- Haskell-Cafe - Haskell.org

Combining stream and list fusion
by Bas van Dijk 12 Oct '11

12 Oct '11

Hello, I'm trying to make the following faster: Data.Vector.Generic.fromList list where 'list' is some expression yielding a list. (For example: map (+1) $ map (*2) [1..1000000]) Note that Data.Vector.Generic.fromList is defined as: fromList :: Vector v a => [a] -> v a {-# INLINE fromList #-} fromList = unstream . Stream.fromList where Stream.fromList is defined in Data.Vector.Fusion.Stream as: fromList :: [a] -> Stream a {-# INLINE fromList #-} fromList = M.fromList where M.fromList is defined in Data.Vector.Fusion.Stream.Monadic as: fromList :: Monad m => [a] -> Stream m a {-# INLINE fromList #-} fromList xs = unsafeFromList Unknown xs where unsafeFromList is defined as: unsafeFromList :: Monad m => Size -> [a] -> Stream m a {-# INLINE_STREAM unsafeFromList #-} unsafeFromList sz xs = Stream step xs sz where step (x:xs) = return (Yield x xs) step [] = return Done I would like to fuse the construction of the list with the construction of the stream as in: import GHC.Base ( build ) {-# RULES "unsafeFromList/build" forall sz (g :: forall b. (a -> b -> b) -> b -> b). unsafeFromList sz (build g) = unsafeFromListF sz g #-} unsafeFromListF :: forall m a. Monad m => Size -> (forall b. (a -> b -> b) -> b -> b) -> Stream m a {-# INLINE unsafeFromListF #-} unsafeFromListF sz g = Stream step st sz where St step st = g c z c :: a -> St m a -> St m a c x st = St (\(St s st') -> s st') (St (\s -> return (Yield x s)) st) z :: St m a z = St (\_ -> return Done) undefined -- Ouch! data St m a = St ((St m a) -> m (Step (St m a) a)) (St m a) Unfortunately, some initial experiments show that this doesn't make it faster. Are there ways to improve this? (Bonus points for getting rid of the undefined!) Regards, Bas

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Re: [Haskell-cafe] The best way to call Java from Haskell?
by Janis Voigtländer 12 Oct '11

12 Oct '11

Ketil Malde <ketil(a)malde.org> writes: >Jerzy Karczmarczuk<jerzy.karczmarczuk(a)unicaen.fr> writes: >> Don't worry, my friend. Haskell is lazy, so there is no problem in >> "handling" those infinite modules. It will just take you an infinite >> amount of time before you get any money from such a work. But this is >> a general problem elsewhere as well. > > I guess you must be thinking of Haskell being increasingly used in > banks? It must have been some bank manager who, after hiring one too > many Haskell programmers, invented a scheme that would generate an > infinite amount of money. He didn't realize before it was too late that > the actual value of the scheme would be bottom... See also: http://www.haskell.org/haskellwiki/Humor/Enron -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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ANNOUNCE: simple-actors 0.1.0 - an eDSL library for actor model concurrency
by Brandon Simmons 12 Oct '11

12 Oct '11

I'm happy to announce the release of my library 'simple-actors', a DSL-style library for more structured concurrent programs based on the Actor Model. It offers an alternative to ad-hoc use of Chans that allows for tight control of side-effects and message passing, and is especially suited to applications such as simulations of communicating processes. You can try it with a $ cabal install simple-actors and view the documentation here: http://hackage.haskell.org/package/simple-actors or check out the repo here: https://github.com/jberryman/simple-actors Here is an example of a system of actors working as a binary tree, supporting insert and query operations: ---- EXAMPLE ---- module Main where import Control.Concurrent.Actors import Control.Applicative import Control.Concurrent.MVar type Node = Mailbox Operation -- operations supported by the network: data Operation = Insert { val :: Int } | Query { val :: Int , sigVar :: MVar Bool } -- the actor equivalent of a Nil leaf node: nil :: Behavior Operation nil = Receive $ do (Query _ var) <- received send var False -- signal that Int is not present in tree return nil -- await next message <|> do -- else, Insert received l <- spawn nil -- spawn child nodes r <- spawn nil branch l r . val <$> received -- create branch from inserted val -- a "branch" node with an Int value 'v' and two children branch :: Node -> Node -> Int -> Behavior Operation branch l r v = loop where loop = Receive $ do m <- received case compare (val m) v of LT -> send l m GT -> send r m EQ -> case m of -- signal Int present in tree: (Query _ var) -> send var True _ -> return () return loop insert :: Node -> Int -> IO () insert t = send t . Insert -- MVar is in the 'SplitChan' class so actors can 'send' to it: query :: Node -> Int -> IO Bool query t a = do v <- newEmptyMVar send t (Query a v) takeMVar v main = do t <- spawn nil mapM_ (insert t) [5,3,7,2,4,6,8] mapM (query t) [1,5,0,7] >>= print ---- END EXAMPLE ---- I need to do some work on the documentation and performance testing. If anyone has anyone questions or comments, I would love to hear them. Thanks, Brandon http://coder.bsimmons.name

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haskell i18n best practices
by Paulo Pocinho 12 Oct '11

12 Oct '11

Hello list. I've been trying to figure a nice method to provide localisation. An application is deployed using a conventional installer. The end-user is not required to have the Haskell runtimes, compiler or platform. The application should bundle ready to use translation data. What I am after is simple; an intuitive way that an interested translator, with little knowledge of Haskell, can look at and create valid translation data. This is what I've been looking at lately. The first thing I noticed was the GNU gettext implementation for Haskell. The wiki page [1] has a nice explanation by Aufheben. The hgettext package is found here [2]. I don't know if this is a bad habit, but I had already separated the dialogue text in the code with variables holding the respective strings. At this time, I thought there could be some other way than gettext. Then I figured how to import localisation data, that the program loads, from external files. The data type is basically a tuple with variable-names associated with strings. This is bit like the file-embed package [3]. Still uncomfortable with i18n, I learned about the article "I18N in Haskell" in yesod blog [4]. I'd like to hear more about it. What is considered the best practice for localisation? -- [1] http://www.haskell.org/haskellwiki/Internationalization_of_Haskell_programs [2] http://hackage.haskell.org/packages/archive/hgettext/ [3] http://hackage.haskell.org/package/file-embed [4] http://www.yesodweb.com/blog/2011/01/i18n-in-haskell

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Hunit Testing
by mukesh tiwari 12 Oct '11

12 Oct '11

Hello everyone I was going through this tutorial http://hunit.sourceforge.net/HUnit-1.0/Guide.html and just wrote some simple code but i am getting error ghci>let test1 = TestCase (assertEqual " equal " 3 ( id 3 )) ghci>let tests = [ TestLabel "test 1" test1 ]ghci>runTestTT tests <interactive>:0:11: Couldn't match expected type `Test' with actual type `[Test]' In the first argument of `runTestTT', namely `tests' In the expression: runTestTT tests In an equation for `it': it = runTestTT tests Could some one please tell me what is wrong with this code. Regards Mukesh Tiwari

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Best bit LIST data structure
by Ryan Newton 12 Oct '11

