- Haskell-Cafe - Haskell.org

Template Haskell tutorials?
by Kenneth Hoste 14 Apr '11

14 Apr '11

Hi, The links to the supposedly brilliant Template Haskell tutorials by Bulat are broken. http://www.haskell.org/bz/thdoc.htm http://www.haskell.org/bz/th3.htm Does anyone know if these tutorials moved to somewhere else? greetings, Kenneth

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Haskell Weekly News: Issue 177
by Daniel Santa Cruz 14 Apr '11

14 Apr '11

Welcome to issue 177 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of April 3 to 9, 2011. You can find a HTML rendition of this issue at: http://bit.ly/hR2sMB Announcements Brent Yorgey [2]sent out a call for contributions for the second edition of Typeclassopedia. Roel van Dijk started what became a popular[3]thread on the mailing list about encodings of Haskell source files. What would you like to see here? I have gotten mixed feedback on what constitutes interesting items for the "announcements" section of the mailing list. Some have pointed out that package announcements tend to be noisy and not all that interesting to the community at large, or that have a bias towards only pointing out packages where the maintainer decides to send an email to the mailing list. What do *you* think? How about the "calls for papers"? Do those catch your attention? Let me hear what you think. Quotes of the Week * ion: Itâs a well-known historical fact cave men writing Haskell hated the ASCII pseudoglyphs. * shapr: I persuaded academics to show up and hang out. [...] I told the academics that there were academics worth chatting with on #haskell :-) * pjscott: If you really want to see something ridiculous, try looking at the auto-generated library documentation for anything involving regular expressions. The type declarations in Text.Regex.TDFA are like something H. P. Lovecraft might write about. * monochrom: the road to haskell is paved with good abstractions Top Reddit Stories * Typeclassopedia second edition: call for contributions From (byorgey.wordpress.com) scored 53 with 1 comments. Read on [4]reddit. Read the [5]original post. * High-Performance Web Applications In Haskell : Gregory Collins : QCon London :: PDF From (qconlondon.com) scored 45 with 8 comments. Read on [6]reddit. Read the [7]original post. * Haskell For the Cloud (new paper by Jeff Epstein, Andrew Black, and and Simon Peyton Jones) From (research.microsoft.com) scored 41 with 23 comments. Read on [8]reddit. Read the [9]original post. * Yhc is dead !? From (yhc06.blogspot.com) scored 32 with 27 comments. Read on [10]reddit. Read the [11]original post. * What is a functional language? :: Robert Harper From (existentialtype.wordpress.com) scored 29 with 35 comments. Read on [12]reddit. Read the [13]original post. * Functions are values From (existentialtype.wordpress.com) scored 27 with 16 comments. Read on [14]reddit. Read the [15]original post. * Understanding practical API design, static typing and functional programming. From (blog.tmorris.net) scored 27 with 2 comments. Read on [16]reddit. Read the [17]original post. * Platform Robots in Nikki and the Robots: a Haskell platform game From (joyridelabs.de), scored 22 with 3 comments. Read on [18]reddit. Read the [19]original post. * A Model for Denotative Continuous-Time Programming (AKA: FRP) From (creativelad.wordpress.com) scored 21 with 1 comments. Read on [20]reddit. Read the [21]original post. * Type Kata: Local data type From (blog.ezyang.com) scored 19 with 3 comments. Read on [22]reddit. Read the [23]original post. Top StackOverflow Questions * [24]Difference between State, ST, IORef, and MVar votes: 16, answers: 3 * [25]Duality approaches in functional programming votes: 11, answers: 1 * [26]What are the main differences of language features between Haskell and the functional part of F#? [closed] votes: 9, answers: 5 * [27]Using the Maybe Monad in "reverse" in Haskell votes: 9, answers: 3 * [28]How to reason about space complexity in Haskell votes: 8, answers: 1 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/18636 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87815 4. http://www.reddit.com/r/haskell/comments/ghs1x/typeclassopedia_second_editi… 5. http://byorgey.wordpress.com/2011/04/03/call-for-contributions-second-editi… 6. http://www.reddit.com/r/haskell/comments/gj7ud/highperformance_web_applicat… 7. http://qconlondon.com/dl/qcon-london-2011/slides/GregoryCollins_HighPerform… 8. http://www.reddit.com/r/haskell/comments/gjb52/haskell_for_the_cloud_new_pa… 9. http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/ 10. http://www.reddit.com/r/haskell/comments/ghtlf/yhc_is_dead/ 11. http://yhc06.blogspot.com/2011/04/yhc-is-dead.html 12. http://www.reddit.com/r/haskell/comments/gk1tp/what_is_a_functional_languag… 13. http://existentialtype.wordpress.com/2011/03/16/what-is-a-functional-langua… 14. http://www.reddit.com/r/haskell/comments/ghdvn/functions_are_values/ 15. http://existentialtype.wordpress.com/2011/04/02/functions-are-values/ 16. http://www.reddit.com/r/haskell/comments/gir70/understanding_practical_api_… 17. http://blog.tmorris.net/understanding-practical-api-design-static-typing-an… 18. http://www.reddit.com/r/haskell/comments/ghs81/platform_robots_in_nikki_and… 19. http://joyridelabs.de/blog/?p=612 20. http://www.reddit.com/r/haskell/comments/ghrmv/a_model_for_denotative_conti… 21. http://creativelad.wordpress.com/2011/04/03/dctp-model/ 22. http://www.reddit.com/r/haskell/comments/gk1w4/type_kata_local_data_type/ 23. http://blog.ezyang.com/2011/04/type-kata-local-data-type/ 24. http://stackoverflow.com/questions/5545517/difference-between-state-st-iore… 25. http://stackoverflow.com/questions/5557810/duality-approaches-in-functional… 26. http://stackoverflow.com/questions/5541487/what-are-the-main-differences-of… 27. http://stackoverflow.com/questions/5606228/using-the-maybe-monad-in-reverse… 28. http://stackoverflow.com/questions/5552433/how-to-reason-about-space-comple…

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Fucntion composing
by Adam Krauze 14 Apr '11

14 Apr '11

Hello, as I am newbie to Haskell and my introductory question is: given functions say f and g with type signatures f :: (Num a) => [a] -> [a] -> [(a,a)] // f takes two lists and zips them into one in some special way g :: (Num a) => a -> [(a,a)] -> [a] // g using some Num value calculates list of singletons from list of pairs of course g 0 :: (Num a) => [(a,a)] ->[a] now I want to create function h :: (Num a) => [a] -> [a] -> [a] in such way that (g 0) consumes output of f. But when I try Prelude> :t (g 0).f I get an error: <interactive>:1:9: Couldn't match expected type `[(a0, a0)]' with actual type `[a1] -> [(a1, a1)]' Expected type: [a1] -> [(a0, a0)] Actual type: [a1] -> [a1] -> [(a1, a1)] In the second argument of `(.)', namely `f' In the expression: (g 0) . f In pointfull representation it works well Prelude> let h x y = (g 0 (f x y)) How to do pointfree definition of h? Ajschylos.

