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ANNOUNCE: version 0.7 of monad-parallel, monad-coroutine and SCC
by Mario Blažević 26 Mar '11

26 Mar '11

Packages monad-parallel [1], monad-coroutine [2] and SCC [3] have been upgraded on Hackage to version 0.7. The monad-parallel library defines two Monad subclasses, MonadParallel and MonadFork, that enable some monadic computations to be executed in parallel and their results combined. The library also exports a subset of the Control.Monad interface (ap, sequence, and related functions), adjusted to exploit the parallelism. The new version of the library adds several more instances of these classes. The monad-coroutine library exports a generic monad transformer Coroutine: Functor s => MonadTrans (Coroutine s). A Coroutine-transformed monad can suspend at any point, returning its resumption wrapped in the functor s. The new release of monad-coroutine makes Coroutine an instance of Functor and Applicative classes. It also adds support for inter-coroutine communication through chunks parsed by the incremental-parser library [4]. The new version of Streaming Component Combinators (a.k.a. SCC) relies the aforementioned incremental-parser support to some good effect. Many coroutine components have been simplified and sped up by ~30% on average. [1] http://hackage.haskell.org/package/monad-parallel-0.7 [2] http://hackage.haskell.org/package/monad-coroutine-0.7 [3] http://hackage.haskell.org/package/scc-0.7 [4] http://hackage.haskell.org/package/incremental-parser-0.1

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ANN: clash-0.1.3.0 (Functional Hardware Descriptions)
by Christiaan Baaij 25 Mar '11

25 Mar '11

Hello, I am pleased to announce an incremental update to CLaSH, version 0.1.3.0. CLaSH can translate a (semantic) subset of Haskell to RTL-style VHDL. Instead of being an embedded DSL like Lava or ForSyDe, CLaSH takes are more 'traditional' approach to synthesis/compilation. It uses the GHC API as a front-end for parsing, type-checking and desugaring the Haskell source. It then exhaustively applies a list of meaning-preserving transformations to the intermediate GHC Core representation until it is in the desired normal form. The normalized Core/System Fc is then 'trivially' translated to RTL-style VHDL. The new version of CLaSH has the following updates: - Support for simulation and synthesis of multi-clock hardware [6] - Significant synthesis speed-up (4x to 10x) CLaSH already supported synthesis of: - User-defined Higher-Order functions [1] - User-defined ADTs (GADTs are not tested, but *might* work) - All of Haskell's choice constructs (Guards, Pattern Matching, etc.) - Lambda-abstraction / Anonymous Functions You can use CLaSH as a library, but the use of the adapted GHC interpreter (added the :vhdl command) is recommended. The interpreter can be found on the CLaSH website [2], where you will also find examples, papers, tutorials, etc. The library can be downloaded from Hackage [3]. I recently gave a demo of CLaSH at the DATE'11 conference in grenoble, this demo already made use of the multi-clock feature (and is actually the only reference if you want to experiment with multi-clock hardware yourself). The source for the demo (audio spectrum analyzer programmed on an Altera Cyclone II FPGA board) can be downloaded from my github page [4]. Do note that CLaSH only works when you have the 6.12.* branch of GHC installed! I am currently analyzing the impact of the GHC API changes made in the 7.0.* brach, and hope to make the transition to the new API within the next month. Although the compiler is already used by 2 other phd's in our group [5], it is not a thoroughly tested product, so your coding style might not be anticipated by the current version of CLaSH ;-) -- Christiaan Baaij [1] There is hard-coded support for a set of recursively defined higher-order functions such as map, fold, zipWith, etc. [2] http://clash.ewi.utwente.nl [3] http://hackage.haskell.org/package/clash-0.1.3.0 [4] http://github.com/christiaanb/DE1-Cyclone-II-FPGA-Board-Support-Package [5] http://caes.ewi.utwente.nl [6] Simulation does not show meta-stability, you will have to take care of synchronization (dual flipflop) yourself

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package cache being out of date
by Wolfgang Jeltsch 25 Mar '11

25 Mar '11

Hi, I have a freshly installed Haskell Platform 2010.2.0.0 on Windows 7. When I run ghc-pkg list, I get the following message (besides the expected package info): WARNING: cache is out of date: C:/Program Files (x86)/Haskell Platform/2010.2.0.0\lib\package.conf.d\package.cache use 'ghc-pkg recache' to fix. What does this mean? Is this harmful? (In principal, I could just run ghc-pkg recache, but the installation is replicated on about 20 machines, and I don’t have admin rights on these.) Best wishes, Wolfgang

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ANNOUNCE: Multiplate 0.0.1
by roconnor＠theorem.ca 25 Mar '11

25 Mar '11

Multiplate is a lightweight generic library for mutually recursive data types that won't make Conor lose his lunch. Multiplate is an alternative extension of the Uniplate/Compos core design to support mutually recursive datatypes in a way that is as powerful as Compos, almost as easy to use as Uniplate, and more portable than both of them. Multiplate does not require you to rewrite your data type, does not require run-time reflection, does not require GADTs, and does not even require multi-parameter type classes. It only requires rank 3 polymorphism. <http://hackage.haskell.org/package/multiplate-0.0.1> A more detailed paper is forthcoming, but the library is available to try right now. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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threading question for ghc 7.0.2
by Oliver Mueller 25 Mar '11

25 Mar '11

hello, i'm a bit lost here so i hope s.o. can help me here: seems i have a problem in my code when compiled with ghc-7.0.2: what i try to do: serverside: -listen on a socket and reply to requests client-side: -send a request to that server -wait for a response (with a timeout) any ideas on how i can make this work again without using the -threaded flag? is there is a better way for what i'm trying to achieve? many thanks, oliver the simplified code is available at https://gist.github.com/882770 explanation: the code forks of a lightweight thread (not an OS thread) that will listen for incoming responses. the main thread will go on to push out a message and then wait for the receiver thread to finish. i used to compile my application with the -threaded option but ran into some obscure FFI problems after upgrading from ghc-6.12.3 to ghc-7.0.2. so i tried to remove the -threaded option. but now the message does not get pushed out until the receiver-thread has finished. what is strange to me is that this worked without problems on 6.12.3 but not on 7.0.2. also the behavior is a little different on mac os X with 7.0.2: here i get a "resource vanished (Connection reset by peer)" see the output of the program below ([S] marks output from the main thread, [R] from the receiver thread) compiled on ghc-7.0.2/ubuntu 64bit, without -threaded "running all things..." [R]listening for response... [R]wait for data with timeout:2000 ms ......................................................................................................................................................................................................... [R]no message available [S]pushing out the message [S]sending --> "\EOT\ETX" "[S]going on..." Nothing compiled on ghc-7.0.2/ubuntu 64bit, using -threaded "running all things..." [S]pushing out the message [R]listening for response... [S]sending --> "\EOT\ETX" [R]wait for data with timeout:2000 ms "[S]going on..." ..................................................Just "\ETX\EOT" and...also very strange: when i introduce a putStrLn in the main thread (not a print !) it will work again...only a lot (see code snipped line38: putStrLn "[S]hickup" "running all things..." [S]hickup [R]listening for response... [S]pushing out the message [S]sending --> "\EOT\ETX" [R]wait for data with timeout:2000 ms "[S]going on..." ..................................................Just "\ETX\EOT"

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STM, newArray, and a stack overflow?
by Ketil Malde 25 Mar '11

25 Mar '11

Hi, I'm seeing some weirdness here. My code does this: omap <- atomically $ (newArray_ (0,n-1) :: STM (TArray Int Int)) This gives a stack overflow when n is one million. The equivalent code in the ST monad doesn't show this behavior, and works as expected. I'm not sure what is going on here, so I thought I'd ask in case anybody can put me on the right track. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Haskell programming 3 month internship in Tokyo
by employment 25 Mar '11

25 Mar '11

Tsuru Capital has an opening for an intern for three months starting next month in Tokyo. We are looking for people with moderate experience with Haskell or another functional programming language. Evidence of work written in Haskell is a big plus. More details can be found here. There is a small coding task on the site. Please send this code with your application. http://www.tsurucapital.com/en/jobs.html Any who have applied before but would like to be considered again, please feel free to send a reminder of your availability along with the code sample. Interested candidates please send resumes with code to jobs(a)tsurucapital.com . The package includes flights, and accommodation plus some salary while here.

