меня не сложилось под виндой, но это >>>>>>> совсем другая история... >>>>>>> >>>>>>> 2016-07-08 12:19 GMT+04:00 Pavel Troev <pavel.troev(a)gmail.com>: >>>>>>> >>>>>>>> было похожее - пакет не собирался на win ghc 7.10.2 и спокойно >>>>>>>> собрался на 7.10.3, >>>>>>>> если используете stack, то можно просто поменять lts на 7.10.3 >>>>>>>> >>>>>>>> 2016-07-08 11:06 GMT+03:00 Anton Kholomiov < >>>>>>>> anton.kholomiov(a)gmail.com>: >>>>>>>> >>>>>>>>> I've got an error that I don't know how to fix with cabal install. >>>>>>>>> Has anyone experienced such a thing? It installs fine on ubuntu but >>>>>>>>> fails to install on windows 7. It's my own package called >>>>>>>>> `sharc-timbre`: >>>>>>>>> >>>>>>>>> ~~~ >>>>>>>>> Building sharc-timbre-0.1... >>>>>>>>> Preprocessing library sharc-timbre-0.1... >>>>>>>>> C:\Program Files\Haskell Platform\8.0.1\bin\ghc.exe: >>>>>>>>> createProcess: does not >>>>>>>>> exist (No such file or directory) >>>>>>>>> cabal: Leaving directory >>>>>>>>> 'C:\Users\антон\AppData\Local\Temp\cabal-tmp-4484\sharc-timbre-0.1' >>>>>>>>> ~~~ >>>>>>>>> >>>>>>>>> Thanks, >>>>>>>>> Anton >>>>>>>>> >>>>>>>>> _______________________________________________ >>>>>>>>> Haskell-Cafe mailing list >>>>>>>>> To (un)subscribe, modify options or view archives go to: >>>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>>>>>>> Only members subscribed via the mailman list are allowed to post. >>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> С уважением, >>>>>>>> >>>>>>>> Павел Троев >>>>>>>> Консультант БерингПойнт >>>>>>>> Проект SOL, Газпром-нефть >>>>>>>> Моб. тел: +7 916 441 48 61 >>>>>>>> pavel.troev(a)gmail.com | troev.pd(a)gazprom-neft.ru >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> >>>>>> >>>>>> >>>>>> -- >>>>>> С уважением, >>>>>> >>>>>> Павел Троев >>>>>> Консультант БерингПойнт >>>>>> Проект SOL, Газпром-нефть >>>>>> Моб. тел: +7 916 441 48 61 >>>>>> pavel.troev(a)gmail.com | troev.pd(a)gazprom-neft.ru >>>>>> >>>>>> >>>>>> >>>>> >>>> >>>> >>>> -- >>>> С уважением, >>>> >>>> Павел Троев >>>> Консультант БерингПойнт >>>> Проект SOL, Газпром-нефть >>>> Моб. тел: +7 916 441 48 61 >>>> pavel.troev(a)gmail.com | troev.pd(a)gazprom-neft.ru >>>> >>>> >>>> >>> >> >> >> -- >> С уважением, >> >> Павел Троев >> Консультант БерингПойнт >> Проект SOL, Газпром-нефть >> Моб. тел: +7 916 441 48 61 >> pavel.troev(a)gmail.com | troev.pd(a)gazprom-neft.ru >> >> >> >

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Natural keys in Haskell data structures
by Lian Hung Hon 10 Jul '16

10 Jul '16

Dear cafe, What is the idiomatic way to "split" records into their natural keys and content in a data structure? For example, given a user: data User = { username :: ByteString, hash :: ByteString, address :: Text, ... } Using map, a first choice would be Map ByteString User, but this leads to duplication of the username. And it is possible to make mistakes, such as insert "John" (User "Jane" ... What does cafe think? Is there any pattern for this? This is probably just a small nit in the overall architecture, but I'm curious to know the clean way to do it. Regards, Hon

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Re: [Haskell-cafe] Haskell Data Structure design
by Will Yager 10 Jul '16