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Service Component Architecture and Distributed Haskell
by James Keats 13 Apr '11

13 Apr '11

Hi all. There's recently been some talk about distributed haskell, but what I wish to see is interest from an experienced member of the Haskell community in implementing Haskell components with SCA/Tuscany. Please see this and consider it, it'd be great to be able to use Haskell in the same project with Java/Scala. Thanks. Link: http://www.haskell.org/pipermail/haskell/2010-April/022004.html

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Higher-kinded Quantification
by Leon Smith 13 Apr '11

13 Apr '11

I have a type constructor (Iterator i o m a) of kind (* -> * -> (* -> *) -> *), which is a monad transformer, and I'd like to use the type system to express the fact that some computations must be "pure", by writing the impredicative type (Iterator i o (forall m. m) a). However I've run into a bit of difficulty expressing this, due to the kind of m. I've attached a minimal-ish example. Is there a way to express this in GHC?

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Generating random graph
by Mitar 13 Apr '11

13 Apr '11

Hi! I have made this function to generate a random graph for Data.Graph.Inductive library: generateGraph :: Int -> IO (Gr String Double) generateGraph graphSize = do when (graphSize < 1) $ throwIO $ AssertionFailed $ "Graph size out of bounds " ++ show graphSize let ns = map (\n -> (n, show n)) [1..graphSize] es <- fmap concat $ forM [1..graphSize] $ \node -> do nedges <- randomRIO (0, graphSize) others <- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ -> randomRIO (1, graphSize) gen <- getStdGen let weights = randomRs (1, 10) gen return $ zip3 (repeat node) others weights return $ mkGraph ns es But I noticed that graph has sometimes same weights on different edges. This is very unlikely to happen so probably I have some error using random generators. Could somebody tell me where? Mitar

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Syntastic support for cabal
by Luke Randall 13 Apr '11

13 Apr '11

Hi all I'm using Haskell mode for Vim, as well as syntastic. I've found syntastic particularly useful as I'm quite new to Haskell and sometimes get the types of my functions wrong. When working on Cabal packages however, syntastic marks import statements which import modules local to the cabal package as being incorrect because the module cannot be found. Has anyone solved this, or do you have pointers on how I can? Thanks -- Luke Randall Sent with Sparrow

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Debugging with gdb?
by Svante Signell 13 Apr '11

13 Apr '11

Hi, As I don't know anything about Haskell, can I make a stupid question: Is there any method to create debug symbols for a Haskell program, and is it possible to debug with gdb? Thanks!

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RFC: Extendable and Easy-To-Use Logger (HLogger)
by Jon Kristensen 13 Apr '11

13 Apr '11

Hello Haskellers! I have developed a very simple logging library to be extendable and easy-to-use. You can see the current release at http://hackage.haskell.org/package/hlogger/. The SimpleHLogger module is an example of what a HLogger implementation could look like. It could be used like this: import System.Log.SimpleHLogger main = do logger<- simpleLogger "test" loggerLog logger (Just ("LoggerTest", "main")) Debug "Test" loggerStop logger As I have now implemented everything that I could think of, I would like to ask the Haskell community to give some feedback on HLogger so that I can continuing improving it. Some questions: 1. Do you have any general/architectural thoughts on things I might be doing wrong? 2. Is the ease-of-use goal contradicting making this library useful in more complex applications? 3. Do I want to redesign HLogger to allow loggers to have a mutable state, and if so, how can I do that? 4. Is there a nice way to provide the module and the function than using (Just ("LoggerTest, "main)) (shortening it somehow, I guess)? Thanks! Warm regards, Jon Kristensen

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Using DPH
by Wilfried Kirschenmann 13 Apr '11

13 Apr '11

Hi, In order to do a performance comparison beetween different approaches for our application, I make different implementation of a simple example (computing the norm of a vector expression. I rely on Repa to do this. However, when I tried to build the parallel version (-threaded -fvectorise -rtsopts), I got an error specifying that dph-par was not available. Indeed, It wasn't. As explained on the dph webpage, I installed a developpment version of ghc (ghc-7.1.20110331). However, when I try to build my application, I get the folowing error: user error (Pattern match failure in do expression at compiler/vectorise/Vectorise/Monad.hs:57:10-20) My source code is attached and compiled with the following line: ghc haskell.hs -O3 -XBangPatterns -msse2 -fforce-recomp -rtsopts -threaded -fvectorise So my question is : is the problem concerning the pattern matching known ? If yes, is there a workaround ? is there an other version of dph that would work with Repa ? Thank you ! ----- Wilfried Kirschenmann "An expert is a person who has made all the mistakes that can be made in a very narrow field."* *Niels Bohr - *Danish physicist (1885 - 1962)*

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ANNOUNCE: TCache 0.8
by Alberto G. Corona 13 Apr '11

13 Apr '11

Tired of your persistence layer? Take persistence the haskelll way. Besides the backward compatible stuff, TCache now defines persistent STM variables (DBRef's) that leverages the traditional haskell reference syntax to perform fast in-memory database transactions and inter-object references. It includes indexing, triggers and an straightforward pure-haskell query language based on record field relations. http://hackage.haskell.org/package/TCache-0.8.0.2 This version includes a lot of changes and must be considered beta. Please submit any bugs, comments and so on. Cheers!

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Request for library recommendations
by Erik de Castro Lopo 13 Apr '11

13 Apr '11

Hi all, I'm currently scoping out a project for work that I'd like to do in Haskell. Basically its a HTTP proxy (ie like the Squid proxy) but with a few wrinkles: - Since the connecting clients are untrusted, the incoming connection is HTTP, the header is modified by the proxy to add BasicAuth (credentials known only to the proxy, not the client) and then forwared upstream with responses pass back to the client. - Needs to work via upstream Squid proxy (so this proxy needs to be able to requirest a CONNECT from the upstream Squid proxy). - In some cases, the client will POST via the proxy and immediately exit and the proxy will be required to re-try the POST if it fails. - Other manipulation of headers and data. I am aware that Squid can satisfy the first two requirements above but not all the others (without actually hacking the Squid code base). I have done stuff like this before in C++ and Ocaml, so the mechanics are not a mystery. However I would like as much as possible to make use of existing Haskell libraries to cut down the development time. Performance is also not critical, so as long as the Haskell version can beat a hypothetical Python implementation I'd be happy. Thanks in advance. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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syntactic sugar for heterogeneous lists
by Alberto G. Corona 13 Apr '11

13 Apr '11

Hi Cafe: http://hackage.haskell.org/trac/ghc/ticket/1245 I also want some kind of syntactic sugar for H. Lists. The absence of syntactic sugar makes heterogeneous list to look scary. People in haskell is accostumed to syntactic sugaring, so people think of not sugared expressions as second class. I´ don't know the details, but it seems that tuples are in the language to cover the heterogeneous flexibility thing that homogeneous lists may not provide, but they introduce its own inflexibilities; These repeating instances for two, three, four and so on tuples makes them artificial and repetitive. In its comparison, a complex but unique H. List instance look elegant. specially if it is sugarized. An alternative to sugarize H.Lists preserving tuples could be to use {} to sugarize H. lists as {x,y,z} and desugarize it into: x :*: y :*: z :*: {} Just like [] means empty list, {} would mean HNil, the empty heterogeneous list. But this alternative , if implemented, would soon render tuples obsolete. These (,,) (,,,) constructors may create marginally faster and compact structures, but they are much less manageable.