12 Oct '11

Hi Cafe, We are lucky to have a plethora of data structures out there. But it does make choosing one off hackage difficult at times. In this case I'm *not* looking for a O(1) access bit vector (Data.Vector.Unboxed seems to be the choice there), but an efficient representation for a list of bits (cons,head,tail). Let's say that you want to represent tree indices as you walk down a binary tree. [Bool] is a simple choice, you only add to the front of the list (0/1 = Left/Right), sharing the tails. But [Bool] is quite space inefficient. Something like [Int] would allow packing the bits more efficiently. A Lazy ByteString could amortize the space overhead even more... but in both cases there's a tiny bit of work to do in wrapping those structures for per-bit access. That's probably the right thing but I wanted to check to see if there's something else recommended, perhaps more off-the-shelf. What about just using the Data.Bits instance of Integer? Well, presently, the setBit instance for very large integers creates a whole new integer, shifts, and xors: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Bits.html#s… (I don't know if it's possible to do better. From quick googling GMP seems to use an array of "limbs" rather than a chunked list, so maybe there's no way to treat large Integers as a list and update only the front...) Advice appreciated! Thanks, -Ryan

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Trouble using State Monad.
by Captain Freako 12 Oct '11

12 Oct '11

Hi all, I'm trying to use the State Monad to help implement a digital filter: 17 newtype Filter e a = F { 18 runFilter :: a -> EitherT e (State FilterState) a 19 } deriving (Monad, MonadState FilterState) but I'm getting these compiler errors: Filter.hs:19:14: Can't make a derived instance of `Monad (Filter e)' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `Filter' Filter.hs:19:21: Can't make a derived instance of `MonadState FilterState (Filter e)' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `Filter' If I change the code to this: 17 newtype Filter e a = F { * 18 runFilter :: EitherT e (State FilterState) a ** * 19 } deriving (Monad, MonadState FilterState) it compiles, but I can't figure out how I'd feed the input to the filter, in that case. In comparing this to the tricks used in constructing the State Monad based version of the `Parser' type, I notice that Parser gets around this issue, by having the input (i.e. - input stream) be a part of the initial state, but I'm not sure that's appropriate for a digital filter. Thanks, -db

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hackage-cabal.tar.gz error
by diazepan 11 Oct '11

11 Oct '11

I've installed hoogle and when i type hoogle data, this comes out Extracting tarball... gzip: ../hackage-cabal.tar.gz: invalid compressed data--format violated hoogle: System command failed: gzip --decompress --force ../hackage-cabal.tar.gz What is going wrong? -- View this message in context: http://haskell.1045720.n5.nabble.com/hackage-cabal-tar-gz-error-tp4893434p4… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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The best way to call Java from Haskell?
by dokondr 11 Oct '11

11 Oct '11

Hi, I need to call Stanford NLP Parser from Haskell (unfortunately Haskell does not have a similar one): http://nlp.stanford.edu/software/lex-parser.shtml What would be the most reliable framework for this? Thanks! Dmitri

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wxhaskell : how to generate an event?
by Johannes Waldmann 11 Oct '11

11 Oct '11

Dear all, in wxHaskell (core) I can set and get event handlers http://hackage.haskell.org/packages/archive/wxcore/0.12.1.7/doc/html/Graphi… but how is it possible to create events (programmatically) and somehow feed them into the "main event handling loop"? I think I need this in an application that needs to handle concurrently events that arrive via the GUI (like mouse clicks) and events from an external source (specifically, alsa-midi input). Of course I could handle those "extra events" separately from wxcore, but they should ultimately result in changes to the GUI state, and it feels dangerous to do this without synchronisation. Any hints welcome. Thanks - J.W.

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Haskell vs. Dart
by Kevin Jardine 11 Oct '11

11 Oct '11

After Google's disappointing Dart announcement yesterday, I decided to tweak them a bit and mention Haskell and functional programming languages as an alternative: https://plus.google.com/u/0/111705054912446689620/posts/UcyLBH7RLXs Comments on the post are welcome!

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Is there a DFA library?
by Alex Rozenshteyn 11 Oct '11

11 Oct '11

I couldn't find one on hackage that isn't better described as a RegEx library. I'm looking for things like minimization, completion, etc. kinda like http://www.cis.upenn.edu/~cis639/docs/xfst.html However, I may have just missed it. -- Alex R

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ANN: ircbot 0.1.1
by Jeremy Shaw 11 Oct '11

11 Oct '11

Hello, I have just released a new library on hackage called ircbot. (Because that is what Haskell really needs -- another irc bot library). http://hackage.haskell.org/package/ircbot A demo app is here: http://patch-tag.com/r/stepcut/ircbot/snapshot/current/content/pretty/demo.… The demo is pretty non-thrilling. 98% of it is parsing command-line arguments, and then it calls a simple bot with two parts: pingPart - part that handles server PING/PONG requests dicePart - a simple part that lets you roll dice Here is an example session: <stepcut> synthea: dice 3d4+12 <synthea> You rolled 3 4-sided dice with a +12 modifier: [1,3,4] => 20 The primary reason this library exists is that a long time ago, I needed a way to log the #happs channel. So I coded up a little logging bot using the irc library from hackage. Then later I decided to fork the irc logging out of the happstack.com server itself and into a resuable irc bot library. The library has the following features: 1. a reconnect loop that tries really hard to make sure the bot notices if it is disconnected and tries to reconnect. 2. a channel logging facility that automatically rolls over to a new log file every day. 3. a BotMonad class and BotPartT monad which make it easy to create handlers for incoming messages: http://patch-tag.com/r/stepcut/ircbot/snapshot/current/content/pretty/Netwo… Each bot part is run in its own thread. Incoming messages are delivered to all the parts. That is useful because some parts, like the seen command, need to see every incoming message, even if they do not actively respond to them. Because the parts are handled by different threads, they do not block each other. By default, each part is single-threaded. That is because some parts may need to see multiple messages and see them in the order they originally arrived. However, a part can easily call forkIO to handle each incoming Message and avoid blocking when appropriate. 4. a wrapper that let's you use Parsec to parse an incoming message. It is used by the dicePart: http://patch-tag.com/r/stepcut/ircbot/snapshot/current/content/pretty/Netwo… Using parsec gives you an easy way to specify syntax errors: <stepcut> synthea: dice <synthea> unexpected end of input; expecting dice <num-dice>d<num- sides>[+<modifier>] Future work: 1. The library is based around the old String based irc library. Would be nice to upgrade to something that used ByteStrings+Text+Builder. Practically speaking.. it's IRC. The maximum line length is 510 characters, and the bot typically needs to handle, at most, a few messages per second. So, space and time issues would only be a practical concern if your bot is joining hundreds of channels. But, that is no excused not to use Text :) Perhaps the fastirc library? 2. The channel logging feature needs some minor changes so that more than one channel at a time can be logged 3. the bot needs to do something sane if the nick it wants is already in use. It should also support nick registration. 4. needs automatic help text generation 5. documentation 6. It should be possible to support dynamically reloadable plugins. Most of the technology exists in happstack-plugins + plugins. We just need to finish splitting happstack-plugins into two packages. 7. clustering support. ircbot does not have any built-in persistent storage. It should work fine with acid-state, SQL, etc. When acid- state gets multimaster replication support, it would be nice if there was an automatic failover system. Basically, you could have two or more bots in the channel using acid-state replication. Queries would normally be answered by the primary bot, but if the primary bot goes offline, the secondary bot could answer the queries instead. 8. windows support - the channel logging feature uses the unix package, which is not supported on Windows. The rest of the code is already portable. Anyway. Enjoy! - Jeremy