10 Jul '16

I believe you could use foldM (\sum student -> (sum +) <$> totalFeesOwed student) 0 students Will > On Jul 9, 2016, at 15:09, Guru Devanla <gurudev.devanla(a)gmail.com> wrote: > > OK. Thank you. That is what I ended up doing after I understood how mapM worked. But, since I was going through 2 steps, I was wondering if using foldM directly was possible in this case. > > >> On Sat, Jul 9, 2016 at 11:25 AM, Will Yager <will.yager(a)gmail.com> wrote: >> fees <- mapM totalFeesOwed students >> let total = sum fees >> >> You can use a fold instead of sum if you want. >> >> Will >> >>> On Jul 9, 2016, at 13:10, Guru Devanla <gurudev.devanla(a)gmail.com> wrote: >>> >>> Say, in the above example, I want to add up values returned by `student_totalFeesOwed` by using foldM operation. Is it possible? >>> >>> For example, here is an expression I have >>> >>> L.foldr (\a b-> (evalState (student_totalFeesOwed a) $ env) + b) 0 [(RowId 1), (RowId 2)] >>> >>>> On Sat, Jul 9, 2016 at 9:15 AM, Will Yager <will.yager(a)gmail.com> wrote: >>>> I did the same thing when I was learning to generalize my understanding of monads! Very common mistake. >>>> >>>> I'm not sure I understand your question about #3. Can you give an example using evalState? We'll tell you if you can do it without evalState. >>>> >>>> I suspect you want something like >>>> >>>> "mapM_ addStudentFee students" >>>> >>>> Will >>>> >>>>> On Jul 9, 2016, at 00:56, Guru Devanla <gurudev.devanla(a)gmail.com> wrote: >>>>> >>>>> William/Tom, >>>>> >>>>> (1) Yes, looking into lens and re-factoring my current experimental project in lens will be my next iteration. For now, I plan not to spend time on it. >>>>> >>>>> (2) Agreed. Not sure how I missed that. >>>>> >>>>> (3) I see how foldM works now. I missed the point that foldM not only is a `map` but also does a `sequence` after that. I got stuck earlier, thinking I will end up with a list of state monads. The sequence steps executes this monadic action. >>>>> >>>>> But, how can I do a foldM in a state monad. Say, I need to map over a list of students and add up all their fees, can I get away not `evalState` inside the foldM step function? >>>>> >>>>> Thanks. this is very exciting as I keep simplifying my code! >>>>> >>>>> Guru >>>>> >>>>> >>>>> >>>>> >>>>>> On Fri, Jul 8, 2016 at 7:55 PM, <amindfv(a)gmail.com> wrote: >>>>>> >>>>>>> On Fri, Jul 8, 2016 at 9:57 PM, Guru Devanla <gurudev.devanla(a)gmail.com> wrote: >>>>>> >>>>>> >>>>>>>> 1. I see that almost in every function I deal with state, I have e <- get , expression in the begining. I always ending up having to use the state to query for different values. I guess this is OK. >>>>>> >>>>>> >>>>>> El 8 jul 2016, a las 22:07, William Yager <will.yager(a)gmail.com> escribió: >>>>>> >>>>>>> For #1, look into using the Lens library's support for the State monad. You can often avoid doing a get, and instead write things like `fees += 5`, which will add 5 to the field in the state called "fees". >>>>>> >>>>>> Lens is a pretty heavy extra thing for a beginner to have to learn -- you'll do fine with the 'modify' function: >>>>>> >>>>>> modify :: (s -> s) -> State s () >>>>>> >>>>>> So instead of writing: >>>>>> >>>>>> do >>>>>> s <- get >>>>>> put (s + 5) >>>>>> >>>>>> You say: >>>>>> >>>>>> modify (+5) >>>>>> >>>>>> >>>>>> Tom >

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Commutative monads vs Applicative functors
by Ronald Guida 09 Jul '16

09 Jul '16

I have a few questions about commutative monads and applicative functors. >From what I have read about applicative functors, they are weaker than monads because with a monad, I can use the results of a computation to select between alternative future computations and their side effects, whereas with an applicative functor, I can only select between the results of computations, while the structure of those computations and their side effects are fixed in advance. But then there are commutative monads. I'm not exactly sure what a commutative monad is, but my understanding is that in a commutative monad the order of side effects does not matter. This leads me to wonder, are commutative monads still stronger than applicative functors, or are they equivalent? And by the way, what exactly is a commutative monad?

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Haskell Data Structure design
by Guru Devanla 09 Jul '16

09 Jul '16

Hello All, I am just getting myself to code in Haskell and would like to design advice. Below, I have a made up example: data ClassRoom = ClassRoom { classRoomNo:: Integer, extra_fees::Float, students: Map StudentId Student} data Student = Student {name::String, feesOwed::Float} data StudentId = Integer get_fees_owed classroom student_id = extra_fees + feesOwed $ (students classroom) M.! studentid Here the `get_fees_owed` needs information from the container 'classroom'. Here is my question/problem: I believe I should model most of my code as expressions, rather than storing pre-computed values such as `fees_owed`. But, defining expressions involve passing the container objects all over. For example, deep down in a function that deals with just one `student`, I might need the fees owed information. Without, having a reference to the container, I cannot call get_fees_owed. Also, I think it hinders composing of functions that just deal with one student at a time, but end up with some dependency on the container. I have several questions related to this design hurdle, but I will start with the one above. Thanks! Guru

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Intanciate data type
by Corentin Dupont 08 Jul '16

08 Jul '16

Hi all, I have a data type looking like this: data Foo e a where Foo :: e → Foo e a I would like to instantiate it to make it equivalent to: data Bar a where A :: String → Bar Int B :: Maybe a → Bar a How can I do that? With a functional dependency? I probably need to change the definition of Foo.