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redefining a type class for one instance?
by Edward Amsden 12 Apr '11

12 Apr '11

I am trying as an experiment to write something as follows: data Wrapper a b where Pure :: (a -> b) -> Wrapper a b Nullable :: (Maybe a -> b) -> Wrapper a b class Liftable a b where liftTo :: (b -> c) -> a b c instance Liftable Wrapper a where liftTo = Pure instance Liftable Wrapper (Maybe a) where liftTo = Nullable Obviously this code (with suitable extensions enabled) complains of overlapping type instances, but is there a way to accomplish redefining a class instance for one particular group of types while having it defined for all others? (Maybe a vs. a in this instance) -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Current heap size and other runtime statistics -- API for accessing in GHC?
by Ryan Newton 12 Apr '11

12 Apr '11

Hi cafe, The rtsopts (-s etc) can provide some nice debugging information regarding memory management. And System.Mem.performGC can initiate garbage collection. But are there APIs for querying the current state of the heap? I've googled and come up dry. In this case I'm running benchmarks and for the sake of fair comparison I want to make sure that everything from a previous run is cleaned up before the next run. Thanks, -Ryan

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ghc6: Re: Update: ... compiling ghc6
by Svante Signell 12 Apr '11

12 Apr '11

Addressing both Haskell and Hurd people here. Any hints by anyone? On Wed, 2011-04-06 at 09:30 +0200, Svante Signell wrote: ... > #5 0x011d3ce0 in __libc_read (fd=DWARF-2 expression error: DW_OP_reg > operations must be used either alone or in conjuction with DW_OP_piece > or DW_OP_bit_piece. > ) at ../sysdeps/mach/hurd/read.c:27 > #6 0x084919c8 in s9qJ_ret () > #7 0x0861f842 in StgRun () > #8 0x087c44e0 in ?? () Looking into this further, this looks like an error. What does it mean? from eglibc-2.11.2/sysdeps/mach/hurd/read.c /* Read NBYTES into BUF from FD. Return the number read or -1. */ ssize_t __libc_read (int fd, void *buf, size_t nbytes) { error_t err = HURD_FD_USE (fd, _hurd_fd_read (descriptor, buf, &nbytes, -1)); return err ? __hurd_dfail (fd, err) : nbytes; } The call is from the haskell binary cabal-bin but I don't have any debug info from that executable. Anybody fluent in Haskell? (How to get debug info in Haskell with gdb?) Below is the (stripped-down) call and the source code cabal-bin.hs: cd libraries/random: ../cabal-bin /usr/bin/ghc6 ../bootstrapping.conf configure --verbose=3 --with-compiler=../../ghc/stage1-inplace/ghc --with-hc-pkg=../../utils/ghc-pkg/install-inplace/bin/ghc-pkg module Main (main) where import Control.Monad import Data.Maybe import Distribution.PackageDescription import Distribution.PackageDescription.Parse import Distribution.Simple import Distribution.Simple.Utils import Distribution.Verbosity import System.Directory import System.Environment import System.FilePath import qualified Distribution.Make as Make import qualified Distribution.Simple as Simple setupProg :: FilePath setupProg = "./Setup" main :: IO () main = do unprocessedArgs <- getArgs let verbosity = verbose case unprocessedArgs of ghc : packageConf : args -> doit verbosity ghc packageConf args _ -> die "Bad args" doit :: Verbosity -> FilePath -> FilePath -> [String] -> IO () doit verbosity ghc packageConf args = do exists <- doesFileExist setupProg if exists then rawSystemExit verbosity setupProg args else do gpdFile <- defaultPackageDesc verbosity gpd <- readPackageDescription verbosity gpdFile let pd = packageDescription gpd case buildType pd of Just Simple -> Simple.defaultMainArgs args Just Make -> Make.defaultMainArgs args Just Configure -> defaultMainWithHooksArgs autoconfUserHooks args _ | packageName pd == PackageName "Cabal" -> -- Cabal is special...*sigh* Simple.defaultMainArgs args | otherwise -> runSetup verbosity ghc packageConf args runSetup :: Verbosity -> FilePath -> FilePath -> [String] -> IO () runSetup verbosity ghc packageConf args = do -- Don't bother building Setup if we are cleaning. If we need to -- build Setup in order to build, and Setup isn't built already, -- then there shouldn't be anything to clean anyway. unless cleaning $ rawSystemExit verbosity ghc ["-package-conf", packageConf, "--make", "Setup", "-o", "Setup"] rawSystemExit verbosity "./Setup" args where cleaning = case args of "clean" : _ -> True _ -> False

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Re: [Haskell-cafe] Assimp FFI Library
by John Lato 12 Apr '11

12 Apr '11

> > From: Joel Burget <joelburget(a)gmail.com> > Subject: [Haskell-cafe] Assimp FFI Library > To: haskell-cafe(a)haskell.org > Message-ID: <BANLkTimW-71NcBRqnr68kRAvKZytE1uVwA(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP > preprocessor > extension. I could reduce the boilerplate by another factor of 3 if I could > recursively call templates but that's not allowed. I would like to have one > template to generate both of these lines: > > > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > > Notice there is an extra !Double in the second. Is there an easy way to do > this? I don't know much about Template Haskell, would that work? Would it > be > easy? > Yes, it works, and it's pretty easy. My adaptive-tuple package, http://hackage.haskell.org/package/adaptive-tuple, provides this (as well as strict vectors up to 20 elements). Tuple is a bit of a misnomer. You'll need to download the source to get the Template Haskell splices because they aren't exported (tarball from hackage, or via http://www.tiresiaspress.us/haskell/adaptive-tuple/) John L.

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Call for Contributions - Haskell Communities and Activities Report, May 2011 edition
by Janis Voigtländer 12 Apr '11

12 Apr '11

Dear all, I would like to collect contributions for the 20th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 May 2011 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2010 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2011/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2011/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please consider using lhs2tex syntax (http://people.cs.uu.nl/andres/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. If you have several entries to submit, it helps if you send them in separate emails. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write "wishlist" items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them.