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Tying the recursive knot
by Joshua Ball 25 Mar '11

25 Mar '11

{- - Hi all, - - I'm having trouble tying the recursive knot in one of my programs. - - Suppose I have the following data structures and functions: -} module Recursion where import Control.Monad.Fix import Data.Map ((!)) import qualified Data.Map as M import Debug.Trace newtype Key = Key { unKey :: String } deriving (Eq, Ord, Show) data Chain = Link Int Chain | Trace String Chain | Ref Key deriving (Show) reduce :: M.Map Key Chain -> Key -> [Int] reduce env k = follow (env ! k) where follow (Link i c) = i : follow c follow (Ref k) = reduce env k follow (Trace message c) = trace message (follow c) -- Now I want a "force" function that expands all of the chains into int sequences. force1, force2 :: M.Map Key Chain -> M.Map Key [Int] -- This is pretty easy to do: force1 mp = M.fromList (map (\k -> (k, reduce mp k)) (M.keys mp)) -- But I want the int sequences to be lazy. The following example illustrates that they are not: example = M.fromList [(Key "ones", Link 1 . Trace "expensive computation here" . Ref . Key $ "ones")] -- Run "force1 example" in ghci, and you will see the "expensive computation here" messages interleaved with an infinite -- list of ones. I would prefer for the "expensive computation" to happen only once. -- Here was my first attempt at regaining laziness: fixpointee :: M.Map Key Chain -> M.Map Key [Int] -> M.Map Key [Int] fixpointee env mp = M.fromList (map (\k -> (k, reduce env k)) (M.keys mp)) force2 env = fix (fixpointee env) main = print $ force2 example {- - However, this gets stuck in an infinite loop and doesn't make it past printing "fromList ". - (It was not difficult for me to see why, once I thought about it.) - - How do I recover laziness? A pure solution would be nice, but in the actual program - I am working on, I am in the IO monad, so I am ok with an impure solution. - It's also perfectly ok for me to modify the reduce function. - - Thanks in advance for you help, - Josh "Ua" Ball -}

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GHC 7/Cabal request for help
by John Goerzen 24 Mar '11

24 Mar '11

Hi folks, I don't have a GHC 7 environment running yet (it's on my list...) but I received a bug report pointing me at this build failure: http://hackage.haskell.org/packages/archive/testpack/2.0.1/logs/failure/ghc… Among other things, this noted: Dependency QuickCheck >=2.1.0.3: using QuickCheck-2.4.0.1 and the errors were: [1 of 3] Compiling Test.QuickCheck.Instances ( src/Test/QuickCheck/Instances.hs, dist/build/Test/QuickCheck/Instances.o ) src/Test/QuickCheck/Instances.hs:39:10: Duplicate instance declarations: instance Arbitrary Word8 -- Defined at src/Test/QuickCheck/Instances.hs:39:10-24 instance Arbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary src/Test/QuickCheck/Instances.hs:42:10: Duplicate instance declarations: instance CoArbitrary Word8 -- Defined at src/Test/QuickCheck/Instances.hs:42:10-26 instance CoArbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary Now, that's fairly standard, and in fact, in my code, is wrapped with: #if MIN_VERSION_QuickCheck(2,3,0) -- we have Word8 instances here #else instance Arbitrary Word8 where arbitrary = sized $ \n -> choose (0, min (fromIntegral n) maxBound) instance CoArbitrary Word8 where coarbitrary n = variant (if n >= 0 then 2 * x else 2 * x + 1) where x = abs . fromIntegral $ n #endif And that code has been working to support modern QuickCheck versions for some time. It would appear that something in Cabal, GHC 7, or QuickCheck is breaking this check. Ideas? -- John

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Blocking problem reading a handle
by Daniel Díaz 24 Mar '11

24 Mar '11

Hi, cafe, all I want is to read the unread portion of a handle, without blocking waiting for an EOF or a newline. My first attempt was to try with hGetBufNonBlocking from System.IO, and other functions that use it in its definition (directly or indirectly). But, unfortunately: hGetBufNonBlocking doesn't work on Windows: http://hackage.haskell.org/trac/ghc/ticket/806 This was a bad surprise. How can achieve my purpose without hGetBufNonBlocking? Thanks in advance, Daniel Díaz

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Haskell Weekly News: Issue 174
by Daniel Santa Cruz 24 Mar '11

24 Mar '11

Welcome to issue 174 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 13-19. You can find the HTML version at: http://bit.ly/hLu36R Announcements JP Moresmau [2]announced a new release of EclipseFP with support for ghc 7! S. Doaitse Swierstra [3]announced the release of version 2.7.0 to the uu-parsinglib library to hackage. Vincent Gerard [4]informed us that he has taken over maintainership of hsmagick, which is "a FFI bindings library to ImageMagick." He is pleased to announce the 0.5 release of the library. Malte Harder [5]announce the first release of the Craftwerk graphics library for 2d vector graphics. Corey O'Connor [6]annouced a new release of his VTY library. Brent Yorgey [7]announced the release of the special Poetry and Fiction Edition of The Monad.Reader. Thanks for putting this together Brent! Gregory D. Weber [8]announced version 1.2.4 of his Sifflet and sifflet-lib. "This is a compatibility release for HXT >= 9.1." Edward Kmett [9]remminded us that the Google Summer of Code is upon us. Follow the link to see the sign up information. Quotes of the Week * ddarius: *ddarius is more of a glass SUM_n (-1)^n full person. * monochrom: some kind of lazy evaluation is already known to highschool kids. teachers tell you that in a*(b+c), "evaluate b+c first", right? well, I challenge you to take 0*(389238493+97283748) and find one single student who faithfully evaluate 389238493+97283748 first. * hpc: in soviet russia, yoda speaks flawless english * lambdabot: trololololo * hpc: jokes are subject to an incompleteness theorem; if you can prove they are funny, they were never good jokes to begin with * hpc: when all you have is Haskell, everything looks like a research paper * Edward Kmett:Clearly we need some sort of xbox live-like achievement system for these. Achievement Unlocked: Stumped Oleg! Top Reddit Stories * Haskell.org has been accepted for the 2011 Google Summer of Code From (twitter.com) scored 48 with 1 comments. Read on [10]reddit. Read the [11]original post. * Haskell's Niche: Hard Problems From (cdsmith.wordpress.com) scored 47 with 17 comments. Read on [12]reddit. Read the [13]original post. * A little care and feeding can go a long way From (serpentine.com) scored 39 with 6 comments. Read on [14]reddit. Read the [15]original post. * The reception of monads circa 1995... From (self.haskell), scored 35 with 1 comments. Read on [16]reddit. * BlazeHtml: a blazingly fast HTML combinator library From (skillsmatter.com) scored 29 with 18 comments. Read on [17]reddit. Read the [18]original post. * Simon Peyton Jones: Managing parallelism: embrace diversity, but control side effects From (skillsmatter.com) scored 28 with 12 comments. Read on [19]reddit. Read the [20]original post. * EZYang: Mutation is frequently not what people expect ; mutation is not modular, and mutation is complicated. From (blog.ezyang.com) scored 26 with 0 comments. Read on [21]reddit. Read the [22]original post. * Functor is to Lens as Applicative is to Biplate: Introducing Multiplate From (front.math.ucdavis.edu) scored 25 with 19 comments. Read on [23]reddit. Read the [24]original post. * Monad.Reader special poetry and fiction edition From (themonadreader.wordpress.com) scored 23 with 1 comments. Read on [25]reddit. Read the [26]original post. * Functional flocks : modelling and visualizing flocking behavior in Haskell From (syntacticsalt.com) scored 22 with 0 comments. Read on [27]reddit. Read the [28]original post. Top StackOverflow Answers * [29]Why does application of `sequence` on List of Lists lead to computation of its Cartesian Product? votes: 12 This works because using lists as monads in Haskell makes them model indeterminism. Consider: sequence [[1,2],[3,4]] By definition this is the same as: do x <- [1,2] y <- [3,4] return [x,y] Just read it as "First a choice between 1 and 2, then a choice between 3 and 4". The list monad will now accumulate all possible outcomes - hence the ... * [30]In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -&ht; ? -> *`? votes: 11 The ? and ?? kinds refer to GHC extensions, specifically unboxed types. http://hackage.haskell.org/trac/ghc/wiki/IntermediateTypes has a diagram showing relationships between the extended kinds ? (all possible types), # (unboxed types), ?? (boxed or normal unboxed types â "least upper bound of # and *"), (#) (unboxed tuples, which can only be used in a ... * [31]How to use (->) instances of Monad and confusion about (->) votes: 11 For a given type r, the function of type r -> a can be thought of as a computation delivering an a using an environment typed r. Given two functions r -> a and a -> (r -> b), it's easy to imagine that one can compose these when given an environment (again, of type r). But wait! That's exactly what monads are about! So we can create an instance ... * [32]How and why is ap defined as liftM2 id in Haskell votes: 10 The type variable a from id can be instantiated at any type, and in this case that type is a -> b. So we are instantiating id at (a -> b) -> (a -> b). Now the type variable a1 from liftM2 is being instantiated at (a -> b), a2 is being instantiated at a, and r is being instantiated at b. Putting it all together, liftM2 is instantiated at ... * [33]What "reify" and "reification" means in the context of (functional?) programming? votes: 8 So I read up on this, and it is pretty much what it means: taking an abstract concept and making it concrete. Or, there is a proxy that represents the abstract concept. For example, in Lisp, the concept of procedure abstraction and application is reified when you use lambdas. Reification by itself is a broad concept and not just applicable to functional ... Top StackOverflow Questions * [34]What "reify" and "reification" means in the context of (functional?) programming? votes: 17, answers: 6 * [35]How to use (->) instances of Monad and confusion about (->) votes: 12, answers: 2 * [36]In GHCi, why does the kind of the function arrow `:kind (->)` include question marks `(->) :: ?? -> ? -> *`? votes: 12, answers: 1 * [37]Haskell-like type system in C votes: 11, answers: 2 * [38]Write a Maximum Monoid using Maybe in Haskell votes: 9, 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://jpmoresmau.blogspot.com/2011/03/eclipsefp-204-released-supports-ghc-… 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87205 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18564 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18568 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18574 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18572 8. http://article.gmane.org/gmane.comp.lang.haskell.general/18584 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87399 10. http://www.reddit.com/r/haskell/comments/g6nwe/haskellorg_has_been_accepted… 11. http://twitter.com/haskellorg/status/48838580029755392 12. http://www.reddit.com/r/haskell/comments/g382d/haskells_niche_hard_problems/ 13. http://cdsmith.wordpress.com/2011/03/13/haskells-niche-hard-problems/ 14. http://www.reddit.com/r/haskell/comments/g6a01/a_little_care_and_feeding_ca… 15. http://www.serpentine.com/blog/2011/03/18/a-little-care-and-feeding-can-go-… 16. http://www.reddit.com/r/haskell/comments/g4f1m/the_reception_of_monads_circ… 17. http://www.reddit.com/r/haskell/comments/g7akc/blazehtml_a_blazingly_fast_h… 18. http://skillsmatter.com/podcast/scala/blazehtml-a-blazingly-fast-html-combi… 19. http://www.reddit.com/r/haskell/comments/g7ajj/simon_peyton_jones_managing_… 20. http://skillsmatter.com/podcast/scala/talk-by-haskell-expert-simon-peyton-j… 21. http://www.reddit.com/r/haskell/comments/g3dsw/ezyang_mutation_is_frequentl… 22. http://blog.ezyang.com/2011/03/killer-mutants-attack-mutation-gone-wrong/ 23. http://www.reddit.com/r/haskell/comments/g56xw/functor_is_to_lens_as_applic… 24. http://front.math.ucdavis.edu/1103.2841 25. http://www.reddit.com/r/haskell/comments/g4r6e/monadreader_special_poetry_a… 26. http://themonadreader.wordpress.com/2011/03/15/special-poetry-and-fiction-e… 27. http://www.reddit.com/r/haskell/comments/g2v75/functional_flocks_modelling_… 28. http://syntacticsalt.com/2011/03/10/functional-flocks/ 29. http://stackoverflow.com/questions/5299295/why-does-application-of-sequence… 30. http://stackoverflow.com/questions/5359707/in-ghci-why-does-the-kind-of-the… 31. http://stackoverflow.com/questions/5310203/how-to-use-instances-of-monad-an… 32. http://stackoverflow.com/questions/5358868/how-and-why-is-ap-defined-as-lif… 33. http://stackoverflow.com/questions/5314884/what-reify-and-reification-means… 34. http://stackoverflow.com/questions/5314884/what-reify-and-reification-means… 35. http://stackoverflow.com/questions/5310203/how-to-use-instances-of-monad-an… 36. http://stackoverflow.com/questions/5359707/in-ghci-why-does-the-kind-of-the… 37. http://stackoverflow.com/questions/5290408/haskell-like-type-system-in-c 38. http://stackoverflow.com/questions/5344164/write-a-maximum-monoid-using-may…