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 155, Issue 8
by Alexander Granin 08 Jul '16

08 Jul '16

Hi Anton! My first thought is to blame Russian letters and/or spaces. Try to move your programs and libraries to plain English path. No guarantees it would help, though. Best, Alexander 2016-07-08 18:00 GMT+06:00 <haskell-cafe-request(a)haskell.org>: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Strange error with cabal install on windows: (Anton Kholomiov) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 8 Jul 2016 12:06:07 +0400 > From: Anton Kholomiov <anton.kholomiov(a)gmail.com> > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Subject: [Haskell-cafe] Strange error with cabal install on windows: > Message-ID: > < > CADDxdqOVLzBe%2BKYiPKieZzjfeZJEkqAHsr1jcdM4pcvfiJ8Qzg(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > I've got an error that I don't know how to fix with cabal install. > Has anyone experienced such a thing? It installs fine on ubuntu but > fails to install on windows 7. It's my own package called `sharc-timbre`: > > ~~~ > Building sharc-timbre-0.1... > Preprocessing library sharc-timbre-0.1... > C:\Program Files\Haskell Platform\8.0.1\bin\ghc.exe: createProcess: does > not > exist (No such file or directory) > cabal: Leaving directory > 'C:\Users\антон\AppData\Local\Temp\cabal-tmp-4484\sharc-timbre-0.1' > ~~~ > > Thanks, > Anton >

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Chair in Information/Computer Security (Sheffield, UK)
by Achim D. Brucker 08 Jul '16

08 Jul '16

(Apologies for duplicates) Dear all, The Computer Science Department of The University Of Sheffield has an open position for a Chair in Computer and Information Security. The new chair will lead a new research group in the Department of Computer Science and establish an agenda for security and privacy research across the wider university. Applications are welcome in all areas of computer and/or information security, including * Human factors and secure systems * Security aspects of distributed and autonomous systems Internet of things, cloud computing * Applications of secure systems Manufacturing, health, transport, robotics * Security analysis Intrusion monitoring, threat detection, links with machine learning and language processing * Secure software engineering (Both empirical and theoretical approaches) * Applied cryptography Applications in IT systems, homomorphic encryption For more information about the position and the Department, please visit http://www.sheffield.ac.uk/dcs/research/groups/security/index Or apply directly at https://goo.gl/FSfk5P, application deadline is 27th July, 2016. Best, Achim -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.uk/ | https://logicalhacking.com/blog

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3rd Call for Papers: OCL and Textual Modeling Tools and Textual Model Transformations (OCL 2016) - Less Than 10 Days Left To Submit Your Paper!
by Achim D. Brucker 08 Jul '16

08 Jul '16

(Apologies for duplicates) Less than 10 days until the deadline! CALL FOR PAPERS 16th International Workshop on OCL and Textual Modeling Co-located with ACM/IEEE 19th International Conference on Model Driven Engineering Languages and Systems (MODELS 2016) October 2, 2016, Saint-Malo, France http://oclworkshop.github.io 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 to 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 or formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- meta-modeling 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 - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - 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 tools that support - in a very broad sense - textual modeling languages (if you have implemented OCL.js to run OCL in a web browser, this is the right workshop to present your work) as well as textual model transformations. Venue ===== The workshop will be organized as a part of MODELS 2016 Conference in Saint-Malo, France. 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, 2013 in Miami, 2014 in Valencia, Spain, and 2015 in Ottawa, Canada. 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 contributions (between 6 and 8 pages) describing new ideas, innovative tools or position papers. * full papers (between 12 and 16 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://easychair.org/conferences/?conf=ocl16) 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 post-conference edition of CEUR (http://www.ceur-ws.org) Important Dates =============== Submission of papers: July 17, 2016 Notification: August 14, 2016 Workshop date: October 2, 2016 Organizers ========== Achim D. Brucker, The University of Sheffield, UK Jordi Cabot, ICREA - Open University of Catalonia, Spain Adolfo Sánchez-Barbudo Herrera, University of York, UK Programme Committee (TBC) ========================= Thomas Baar, University of Applied Sciences Berlin, Germany Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Domenico Bianculli, University of Luxembourg Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Indra Sistemas S.A., Spain Geri Georg, Colorado State University, USA Martin Gogolla, University of Bremen, Germany Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Massimo Tisi, Mines de Nantes, France Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ. Paris-Sud - LRI, France Steffen Zschaler, King's College, UK -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.uk/ | https://logicalhacking.com/blog