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Assimp FFI Library
by Joel Burget 12 Apr '11

12 Apr '11

Hello, I've been working on an ffi library for the Assimp asset import library( http://assimp.sourceforge.net) It should be useful for people doing graphics in Haskell. I've been working on it so I can import models into a ray-tracer I've been working on. My current progress is here: https://github.com/joelburget/assimp. A couple weeks ago I managed to import models and do some simple (and slightly buggy) renders, but since then I've been redoing a lot of the guts of the library. At this moment it doesn't build because of the changes I'm in the middle of. Hopefully I'll be able to release the first version in the next week or so. I've run into a few roadblocks that I would be thankful for help with. 1. In Graphics.Formats.Assimp.Vec I'm trying to get the fastest and smallest possible vector types since a program could easily have millions of vectors in memory and operations on them take a huge fraction of the time in an average program. One problem I ran into is how to make the vectors type-safe. The assimp library has separate data types for colors and vectors, but represented the same in memory. I have a few ideas about how to represent these: a. The way I've been doing it: >data N2 >data N3 >data N4 >data Color >class Num a => Vector n a where > data Vec n a :: * -> * > ... rest of class >-- A 3-dimensional vector of doubles. t is the (phantom) tag type, like Color, Direction, etc >-- This is meant to improve type safety >instance Vector N3 Double where > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > ... rest of instance This seems very close to what I want, but it presents a few problems. First, I create matrices from vectors, so the matrix must have an implicit type, the type of the tags from the vectors it is made of. I made this () by convention but then to multiply a vector by this matrix I have to use the dot product and other vector operations, but then the tag type from the matrix is different from that of the matrix I am multiplying. This leads to the type `dot :: Vec n a t1 -> Vec n a t2 -> a`. Notice how there are two different tag types, reducing type safety. Another problem is importing vectors from the library. I would like to have different tags for distance and direction, again to increase type safety, but they are both represented the same way in the library, so the Storable instance can't always know what type to create. I would probably always have to return a vector with tag type (). b. Just create separate Vec and Color datatypes. I would be mirroring the library directly. Both would be instances of Vector. c. Use newtype wrappers around Vec to represent the different types. I don't really like this solution because the newtypes are too painful to use. d. I've thought about some other solutions that I've forgotten now. Maybe you have something better! Remember, I would like to have a high degree of type safety if possible, but the top consideration is performance. Another thing I've been wondering about: could I use overlapping instances to allow the vectors to be of any numeric type, but specialize and unbox them for the common ones? I'm assuming unboxing can't be done unless we have a specific type because the compiler won't know how large the type is. 2. How do people feel about the vector and matrix operators (eg |*|, |*||, ||*)? I like them a lot but I would like to see how other people feel about them. 3. Do I have to worry about marshalling float, double, and int straight to Haskell? The report says Float and Double should conform to the IEEE standard. I'm more interested in Int since the report only requires a 30 bit Int. On my computer they appear to be 64 bits. Does this vary on 32 bit computers? 4. There are several places the original library uses unsigned ints. Obviously they can't be negative so I used CUInt in the Haskell code. I never see CUInt instead of Int in real code. Should I change these? 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP preprocessor extension. I could reduce the boilerplate by another factor of 3 if I could recursively call templates but that's not allowed. I would like to have one template to generate both of these lines: > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) Notice there is an extra !Double in the second. Is there an easy way to do this? I don't know much about Template Haskell, would that work? Would it be easy? I should mention that I'm going to convert all the Storable instances from something like this: > peek p = do > w <- (#peek aiQuaternion, w) p > x <- (#peek aiQuaternion, x) p > y <- (#peek aiQuaternion, y) p > z <- (#peek aiQuaternion, z) p > return $ Quaternion w x y z to something like this: > peek p = Quaternion <$> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p So there's no need to tell me to change those. But please let me know of other places that are in poor style. Oh yes, patches and collaborators are welcome. Thanks! Joel

4 3

Stateful iteratees
by Ertugrul Soeylemez 12 Apr '11

12 Apr '11

Hello fellow Haskellers, I'm trying to solve a very practical problem: I need a stateful iteratee monad transformer. Explicit state passing is very inconvenient and would destroy the elegance of my library. There are two approaches to this: 1. type MyT a m = Iteratee a (StateT MyConfig m) 2. type MyT a m = StateT MyConfig (Iteratee a m) Both work well except in two very specific corner cases: - I need to convert the transformer to 'Iteratee a m', i.e. remove the state layer. This is obviously trivial with the second variant, but seems very difficult with the first one, if it's possible at all. - I need to use control structures of Iteratee like catchError. This is obviously trivial with the first variant, but very inconvenient with the second, because I would need to reinvent many wheels. Does someone know a cleaner, more elegant solution? Encapsulating the state in the iteratee's input type is not an option. Many thanks in advance. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

3 6

Re: [Haskell-cafe] Foreign function performance: monadic vs pure
by John Lato 12 Apr '11

12 Apr '11

> > From: Gregory Collins <greg(a)gregorycollins.net> > > On Mon, Apr 11, 2011 at 3:55 PM, Serguei Son <serguei.son(a)gmail.com> > wrote: > > So if I must use a safe function returning IO a, > > there is no way to improve its performance? To give you > > a benchmark, calling gsl_ran_ugaussian a million times > > in pure C takes only a second or two on my system. > > In the C version, are you also producing a linked list containing all > of the values? Because that's what mapM does. Your test is mostly > measuring the cost of allocating and filling ~3 million machine words > on the heap. Try mapM_ instead. > I've done some rough tests of a few versions, and I believe Gregory's analysis is correct. These are for calculating a sum of sines of 1..10^7 with ghc-7.0.3. version execution time IO, safe - 4.08s Pure, safe - 3.07s IO, unsafe - 2.77s Pure, unsafe - 1.95s For the IO versions, I used the following: main = foldM (\ !acc n -> fmap (acc +) $ c_sin_io n) [1..10^7] >>= print (this can possibly be improved; it was the first thing I thought of) The most important factor (as often the case with Haskell) is to pay attention to the space properties of your algorithm. Instead of using mapM, which forces the spine of the list, it's much better to interleave the fold step with the other IO. Note that the "acc" parameter is strict, this is necessary to avoid building a thunk. John L.

1 0

Tuple
by Anwar Bari 12 Apr '11

12 Apr '11

HI Cafe I have to make a function to check that I have one occurrence of the last element (z) of the same list [a,b] in the tuple [([a,b],z)] For example [([1,2],3),([1,1],5),([1,3],6).......] this is true because there is one single z for each single list. while this one is false [([1,2],3),([1,2],5),([1,3],6).......] because 3&5 were found for the same list [1,2] any Idea how to code this Fn. Thanks

5 4

Stacking data types
by Yves Parès 12 Apr '11

12 Apr '11

Hello Café, I'm trying to get some modular data types. The idea that came to me is that I could stack them, for instance : data Character a = Character { life :: Int, charaInner :: a } data Gun a = Gun { firepower :: Int, gunInner :: a } data Armor a = Armor { resistance :: Int, armorInner :: a } Then a character with a gun and an armor can be build this way: chara = Character 100 $ Armor 40 $ Gun 12 The idea now is to be able to get some part of the character: itsGun :: Character ?? -> Gun ?? itsGun = content Then content would be a class method: class Has b a where content :: a -> b And it would be recursively defined so that: instance (Has c b, Has b a) => Has c a where content = (content :: b -> c) . (content :: a -> b) Then itsGun would be more like: itsGun :: (Has Gun a) => a -> Gun ?? itsGun = content But after some juggling with extensions (ScopedTypeVariables, UndecidableInstances, IncoherentInstances...) I can't get it working. Has someone a simpler way to achieve modular types?

6 8

Foreign function performance: monadic vs pure
by Serguei Son 11 Apr '11

11 Apr '11

Consider two versions of sin wrapped: foreign import ccall "math.h sin" c_sin_m :: CDouble -> IO CDouble and foreign import ccall "math.h sin" c_sin :: CDouble -> CDouble One can invoke them so: mapM c_sin_m [1..n] mapM (return . c_sin) [1..n] On my computer with n = 10^7 the first version never finishes, whereas the second one calculates the result within seconds. To give you my context, I need to call a random variable generator multiple times, so that it must return IO a. Any explanation for this behavior?