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How to use roots package?
by Artyom Kazak 24 Mar '11

24 Mar '11

Hi Café! roots (http://hackage.haskell.org/package/roots) is a package to solve equations like "f(x)==0". In RootFinder class there is an 'defaultNSteps' value, which is used as maximal count of iterations functions like findRoot and traceRoot can make. By default it is 250, but sometimes it's not enough. How can I use another value instead of 250? Should I write my own RootFinder instance, or findRoot function? Thanks in advance. — Artyom.

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a finite free category
by Vasili I. Galchin 24 Mar '11

24 Mar '11

Hello, Does there exist Haskell to generate a finite free category from a finite multipath graph with loops? Thank you, Vasilki

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Installation of Haskell Platform on CentOS 5.5
by frode k 23 Mar '11

23 Mar '11

Hi. I've spent some time trying to setup the Haskell Platform on CentOS 5.5: # cat /proc/version Linux version 2.6.18-028stab070.14 (root@rhel5-build-x64) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)) #1 SMP Thu Nov 18 16:04:02 MSK 2010 I've compiled from source, similar to what is done here: http://www.haskell.org/pipermail/haskell-cafe/2010-April/076489.html Installation of GHC 7.0.2 failed: # ./configure --prefix=/usr/haskell-platform-2011.2.0.0/ checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I had to use GHS 6.12.3 which compiled without errors. However when I tried installing the Haskell Platform I ran into problems. Briefly, this is what I did and what happened trying to "make" it: wget http://hackage.haskell.org/platform/2010.2.0.0/haskell-platform-2010.2.0.0.… unpack... ./configure --prefix=/usr/haskell-platform-2010.2.0.0/ make # make scripts/build.sh ************************************************** Scanning system for any installed Haskell Platform components... Found:None. New packages to install: None! All done. ************************************************** Building "/usr/haskell-platform-2010.2.0.0/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.8.0.6" Linking Setup ... collect2: ld terminated with signal 15 [Terminated] Error: Compiling the Setup script failed make: *** [build.stamp] Error 2 I first thought it was related to the fact that Cabal was using the /tmp directory for executables, but the alias trick (alias cabal="TMPDIR=/foo cabal") did not help. Anyone have got any ideas on what might be causing this? Can it me any missing dependencies? I've gone through the list described at http://www.vex.net/~trebla/haskell/haskell-platform.xhtml and used "yum install" to get the software mentioned. Regards, Frode

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Haskell internship in the south of France
by David Waern 23 Mar '11

23 Mar '11

To all undergrads on this list: Are you a student looking for an internship? Do you want to work with Haskell in the south of France? My company, Amadeus, is able to offer an internship on the subject of property-based testing of XML schema based web service APIs. The project goal is to write an XML test case generation tool in Haskell. The exact length of the intership can be discussed. It may be six months or a full year. XML based web services may not be the most fashionable technology, but this is a chance to apply Haskell to real world problems! Also, you get to work in Sophia-Antipolis, a beautiful technology park close to Nice on the French riviera. Contact me if you're interested! David

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Looking for feedback on my attempt at a tree construction edsl
by C K Kashyap 23 Mar '11

23 Mar '11

Hi, With my "edsl", one can describe a tree like this - import TreeEdsl import Data.Tree createTree :: TreeContext String () createTree = do insertSubTree "Fruits" $ do insertLeaf "Apple" insertLeaf "Mango" insertSubTree "Arbitrary" $ do insertSubTree "Numbers" $ do insertLeaf "1" insertLeaf "2" insertLeaf "3" insertSubTree "Letters" $ do insertLeaf "A" insertLeaf "B" insertLeaf "C" return () main = do tree <- process "root" createTree putStrLn (drawTree (fmap show tree)) return () and get a tree like this - "root" | +- "Arbitrary" | | | +- "Letters" | | | | | +- "C" | | | | | +- "B" | | | | | `- "A" | | | `- "Numbers" | | | +- "3" | | | +- "2" | | | `- "1" | `- "Fruits" | +- "Mango" | `- "Apple" My code is here https://github.com/ckkashyap/LearningPrograms/blob/master/Haskell/edsl/Tree… I'd appreciate your feedback on this. Does this qualify to be a edsl? Regards, Kashyap

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Help optimising a Haskell program
by David MacIver 23 Mar '11