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PEPM'12 Deadline Extension
by Simon Thompson 11 Oct '11

11 Oct '11

Apologies for multiple postings of this announcement. PEPM'12 - DEADLINE EXTENSION Due to a number of requests for extensions, the deadline for submission to PEPM'12 has been extended until 23:59 GMT on Sunday 16 October. We would like to remind you about three aspects of PEPM. Journal Special Issue There will be a journal special issue for PEPM'12. PEPM'09 has already appeared in Higher-order and Symbolic Computation (HOSC, online-first edition this year), and the special issue based on PEPM'10 papers has just been delivered to the HOSC editorial team. Short papers We're looking for short papers (up to 4pp) and tool papers as well as full research papers. All categories of papers will appear in the formal ACM proceedings. Tool papers The main purpose of a tool paper is to display other researchers in the PEPM community a completed, robust and well-documented tool; more guidance on the format and expected contact is given on the PEPM 2012 web site. In contrast with regular PEPM submissions, PEPM tool demo papers may include work that has been published elsewhere. In the ideal case, the technical foundations of the tool will have been published previously, and the submitted PEPM tool paper will report on follow-on work that has produced a robust tool that has been applied to interesting examples. The PEPM program committee will consider accepting tool demo papers that describe tools that have been presented at other conferences/ workshops if these conferences/ workshops belong to a different community (the authors should acknowledge the previous demos and justify the benefits of presenting the tool again for the PEPM audience). Call For Papers ACM SIGPLAN 2012 Workshop on Partial Evaluation and Program Manipulation January 23-24, 2012. Philadelphia, Pennsylvania, USA (co-located with POPL'12) http://www.program-transformation.org/PEPM12 Paper submission deadline: Sunday, October 16, 2011, 23:59, GMT The PEPM Symposium/Workshop series aims to bring together researchers and practitioners working in the broad area of program transformation, which spans from refactoring, partial evaluation, supercompilation, fusion and other metaprogramming to model-driven development, program analyses including termination, inductive programming, program generation and applications of machine learning and probabilistic search. PEPM focuses on techniques, supporting theory, tools, and applications of the analysis and manipulation of programs. Each technique or tool of program manipulation should have a clear, although perhaps informal, statement of desired properties, along with an argument how these properties could be achieved. Topics of interest for PEPM'12 include, but are not limited to: - Program and model manipulation techniques such as: supercompilation, partial evaluation, fusion, on-the-fly program adaptation, active libraries, program inversion, slicing, symbolic execution, refactoring, decompilation, and obfuscation. - Program analysis techniques that are used to drive program/model manipulation such as: abstract interpretation, termination checking, binding-time analysis, constraint solving, type systems, automated testing and test case generation. - Techniques that treat programs/models as data objects including metaprogramming, generative programming, embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. - Application of the above techniques including case studies of program manipulation in real-world (industrial, open-source) projects and software development processes, descriptions of robust tools capable of effectively handling realistic applications, benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, resource-limited computation, and security. To maintain the dynamic and interactive nature of PEPM, we will continue the category of `short papers' for tool demonstrations and for presentations of exciting if not fully polished research, and of interesting academic, industrial and open-source applications that are new or unfamiliar. Student attendants with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses and other support. All accepted papers, short papers included, will appear in formal proceedings published by ACM Press and will be included in the ACM Digital Library. Selected papers may later on be invited for a journal special issue dedicated to PEPM'12. Submission Categories and Guidelines Authors are strongly encouraged to consult the advice for authoring research papers and tool papers before submitting. The PC Chairs welcome any inquiries about the authoring advice. Regular research papers must not exceed 10 pages in ACM Proceedings style. Short papers are up to 4 pages in ACM Proceedings style. Authors of tool demonstration proposals are expected to present a live demonstration of the described tool at the workshop (tool papers should include an additional appendix of up to 6 extra pages giving the outline, screenshots, examples, etc. to indicate the content of the proposed live demo at the workshop). Important Dates - Paper submission: Sunday, October 16, 2011, 23:59, GMT - Author notification: Tue, November 8, 2011 - Workshop: Mon-Tue, January 23-24, 2012 Invited Speakers - Markus Pueschel (ETH Zurich, Switzerland) - Martin Berger (University of Sussex, UK) Program Chairs - Oleg Kiselyov (Monterey, CA, USA) - Simon Thompson (University of Kent, UK) Program Committee Members - Emilie Balland (INRIA, France) - Ewen Denney (NASA Ames Research Center, USA) - Martin Erwig (Oregon State University, USA) - Sebastian Fischer (National Institute of Informatics, Japan) - Lidia Fuentes (Universidad de Malaga, Spain) - John Gallagher (Roskilde University, Denmark and IMDEA Software, Spain) - Dave Herman (Mozilla Research, USA) - Stefan Holdermans (Vector Fabrics, the Netherlands) - Christian Kaestner (University of Marburg, Germany) - Emanuel Kitzelmann (International Computer Science Institute, USA) - Andrei Klimov (Keldysh Institute of Applied Mathematics, Russian Academy of Sciences) - Shin-Cheng Mu (Academia Sinica, Taiwan) - Alberto Pardo (Universidad de la Repu'blica, Uruguay) - Kostis Sagonas (Uppsala University, Sweden and National Technical University of Athens, Greece) - Anthony M. Sloane (Macquarie University, Australia) - Armando Solar-Lezama (MIT, USA) - Aaron Stump (The University of Iowa, USA) - Kohei Suenaga (University of Kyoto, Japan) - Eric Van Wyk (University of Minnesota, USA) - Kwangkeun Yi (Seoul National University, Korea) Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Three questions to graphviz
by kaffeepause73 11 Oct '11

11 Oct '11

First of all - thanks a lot for this package, graphviz is an awesome tool and having this interface library is really convenient. There a three point where I could use some help: 1. when I try to create a label with e.g.: textLabelValue "Hallo" - the compiler complains he can't match string with data.text.lazy.internal -- I have idea how to create lazy internal text ! 2. I know how to rotate the whole diagram (with landscape or rotate 90), but not how to keep all the text in unrotated position -- is there a command to do this ? 3. When I create a symmetric tree with two directions on the two sides. The tree gets completely messed up when I enter the right directions. (left graph ok but wrong edge dirs, right graph with correct dirs but gemetry scambled). - It has todo with ranking order which is based on the direction of the edges. I can fake it with reversing the arrows in the diagram, but my original graph data is coming directed ... Cheers Phil http://haskell.1045720.n5.nabble.com/file/n4888818/GraphA.jpg http://haskell.1045720.n5.nabble.com/file/n4888818/GraphC.jpg -- View this message in context: http://haskell.1045720.n5.nabble.com/Three-questions-to-graphviz-tp4888818p… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Implementing a new primtype
by Paul Monday 11 Oct '11