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Strange error with cabal install on windows:
by Anton Kholomiov 08 Jul '16

08 Jul '16

I've got an error that I don't know how to fix with cabal install. Has anyone experienced such a thing? It installs fine on ubuntu but fails to install on windows 7. It's my own package called `sharc-timbre`: ~~~ Building sharc-timbre-0.1... Preprocessing library sharc-timbre-0.1... C:\Program Files\Haskell Platform\8.0.1\bin\ghc.exe: createProcess: does not exist (No such file or directory) cabal: Leaving directory 'C:\Users\антон\AppData\Local\Temp\cabal-tmp-4484\sharc-timbre-0.1' ~~~ Thanks, Anton

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Resumable applicative parsing from tail end?
by martin 07 Jul '16

07 Jul '16

Hello all, in my attempt to write a simulation program, I am facing the following problem: as the simulation progresses, I collect "primary" log entries, each one associated with a timestamp. These entries carry the information I am interested in (and not so much the final state). Currently I am using a simple list to collect log entries. Since (:) is so much easier than (++) I prepend new entries to the head of the list. So my log runs backwards in time with the oldest entries at the tail end. Because the log-entries are typically too fine-grained to be useful, I want to aggregate several of them into a single log entry in a secondary log. I want to do this "as I go" and discard primary log entries which are already aggregated. Thus I can keep the primary log reasonably small. I thought that aggregation is just a special case of parsing. And indeed, I could write an applicative parser PrimaryLog -> (PrimaryLog, SecondaryLog) which does the trick, except I had to reverse the ordering of the primary log. This is because the parser/aggregator must parse from past to future, because there may be new log entries coming up in the future. I need to be able to resume parsing where I left off whenever new primary log entries are entered, becuase I may then have just enough unconsumed primary log entries to create another secondary entry. Currently I'm trying Data.Sequence to let the primary log grow to the left and let parsing to begin at the far-right. But I have a bad feeling about it. Particularly, I don't have any hard reasons why a simple List wouldn't be sufficient. But I just wasn't able to write the code. What do you think? How would you parse a list from the tail end? Or how else would you deal with this problem?

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[ANN] processing-for-haskell. New graphics and animation package
by Anton Kholomiov 06 Jul '16

06 Jul '16

I'd like to announce a graphics libary processing-for-haskell (available on Hackage). It tries to implement Processing language with Haskell. Processing is an imperative DSL for graphics and animation. It's very easy and fun to use. Intended to be used by non-programmers it has very small but powerful core of functions. It's a shallow EDSL. The Processing is implemented with OpenGL and GLUT. That gives us an opportunity to use all Haskell IO-functions inside the animation loop. Enjoy! github: https://github.com/anton-k/processing-for-haskell hackage: http://hackage.haskell.org/package/processing-for-haskell

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Haskell in Leipzig 2016: Another Call for Papers (deadline extended)
by Joachim Breitner 05 Jul '16