23 Mar '11

Hi, I have a Haskell program I'm trying to optimise, and could use some assistance. It's for rank aggregation - taking a bunch of partial rankings of some items from users and turning them into an overall ranking (aka "That thing that Hammer Principle does"). The code is here: https://github.com/DRMacIver/hs-rank-aggregation. The interesting code lives in the Algorithms.RankAggregation module. The sample data I'm running it on is samples/electornot (voting data from http://electornot.org.uk/) Here's a profile: https://gist.github.com/1ee9356f45330e9f8caa Here's a recent profile: https://gist.github.com/1ee9356f45330e9f8caa It's doing pretty well on this data, in that it's taking 8 seconds for 180k records (down from about 80 seconds earlier today) on my computer, but I'd like it to be faster - ideally under a second. Admittedly this is an arbitrary figure and I will just move the goal posts if I achieve it, but it would still be nice. :-) I suspect that it may be hitting the limits of what can be improved without actually changing the algorithm (which I'm open to doing, but not if it makes the results significantly worse), but the last 3 or 4 times I've thought that it was shortly followed by an optimisation which halved the runtime or better. In particular the most likely culprit for improvement is to deal with the fact that it allocates nearly 5GB of data (that appears to be mostly transient - watching the process as it runs the memory usage never seems to raise above 150MB, which is large but unproblematic). The biggest culprit for this is divideAndConquer which is probably generating a silly number of intermediate lists, but it's not obvious how to fix that - maybe by making the tree structure in the recursion more explicit? Don't know. Suggestions very welcome. Regards, David

4 6

Computing the multiplication table of a group using the GHC inliner ; )
by Daniel Schüssler 23 Mar '11

23 Mar '11

Hello, turns out that you can define the group operation of the symmetric group on 3 elements in this abstract way (via the isomorphism to the group of bijective functions from a three-element type to itself): s3mult g2 g1 = fromFun (toFun g2 . toFun g1) and convince GHC to compile it down to a nested case statement. It even somehow made the left multiplication with the identity non-strict. Just thought it's neat ;) $ ghc-core S3.hs -funfolding-use-threshold=64 ... -- identifiers manually un-qualified for readability s3mult = \ (g2_ahA :: S3) (g1_ahB :: S3) -> case g2_ahA of _ { S3abc -> g1_ahB; S3bca -> case g1_ahB of _ { S3abc -> S3bca; S3bca -> S3cab; S3cab -> S3abc; S3acb -> S3bac; S3bac -> S3cba; S3cba -> S3acb }; S3cab -> case g1_ahB of _ { S3abc -> S3cab; S3bca -> S3abc; S3cab -> S3bca; S3acb -> S3cba; S3bac -> S3acb; S3cba -> S3bac }; S3acb -> case g1_ahB of _ { S3abc -> S3acb; S3bca -> S3cba; S3cab -> S3bac; S3acb -> S3abc; S3bac -> S3cab; S3cba -> S3bca }; S3bac -> case g1_ahB of _ { S3abc -> S3bac; S3bca -> S3acb; S3cab -> S3cba; S3acb -> S3bca; S3bac -> S3abc; S3cba -> S3cab }; S3cba -> case g1_ahB of _ { S3abc -> S3cba; S3bca -> S3bac; S3cab -> S3acb; S3acb -> S3cab; S3bac -> S3bca; S3cba -> S3abc } } -- inverse s3inv = \ (g_ahC :: S3) -> case g_ahC of _ { S3abc -> S3abc; S3bca -> S3cab; S3cab -> S3bca; S3acb -> S3acb; S3bac -> S3bac; S3cba -> S3cba } --- end core --- --- source --- module S3 where -- | Symmetric group / permutation group on 3 elements data S3 = S3abc | S3bca | S3cab | S3acb | S3bac | S3cba deriving(Eq) -- | Returns an element of S3 satisfying the given predicate s3the :: (S3 -> Bool) -> S3 s3the p | p S3abc = S3abc | p S3acb = S3acb | p S3bac = S3bac | p S3bca = S3bca | p S3cba = S3cba | p S3cab = S3cab | otherwise = error "s3the: no element satisfies the predicate" data ABC = A | B | C deriving(Eq) toFun :: S3 -> ABC -> ABC toFun g = case g of S3abc -> mkFun A B C S3bca -> mkFun B C A S3cab -> mkFun C A B S3acb -> mkFun A C B S3bac -> mkFun B A C S3cba -> mkFun C B A where mkFun imA _ _ A = imA mkFun _ imB _ B = imB mkFun _ _ imC _ = imC fromFun :: (ABC -> ABC) -> S3 fromFun f = s3the (\g -> toFun g A == f A && toFun g B == f B) s3mult :: S3 -> S3 -> S3 s3mult g2 g1 = fromFun (toFun g2 . toFun g1) s3inv g = s3the (\g' -> s3mult g' g == S3abc)

2 1

Install GTK on Windows
by Daniel Díaz 23 Mar '11

23 Mar '11

Hi, cafe, I know this has been treated many times, but I've tried install GTK so hard without any success that I decided to ask for help. Firstly, I followed the instructions listed in HaskellWiki [1]. No results. Then, I followed the instruction from the Mark Shroyer blog [2]. No results. I did some mixing of the two guides, too, but my error was always the same: Configuring cairo-0.12.0... [...] setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: z, cairo Configuring glib-0.12.0... [...] setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, gthread-2.0, glib-2.0, intl I've searched for these libraries, and, with --extra-include-dirs=, I've included them. But the same error was thrown. I'm sure I'm doing something wrong or, perhaps, I'm missing something. Any help is welcomed! Thanks in advance, Daniel Díaz References: [1] http://www.haskell.org/haskellwiki/Gtk2Hs [2] http://markshroyer.com/2010/10/gtk2hs-on-windows/

5 12

Haskell and friends in Spanish
by Manuel Hernandez 23 Mar '11

23 Mar '11

Dear Haskellers, I have tried to compile from my own work and from several sources a university-level course in functional and logic programming, and the result is a prospect of a book (*right now only in Spanish*), which I have entitled "Programación declarativa". Perhaps someone could be interested in giving it a look. The book is big (more than 400pp and 2Mb) and eclectic wrt declarative topics (ranging from Scheme to Prolog), but Haskell has an ample treatment. Some parts of the book are even unfinished, but I think the book may be valuable for some Spanish-speaking people wishing to learn declarative programming through functional programming and Prolog. The book can be downloaded from: http://sites.google.com/site/writingsmhg/haskell/SpanishDeclarativeProgramm… Corrections and suggestions are welcome. Bests, -- Dr Manuel Hernández Gutiérrez Profesor-Investigador, Oficina num. 14 Coordinador del Doctorado en Tecnologías de Cómputo Aplicado. Área de Estudios de Posgrado. Instituto de Computación Universidad Tecnológica de la Mixteca Km 2.5 de la Carretera a Acatlima Huajuapan de León, Oaxaca, México 69000 Tel. +52 (953) 53 2 03 99 extensión 200 Fax +52 (953) 53 2 02 14 www.utm.mx

1 0

Bamse package virus false positive
by Isaac Potoczny-Jones 22 Mar '11

22 Mar '11

We have realized that a file in the Bamse distribution in Hackage is being flagged by some virus scanners as malicious, including Symantec and McAfee. We have worked with Microsoft, Symantec, and McAfee to analyze the data. All parties have concluded that this file is not malicious, but we wanted to let the community know in case you run across a similar issue. We have notified the maintainer. Symantec and McAfee have updated their rules accordingly and will no longer flag this file, but other virus scanners might continue to flag it. Note that if you use Bamse to build MSI packages, inclusion of this file could cause some scanners to flag the package. The details of this file are as follows: File name: Folder.exe File Size: 83249 byte MD5: 9f939a06344af9b242a7b43849defb40 SHA1: a9faf0915d1ea142b161bec57792ef2ca379ce5b URL: http://hackage.haskell.org/package/bamse From the package description: "Bamse is a framework for building Windows Installers for your Windows applications, giving you a comprehensive set of features to put together MSIs using Haskell." peace, isaac

1 0

wrong backquote in haskell 2010 report Prelude
by Albert Y. C. Lai 22 Mar '11

22 Mar '11

Haskell 2010 report chapter 9 Standard Prelude uses a wrong backquote, e.g., infixl 7 ⋆, /, ‘quot‘, ‘rem‘, ‘div‘, ‘mod‘ Those are U+2018. The grammar (Chapter 3) requires U+0060 and accepts no substitutes, e.g., varop → varsym | ` varid `

2 1

Uncertainty analysis library?
by Edward Amsden 22 Mar '11

22 Mar '11

Hi cafe, I'm looking for a library that provides an instance of Num, Fractional, Floating, etc, but carries uncertainty values through calculations. A scan of hackage didn't turn anything up. Does anyone know of a library like this? Thanks! -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