11 Oct '11

There seems to be plenty of documentation around on implementing a new primop, much of it needs some tweaking as files have moved and such. I can't seem to find any documentation about how to implement primtypes though. For example, I want to experiment with a new primtype "DV#", my assumption that the type was first declared in the primops.txt.pp seems incorrect. So I'm trying to backtrack a bit and see where primtypes first appear, I'm hoping someone can at least give me a pointer :-) For example: ./compiler/prelude/primops.txt.pp Add the following: primtype DoubleVec# a Compiles fine Add a primop on the type: primop ExtractDoubleVecOp "extractDoubleVec#" GenPrimOp DoubleVec# -> Int# -> Double# And suddenly it's not as friendly. I expected a compile issue since I hadn't added it to CgPrimOps.hs yet, but this is not that compile problem: make -r --no-print-directory -f ghc.mk phase=1 phase_1_builds /usr/bin/gcc -E -undef -traditional -P -Iincludes -x c compiler/prelude/primops.txt.pp | grep -v '^#pragma GCC' > compiler/prelude/primops.txt "inplace/bin/genprimopcode" --data-decl < compiler/prelude/primops.txt > compiler/primop-data-decl.hs-incl "inplace/bin/genprimopcode" --primop-tag < compiler/prelude/primops.txt > compiler/primop-tag.hs-incl "inplace/bin/genprimopcode" --primop-list < compiler/prelude/primops.txt > compiler/primop-list.hs-incl "inplace/bin/genprimopcode" --has-side-effects < compiler/prelude/primops.txt > compiler/primop-has-side-effects.hs-incl "inplace/bin/genprimopcode" --out-of-line < compiler/prelude/primops.txt > compiler/primop-out-of-line.hs-incl "inplace/bin/genprimopcode" --commutable < compiler/prelude/primops.txt > compiler/primop-commutable.hs-incl "inplace/bin/genprimopcode" --code-size < compiler/prelude/primops.txt > compiler/primop-code-size.hs-incl "inplace/bin/genprimopcode" --can-fail < compiler/prelude/primops.txt > compiler/primop-can-fail.hs-incl "inplace/bin/genprimopcode" --strictness < compiler/prelude/primops.txt > compiler/primop-strictness.hs-incl "inplace/bin/genprimopcode" --primop-primop-info < compiler/prelude/primops.txt > compiler/primop-primop-info.hs-incl genprimopcode: ppType: can't handle: TyApp "DoubleVec#" [] make[1]: *** [compiler/primop-primop-info.hs-incl] Error 1 make[1]: *** Deleting file `compiler/primop-primop-info.hs-incl' make: *** [all] Error 2 So, I'm thinking the type needs to be better defined but I haven't tracked it down yet (I'm working on it though ;-) Any clues? Paul Monday Parallel Scientific, LLC. paul.monday(a)parsci.com

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Question: Lazy Incremental Evaluation and Haskell?
by Benjamin Redelings I 10 Oct '11

10 Oct '11

Hi all, I'm not sure this is the right forum for this question. If so, please let me know where else might be more appropriate. My question is, roughly, is there already an existing framework for incremental evaluation in Haskell? That is, if I write a program using Haskell, and then change a small part of this program, can the modified program re-use any results which are calculated by the first, unmodified program? This would be really useful for CPU-intensive statistical software and other scientific computing. For example, if I have the expression (x+y)+z, where x=1, y=2, and z=4, then I need not recalculate (x+y) when z changes. However, (x+y)+z must still be recalculated. This is useful in speeding up statistical software that optimizes a complex function of many arguments, when only one argument changes at a time. It is also useful for Markov chain Monte Carlo (MCMC) since usually one changes only one argument at a time there as well. I haven't seen much work on this using the lambda calculus for this since JH Field's Ph.D. Thesis "Incremental Reduction in the Lambda Calculus". There are a number of programs that represent the calculation as a static DAG (directed, acyclic graph), but this can't handle functions very well. I'm currently trying to write an interpreter that could do this correctly, but I don't want to reinvent the wheel. Can anyone point me to where I could find more information about how to do this in a Haskell framework? thanks! -BenRI -- Benjamin Redelings

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Question on definition of `parse' function in Parsec library.
by Captain Freako 10 Oct '11

10 Oct '11

Hi all, In this definition from the Parsec library: parse :: (Stream s Identity t) => Parsec s () a -> SourceName -> s -> Either ParseError aparse p = runP p () what's the significance of `Identity t'? (`t' isn't used anywhere.) Thanks, -db

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Seeking Feedback on Crypto Learning Project Ideas
by Daniel Kelly 10 Oct '11

10 Oct '11

Hi, I am a Haskell newbie looking for a small to medium size Haskell project that I can use as a learning tool, and would appreciate some feedback. I want to design and build something new, from scratch, that could actually be of practical use and that hasn't been done before (in Haskell, that is). I want to end up with a library package that could be uploaded to Hackage. I also want it to be something cryptography related, since this is a strong interest of mine. I'm particularly interested in exploring the possibilities of being able to pass basic crypto algorithms, to very generic higher-order functions quite naturally in Haskell. I would like to know a) if these ideas have been done (in Haskell) that I'm not aware of, b) which would be most useful/interesting to fellow Haskellers, c) guesstimates of the amount of work involved in each, d) suitability for learning language concepts, and e) any other helpful suggestions you might have Some specific project ideas I have been kicking around that i would appreciate feedback on: 1. An implementation of Shamir's Secret Sharing: http://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing 2. A Haskell binding to LibTomCrypt (Opinions for/against LibTom? - it looks unusually complete.) 3. A generic secure channel, where you pass your choice of basic algorithms (hash, symmetric, asymmetric, compression, etc) to higher order function 4. A Haskell implementation of Trevor Perrin's CryptoID's: http://trevp.net/cryptoID/index.html 5. A Haskell implementation of a generic Proof of Work system: http://en.wikipedia.org/wiki/Proof-of-work_system as a building block for anti-spam/anti-denial-of-service systems such as hashcash. 6. Some other generic high level interface to basic algorithms (looking for ideas here). I would also welcome any other project ideas along these lines that you might have. Thank you in advance. Dan Kelly

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Simple design question using Wai
by Arnaud Bailly 10 Oct '11

10 Oct '11

Hello, I am trying to move a web application I wrote that initially used raw sockets for doing HTTP by "hand" to a more sensible Wai-based framework, and I am running into a design issue. I initially thought it would be a good idea to be able to support various I/O methods so I defined a layer called CommandIO (this is a game application) to insulate business-level code from the detials of reading commands and outputting results to the outside world. Here is the signature for this simple monad: class (Monad io) => CommandIO io where readCommand :: io Command writeResult :: CommandResult -> io () then I have an interpret function which looks like: interpret :: (CommandIO io, Map t) => Commands t io CommandResult which reads a command from the io, execute it producing a CommandResult, then request output from the io using writeResult. This works nicely using Reader-based data containing a Handle, both for HTTP-based and console-based I/O. But when I try to move to WAI, I am a little bit stuck. The core of Wai is an application which rougly has type Request -> Response and this basically breaks my (simple) model of read commands / write results. I suspect there might be a way to stick to my current model using enumerators/streams for I/O and a custom type for encapsulating the underlying state of the HTTP exchange. I also thought about reversing the dependency : ensuring that interpret does not need to know about low-level I/O and only dealing with Command/CommandResult, with I/O being handled by wrappers. Thanks in advance for any insights, Arnaud PS: the code is on github: http://github.com/abailly/crete1941. Not that I deem it ready for prime-time though...