05 Jul '16

Haskell in Leipzig September 14-15, 2016 HTKW Leipzig, Germany http://hal2016.haskell.org/ We have received good and interesting proposals, but our schedule still has room for more. So whether you wanted to submit, but just did not make the first deadline, or whether you only now decide to come to Leipzig, this is your chance: Please submit your short abstract until Friday, July 15th. Do you need ideas? Here some possible directions: * Very fancy type tricks using all of GHC’s extensions. * Very fancy type tricks using none of GHC’s extensions. * Getting Haskell to run really really fast. * Using Haskell to make real money, in real life. * Showing how this one odd Haskell library make doing something appear rally simple. * Stuff that is really horrible with Haskell (and how to do it better) * What is cool in other functional programming language right now. We are particularly interested in good tutorials. These could range from impressing beginners with what you can do with Haskell to teaching new tricks to the experts. So if you think you have something useful to teach, please submit a proposal. Please share this CfP with whoever might be interested. == About HaL == The workshop series “Haskell in Leipzig”, now in its 11th year, brings together Haskell developers, Haskell researchers, Haskell enthusiasts and Haskell beginners to listen to talks, take part in tutorials, and join in interesting conversations. Everything related to Haskell is on topic, whether it is about current research, practical applications, interesting ideas off the beaten track, education, or art, and topics may extend to functional programming in general and its connections to other programming paradigms as well. This year, HaL is colocated with two related conferences, * the Workshop on Functional and (Constraint) Logic Programming (WFLP) and * the Workshop on (Constraint) Logic Programming (WLP), to form the Leipzig Week of Declarative Programming (L-DEC): http://nfa.imn.htwk-leipzig.de/LDEC2016/ In order to accommodate and welcome a more international audience, this year’s HaL will be held in English. == Invited Speaker == Unsure about whether HaL is interesting to you? No need to worry: Alejandro Russo (from Chalmers) is our keynote speaker and will talk about his work on information-flow control (i.e. SecLib, LIO, MAC, HLIO) – a great topic that is of interest to researchers, practitioners and beginners alike. == Submissions == Contributions can take the form of * talks (about 30 minutes), * tutorials (about 90 minutes), * demonstrations, artistic performances, or other extraordinary things. Please submit an abstract that describes the content and form of your presentation, the intended audience, and required previous knowledge. We recommend a length of 2 pages, so that the PC and the audience get a good idea of your submission, but this is not a hard requirement. You can submit your abstract, as an PDF document, at https://easychair.org/conferences/?conf=hal2016 until Friday, July 15, 2016. You will be notified by July 30, 2016. == Program committee == * Andreas Abel, Chalmers and Gothenburg University, Sweden * Heinrich Apfelmus, Leipzig, Germany * Joachim Breitner, Karlsruhe Institute of Technology, Germany (Chair) * Matthias Fischmann, Zerobuzz, Germany * Petra Hofstedt, BTU Cottbus-Senftenberg, Germany * Wolfgang Jeltsch, Institute of Cybernetics at Tallinn University of Technology, Estonia * Andres Löh, Well-Typed LLP, Germany * Alejandro Serrano Mena, Universiteit Utrecht, Netherlands * Neil Mitchell, Standard Chartered Bank, UK * Katie Ots, Facebook, UK * Peter Stadler, University of Leipzig, Germany * Henning Thielemann, Freelancer, Germany * Niki Vazou, University of California, San Diego, USA If you have any questions, please do not hesitate to contact Joachim Breitner <breitner(a)kit.edu>. -- Dr. rer. nat. Joachim Breitner Wissenschaftlicher Mitarbeiter http://pp.ipd.kit.edu/~breitner

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2nd CfP: IFL 2016 (28th Symposium on Implementation and Application of Functional Languages)
by publicityifl＠gmail.com 05 Jul '16

05 Jul '16

Hello, Please, find below the second call for papers for IFL 2016. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- IFL 2016 - 2nd Call for papers 28th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2016 KU Leuven, Belgium In cooperation with ACM SIGPLAN August 31 - September 2, 2016 https://dtai.cs.kuleuven.be/events/ifl2016/ Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2016 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Peer-review Following the IFL tradition, IFL 2016 will use a post-symposium review process to produce the formal proceedings. All participants of IFL 2016 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings are not subject to the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. The formal proceedings will appear in the International Conference Proceedings Series of the ACM Digital Library. Important dates August 1: Submission deadline draft papers August 3: Notification of acceptance for presentation August 5: Early registration deadline August 12: Late registration deadline August 22: Submission deadline for pre-symposium proceedings August 31 - September 2: IFL Symposium December 1: Submission deadline for post-symposium proceedings January 31, 2017: Notification of acceptance for post-symposium proceedings March 15, 2017: Camera-ready version for post-symposium proceedings Submission details Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must use the new ACM two columns conference format, which can be found at: http://www.acm.org/publications/proceedings-template For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. Authors submit through EasyChair: https://easychair.org/conferences/?conf=ifl2016 Topics IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2016, please contact the PC chair at tom.schrijvers(a)cs.kuleuven.be. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee Chair: Tom Schrijvers, KU Leuven, Belgium - Sandrine Blazy, University of Rennes 1, France - Laura Castro, University of A Coruña, Spain - Jacques, Garrigue, Nagoya University, Japan - Clemens Grelck, University of Amsterdam, The Netherlands - Zoltan Horvath, Eotvos Lorand University, Hungary - Jan Martin Jansen, Netherlands Defence Academy, The Netherlands - Mauro Jaskelioff, CIFASIS/Universidad Nacional de Rosario, Argentina - Patricia Johann, Appalachian State University, USA - Wolfram Kahl, McMaster University, Canada - Pieter Koopman, Radboud University Nijmegen, The Netherlands - Shin-Cheng Mu, Academia Sinica, Taiwan - Henrik Nilsson, University of Nottingham, UK - Nikolaos Papaspyrou, National Technical University of Athens, Greece - Atze van der Ploeg, Chalmers University of Technology, Sweden - Matija Pretnar, University of Ljubljana, Slovenia - Tillmann Rendel, University of Tübingen, Germany - Christophe Scholliers, Universiteit Gent, Belgium - Sven-Bodo Scholz, Heriot-Watt University, UK - Melinda Toth, Eotvos Lorand University, Hungary - Meng Wang, University of Kent, UK - Jeremy Yallop, University of Cambridge, UK Venue The 28th IFL will be held in association with the Faculty of Computer Science, KU Leuven, Belgium. Leuven is centrally located in Belgium and can be easily reached from Brussels Airport by train (~15 minutes). The venue in the Arenberg Castle park can be reached by foot, bus or taxi from the city center. See the website for more information on the venue.