5 7

Emscripten: compiling LLVM to JavaScript
by Sönke Hahn 11 Apr '11

11 Apr '11

Hi all! I haven't tried the tool myself, but it seems interesting to the Haskell efforts to compile to JavaScript: http://syntensity.blogspot.com/2011/04/emscripten-10.html Cheers, Sönke

6 5

Re: [Haskell-cafe] already installed packages alerted as not being installed
by Kazu Yamamoto 11 Apr '11

11 Apr '11

Hello, > thanks I wanted to mention that the "unknown symbol" error is very > likely not related to the cab tool as the same error appears, when > using the cabal - tool. I guess we can ignore it even in the context > of my main question, sorry for being to verbose. What I found more > interesting is, that cab doesn't try to re-install an installed > package - already in the database too, as ghc-pkg output pretends - > which is expected behaviour. The cabal system frontend obviously does > trying to reinstall the package though. This seems like different > model of reality... This is not true. cab install re-installs some packages if necessary, unfortunately. That's why I recommend to use "cab install -n". --Kazu

2 1

how to optmize this code?
by Gilberto Garcia 11 Apr '11

11 Apr '11

Hi Haskellers, I was solving this problem from project euler to study haskell. I came up whit the following solution and I was wondering if there is a more optimized and concise solution. fkSum :: Int -> [Int] -> Int fkSum a [] = 0 fkSum a (b) = foldl (+) 0 (filter (\x -> isMultiple x b) [1..a]) isMultiple :: Int -> [Int] -> Bool isMultiple a [] = False isMultiple a (x:xs) = if (mod a x == 0) then True else isMultiple a xs Thanks in advance ggarcia

8 12

Re: [Haskell-cafe] Fucntion composing
by Adam Krauze 11 Apr '11

11 Apr '11

Thanks all! You helped me a lot. Adam.

1 0

Parsing HTML tables with HXT
by Dmitry Simonchik 11 Apr '11

11 Apr '11

Dear Cafe, Can someone please help me with getting the value of the table cell with HXT in the following html: <table class="tblc"> <tr> <td class="tdc">x</td> <td>y</td> </tr> <tr> <td class="tdc">a</td> <td>b</td> </tr> </table> I need the value of the second cell in a row that has first cell with some predefined value (in the example above it can be x or a) I need the arrow of the type (IOSArrow XmlTree String) How to write it? Thanks

2 2

already installed packages alerted as not being installed
by Daniel Kahlenberg 11 Apr '11

11 Apr '11

Hello cafe-readers, does anyone of you observe similar problems e. g. on a Windows with ghc-7.0.2 setup: When I'm trying cabal install threadscope (as an example package depending on gtk2hs, latter which I've installed using the stepwise approach of cabal unpack first, then cabal configure --user etc.) it complains about cairo etc. not being installed and tries to re-install it on-the-fly, failing due to whats described her: http://hackage.haskell.org/trac/gtk2hs/ticket/1203! When I install cabal-dev and cab first and then re-install everything with cab instead of cabal the issue with re-installing already installed packages described above disappears and only an "unknown symbol" message related to the correctly found installed cairo package remains. So is there an error in package database handling somewhere or changed semantics in cabal | ghc-pkg | (even) pkg-config flags I missed? Cheers Daniel

2 2

CFP: APLAS 2011, Kenting, Taiwan
by Shin-Cheng Mu 11 Apr '11

11 Apr '11

========================================================== APLAS 2011 Call For Papers Ninth Asian Symposium on Programming Languages and Systems Kenting, Taiwan, December 5--7, 2011 (co-located with CPP 2011) http://flolac.iis.sinica.edu.tw/aplas11/ ========================================================== BACKGROUND: APLAS aims at stimulating programming language research by providing a forum for the presentation of latest results and the exchange of ideas in topics concerned with programming languages and systems. APLAS is based in Asia, but is an international forum that serves the worldwide programming language community. APLAS is sponsored by the Asian Association for Foundation of Software (AAFS) founded by Asian researchers in cooperation with many researchers from Europe and the USA. The past APLAS symposiums were successfully held in Shanghai ('10), Seoul ('09), Bangalore ('08), Singapore ('07), Sydney ('06), Tsukuba ('05), Taipei ('04) and Beijing ('03) after three informal workshops. Proceedings of the past symposiums were published in Springer-Verlag's LNCS 6461, 5904, 5356, 4807, 4279, 3780, 3302, and 2895. The 2011 edition will be held at Kenting, a seaside resort in Southern Taiwan. TOPICS: The symposium is devoted to both foundational and practical issues in programming languages and systems. Papers are solicited on, but not limited to, the following topics: + semantics, logics, foundational theory; + design of languages and foundational calculi; + type systems; + compilers, interpreters, abstract machines; + program derivation and transformation; + program analysis, verification, model-checking, software security; + concurrency, constraints, domain-specific languages; + tools for programming, verification, implementation. APLAS 2011 is not limited to topics discussed in previous symposiums. Papers identifying future directions of programming and those addressing the rapid changes of the underlying computing platforms are especially welcome. Demonstration of systems and tools in the scope of APLAS are welcome to the System and Tool presentations category. Authors concerned about the appropriateness of a topic are welcome to consult with Program Chair prior to submission. IMPORTANT DATES: Abstract Deadline: June 13, 2011 Submission Deadline: June 17, 2011 (GMT) Notification: August 29, 2011 Camera-Ready: September 19, 2011 INVITED SPEAKERS: Nikolaj Bjorner, Microsoft Research Redmond Ranjit Jhala, UC San Diego Peter O'Hearn, Queen Mary U. London Sriram Rajamani, Microsoft Research India SUBMISSION INFORMATION: We solicit submissions in two categories: Regular research papers, describing original research results, including tool development and case studies, from a perspective of scientific research. Regular research papers should not exceed 16 pages in the Springer LNCS format, including bibliography and figures. They should clearly identify what has been accomplished and why it is significant. Submissions will be judged on the basis of significance, relevance, correctness, originality, and clarity. In case of lack of space, proofs, experimental results, or any information supporting the technical results of the paper could be provided as Appendix or a link to a web page. System and Tool presentations, describing systems or tools that support theory, program construction, reasoning, and/or program execution in the scope of APLAS. Unlike presentations of regular research papers, presentation of accepted papers in this category is expected to be centered around a demonstration. The paper and the demonstration should identify the novelties of the tools and use motivating examples. System and Tool papers should not exceed 8 pages in the Springer LNCS format, including bibliography and figures. Submissions will be judged based on both the papers and the systems or tools as described in the papers. It is highly desirable that the tools are available on the web. Papers should be submitted electronically via the submission web page at http://www.easychair.org/conferences/?conf=aplas2011. Acceptable formats are PostScript or PDF, viewable by Ghostview or Acrobat Reader. Submitted papers must be unpublished and not submitted for publication elsewhere. Papers must be written in English. The proceedings are planned to be published as a volume in Springer-Verlag's Lecture Notes in Computer Science series. Accepted papers must be presented at the conference. GENERAL CHAIR: Tyng-Ruey Chuang, Academia Sinica, Taiwan PROGRAM CHAIR: Hongseok Yang, Queen Mary U. London aplas2011(a)easychair.org PROGRAM COMMITTEE: Lars Birkedal (ITU, Denmark) James Brotherston (Imperial College, UK) Kung Chen (National Chengchi U., Taiwan) Wenguang Chen (Tsinghua U., China) Wei-Ngan Chin (NUS, Singapore) Javier Esparza (TUM, Germany) Xinyu Feng (USTC, China) Jerome Feret (INRIA, France) Matthew Fluet (RIT, USA) Rajiv Gupta (UC Riverside, USA) Masahito Hasegawa (Kyoto U., Japan) Radha Jagadeesan (Depaul U., USA) Naoki Kobayashi (Tohoku U., Japan) Daniel Kroning (U. Oxford, UK) Rupak Majumdar (MPI-SWS, Germany) Andrzej Murawski (U. Leicester, UK) Paulo Oliva (Queen Mary U. London, UK) Doron Peled (Bar Ilan U., Israel) Sukyoung Ryu (KAIST, Korea) Sriram Sankaranarayanan (U. Colorado, USA) Armando Solar-Lezama (MIT, USA) Sam Staton (U. Cambridge, UK) Viktor Vafeiadis (MPI-SWS, Germany) Kapil Vaswani (MSR, India) Martin Vechev (IBM, USA) Peng Wu (IBM, USA) Hongseok Yang (Queen Mary U. London, UK) Pen-Chung Yew (Academia Sinica, Taiwan) ORGANIZERS: Mike Dodds, U. Cambridge (Poster chair) Shin-Cheng Mu, Academia Sinica, Taiwan (Local arrangement chair) Noam Rinetzky, Queen Mary U. London (Publicity chair) Yih-Kuen Tsay, National Taiwan University (Finance chair)