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thespian thoughts
by Simon Michael 09 Oct '11

09 Oct '11

Hi Alexander, I went looking for something to clean up rss2irc's thread management, and your recently released thespian package looks like the simplest, most practical actors/erlang/OTP-ish lib for haskell so far. Thanks! I need to restart threads (actors) in a controlled way when they die or hang. I think some of all of that is also part of Erlang/OTP, and maybe it's already on your todo list. Here are some notes I made, for your interest and in case you have any further thoughts. I thought of adding some actor-transforming combinators like these: -- | Spawn the specified action and restart it whenever it dies, unless -- the restart frequency exceeds any of the given maximum frequencies in -- which case throw an exception. restart :: [Frequency] -> IO a -> IO Address restart fs action = spawn $ do setFlag TrapRemoteExceptions let start times = [] forever t <- getCurrentTime update start times when (any fs exceeded by start times) throw error worker <- spawn action monitor worker receive worker [Case _ -> return ()] -- | Convert hangs to exceptions using a watchdog timer: spawn the -- specified action under the control of a watcher which monitors it and -- also kills it and throws an exception if the worker ever fails to -- send (something) to the watcher within the specified interval. watchdog :: Seconds -> (Address -> IO a) -> IO Address watchdog t action = spawn $ do watcher <- self worker <- spawn $ action watcher monitor worker forever receiveTimeout t [Case _ -> return () Default -> throw error ] which I would use something like: main = do (feed, bot) <- ... spawn $ do reader <- restart [MaxPerHour 2] $ watchdog 60 $ feedReader feed bot announcer <- restart [MaxPerHour 2] $ watchdog 60 $ ircAnnouncer bot responder <- restart [MaxPerHour 2] $ watchdog 60 $ ircResponder bot monitor reader monitor announcer monitor responder setFlag TrapRemoteExceptions receive [Case e :: RemoteException -> exitFailure (show e) -- one of the above died or hung too frequently ] but the types aren't right. I'm not sure if these combinators are possible with the current thespian api. Also they would complicate things, eg what is the address of a restarted actor ? On second thoughts it might be better if these features were built in to the library. I imagine something like: main = do ... reader <- spawnWith [RestartSpec [MinInterval (minutes 5), MaxPerHour 3, MaxPerWeek 6], Watchdog (seconds 60)] $ feedReader feed bot feedReader = do forever poll feed, do stuff resetWatchdog What do you think ? Best - Simon

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Parsec and Text
by Yves Parès 09 Oct '11

09 Oct '11

Hallo Café, Is there a package that allows parsing Text with parsec, or do you have to switch to attoparsec-text or convert your Text to a ByteString? Thanks!

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ANNOUNCE: knob, a library for memory-backed handles
by John Millikin 09 Oct '11

09 Oct '11

http://hackage.haskell.org/package/knob This is a small package which allows you to create memory-backed handles. I found it as a pattern in a few of my test suites, so I spent a day or so polishing it up before posting it to the internet. Feel free to play around with it and tell me about any issues that pop up, or things you wish it could do. The interface is very simple; here's a taste: -------------------------------------------------------------------------- import Data.ByteString (pack) import Data.Knob import System.IO main = do -- for this example, we'll start with an empty knob knob <- newKnob (pack []) -- write to it withFileHandle knob "test.txt" WriteMode $ \h -> do hPutStrLn h "Hello world!" bytes <- Data.Knob.getContents knob putStrLn ("Wrote bytes: " ++ show bytes) -- read from it withFileHandle knob "test.txt" ReadMode $ \h -> do line <- hGetLine h putStrLn ("Got line: " ++ show line) --------------------------------------------------------------------------

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Call for Contributions - Haskell Communities and Activities Report, November 2011 edition
by Janis Voigtländer 08 Oct '11

08 Oct '11

Dear all, I would like to collect contributions for the 21st edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 November 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 May 2011 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/11-2011/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2011/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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ANNOUNCE: Parsec 3.1.2
by Antoine Latter 08 Oct '11

08 Oct '11

Parsec is a monadic combinator library that is well-documented, simple to use, and produces good error messages. This is a minor point release over Parsec 3.1.1. The changes in this release are: >From Roman Cheplyaka: - 'lookAhead' no longer counts as 'consuming input', even when it succeeds. - Fixes for 'try' related to error position reporting - Fix for merging error messages Other changes: - Fix reserved name recognition for case-insensitive languages - Add instances for 'Stream Text' (both lazy Text and strict Text), allowing Char parsers to run on the Text type. Link on Hackage: http://hackage.haskell.org/package/parsec-3.1.2 Notes: The new instances are orphaned in their own new modules, so there shouldn't be conflicts for projects defining their own orphan instances. Thanks to: Roman Cheplyaka for development, Andrew U. Frank for a bug report. Antoine

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Problem on using class as type.
by Magicloud Magiclouds 08 Oct '11

08 Oct '11

Hi, I have a function: post :: (ToJson p, FromJson q) => String -> String -> String -> Map.Map String p -> IO q Now I'd like to call it like: r <- post site token "user.addMedia" (Map.fromList [ ("users", users :: ToJson) , ("medias", medias :: ToJson) ]) So I got the problem. If I used things like "users :: ToJson", then class used as a type error occurred. But if I did not use them, since users and medias were actually different types, then fromList failed, required the type of medias the same with users. How to resolve the conflict? -- 竹密岂妨流水过山高哪阻野云飞

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Re: [Haskell-cafe] Composing a list of Enumeratees
by oleg＠okmij.org 08 Oct '11