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Packaging a library with a separate shared object
by Bryan Gardiner 05 Jul '16

05 Jul '16

Hi folks! I'm working on packaging Qtah (my Qt bindings) for upload to Hackage. This is somewhat complicated, as I have some C++ glue code that I build into a shared library that the Haskell library needs. Currently, my build hooks are as follows, in: https://gitlab.com/khumba/qtah/blob/packaging/qtah/Setup.hs - postConf: Generate Haskell code in src/ and C++ code in cpp/. - preBuild: Add cpp/ to HookedBuildInfo.extraLibDirs because the build phase needs this needs this. - buildHook: Run make to build the C++ code into libqtah.so before the Haskell code builds. - copyHook and instHook: Copy libqtah.so into InstallDirs.libdir (~/.cabal/lib/.../qtah-*/). This installs successfully, but ultimately doesn't work because the Haskell libHSqtah*.so that gets built can't find libqtah.so. The installation directory ~/cabal/lib/.../qtah-*/ isn't on libHSqtah*.so's RUNPATH (which makes sense, it's not known at link time). I could create a separate package for the C++ bits, but I'd rather not do that unless necessary (there's already a qtah-generator package as well). I can't see how I can avoid doing this though. Are there any Cabal experts that could point me in the right direction, or confirm that I'll need to split the package? Many thanks, Bryan

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Interruptible exception wormholes kill modularity
by Edward Z. Yang 04 Jul '16

04 Jul '16

In 2010, in the thread "Asynchronous exception wormholes kill modularity" [1], Bas van Dijk observed that 'unblock :: IO a -> IO a' broke modularity, as the sequence of calls 'block . block . unblock $ io' would result in 'io' being run with asynchronous exceptions unblocked, despite the outer 'block' "expecting" that asynchronous exceptions cannot be thrown. I would like to make two claims: 1. The new mask/restore interface is insufficient to "solve" this modularity problem, as *interruptible* operations can still be used to catch asynchronous exceptions. 2. Thus, we should provide an unblock combinator which can be used to catch asynchronous exceptions from a 'mask' (though not an 'uninterruptibleMask')--though it is doubtful if anyone should ever use 'mask' in the first place. Claim 1: Here is some code which reimplements 'unblock': import Control.Exception import Control.Concurrent import Control.Concurrent.MVar unblock :: IO a -> IO a unblock io = do m <- newEmptyMVar _ <- forkIO (io >>= putMVar m) takeMVar m The main idea is that 'takeMVar' is an interruptible operation: when it blocks, the thread can now receive asynchronous exceptions. In general, a thread can unmask exceptions by blocking. Here is a simple test-case: main = do let x = 10000000 -- Just do a bit of work tid <- myThreadId forkIO $ (threadDelay 10000 >> killThread tid) r <- mask $ \restore -> do -- restore $ do -- unblock $ do -- do something non-blocking evaluate (f x []) -- If the exception is delivered in a timely manner, -- shouldn't get here. print r f 0 r = r f n r = f (n-1) (n:r) With both restore and unblock commented, the ThreadKilled exception is delayed; uncommenting either restore or unblock causes the exception to be delivered. This admonition does not apply to uninterruptibleMask, for which there are no interruptible exceptions. Claim 2: Thus, I come to the conclusion that we were wrong to remove 'unblock', and that it is no worse than the ability for interruptible actions to catch asynchronous exceptions. You could very well argue that interruptible actions are a design flaw. Then you should use 'uninterruptibleMask' instead, which effectively removes the concept of interruptibility--and is thus modular. Indeed, Eyal Lotem proposed [2] that 'bracket' should instead use 'uninterruptibleMask', for precisely the reason that it is too easy to reenable asynchronous exceptions in 'mask'. But assuming that interruptible masks are a good idea (Simon Marlow has defended them as "a way avoid reasoning about asynchronous exceptions except at specific points, i.e., where you might block"), there should be an 'unblock' for this type of mask. It should be said that the absence of 'unblock' for 'uninterruptibleMask' only implies that a passed in IO action (e.g., the cleanup action in bracket) does not have access to the exceptions thrown to the current thread; it doesn't actually guarantee uninterruptibility, since the passed in IO action could always raise a normal exception. Haskell's type system is not up to the task of enforcing such invariants. Cheers, Edward [1] https://mail.haskell.org/pipermail/libraries/2010-March/013310.html https://mail.haskell.org/pipermail/libraries/2010-April/013420.html [2] https://mail.haskell.org/pipermail/libraries/2014-September/023675.html P.S. You were CC'ed to this mail because you participated in the original "Asynchronous exception wormholes kill modularity" discussion. P.P.S. I have some speculations about using uninterruptibleMask more frequently: it seems to me that there ought to be a variant of uninterruptibleMask that immediately raises an exception if the "uninterruptible" action blocks. This would probably of great assistance of noticing and eliminating blocking in uninterruptible code.