1 0

Re: [Haskell-cafe] [Haskell-beginners] Calling a foreign function: superlinear comlexity
by Antoine Latter 11 Apr '11

11 Apr '11

On Sun, Apr 10, 2011 at 6:26 PM, Serguei Son <serguei.son(a)gmail.com> wrote: > I call GSL's gsl_ran_ugaussian function in the following way (using > bindings-gsl): > > module Main where > > import Bindings.Gsl.RandomNumberGeneration > import Bindings.Gsl.RandomNumberDistributions > import Foreign > import Control.Monad > import Data.List > > main = do > let n = 100000 > p <- peek p'gsl_rng_mt19937 > rng <- c'gsl_rng_alloc p > lst <- replicateM n $ c'gsl_rng_uniform rng > print $ sum lst > > As I increase n from 10^4 to 10^5 to 10^6 execution time grows superlinearly. > Not sure if it is related, but this thread also documents issues folks had with replicateM and performance: http://www.haskell.org/pipermail/haskell-cafe/2011-March/090419.html Antoine > To forestall the answer that the reason is the overhead of List, > this code scales approximately linearly: > > module Main where > > > import Foreign > import Control.Monad > import Data.List > > main = do > let n = 100000 > let lst = map sin [1..n] > print $ sum lst > > Another interesting observation: when I wrap the sin function > of math.h with signature CDouble -> IO CDouble calling it > repeatedly scales superlinearly, whereas when I wrap it as a pure > function calling it repeatedly scales linearly. > > What is the reason for this performance and how can > I make the first code scale linearly in execution time? > > > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

1 0

haskell-src-meta installation failed
by Qiuchi Jian 10 Apr '11

10 Apr '11

Hi guys, I tried to install haskell-src-meta and got the error below. I checked all dependencies which are listed on the hackage page, they are all installed with the right version. Can anyone tell me what is the problem? Thanks a lot. jqc@ubuntu:~$ cabal install haskell-src-meta Resolving dependencies... Configuring haskell-src-meta-0.3... Preprocessing library haskell-src-meta-0.3... Building haskell-src-meta-0.3... [1 of 7] Compiling Language.Haskell.Meta.Syntax.Translate ( src/Language/Haskell/Meta/Syntax/Translate.hs, dist/build/Language/Haskell/Meta/Syntax/Translate.o ) src/Language/Haskell/Meta/Syntax/Translate.hs:19:0: Bad interface file: /home/jqc/.cabal/lib/haskell-src-exts-1.9.6/ghc-6.12.3/Language/Haskell/Exts/Syntax.hi Data.Binary.getWord8: end of file cabal: Error: some packages failed to install: haskell-src-meta-0.3 failed during the building phase. The exception was: ExitFailure 1 Regards, Jian

2 1

CFP -- Haskell Symposium 2011
by Haskell Symposium 10 Apr '11

10 Apr '11

"Haskell 2011" ACM SIGPLAN Haskell Symposium 2011 Tokyo, Japan 22nd September, 2011 CALL FOR PAPERS http://www.haskell.org/haskell-symposium/2011/ The ACM SIGPLAN Haskell Symposium 2011 will be co-located with the 2011 International Conference on Functional Programming (ICFP), in Tokyo, Japan. The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. The scope of the symposium includes all aspects of the design, semantics, theory, application, implementation, and teaching of Haskell. Topics of interest include, but are not limited to: * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal treatments of the semantics of the present language or future extensions, type systems, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management as well as foreign function and component interfaces; * Tools, in the form of profilers, tracers, debuggers, pre-processors, testing tools, and suchlike; * Functional Pearls, being elegant, instructive examples of using Haskell; * Applications, using Haskell for scientific and symbolic computing, database, multimedia, telecom and web applications, and so forth; * Practice and Experience, general experience with Haskell in education and industry. Papers in the latter three categories need not necessarily report original research results; they may instead, for example, report practical experience that will be useful to others, reusable programming idioms, or elegant new ways of approaching a problem. (More advice appears on the symposium webpage.) The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a program! Submission Details * Submission Deadline: Monday, 6th June 2011 * Author Notification: Friday, 1st July 2011 * Final Papers Due : Sunday, 10th July 2011 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (http://www.acm.org/sigs/sigplan/authorInformation.htm) The text should be in a 9pt font in two columns; the length is restricted to 12 pages, except for "Applications, Practice, and Experience" papers, which are restricted to 6 pages. Each submission must adhere to SIGPLAN's republication policy, as explained on the web. Violation risks summary rejection of the offending submission. Accepted papers will be published by the ACM and will appear in the ACM Digital Library. In addition, we solicit proposals for system demonstrations, based on running (perhaps prototype) software rather than necessarily on novel research results. Proposals are limited to 2-page abstracts, in the same ACM format as papers, and should explain why a demonstration would be of interest to the Haskell community. They will be assessed for relevance by the PC; accepted proposals will be published on the Symposium website, but not formally published in the proceedings. Links * http://www.haskell.org/haskell-symposium, the permanent homepage of the Haskell Symposium. * http://www.haskell.org/haskell-symposium/2011, the 2011 Haskell Symposium web page. * http://www.icfpconference.org/icfp2011, the ICFP 2011 web page. Programme Committee * Koen Claessen, Chalmers University of Technology (chair) * Conal Elliott, LambdaPix * Andy Gill, University of Kansas * Ralf Hinze, Oxford University * Graham Hutton, University of Nottingham * John Launchbury, Galois, Inc. * Sam Lindley, University of Edinburgh * Rita Loogen, Philipps-Universität Marburg * Neil Mitchell, Standard Chartered * Matthew Naylor, University of York * Bruno Oliveira, Seoul National University * Dimitrios Vytiniotis, Microsoft Research * Steve Zdancewic, University of Pennsylvania