08 Oct '11

Roma'n Gonza'lez wrote > how can one compose a list of enumeratees, is it even possible? It is possible. Composition of enumeratees is interesting because -- there are several, distinct and useful ways of doing it, -- one of the composition methods, shell-like pipelining, requires higher-rank types. Let us step back to Enumerators and Iteratees. Iteratee is a monad and composes as a monad. A list of Iteratees can be fused into a single Iteratee using the ordinary Control.Monad.sequence (or sequence_). Enumerator, an iteratee transformer, is a function of the type Iteratee el m a -> m (Iteratee el m a) which fits the patters (t -> m t) for some monad m. Enumerators thus compose as ordinary `monadic functions' using Kleisli composition: > (>>>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c) > f >>> g = \x -> f x >>= g (the operator (>>>) is defined in Control.Category). A list of enumerators is composed with foldl1 (>>>), which is the same as foldr1 (>>>) since (>>>) is associative. As a running example of the list of enumeratees to compose we take > takes = [take 2, take 3, take 4] Granted, the example is a bit contrived: normally we want to compose a variety of enumeratees rather than the single take. On the other hand, `take n' makes a neater example. Enumeratees are enumerators, and so compose as enumerators, through Kleisli composition: > c1 :: Monad m => Enumeratee el el m a > c1 = foldl1 (>>>) takes Composing enumerators `concatenates' their sources. Therefore, c1 is equivalent to take (2+3+4): > c1r = runIdentity $ run =<< run =<< enum_pure_1chunk [1..15] (c1 stream2list) > -- [1,2,3,4,5,6,7,8,9] If e_j is an enumeratee and i is an iteratee (for the inner stream), (e_i i) is an iteratee for the outer stream. We may use various iteratee compositions to compose (e_j i). For example, we use sequential composition > c2s :: Monad m => Iteratee el m a -> Iteratee el m [a] > c2s = \i -> sequence $ map (\e -> runI =<< e i) takes > c2sr = runIdentity $ run =<< enum_pure_1chunk [1..15] (c2s stream2list) > -- [[1,2],[3,4,5],[6,7,8,9]] or parallel composition > c2p :: Monad m => Iteratee el m a -> Iteratee el m [a] > c2p = \i -> parallel $ map (\e -> runI =<< e i) takes > where > parallel [i] = liftM (: []) i > parallel (i:t) = do > (iv,tv) <- enumPair i (parallel t) > return (iv:tv) > > c2pr = runIdentity $ run =<< enum_pure_1chunk [1..15] (c2p stream2list) > -- [[1,2],[1,2,3],[1,2,3,4]] Finally, we can compose enumeratees `telescopically': enumeratees are stream converters, which can be arranged into a pipeline -- very much like the Unix Shell pipeline -- to convert the elements of the outer stream further and further. > pipe :: Monad m => > (forall a. Enumeratee el1 el2 m a) -> Enumeratee el2 el3 m a -> > Enumeratee el1 el3 m a > pipe e12 e23 = \i3 -> e12 (e23 i3) >>= runI (see the source code for the step-wise derivation of that composition). > c4 :: Monad m => Enumeratee el el m a > c4 = take 4 `pipe` (take 3 `pipe` take 2) Piping 'take n_j' into each other is not very interesting. It is easy to see that the result is equivalent to the single take (minimum [n_1,...n_j]). > c4r = runIdentity $ run =<< enum_pure_1chunk [1..15] (runI =<< c4 stream2list) > -- [1,2] > -- indeed, c4 behaves like take (minimum [2,3,4]) == take 2 To perform the pipeline composition on the list of enumeratees via fold we need impredicative polymorphism -- or at least its emulation, using the newtype trick. The trick makes type abstractions and applications explicit. > newtype EI el1 el2 m = EI{unEI :: forall a. Enumeratee el1 el2 m a} > takes' = [EI (take 2), EI (take 3), EI (take 4)] > c4' = foldl1 (\e1 e2 -> EI (unEI e1 `pipe` unEI e2)) takes' > c4'' = foldr1 (\e1 e2 -> EI (unEI e1 `pipe` unEI e2)) takes' > -- both are equivalent to take (minimum [2,3,4]) The complete source code for the article is available at http://okmij.org/ftp/Haskell/Iteratee/Compose.hs

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SplitObjs and LLVM
by Mathijs Kwik 07 Oct '11

07 Oct '11

Hi all, Is it a known issue that -fllvm doesn't produce split objs? I tried ghc 7.0.3 with llvm 2.8 and ghc 7.2.1 with llvm 2.9. Thanks, Mathijs

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Parallel Haskell Digest 6
by Eric Y. Kow 06 Oct '11