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How to make Repa do stencils unboxed?
by William Yager 04 Jul '16

04 Jul '16

Hello all, TL;DR: Repa appears to be calculating my stencil operation using boxed arithmetic, which is very slow. How do I make it do unboxed arithmetic? I am currently working on a project where I need to simulate a 2D environment that exhibits wave behavior. I'm accomplishing this by directly discretizing the wave equation and turning it into a cellular automaton. The math here is relatively simple. Let's specify the example to the physical system of a rubber sheet, which exhibits wave behavior. To simulate a rubber sheet, you divide it up into grids. Then, you keep track of the velocity and height of each grid. At each iteration of the simulation, you update the velocity of each grid based on the force exerted upon it by its neighboring grids. This is simply a linear multiple of the distance between the grid and its neighbors (i.e. how stretched the sheet is). For example, if cell x is at height X, and its neighbors a,b,c,d are at height A,B,C,D, the tension on X is, for some constant L, T = L*((A-X)+(B-X)+...) = L*(A+B+C+D-4X). You can calculate this efficiently via a matrix convolution, in particular by a convolution with the matrix [[ 0 1 0 1 -4 1 0 1 0 ]] You can be more physically accurate by including the diagonals (weighted down a bit). A popular stencil for such purposes is [[ 1 2 1 2 -12 2 1 2 1 ]] Technically the off-diagonal elements should be something like sqrt(2), but this is sufficient for most purposes. After calculating the tension matrix, you divide it by the mass of each grid. This is the acceleration on each grid. Multiply it by the length of your time step, and you get the change in velocity of each grid. Add this to the velocity matrix. Then, multiply the velocity matrix by the time step, and that's the change in height. Add this to the height matrix. Boom, you've simulated a single time step of a rubber sheet. Keep doing this over and over to get the time-evolution of the rubber sheet. My code to do this is as follows: https://gist.github.com/wyager/8c468c9d18ad62aff8bc9738aa947ea4 The imports are messy; sorry about that. You'll notice I'm using the Dimensional library, which provides statically typed physical dimensions over any numeric value using a newtype wrapper with typelits. Hopefully this isn't slowing things down somehow. According to profiling via "ghc -Odph -rtsopts -threaded -fno-liberate-case -funfolding-use-threshold1000 -funfolding-keeness-factor1000 -optlo-O3 -fforce-recomp -prof -fprof-auto -fexternal-interpreter RepaSim.hs" (whew), fully 80% of time and 84% of allocation comes from the stencil operation. The simulation starts with a rubber sheet with a smooth bump in the middle, simulates 100ms of time steps, saves a .png file of the sheet, and then repeats n times (n is typed in via standard input). How can I make Repa do this stencil unboxed, and are there other things I should be doing to get better performance? Thanks, Will

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Octree and Bounding Boxes
by Michael Litchard 04 Jul '16

04 Jul '16

I've forked the octree library and added some bounding box functionality. The problem is, it turns out kd-trees are better for my use case. I'd rather not junk the code, but I can't see a use case for the octree library as it stands. Below is the link to the branch where the code is, is there a use case for point octrees needing bounding boxes? https://github.com/mlitchard/octree/tree/test-reorg/Data/Octree/BoundingBox

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 154, Issue 27
by Vlad Lopatin 02 Jul '16

02 Jul '16

Re: up-to-date glfw examples (briand(a)aracnet.com) Hey, Brian Here's a bare-bones example of modern OpenGL(4.5) (VBO's, vertex/fragment shaders), using GLFW-b to handle windows/input and Yampa for animation. Shader-loading is handled by Sven Panne's code: https://github.com/madjestic/Haskell-OpenGL-Tutorial/tree/master/Mandelbrot… Cheers, Vlad