5 13

ANNOUNCE: incremental-parser library package
by Mario Blažević 22 Mar '11

22 Mar '11

The first version of incremental-parser has been released on Hackage [1]. It's yet another parser combinator library, providing the usual set of Applicative and Monad combinators. Apart from this, it has three twists that make it unique. First, the parser is incremental. That means it can be fed its input in chunks, and in proper circumstances it can also provide the parsed output in chunks. For this to be possible the result type must be a Monoid. The complete parsing result is then a concatenation of the partial results. In order to make the incremental parsing easier, the combinator set is optimized for monoidal results. The usual combinator many1, for example, assumes the result type is a monoid and concatenates its components instead of constructing a list. In Parsec: > many1 :: Stream s m t => ParsecT s u m a -> ParsecT s u m [a] In incremental-parser: > many1 :: (Monoid s, Monoid r) => Parser s r -> Parser s r The second weirdness is that the the parser is generic in its input stream type, but this type is parameterized in a holistic way. There is no separate token type. Primitive parsers that need to peek into the input require its type to be an instance of a monoid subclass. In Parsec: > string :: Stream s m Char => String -> ParsecT s u m String > char :: Stream s m Char => Char -> ParsecT s u m Char > anyToken :: (Stream s m t, Show t) => ParsecT s u m t In Attoparsec: > string :: ByteString -> Parser ByteString > word8 :: Word8 -> Parser Word8 > anyWord8 :: Parser Word8 In incremental-parser: > string :: (LeftCancellativeMonoid s, MonoidNull s) => s -> Parser s s > token :: (Eq s, FactorialMonoid s) => s -> Parser s s > anyToken :: FactorialMonoid s => Parser s s The monoid subclasses referenced above provide methods for analyzing and subdividing the input stream. The classes are not particularly demanding, and any reasonable input stream should be able to accommodate them easily. The library comes with instances for lists, ByteString, and Text. > class Monoid m => MonoidNull m where > mnull :: m -> Bool > class Monoid m => LeftCancellativeMonoid m where > mstripPrefix :: m -> m -> Maybe m > class Monoid m => FactorialMonoid m where > factors :: m -> [m] > primePrefix :: m -> m > ... Finally, the library being implemented on the basis of Brzozowski derivatives, it can provide both the symmetric and the left-biased choice, <|> and <<|>. This is the same design choice made by Text.ParserCombinators.ReadP and uu-parsinglib. Parsec and its progeny on the other hand provide only the faster left-biased choice, at some cost to the expressiveness of the combinator language. [1] http://hackage.haskell.org/package/incremental-parser-0.1

4 4

Re: [Haskell-cafe] [Haskell] ANNOUNCE: incremental-parser library package
by Mario Blažević 22 Mar '11

22 Mar '11

2011/3/22 Philippa Cowderoy <postmaster(a)flippac.org> > This is what newtypes are for, no? > I did not think of that approach. I'm not sure how well it would work out, but it would solve another problem I have, which is the duplication of combinators many, some, and optional. Each of these could exist in two forms, the lazy one and the greedy one, and the only difference is the underlying choice combinator, (<|>) vs. (<<|>). I'm not aware of any other parsing library taking this road, though, and there must be a good reason. I'll try and see. > 2011/3/22 Mario Blažević <mblazevic(a)stilo.com> > >> >> This seems very interesting. One question: >>> >>> > The MonadPlus and the Alternative instance differ: the former's mplus >>> > combinator equals the asymmetric <<|> choice. >>> >>> Why? >>> >> >> >> Good question. Basically, I see MonadPlus as a union of Monad and >> Alternative. The class should not exist at all. But as long as it does, I >> figured I should provide an instance, and I made it different from the >> Monoid+Alternative combination because otherwise it would be useless. My >> second choice would be to remove the instance completely. >> > > I have to admit I really do not like having Applicative and MonadPlus with > different behavior. Yes, one is redundant, but that is more an artifact of > language evolution, than an intentional opportunity for diverging behavior. > > Every library I am aware of to date, save of course this one, has > maintained their compatibility. > > If the instance for Alternative satisfies the underspecified MonadPlus > laws, I'd just as soon have the 'useless redundant' instance. The power of > MonadPlus is in the combinators that are built on top of it. Not in the > primitives themselves. > > If the Alternative instance would not be a legal MonadPlus instance, then > I'd feel much less queasy with your second scenario, and it simply removed. > > -Edward >

1 0

Berlin Haskell Meeting
by Sönke Hahn 22 Mar '11

22 Mar '11

Hi all! It's been some time since the last Berlin Haskell Meeting, but we're doing it again: Date: Wednesday, March 30th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin If you're in Berlin and interested in Haskell, save the date! Cheers, Sönke

1 0

Loops and ray casting with GPipe?
by Jarkko Vilhunen 22 Mar '11

22 Mar '11

Greetings, With OpenGL shaders it is possible to write code utilizing loops with unknown number of iterations, I'm wondering if the same can be done with GPipe? Fractals would be one use case and implicit surfaces another. Fragment Floats had to be compared with IfB from Data.Boolean, so I tried something recursive like foo x = boolean x (foo (x * 0.9)) (x <* 1) which compiles fine but seems to concentrate on eating stack space while running :)

3 2

Mime / Mail library
by Pieter Laeremans 22 Mar '11

22 Mar '11

Hi all, The MIME package that can be found on hackage, uses String as input. Would i be considered better if there would be a version based on Text, or ByteString ? kind regards, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

4 3

Anyone recommend a VPS?
by Michael Snoyman 21 Mar '11

21 Mar '11

Hi all, I'm working with a client right now on deploying an app. I was wondering if anyone had some recommendations for a Haskell-friendly host. I'm inclined to go with a VPS for this setup, but if there's reliable shared hosting, that would do as well. This client is fairly price-sensitive, so I don't think anything past the $20/month range to start off with. The only other requirement for this project is PostgreSQL. As far as wants: * Linux is preferable * A server that isn't on everyone's spam list would be better. Of course, I can route through an outside e-mail provider, but local would be nicer. * Have room to grow. For now, this is a fairly small app, but the hope of course is for it to expand. Thanks in advance. Michael