1 0

ImProve Tutorial
by Tom Hawkins 10 Apr '11

10 Apr '11

ImProve is a Haskell eDSL for embedded control systems -- we use it for automotive and off-highway powertrain control. I've started writing a tutorial for ImProve. So far it has a basic tour of the language and a handful of examples. Comments and suggestions are welcome. In other recent news, ImProve can now generate models in Simulink and Modelica. -Tom [1] http://github.com/tomahawkins/improve/wiki/ImProve

1 0

Deciding equality of functions.
by Grigory Sarnitskiy 10 Apr '11

10 Apr '11

I guess that deciding whether two functions are equal in most cases is algorithmically impossible. However maybe there exists quite a large domain of decidable cases? If so, how can I employ that in Haskell? It is a common situation when one has two implementations of the same function, one being straightforward but slow, and the other being fast but complex. It would be nice to be able to check if these two versions are equal to catch bugs in the more complex implementation.

7 6

ANNOUNCE: hplaylist-0.1
by Tim Chevalier 10 Apr '11

10 Apr '11

Hello, hplaylist is a very simple command-line utility, written in Haskell, for automatically syncing an iTunes playlist to a non-Apple-manufactured MP3 player (anything that mounts as a USB drive should work in principle). For example, if I have an iTunes playlist called "Songs about cheese", hplaylist allows me to type: $ hplaylist "Songs about cheese" which automatically copies the named playlist, and all music files included in the playlist, to my device. Download at: http://hackage.haskell.org/package/hplaylist-0.1 or https://github.com/catamorphism/hplaylist Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc/ * Often in error, never in doubt "an intelligent person fights for lost causes,realizing that others are merely effects" -- E.E. Cummings

1 0

Testing Implementation vs Model - Records or Type Classes?
by Heinrich Apfelmus 09 Apr '11

09 Apr '11

Hello, I'm writing a small Haskell library for functional reactive programming. The core of the library consists of two data types and several primitives. However, I have programmed this core *twice*: once as a *model* that displays the intended semantics, and once as the actual *implementation* to be used in production code. Of course, I would like to use QuickCheck to test that the implementation gives the same results as the model. My problem is: how to organize this with the minimum amount of boilerplate? It appears that I can use *multiparameter type classes* to solve this, but I'm not sure I'm happy with this, in particular because it makes the generated Haddock less readable (see 3) below). On the other hand, I could also use *record wildcards* to conveniently reify a module into a record data structure. But that will give me problems with combinators that are derived from the core combinators (see 2) below). Haskell Café, what are your suggestions and ideas? In particular, I wish to: 1) Write examples to be QuickChecked only once. I imagine that my main function for testing looks like this test_equal example = forAll $ \input -> example model input == example implementation input where example an expression involving the combinators from my library. The point is that I don't want to write example twice, once for each version. 2) Use derived combinators. For reference, here the full signature of the core combinators: data Event a data Behavior a instance Functor Behavior instance Applicative Behavior instance Functor Event instance Monoid (Event a) filter :: (a -> Bool) -> Event a -> Event a apply :: Behavior (a -> b) -> Event a -> Event b accumB :: a -> Event (a -> a) -> Behavior a When writing tests, I also want to use common derived combinators, like, say filterJust :: Event (Maybe a) -> Event a filterJust = fmap fromJust . filter isJust without implementing them twice. 3) Obtain readable Haddock. I like the simplicity and readability of an ordinary module signature. In contrast, libraries with many type class scare me. Or is there a way to make this less scary? I'm not sure about the last one: 4) Make both model and implementation available to the user, so that he can QuickCheck his own programs as well. Since the implementation makes heavy use of IO, it is a bit harder to test automatically, so the model might be useful to have. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

3 3

ANNOUNCE: normaldistribution-1.1 – Minimum fuss normally distributed random values.
by Bjorn Buckwalter 09 Apr '11

09 Apr '11

Dear all, Thanks to enlightening advice from Tom Nielsen and Zoltan Novak normaldistribution now generates normally distributed values using the Box-Muller method instead of the Central Limit Theorem. This change provides a 5-10x speedup. The only API impact is that the random values must now be instances of Floating rather than just Fractional. (The pre-canned Random instances that satisfy Fractional, i.e. Double and Float, also satisfy Floating.) Cheers, Bjorn ---------- Forwarded message ---------- From: Bjorn Buckwalter <bjorn.buckwalter(a)gmail.com> Date: Sat, Apr 9, 2011 at 15:21 Subject: ANNOUNCE: normaldistribution-1.0 – Minimum fuss normally distributed random values. Dear all, I'm pleased to announce the immediate availability of the normaldistribution library on hackage: http://hackage.haskell.org/package/normaldistribution This purpose of this library is to have a simple API and no dependencies beyond Haskell 98 in order to let you produce normally distributed random values with a minimum of fuss. This library does not attempt to be blazingly fast nor to pass stringent tests of randomness. It attempts to be very easy to install and use while being "good enough" for many applications (simulations, games, etc.). The API builds upon and is largely analogous to that of the Haskell 98 Random module (more recently System.Random). Pure: > (sample,g) = normal myRandomGen -- using a Random.RandomGen > samples = normals myRandomGen -- infinite list > samples2 = mkNormals 10831452 -- infinite list using a seed In the IO monad: > sample <- normalIO > samples <- normalsIO -- infinite list With custom mean and standard deviation: > (sample,g) = normal' (mean,sigma) myRandomGen > samples = normals' (mean,sigma) myRandomGen > samples2 = mkNormals' (mean,sigma) 10831452 > sample <- normalIO' (mean,sigma) > samples <- normalsIO' (mean,sigma) Internally the library uses the Central Limit Theorem to approximate normally distributed values from multiple uniformly distributed random values. Enjoy, Bjorn

1 0

ANNOUNCE: normaldistribution-1.0 – Minimum fuss normally distributed random values.
by Bjorn Buckwalter 09 Apr '11

