July 2016 - Haskell-Cafe

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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