12 15

Subsets and supersets
by Andrew Coppin 21 Mar '11

21 Mar '11

The other day I saw the "green field Haskell" discussion on Reddit. Of course, I could give you a very long list (ha!) of things that I would change about Haskell. (This would probably begin with expunging Monad.fail with extreme prejudice...) But there's one particular feature which is non-trivial enough to be worth discussing here... Haskell has ADTs. Most of the time, these work great. As I've written in several other places (but possibly not here), OO languages tend to "factor" the problem the other way. That is, if I want a binary tree, an OO language makes me split the type and all of its operations into three parts (an abstract base class, a branch subclass, and a leaf subclass). Adding each new operation requires adding an abstract version of it to the abstract base class, and putting half of the implementation into each concrete subclass. Haskell is factored the other way. The definition of the type goes in one place. The definition of each operation over it goes in one place. More can be added at any time. In particular, the OO approach doesn't let you add new tree operations without altering the source code for the tree classes. The ADT version *does* let you add new operations - provided the tree structure is visible. This is not to say that the OO approach is wrong, of course. The important thing about a binary tree is that there are exactly two kinds of nodes - leaves and branches - and there will *always* be exactly two kinds. That's what makes it a binary tree! [OK, no it isn't, but bare with me.] If, instead of a binary tree, or an abstract syntax tree, or some other fixed, unchanging structure, we wanted to deal with, say, bank accounts... Well, there are a bazillion kinds of bank account. And more might be added at any time. Here an ADT is utterly the wrong thing to do. It would have a bazillion constructors, and all the processing for a particular bank account type would be split between dozens of seperate functions. By contrast, an OO language would put each account type and all its associated processing in the same place - the class definition. In summary, problems can be factored two ways. ADTs do it one way, classes do it the other way, and both can be appropriate. OO languages use classes, but, happily, Haskell has ADTs *and* classes! :-D And not classes in the same sense as OO classes; the way Haskell does it is actually better, IMHO. Anyway, coming back on-topic... ADTs are great, but sometimes they don't let me easily express exactly what I want. Specifically, sometimes I have one type which is really a subset of another type, or perhaps a superset of several times. You can build subsets using GADTs, after a fashion: data Foo data Bar data Foobar x where Foobar1 ... :: Foobar Foo Foobar2 ... :: Foobar Bar Foobar3 ... :: Foobar x Foobar4 ... :: Foobar Bar Foobar5 ... :: Foobar Foo Now any expression of type Foobar Foo is guaranteed to contain only Foobar1, Foobar3 or Foobar5, while any expression of type Foobar Bar is guaranteed to contain only Foobar2, Foobar3 or Foobar4. As the tangle of sets becomes more complicated, this approach becomes more difficult to apply. You can of course build supersets using an even more mundane Haskell construct. The canonical example is Either. If I want a field that holds a Foo *or* a Bar, I can trivially write Either Foo Bar. The trouble is, this entails an extra level of indirection. In order to access a Foo or a Bar, I now have to strip off the Left or Right constructor to get to it. To construct a value, I have to stick in the constructor. Sometimes this is just fine, even desirable. For example, I recently wrote a function who's type is spans :: (x -> Bool) -> [x] -> [Either [x] [x]] Here the Left and Right constructors are actually /telling/ me something. They convey /information/. On the other hand, in the case of Either Foo Bar, the constructor tells me nothing. Consider: data Foo = Foo1 | Foo2 | Foo3 data Bar = Bar1 | Bar2 | Bar3 foobar :: Either Foo Bar -> x foobar e = case e of Left Foo1 -> ... Left Foo2 -> ... Left Foo3 -> ... Right Bar1 -> ... Right Bar2 -> ... Right Bar3 -> ... There is, logically, no particular reason why we shouldn't simplify the function to just foobar Foo + Bar -> x foobar fb = case fb of Foo1 -> ... Foo2 -> ... Foo3 -> ... Bar1 -> ... Bar2 -> ... Bar3 -> ... The set of possible constructors for Foo and for Bar are disjoint. There is no possibility of ambiguity here. The type system itself even /uses/ this fact to infer the type of literal constructor applications! And yet, you cannot write code such as the above. Not in Haskell, anyway. So what is the use case here? Well, consider for a moment the problem of parsing Haskell source code. Haskell contains both "patters" and "expressions". Now actually, it turns out that a pattern and an expression have a very similar structure. The set of possible patterns is /almost/ a subset of the set of possible expressions. (But not quite.) That gives me design choices: * Design one ADT which encodes both patterns and expressions. Since this removes the ability to statically distinguish between them, this is almost certainly the wrong choice. * Design one ADT for patterns and a seperate, unrelated ADT for expressions. I have now violated the DRY principle. And, hypothetically, if I wanted to treat a pattern as an expression, I would have to write a lot of boilerplate code that does nothing more than replace one constructor name with another (recursively). * Do something with GADTs, and hope the type system doesn't explode. Note that a literal value such as 5 or "foo" is both a valid expression and a valid pattern. In fact, I probably want an ADT just for literal values (since there's quite a few possible forms, and often I don't care which). The net result of all this is usually something like data Literal = LInteger Integer | LChar Char | LString String | ... data Pattern = PLiteral Literal | PConstructor Name [Pattern] | PVariable Name | PWildcard | ... data Expression = ELiteral Literal | EConstructor Name [Expression] | EVariable Name | ELambda [Pattern] Expression | ... expr1 = EConstructor "Foo" [ELiteral (LString "bar")] patt1 = PConstructor "Foo" [PLiteral (LString "bar")] The entire Literal type just looks plain wrong. The set of possible Integer values and the set of possible Char values are utterly disjoint. Really I ought to be able to just say which types Literal is the union of, and not worry about it any further. (There is the minor detail that the set of Integer values and Double values are *not* disjoint. There is also the minor detail that if any valid Char is also a valid Literal, then the type of 'x' is now no longer unambiguously Char. This makes type inference "interesting".) The case of patterns and expressions is more interesting, since here we have two overlapping sets, yet neither is strictly a superset of the other. Today we could probably solve this as just data Haskell x where HLiteral Literal :: Haskell x HConstructor Name [Haskell x] :: Haskell x -- Is this right? HVariable Name :: Haskell x HWildcard :: Haskell Pattern HLambda [Haskell Pattern] (Haskell Expression) :: Haskell Expression ... In other words, by carefully applying GADTs, the pattern/expression problem is probably solveable today. But it's tricky. And it requires me to create two useless nullary types that exist only to let the type checker distinguish patterns from expressions. (Fortunately empty data declarations can be enabled quite easily.) It would just be nice if I would trivially and easily take the union of several types, and if this was a fundamental operation. Then we would probably build type hierachies in a quite different way. Of course, there are several "minor details" which would need to be overcome first: 1. The Haskell 2010 type system guarantees that 'x' has type Char. If we let people define supersets of types, how is the poor type checker supposed to know exactly which type 'x' is meant to belong to? 2. I might not want to explicitly notate every single pattern and expression with its exact type, but /the compiled code/ needs to know! When a Char value becomes a Literal value, how does a case expression handle that? Currently (AFAIK) each constructor in a type is assigned a unique ID, and case expressions match against that. But if you take the union of two types, now what? Constructor IDs are only unique within a single type, not within some arbitrary union of types. 3. It strikes me that the RTS (particularly the GC) is going to need to know what the hell we're doing too. The most obvious solution to #1 is that the type of every term is the simplest possible type, unless you try to unify it with some other type, at which point the type for both such terms becomes the union of types. I rather suspect that this will instantly make every possible program well-typed, however. I suppose an alternative is to require explicit type signatures, but then we just lost the entire advantage of this construction. As to #2, maybe this whole type union insanity could be considered merely syntax sugar, with the whole edifice being desugared into the multiply constructor wrapping craziness we have now. But it seems a pitty. If we could find a way to assign a globally unique ID to every constructor (code address pointer perhaps?), then we could increase program efficiency at the same take as making code less verbose. As to #3... my name is not Simon.

4 3

ANN: Monad.Reader special Poetry and Fiction Edition
by Brent Yorgey 21 Mar '11

21 Mar '11

I am pleased to announce that the special Poetry and Fiction Edition of The Monad.Reader is now available [1]. Enjoy! Also, the submission deadline for Issue 18 has been extended one week, to Friday, April 8. Please get in touch if you would like to submit something! -Brent [1] http://themonadreader.files.wordpress.com/2011/03/specialissue.pdf

6 8

unregisterised plan9 port
by * 21 Mar '11

21 Mar '11

Hi there. I am trying to get an unregisterised build of ghc 7.0.2 working and am having problems turning mangling off. When I build the compiler with GhcUnregisterized = YES GhcWithNativeCodeGen = NO SplitObjs = NO BeConservative = YES The 'evil mangler' is still used. When I disable it at the command line with -fno-asm-mangling the FB_ macro is still put into the generated C. I just want to turn everything related to this dreaded "evil mangler" off, and still produce working executables. Are there things that I could set in the build.mk file to do this, or is it more involved? I have read some things in the mailing list and the bug tracker that (to my ignorant and slightly confused ears) seem to suggest that this unregisterised is problematic or depreciated or something. If this is the case I would be perfectly happy using an older version of the compiler, and would be grateful if someone could point me to a specific older version number where this stuff builds with less hassel. I don't care how out of date it is. In case you are wondering, I am attempting to get an unregisterised port of ghc working in plan9, an operating system that has yet to be blessed with a haskell implementation (hugs doesn't count :P) Plan 9 has a gcc port, an older one, but it should be fine.

2 1

Re: [Haskell-cafe] Mime / Mail library
by John Lato 21 Mar '11

21 Mar '11

> From: Christopher Done <chrisdone(a)googlemail.com> > > On 20 March 2011 15:05, Pieter Laeremans <pieter(a)laeremans.org> wrote: > > > Hi all, > > > > The MIME package that can be found on hackage, uses String as input. > > Would i be considered better if there would be a version based on Text, > or > > ByteString ? > > > > I think the solution to this problem is a generic `string' package which > just provides a few classes. The MIME library would export an interface > that > only deals with instances of these classes, and whether you're using Text, > String, ByteString/Lazy/Char8, ropes, whatever, it's not the library > writer's concern or assumptions to make. > > We already have: > > http://hackage.haskell.org/packages/archive/string-combinators/0.6/doc/html… > < > http://hackage.haskell.org/packages/archive/string-combinators/0.6/doc/html… > > > Which works on Monoid and IsString, but there needs to be a class like "can > be read/outputted via IO" and one for read/show/serialize, both of which > are > important for speed. > > Anyone already done work on this? The Data.ByteString modules could export > some instances. It's fair enough that people who need fine-grained speed > use > the concrete types and all their special-purposes functions, but in the > general having to choose arbitrarily (often defaulting to String) is a > burden. Can't we standardize on a default set of String classes to use? > > I suppose I could try reducing Data.ByteString's function set into a set of > essential core methods, seeing as the ByteString modules are a very good > spec of what's required for a String type to be completely usable. > Data.ListLike (http://hackage.haskell.org/package/ListLike) provides this via the "StringLike" class (and StringLikeIO). There are built-in instances for Strings and ByteStrings, with orphan Text instances in http://hackage.haskell.org/package/listlike-instances. Probably the StringLike class could be extended to provide more (high-performance) methods, but I don't have a good idea of what's necessary. Suggestions are welcome! John L.