09 Apr '11

Dear all, I'm pleased to announce the immediate availability of the normaldistribution library on hackage: http://hackage.haskell.org/package/normaldistribution This purpose of this library is to have a simple API and no dependencies beyond Haskell 98 in order to let you produce normally distributed random values with a minimum of fuss. This library does not attempt to be blazingly fast nor to pass stringent tests of randomness. It attempts to be very easy to install and use while being "good enough" for many applications (simulations, games, etc.). The API builds upon and is largely analogous to that of the Haskell 98 Random module (more recently System.Random). Pure: > (sample,g) = normal myRandomGen -- using a Random.RandomGen > samples = normals myRandomGen -- infinite list > samples2 = mkNormals 10831452 -- infinite list using a seed In the IO monad: > sample <- normalIO > samples <- normalsIO -- infinite list With custom mean and standard deviation: > (sample,g) = normal' (mean,sigma) myRandomGen > samples = normals' (mean,sigma) myRandomGen > samples2 = mkNormals' (mean,sigma) 10831452 > sample <- normalIO' (mean,sigma) > samples <- normalsIO' (mean,sigma) Internally the library uses the Central Limit Theorem to approximate normally distributed values from multiple uniformly distributed random values. Enjoy, Bjorn

2 1

Is there a way to find out the type inferred for a local function inside another function? :)
by KC 09 Apr '11

09 Apr '11

-- -- Regards, KC

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Patch: adding CTR, CMAC and SIV modes to the Crypto API
by klondike 09 Apr '11

09 Apr '11

Hi, After a few weeks of hard work I have managed to add the CTR, CMAC and SIV modes of operation. The implemententation passes all the KATS I could find and is made as efficiently as possible without forgetting the risk of timing attacks over them. The patch is attached here and you can free it along with your library with a BSD-3 license as long as you recognize the authorship. Francisco Blas Izquierdo Riera (klondike) PS: Attached is also a set of commands I used with ghci to check quickly the implementation and which contain parts of the KATS I used, feel free to use them.

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erratic behavior for System.Time.diffClockTimes
by Warren Harris 09 Apr '11

09 Apr '11

I'm trying out GHC-7.0.3-x86_64 for Mac OS X and see what seems to be a bug in System.Time.diffClockTimes. The TimeDiff tdPicosec field returns values that seem to jump around erratically: test = do startTime <- System.Time.getClockTime endTime <- System.Time.getClockTime let dt = diffClockTimes endTime startTime putStrLn $ show dt Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 0} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 3000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = -3329012246138323150000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 3000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = -3329012246138322851000000} Prelude Test> I can file a bug, unless I'm overlooking something here. Warren

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fmap and LaTeX
by Mitar 08 Apr '11

08 Apr '11

Hi! Is there for <$> (fmap) operator some nice looking symbol in mathematics, LaTeX? I am looking here: http://www.soi.city.ac.uk/~ross/papers/Applicative.html http://www.haskell.org/ghc/docs/6.12.1/html/libraries/base/Control-Applicat… but only for other operators there are nice symbols, and <$> is left as it is. I have also tried \mathbin{<\$>} but it adds space around $ (at least in lhs2tex), so it looks ugly. Mitar

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FFI for a beginner
by Andrew Pennebaker 08 Apr '11

08 Apr '11

ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us

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cab 0.1.2
by Kazu Yamamoto 08 Apr '11

08 Apr '11

Hello cafe, Let me announce "cab version 0.1.2". This version integrates "cabal-dev" as well as "cabal/ghc-pkg". So, you can use a sandbox for your development. http://www.mew.org/~kazu/proj/cab/en/ Here is the short explanation from the page above: ---- "cab" is a MacPorts-like maintenance command of Haskell cabal packages. Some parts of this program are a wrapper to "ghc-pkg", "cabal", and "cabal-dev". If you are always confused due to inconsistency of "ghc-pkg" and "cabal", or if you want a way to check all outdated packages, or if you want a way to remove outdated packages recursively, this command helps you. "cab" is designed to maintain user packages only assuming that users use a Haskell Platform and don't want to touch the global packages. ---- --Kazu

1 0

DSL for task dependencies
by Serge Le Huitouze 07 Apr '11

07 Apr '11

Hi Haskellers! I think I remember reading a blog post or web page describing a EDSL to describe tasks and their dependencies "a la" make. Can anyone point me to such published material? Thanks in advance. --serge

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'Progress bar' enumeratee
by David Hotham 07 Apr '11

07 Apr '11

Hello, I've spent some time over the last couple of days trying to write an enumeratee that prints a "." every n bytes (with obvious intended use as a progress tracker). Seems like it oughtn't be hard, but it has been a steep learning curve... I have come up with something that seems to do the job but I don't know that I'm completely happy with it (or even that I completely understand it, to be honest). If anyone more expert would be kind enough either to reassure me that I'm doing it right or - more likely - to offer improvements / suggestions on what obvious simplifications I have overlooked, I'd be grateful. Thanks David import qualified Data.ByteString.Char8 as B import qualified Data.ByteString.Lazy.Char8 as BL import qualified Data.Enumerator as E import qualified Data.Enumerator.Binary as EB import Control.Monad.IO.Class (liftIO, MonadIO) import System.IO (hFlush, stdout) dotEvery :: MonadIO m => Integer -> E.Enumeratee B.ByteString B.ByteString m b dotEvery count = E.checkDone $ E.continue . dotIn count where dotIn need k E.EOF = E.yield (E.Continue k) E.EOF dotIn need k (E.Chunks []) = E.continue (dotIn need k) dotIn need k (E.Chunks xs) = iter where lazy = BL.fromChunks xs len = toInteger $ BL.length lazy iter = if len < need then k (E.Chunks xs) E.>>== E.checkDone (E.continue . dotIn (count - len)) else let (x1, x2) = BL.splitAt (fromInteger need) lazy s1 = E.Chunks $ BL.toChunks x1 s2 = E.Chunks $ BL.toChunks x2 enumee = E.checkDoneEx s2 (\k' -> dotIn count k' s2) in E.Iteratee $ do newStep <- E.runIteratee $ k s1 liftIO $ putStr "." >> hFlush stdout E.runIteratee $ enumee newStep PS Implementations which involve "EB.take count" seem to me unsatisfactory; one surely oughtn't need to have a large buffer to solve this problem PPS I did find an implementation via mapAccumM which I consider pleasing enough from an aesthetic point of view - but which runs 30x slower dotAt :: MonadIO m => Integer -> Integer -> Word8 -> m (Integer, Word8) dotAt n s w | s >= n = do liftIO $ putStr "." >> hFlush stdout return (1, w) | otherwise = return (s+1, w) dotEvery' :: MonadIO m => Integer -> E.Enumeratee B.ByteString B.ByteString m b dotEvery' n = EB.mapAccumM (dotAt n) 1

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parsec - source position information
by Ozgur Akgun 07 Apr '11

07 Apr '11

Hi, Is there a way to get source position[1] information from parsec while defining a parser? It surely knows about source positions, as they are used while reporting a parsing error. data Identifier = Identifier String SourcePos pIdentifier :: Parser Identifier pIdentifier = do pos <- ?? s <- pString return (Identifier s pos) [1] http://hackage.haskell.org/packages/archive/parsec/3.0.0/doc/html/Text-Pars… Thanks in advance, Ozgur

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Local copy of hackageDB
by José Pedro Magalhães 07 Apr '11

07 Apr '11

Hi all, I want to use cabal-install on a machine without internet access. I tried downloading http://hackage.haskell.org/cgi-bin/hackage-scripts/archive.tar, unpacking it and setting the local-repo field in the config file to this location but that doesn't work, as cabal-install says that it is missing the package list for the local repo. What should I do? Thanks, Pedro

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