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haskell-browser-mods - v0.1
by Erik Rantapaa 02 Jul '16

02 Jul '16

Hi everyone, I've created a browser extension to aid in navigating around Haskell-related websites. The extension adds information to visited pages, create links to other relevent pages and adds keyboard shortcuts and conveniences. The sites the extension knows about include: - haskellnews.org - hdiff.luite.com - hackage.haskell.org - packdeps.haskellers.com - hayoo.fh-wedel.de The extension is currently available for Safari and Chrome. For more information and downloads visit: https://github.com/erantapaa/haskell-browser-mods Feedback, suggestions, bug reports and PRs are very welcome. Erik

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type level Min function for GHC.TypeLits?
by Baojun Wang 01 Jul '16

01 Jul '16

Hi Cafe, I'm trying to do some basic type level tricks, I wonder whether it is possible to get ``vZipWith`` in below code snippet working: {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UndecidableInstances #-} {-# OPTIONS_GHC -fplugin GHC.TypeLits.Normalise #-} import GHC.TypeLits import Data.Proxy data Vect :: (Nat -> * -> *) where VNil :: Vect 0 a VCons :: a -> Vect n a -> Vect (1+n) a deriving instance (Show a) => Show (Vect n a) vlength :: forall n a . KnownNat n => Vect n a -> Integer vlength _ = natVal (Proxy :: Proxy n) vconcat :: Vect n a -> Vect m a -> Vect (n+m) a -- Need type checker plugin vconcat VNil ys = ys vconcat xs VNil = xs vconcat (VCons x xs) ys = VCons x (vconcat xs ys) type family Min3 (t :: Bool) (m :: Nat) (n :: Nat) :: Nat type instance Min3 'True m n = m type instance Min3 'False m n = n type Min m n = Min3 (m <=? n) m n vZipWith :: (a -> b -> c) -> Vect m a -> Vect n b -> Vect (Min m n) c vZipWith f VNil _ = VNil vZipWith f _ VNil = VNil vZipWith f (VCons x xs) (VCons y ys) = f x y `VCons` vZipWith f xs ys As I got errors: /home/wangbj/downloads/stack/vect/src/Vect.hs:51:21: Could not deduce (Min3 (m <=? 0) m 0 ~ 0) from the context (n ~ 0) bound by a pattern with constructor VNil :: forall a. Vect 0 a, in an equation for ‘vZipWith’ at /home/wangbj/downloads/stack/vect/src/Vect.hs:51:14-17 Expected type: Vect (Min m n) c Actual type: Vect 0 c Relevant bindings include vZipWith :: (a -> b -> c) -> Vect m a -> Vect n b -> Vect (Min m n) c (bound at /home/wangbj/downloads/stack/vect/src/Vect.hs:50:1) In the expression: VNil In an equation for ‘vZipWith’: vZipWith f _ VNil = VNil /home/wangbj/downloads/stack/vect/src/Vect.hs:52:40: Could not deduce (Min3 (m <=? n) m n ~ (1 + Min3 (n1 <=? n2) n1 n2)) from the context (m ~ (1 + n1)) bound by a pattern with constructor VCons :: forall a (n :: Nat). a -> Vect n a -> Vect (1 + n) a, in an equation for ‘vZipWith’ at /home/wangbj/downloads/stack/vect/src/Vect.hs:52:13-22 or from (n ~ (1 + n2)) bound by a pattern with constructor VCons :: forall a (n :: Nat). a -> Vect n a -> Vect (1 + n) a, in an equation for ‘vZipWith’ at /home/wangbj/downloads/stack/vect/src/Vect.hs:52:26-35 Expected type: Vect (Min m n) c Actual type: Vect (1 + Min n1 n2) c Relevant bindings include ys :: Vect n2 b (bound at /home/wangbj/downloads/stack/vect/src/Vect.hs:52:34) xs :: Vect n1 a (bound at /home/wangbj/downloads/stack/vect/src/Vect.hs:52:21) vZipWith :: (a -> b -> c) -> Vect m a -> Vect n b -> Vect (Min m n) c (bound at /home/wangbj/downloads/stack/vect/src/Vect.hs:50:1) In the expression: f x y `VCons` vZipWith f xs ys In an equation for ‘vZipWith’: vZipWith f (VCons x xs) (VCons y ys) = f x y `VCons` vZipWith f xs ys Failed, modules loaded: none. Also I tried https://github.com/yav/type-nat-solver but got error like: *** Exception: user error (Unexpected result from the SMT solver: Expected: success Result: (error "line 57 column 16: unknown constant tn_0" ) Which I have no idea what it really means. Is there any way to get ``vZipWith`` working without too much hassle? Thanks baojun

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