1 0

ANN: unix-bytestring: Unix/Posix-specific functions for ByteStrings.
by wren ng thornton 20 Mar '11

20 Mar '11

-------------------------------------------- -- unix-bytestring 0.3.2 -------------------------------------------- The unix-bytestring package offers a full selection of Unix/Posix-specific functions for reading and writing ByteStrings to Fds. -------------------------------------------- -- The Story Behind It All -------------------------------------------- A while back I needed a ByteString-based version of the System.Posix.IO String-based API. I coded up the new versions and submitted a patch to the unix package for adding them in[1]. But apparently, noone's much pleased with the System.Posix.IO API! In the ensuing discussion folks brought up their specific complaints and offered suggestions. So now, the couple of functions have become enough to release as its own package--- with the hope that one day it'll be rolled into the unix package. That's where you come in. The current API is more than good enough for anything I've needed, so I need your feedback on what you want from a ByteString-based Unix/Posix I/O library. I've looked through Roel van Dijk's reverse dependency analysis[2] to find the folks currently using the unix package. The maintainers of the following packages may be particularly interested in unix-bytestring: * HFuse * iteratee * iteratee-mtl * liboleg HFuse has complained about the lack of bindings for pread(2) and pwrite(2), which are included in unix-bytestring. The only standing complaint I haven't addressed is one expressed in the iteratee, iteratee-mtl, and liboleg packages which would prefer a return type of (Either Errno _) instead of throwing exceptions. [1] http://article.gmane.org/gmane.comp.lang.haskell.libraries/15390 [2] http://bifunctor.homelinux.net/~roel/cgi-bin/hackage-scripts/package/unix -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unix-bytestring Darcs: http://community.haskell.org/~wren/unix-bytestring Haddock (Darcs version): http://community.haskell.org/~wren/unix-bytestring/dist/doc/html/unix-bytes… -- Live well, ~wren

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Re: [Haskell-cafe] How to use roots package?
by Artyom Kazak 20 Mar '11

20 Mar '11

Oh. I have taken a wrong approach to the problem. I have written Newton method with cutting precision if it's more than N digits, and it finds an answer practically in no time. But still, it's very good, thank you!

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Downloading Haskell repos from GitHub
by Gwern Branwen 20 Mar '11

20 Mar '11

Along the lines of http://blog.patch-tag.com/2010/03/13/mirroring-patch-tag/ for downloading all patch-tag.com repositories, I've begun to wonder how to download all Github repositories since more and more people seem to be using it. Nothing in http://develop.github.com/ seems especially useful for grabbing the git:// URLs of all repos by language - just by user. The only real list of repos by language seems to be gotten at via http://github.com/languages/Haskell/updated or http://github.com/languages/Haskell/created . (You might think http://github.com/languages/Haskell would be good, but no, it's just a few random repos by interest and not a full listing.) I looked at the HTML, and it looks possible to use tagsoup to get all 98 pages and then parse the entries to get the HTTP URLs of the repos, and then turn *that* into git:// URLs suitable for shelling out to 'git clone', but I can't help but wonder if maybe there's a better approach someone more familiar with Github would know. -- gwern

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Gtk2hs multiple column TreeView with ListStore issue
by Кирилл Березин 20 Mar '11

20 Mar '11

I cannot force GTK to render data in TreeView with ListStore model with multiple columns through Haskell. I have the following code addTextColumn view name = do col <- treeViewColumnNew rend <- cellRendererTextNew treeViewColumnSetTitle col name treeViewColumnPackStart col rend True treeViewColumnSetExpand col True treeViewAppendColumn view col prepareTreeView view = do addTextColumn view "column1" addTextColumn view "column2" --adding data here Then I try to add some data, and there are problems. I tried these: --variant 1 (data TRow = TRow {one::String, two::String} model <- listStoreNew ([] :: [TRow]) listStoreAppend model $ TRow { one = "Foo", two = "Boo" } treeViewSetModel view model --variant 2 model <- listStoreNew ([] :: [[String]]) listStoreAppend model ["foo","boo"] treeViewSetModel view model --variant 3 model <- listStoreNew ([] :: [(String, String)]) listStoreAppend model ("foo", "boo") treeViewSetModel view model But in all cases I see the table with column header and one blank row inserted. Any help will be appreciated.

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Defining subtraction for naturals
by wren ng thornton 19 Mar '11

19 Mar '11

Another question on particulars. When dealing with natural numbers, we run into the problem of defining subtraction. There are a few reasonable definitions: (1) If the result would drop below zero then throw an overflow error; (2) Use saturating subtraction, i.e. if the result would drop below zero then return zero; (3) Use type hackery to disallow performing subtraction when the result would drop below zero, e.g. by requiring a proof that the left argument is not less than the right. Dependently typed languages tend to use the second definition because it gives a pure total function (unlike the first) and because it's less obnoxious to use than the third. In Haskell the third would be even more obnoxious. So my question is, mathematically speaking, is there any reason to prefer the second option over the first or third? Purity aside, I mean; do the natural numbers with saturating subtraction form any interesting mathematical object, or is it just what we're stuck with? In a similar vein. Given a finite set (e.g., the natural numbers with some finite upper bound) and assuming we've already decided not to use modular arithmetic, is there any mathematical reason to prefer saturating addition and/or multiplication instead of throwing an overflow error? -- Live well, ~wren

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Re: [Haskell-cafe] Trac login problem
by Jason Dagit 19 Mar '11

19 Mar '11

I've experienced very similar problems several times on both the ghc and cabal trac instances. I think resetting your password is (or was) breaking accounts. Now I just stick to guest login accounts on all the haskell.org trac instances. Ideally, all the trac instances, haskell wiki and etc, used a common identity system such as openid so we wouldn't need so many accounts. On Mar 19, 2011 11:28 AM, "Mihai Maruseac" <mihai.maruseac(a)gmail.com> wrote: Hi, I have a problem while trying to login to Haskell Trac instance. Entering the username and password that I know are valid gives me a login error. Trying to recover the password with the mail address that I use gives another error saying that there is no combination between the username and the mail. And, when trying to recreate the account if correctly complains that there is a valid username already. Can someone, please, look into it? Thanks, -- Mihai _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Trac login problem
by Mihai Maruseac 19 Mar '11

19 Mar '11

Hi, I have a problem while trying to login to Haskell Trac instance. Entering the username and password that I know are valid gives me a login error. Trying to recover the password with the mail address that I use gives another error saying that there is no combination between the username and the mail. And, when trying to recreate the account if correctly complains that there is a valid username already. Can someone, please, look into it? Thanks, -- Mihai

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[haskell-gsoc] Haskell.org Google Summer of Code 2011
by Edward Kmett 19 Mar '11

19 Mar '11

It is that time of year again. The Google Summer of Code is upon us! *Sign Up Information* If you are a student and want to sign up to make $5,000 for hacking on the code you love over the summer or are willing to help out as a mentor, now is the time to act. Please sign up by adding your name to the list at: http://hackage.haskell.org/trac/summer-of-code/wiki/People2011 Our main wiki entry for the Summer of Code 2010 is now online: http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2011 If you are interested in applying as a student, here are some guidelines: http://hackage.haskell.org/trac/summer-of-code/wiki/StudApply2011 The official deadline for student applications with Google is April 8th, but if you are interested please add your name to the People2010 page above, as soon as possible. *Interested in helping out, but not sure how? * There are a number of ideas that we've been collecting for projects available through the trac http://hackage.haskell.org/trac/summer-of-code/report/1 and which have been collecting over time in a reddit: http://www.reddit.com/r/haskell_proposals/top/ <http://www.reddit.com/r/haskell_proposals/top/>Unfortunately, reddit doesn't let you comment on topics after they become archived, and most of the topics in the haskell_proposals reddit have been. While the reddit is a great place to discuss proposals, we had to choose one mechanism to supply Google with our ideas officially, so please submit proposals <http://hackage.haskell.org/trac/summer-of-code/newticket> to the trac once you have hashed out something you like to work on, or which would be an appropriately beneficial project for a summer worth of work. *Helping Out Darcs * Haskell.org <http://haskell.org/> is going to continue to act as an umbrella organization covering darcs <http://darcs.net/> for the Summer of Code once more. If you are interested in helping them out, they also have a page full of project ideas as well: http://wiki.darcs.net/GoogleSummerOfCode Again, please submit proposals<http://hackage.haskell.org/trac/summer-of-code/newticket> as tickets to the trac if you can think of something you'd like to work on. *More Information* Here are a bunch of relevant links that may be able to answer your questions: Google's Summer of Code Homepage <http://socghop.appspot.com/> Google's Summer of Code 2011 FAQ<http://socghop.appspot.com/document/show/gsoc_program/google/gsoc2011/faqs> Google's Summer of Code 2011 Timeline<http://socghop.appspot.com/document/show/gsoc_program/google/gsoc2011/timel…> Google's Summer of Code Wiki Knowledge Base<http://code.google.com/p/google-summer-of-code/wiki/WikiStart> Of course, you can always feel free to email me <ekmett(a)gmail.com> or inquire on irc.freenode.net #haskell-soc or with the darcs folks over on #darcs for more information. If you don't have an IRC client, you can get there from: http://webchat.freenode.net Thank you all very much and we look forward to another successful Summer of Code! -Edward Kmett

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Cabal test interface, what/where is it?
by Magnus Therning 19 Mar '11

19 Mar '11

After upgrading to ghc7 and Cabal 1.10 I get the following message when running 'Setup configure' on one of my packages: Setup.hs:3:1: Warning: In the use of `runTests' (imported from Distribution.Simple, but defined in Distribution.Simple.UserHooks): Deprecated: "Please use the new testing interface instead!" I usually turn to the Cabal docs at [1], but the new testing interface isn't mentioned. Looking through the haddock API reference doesn't offer any obvious pointers. So, where should I turn to get rid of the warning? /M [1] http://www.haskell.org/ghc/docs/latest/html/Cabal/ -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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Missing e-mail @haskell-cafe
by Mario Blažević 19 Mar '11

19 Mar '11

I've sent an e-mail to Haskell Café this morning about my troubles with Trac confirmation e-mails. The e-mail must have reached the server, because it showed up in the mailing list archive: http://www.haskell.org/pipermail/haskell-cafe/2011-March/090311.html This other archive, however, doesn't list it: http://www.mail-archive.com/haskell-cafe@haskell.org/ I thereby infer that the message was never forwarded to the mailing list. Is that correct, and does anybody have a clue why?

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Wanted: composoable parsers from haskell-src-exts
by J. Waldmann 18 Mar '11

18 Mar '11

Hi. I want to use parsers from haskell-src-exts as sub-parsers, which does not seem to work since they insist on consuming the input completely. I would need them to parse a maximal prefix, and return the (unconsumed) rest of input as well (cf. http://hackage.haskell.org/packages/archive/parsec/3.1.1/doc/html/Text-Pars… ) I figure that happy has the %partial directive for that, but the description http://www.haskell.org/happy/doc/html/sec-directives.html#sec-partial-parse… does not really tell me how to obtain the rest of the input. Any hints (or code samples) appreciated. Thanks - J.W.

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Could not deduce ... using functional dependencies with GHC7
by JP Moresmau 18 Mar '11

18 Mar '11

Hello, when moving to GHC7 a body of code that I'm not immensely familiar with, I got an error compiling with GHC 7.0.2. Can somebody kindly explain to me what it means and how to get around it? This is in the scion code base so is using some GHC types. I have a class with a functional dependency (I think :-p): class Search id a | a -> id where search :: (SrcSpan -> Bool) -> SrcSpan -> a -> SearchResults id And some instances, notably: instance Search Id Id where search _ _ i = only (FoundId i) And the error occurs on another instance: instance (Search id id) => Search id (Sig id) where search p s (IdSig i) = search p s i ... other cases here, that work The error is: Could not deduce (id ~ Id) from the context (Search id id) bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:10-45 `id' is a rigid type variable bound by the instance declaration at lib\Scion\Inspect\Find.hs:477:18 When using functional dependencies to combine Search Id Id, arising from the dependency `a -> id' in the instance declaration at lib\Scion\Inspect\Find.hs:183:10 Search id Id, arising from a use of `search' at lib\Scion\Inspect\Find.hs:479:32-37 In the expression: search p s i In an equation for `search': search p s (IdSig i) = search p s i If I replace the call to search with the actual implementation of search for Id, it compiles, so at some level the code "makes sense". I much prefer when error messages end with "Possible fix:..."! Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] trac.haskell.org problem
by Mario Blažević 18 Mar '11

18 Mar '11

I've tried to send the verification e-mail from trac.haskell.org to three different e-mail accounts in the last 24 hours. None arrived. Note that I'm not trying to create a new account, I'm trying to verify my e-mail address in order to edit the Wiki pages of my projects. Can anybody else confirm this problem? > Ah, I suppose that's to do with the trac server itself, that's beyond me. > > Moving to café: Does anyone know what's up with trac.haskell.org not > sending > out verification emails? > > Cheers, > > /Niklas > > ________________________________________ > From: Jurriaan Hage [jur at cs.uu.nl] > Sent: Monday, February 28, 2011 8:41 AM > To: Niklas Broberg > Subject: Re: Again a question about haskell-src-exts. > > On Feb 28, 2011, at 8:39 AM, Niklas Broberg wrote: > > > I suppose I do - to the extend I can. What's the issue? :-) > The system does not send out verification e-mails, so I can not really > register completely and submit new issues. > I have tried it with two different e-mail accounts, and somebody else has > tried too. > They also do not show up in spam folders. > > Jur >

1 0

Data constructor synonyms
by Grigory Sarnitskiy 18 Mar '11

18 Mar '11

Hello! Assume I have a type MyType with the constructor GeneralConstructor: data MyType = GeneralConstructor [Double] but I also want to have a separate name for special case of this constructor: SpecialConstructor Double so SpecialConstructor a = GeneralConstructor (a:[]) that is SpecialConstructor 5 was exactly the same as GeneralConstructor [5]. And for example instead of writing GeneralConstructor [0] I would like to use constructor Zero. It's all just for convenience. How can I achieve this? Well, of course I always can use sed to replace SpecialConstructor 5 with GeneralConstructor [5] in program sources, but it's not convenient.

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tplot and splot - analyst's swiss army knifes for visualizing log files
by Eugene Kirpichov 18 Mar '11

18 Mar '11

Hi cafe, I've published a large presentation about two Haskell-based tools of mine - tplot and splot. Their motto is "visualize system behavior from logs with a shell one-liner". Based on my experience, they usually seem to live up to this motto. http://www.slideshare.net/jkff/two-visualization-tools [attention attractor: the presentation has *really a lot* of pictures] hackage: http://hackage.haskell.org/package/timeplot http://hackage.haskell.org/package/splot other docs: http://www.haskell.org/haskellwiki/Timeplot http://www.haskell.org/haskellwiki/Splot -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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order of arguments matters
by Peter Padawitz 18 Mar '11

18 Mar '11

Why does only tr2 work, although the only difference between tr1 and tr2 is the order of arguments? import Data.Tree (Tree(..)) data Type a where Tree :: Type a -> Type (Tree a) Int :: Type Int String :: Type String type Traversal1 = forall a.a -> Type a -> a type Traversal2 = forall a.Type a -> a -> a tr1 :: Traversal1 tr1 (Node _ (t:_)) (Tree Int) = Node 1 [t] tr1 n Int = n+n tr1 s String = s++s tr2 :: Traversal2 tr2 (Tree Int) (Node _ (t:_)) = Node 1 [t] tr2 Int n = n+n tr2 String s = s++s Couldn't match expected type `a' against inferred type `Tree Int' `a' is a rigid type variable bound by the type signature for `tr1' at tratest.hs:9:25 In the pattern: Node _ (t : _) In the definition of `tr1': tr1 (Node _ (t : _)) (Tree Int) = Node 1 [t]

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How to contact OpenGL package maintainer (where is Sven?)
by Jason Dagit 18 Mar '11

18 Mar '11

Hello, It looks like the OpenGL packages on hackage[1,2,3,4] have not been updated in some time. No updates later than Oct 2009. I tried to email Sven directly using the email address listed on hackage but after about a week I still haven't heard from him. I sent some patches to the opengl list yesterday, and then today I noticed that Sven hasn't posted on that list since Oct 2009 when he released the current version of OpenGLRaw. None of the public darcs repos have patches newer than Oct 2009. Also, the homepage url listed in the packages is a 404: http://www.haskell.org/HOpenGL/ My concern is that Sven has disappeared for some reason. I hope he's well. He has always done top notch work in the past maintaining these libraries. Perhaps he's simply busy or lost interest? Does anyone know if he was looking for a new maintainer? Perhaps you've heard from him more recently than Oct 2009? If a new maintainer is needed, I would consider nominating myself :) Thanks, Jason [1] http://hackage.haskell.org/package/OpenGLRaw [2] http://hackage.haskell.org/package/OpenGL [3] http://hackage.haskell.org/package/GLURaw [4] http://hackage.haskell.org/package/GLUT

1 0