06 Oct '11

Parallel Haskell Digest ======================= Edition 6 2011-10-06 Hello Haskellers! This edition of the Parallel Haskell Digest kicks off with some [warm feelings][n1] from BioHaskeller Ketil Malde > Blessing of the day goes to the authors of threadscope. In fact, let me look > it up: Donnie Jones, Simon Marlow, and Satnam Singh. I had an old STM > experiment lying around, but couldn't quite make it go faster than the single > threaded version. > > Today I installed threadscope, and a glance at the output sufficed to > identify the problem, and now at least it's faster. Great tool! (But you all > knew that, of course) Thanks indeed to the original authors of ThreadScope! Ketil made good use of the original ThreadScope. Since the original release, the folks at Well-Typed have taken over development on ThreadScope (Andres, Duncan, and Mikolaj) and are working to extend it so that it becomes even easier to squash those pesky performance bugs. These extensions, much like this digest, are made possible by the [Parallel GHC Project][n0]. We hope you'll love the results. News ---------------------------------------------------------------------- [Data Parallelism in Haskell][n2] slides and video Manuel Chakravarty came all the way from Sydney (that's 931 km!) to tell the Brisbane FP Group about work going on at UNSW on data parallel programming in Haskell. The talk covers a lot of ground. It's an hour and a half long and > [It] motivates the use of functional programming for parallel, > and in particular, data parallel programming and explains the > difference between regular and irregular (or nested) data > parallelism. It also discusses the Repa library (regular data > parallelism for multicore CPUs), the embedded language Accelerate > (regular data parallelism for GPUs), and Data Parallel Haskell > (nested data parallelism for multicore CPUs). Both video and slides are available, so check it out! Word of the Month ---------------------------------------------------------------------- The word of the month is *dataflow* as in dataflow parallelism. (And if you've just seen Manuel's talk, not to be confused with data paralellism). With dataflow, we will be wrapping up our now four-part series on parallel programming in Haskell. In recent words of the month, we've been looking at various approaches for parallelising your way to faster code. We started with the parallel arrays library Repa, then moved on to the Haskell `par` and `pseq` primitives, and built from there to get Strategies. Now we'll be looking at `monad-par`, a library for *dataflow* parallelism. First, the bigger picture: why do we have another way of doing parallel programming in Haskell when we already have parallel arrays and Strategies? Part of the answer is parallelism is still a research topic, with new ideas coming out from time to time and some old ideas slowly withering away. For another part of the answer, it may help to think in terms of a trade-off between implicitness and performance, in other words, between Easy and Fast. ![Degrees of implicitness](parallelism.png) Not all problems are subject to the trade-off in the same way. It's also not always clear how they are. As a rough sketch, problems that require applying the same operation on a large number of simple object can get fast performance from a parallel arrays library. This is a form of data parallelism. If the problem does not lend itself to data parallelism, the next port of call is likely Strategies. If Strategies are not adequate for the desired speedups, the problem may require an even more explicit approach. Until recently, this meant “rolling your own” parallelism by using Concurrent Haskell: forking off threads, assigning tasks to them, and communicating finished work between them. Using concurrency for DIY parallelism is risky. It means venturing into IO, giving up determinism, exposing ourselves to all manner of side-effects, not to mention subtle concurrency bugs like deadlock. As Haskellers, we should demand better: safer, more predictable and easier to reason about. We want to have explicit fine-grained control without all the nasty side-effects. And now we can have it! The latest addition to the parallel Haskell quiver is the monad-par library. Programming in monad-par looks a lot like programming in Concurrent Haskell: data Par a instance Monad Par runPar :: Par a -> a fork :: Par () -> Par () data IVar a new :: Par (IVar a) put :: NFData a => IVar a -> a -> Par () get :: IVar a -> Par a Instead of using `forkIO`, we use `fork` to create threads (not sparks!) and instead of using `MVar`s to communicate, we use `IVar`s. `MVar` and `IVar` behave in somewhat similar ways, in that any attempt to read from an `IVar` will wait until it has been filled. But this is also where differences emerge. Unlike their concurrent counterparts `IVar` may only be written to once; subsequent writes result in an error. This aligns with our desire for determinism. We never have to worry about the `IVar` changing values over time. Moreover, the `Par` monad does not allow for allow for any `IO` (feature! not a bug) and computations in the `Par` monad can always be extracted with a simple `runPar`. But this doesn't really tell us all that much about how to use this library. For now we come back to our word *dataflow*. The dataflow model treats program as a little black box, an input goes in, an output comes out, and what happens in between is immaterial: ![Input goes in, output comes out](dataflow-basic.png) The program is represented as a directed graph (a *dataflow network*), with each node representing an individual operation (black boxes in their own right), and connections between the nodes expressing dependencies. In the graph above, `g` and `h` depend on the output of `f` (the same output - there is only one) and in turn, `i` depends on the output of both `g` and `h`. ![Dataflow network](dataflow-network.png) Dataflow networks are handy thinking tools because one of the things that makes it hard to get fast parallel code is the inherent sequentially that dependencies introduce. A dataflow network nicely captures which parts of the program are parallel (`g` and `h` are independent of each other) and which parts are sequential (`g` depends on `f`). This is a sort of parallelism topline; we can't go any faster than what this network allows, but at the very least we should take the shape of the network into account so we don't make matters worse and find ourselves waiting a lot more on dependencies than is strictly necessary. Using the `Par` monad essentially consists in translating dataflow networks into code. Suppose we have functions f :: In -> A g :: A -> B h :: A -> C j :: B -> C -> Out For the graph above, we might say: network :: IVar In -> Par Out network inp = do [vf,vg,vh] <- sequence [new,new,new] fork $ do x <- get inp put vf (f x) fork $ do x <- get vf put vg (g x) fork $ do x <- get vf put vh (h x) x <- get vg y <- get vh return (j x y) It's useful to note that here dataflow graphs need not be static; they may be dynamically generated to fit the shape of the input. To see this and also a more realistic taste of monad-par, check out Simon Marlow's [Parallel and Concurrent Programming Haskell][sm-tutorial] tutorial. It shows how a parallel type inferencer might be implemented, its dataflow graph arising from the set of bindings it receives as input. Finally what about Strategies? How do we choose between the two? Both are IO-free and deterministic, both require some but not a whole lot of comfort with monads. `Par` may be easier to use correctly than Strategies because it is strict and forces you to explicitly delineate the dependencies in parallel computations. This also makes it more verbose on the other hand. Aside from being more verbose, `Par` can be a little bit “messier” than Strategies because you don't get the same separation between algorithm and parallelism. And since we're ultimately concerned with performance, it's worth knowing that the `Par` monad currently involves more overhead than the `Eval` monad from Strategies. The latter may be more appropriate at finer granularities. Perhaps one way to go about this is to start off with Strategies as they have been around longer and may require less invasive modifications to your code. But if you find Strategies to be difficult, or you just can't get the performance you want, don't reach for those `forkIO` yet. Give `Par` a try. For more about monad-par, do visit [Simon's tutorial][sm-tutorial] and the [API on hackage][mp-haddock]. To dig deeper, you may also find it instructive to read [A Monad of Deterministic Parallelism][mp-paper] (Marlow et al), which was presented at this year's Haskell Symposium. Related to this are his CUFP [monad-par slides][mp-cufp] tutorial slides. This concludes our tour of current approaches to Haskell parallelism. There is some new technology on the horizon which we may cover in a later digest, particularly Data Parallel Haskell and nested data parallelism (*data* parallelism not to be confused with *dataflow* parallelism), but for next month, let's take a break from parallelism and look at something a little different. Parallel GHC Project Update ---------------------------------------------------------------------- The Parallel GHC Project is an MSR-funded project, run by Well-Typed, with the aim of demonstrating that parallel Haskell can be employed successfully in real world projects. This has been a month for releasing software. We've recently put on Hackage updates to ThreadScope and some helper libraries * ThreadScope (0.2.0), with a new spark profiling visualisation, bookmarks, and other enhancements * gtk2hs (0.12.1), adding GHC 7.2 compatibility, * ghc-events (0.3.0.1), adding support for spark events (GHC 7.3 needed for this) Duncan Coutts presented [our recent work on ThreadScope][ts-talk] at the Haskell Implementors Workshop in Tokyo (23 Sep). He talked about the new spark visualisation feature which show a graphical representation of spark creation and conversion statistics with a demo inspired by a program we are working on for Parallel GHC. Meanwhile, we have also completed a pure Haskell implementation of the “Modified Additive Lagged Fibonacci” random number generator. This generator is attractive for use in Monte Carlo simulations because it is splittable and has good statistical quality, while providing high performance. The LFG implementation will be released on Hackage when it has undergone more extensive quality testing. Blogs, Papers, and Packages ---------------------------------------------------------------------- * [First Class Concurrency in Haskell][b3] (PDF) (31 Aug) Keegan McAllister posted slides from a talk he gave last year at the Boston Area Haskell Users' Group. His slides give an overview of topics in concurrent imperative programming: - Using closures with concurrency primitives to improve your API - Using MVar to retrieve thread results - How System.Timeout works - Implementing IORef and Chan in terms of other primitives - Implementing the π-calculus, and compiling the λ-calculus to it You might find Keegan's slides to be a useful introduction to Haskell Concurrency. * [Lambda to pi][b1] (9 Sep) “If the λ-calculus is a minimal functional language, then the π-calculus is a minimal concurrent language”. Following up on his recent slides, Keegan wrote a follow-up post elaborating on his pi-calculus interpreter and lambda-calculus to pi-calculus compiler. If you want to run his blog post just pass the [lhs file][lam2pi] to GHCi. (note that I had to use `-hide-package=monads-tf`) * [Sirkle][b4] (17 Sep) Morten Olsen Lysgaard released Sirkle. It provides an implementation of the Chord [Distributed Hash Table][DHT], and a simple storage layer with fault tolerance and replication similar to DHash. The Sirkle DHT implementation uses Cloud Haskell, which we saw some interest about in the [last edition][phd5] of this digest. Sirkle is available on hackage and GitHub. “If you think distributed computing is fun”, Morten invites us, “and would love a hobby project, join me in discovering what can be done with a DHT in Haskell!” * [ThreadScope][b6] (5 Sep) Duncan Coutts released ThreadScope 0.2.0, the performance visualiser that we saw Ketil used to make his STM experiment faster. The updated ThreadScope has a new spark profiling visualisation, bookmarks, and other enhancements. Now if only we had some sort of user documentation… Mailing list discussions --------------------------------------------------------------------- * [Performance of concurrent array access][m1] (23 Aug) As a first step to his foray into writing concurrent parallel Haskell programs Andreas Voellmy wrote a bit of code to read that reads and writes concurrently to an IOArray. Unfortunately, the performance with multiple cores was not very good and using ThreadScope did not reveal anything obvious. What did help on the other hand was to switch to IOUArray, which puzzled Andreas. Why would this improve scalability to multiple threads and cores? Andrew Coppin and Brandon Allbery suggested it may have something to do with strictness. Johan Tibbell pointed out some places where Andreas would likely want to add a bit of strictness to his original IOArray version. But this only seems to have helped slightly. Anybody else have an idea? * [(Repa) hFlush: illegal operation][m2] (24 Aug) Michael Orlitzky is using Repa to process MRI data, but when writing the results to file, he gets the error message `spline3: output.txt: hFlush: illegal operation (handle is closed)` Trial and error is a bit hard to do here because it takes 45 minutes to get to the point of failure. Ben Lippmeier offered to have a look if Michael could provide some data to go with the code he posted. Ben also pointed out that the implementation of Repa's text IO functions is naive, and may have problem with massive files. If the data is in some standard binary form, it may be worthwhile to write a Repa loader for it. * [how to read CPU time vs wall time report from GHC?][m3] (14 Aug) Wishnu Prasetya is just getting started with Parallel Haskell. He's having some trouble interpreting runtime statistics from the RTS (eg. `8.97s ( 2.36s elapsed)`). > SPARKS: 5 (5 converted, 0 pruned) > > INIT time 0.02s ( 0.01s elapsed) > MUT time 3.46s ( 0.89s elapsed) > GC time 5.49s ( 1.46s elapsed) > EXIT time 0.00s ( 0.00s elapsed) > Total time 8.97s ( 2.36s elapsed) Why does the CPU time on the left exceed the wall clock time on the right? Iustin Pop pointed out that CPU time accumulates over the mulitple CPUs. The roughly 4:1 ratio suggest Wish must be making 95% use of 4 CPUs (or partial use of more CPUs). This may sound efficient except that at the end of day what counts is the difference in wall clock times. Iustin points out that the MUT/GC times show Wish spending 60% of his time in garbage colection, perhaps a sign of a space leak or an otherwise inefficient algorithm. * [Can't link OpenCL on Windows][m4] (12 Sep) OpenCL (Open Computing Language) is a framework for writing parallel programs that run on GPUs (more generally, on heterogeneous systems with perhaps a mix of CPUs and other processors). Jason Dagit is trying to get the OpenCLRaw bindings to work on Windows. On link time, however, he gets undefined symbol errors for everything he uses in the OpenCL API. Mystery solved: Jason later reported that the binding was set to use ccall whereas OpenCL uses stdcall. * [Turn GC off][m5] (14 Sep) and [Build local-gc branch of GHC][m6] (26 Sep) Andreas Voellmy (who we saw above with IOArray woes) is writing a server handling multiple long-lived TCP connections with a bit of shared state among connections. Ideally, it'd be a case of more cores = more clients, but the current stop-the-world garbage collection in GHC makes this expensive. His first thought was to completely disable garbage collection, and just run until he is out of memory. Perhaps there is an alternative. David Barbour commented that controlling the amount of live memory is important here, much more than avoiding allocations. Austin Seipp pointed us to recent work on the [local-gc branch of GHC][local-gc] (see the paper [Multicore Garbage Collection with Local Heaps][local-gc-paper]). This work has not been merged into GHC though (as it's not clear if the improvements are worth the complexity increase). Andreas decided to give it a shot. Unfortunately, he can't get it to buid. Help? * [A Missing Issue on Second Generation Strategies][m7] (24 Sep) Burak Ekici is trying to parallelize RSA encryption and decryption with Strategies, but all his sparks keep getting pruned! Antoine Latter noticed that Burak's strategy wasn't actually doing anything to its input; it simply sparks off computations (themselves evaluated with `rdeepseq`) that nobody ever looks at. Daniel Fischer made a similar point, providing some improved code with a refactor and clean up of repeated code along the way. * [Parallel Haskell Digest 5 followup][p1] Following up on the last word of the month (Strategies). Kevin Hammond observed that the first `parMap` example interweaves behavioural and functional code, defeating the point of strategies. The issue here is pedagogical: although I later follow up with the version using `parList rseq`, I run the risk of people simply copy and pasting while missing out on the benefits. Thanks for the feedback, Kevin! I'll see if I can avoid the try-the-wrong-way-first style in the future. As a more general note, feedback people give, particularly criticism and suggestions for improvement are very valuable, so keep it coming, folks! Stack Overflow and Reddit ---------------------------------------------------------------------- * [Concurrent reading and writing to IOArray in Haskell][s1] * [What's the best way to write some semaphore-like code in Haskell?][s2] * [Help understanding MVar example in Haskell][s3] * [How to exit from Hackstack Server App?][s4] * [The most useful error message evar! : haskell][r1] * [New IBM CPU has transactional memory built in : haskell][r2] Help and Feedback ---------------------------------------------------------------------- If you'd like to make an announcement in the next Haskell Parallel Digest, then get in touch with me, Eric Kow, at <parallel(a)well-typed.com>. Please feel free to leave any comments and feedback! [sm-tutorial]: http://community.haskell.org/~simonmar/par-tutorial.pdf [mp-paper]: http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/monad… [mp-haddock]: http://hackage.haskell.org/package/monad-par [mp-cufp]: http://community.haskell.org/~simonmar/slides/CUFP.pdf [lam2pi]: http://ugcs.net/~keegan/code/pi-calc.lhs [DHT]: http://en.wikipedia.org/wiki/Distributed_hash_table [phd5]: http://www.well-typed.com/blog/58 [ts-talk]: http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2011/Coutts [local-gc]: https://github.com/ghc/ghc/tree/local-gc [local-gc-paper]: http://research.microsoft.com/~simonpj/papers/parallel/local-gc.pdf [n0]: http://www.haskell.org/haskellwiki/Parallel_GHC_Project [n1]: https://plus.google.com/108473609018577106562/posts/QHYorSiZr89 [n2]: http://justtesting.org/video-and-slides-of-data-parallelism-in-haske [b1]: http://mainisusuallyafunction.blogspot.com/2011/09/lambda-to-pi.html [b3]: http://ugcs.net/~keegan/talks/first-class-concurrency/talk.pdf [b4]: http://mortenlysgaard.com/?p=35 [b5]: http://bos.github.com/strange-loop-2011/talk/talk.html [b6]: http://hackage.haskell.org/package/threadscope [m1]: http://www.haskell.org/pipermail/haskell-cafe/2011-August/094814.html [m2]: http://www.haskell.org/pipermail/haskell-cafe/2011-August/094863.html [m3]: http://www.haskell.org/pipermail/haskell-cafe/2011-August/094639.html [m4]: http://www.haskell.org/pipermail/haskell-cafe/2011-September/095289.html [m5]: http://www.haskell.org/pipermail/haskell-cafe/2011-September/095344.html [m6]: http://www.haskell.org/pipermail/haskell-cafe/2011-September/095616.html [m7]: http://www.haskell.org/pipermail/haskell-cafe/2011-September/095589.html [p1]: https://groups.google.com/d/topic/parallel-haskell/dhMkpgdswt0/discussion [s1]: http://stackoverflow.com/questions/7194826/concurrent-reading-and-writing-t… [s2]: http://stackoverflow.com/questions/7216149/whats-the-best-way-to-write-some… [s3]: http://stackoverflow.com/questions/7256860/help-understanding-mvar-example-… [s4]: http://stackoverflow.com/questions/7305625/how-to-exit-from-hackstack-serve… [r1]: http://www.reddit.com/r/haskell/comments/k764g/the_most_useful_error_messag… [r2]: http://www.reddit.com/r/haskell/comments/jperf/new_ibm_cpu_has_transactiona… -- Eric Kow <http://erickow.com>

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Fwd: Is it possible to represent such polymorphism?
by Alberto G. Corona 06 Oct '11

06 Oct '11

if Hlist is sugarized as variable length tuples, then the initial code would compile without noticing the use of HList... 2011/10/5 Felipe Almeida Lessa <felipe.lessa(a)gmail.com> > On Wed, Oct 5, 2011 at 8:45 AM, Alberto G. Corona <agocorona(a)gmail.com> > wrote: > > If a newbie considers this as something natural, this is another reason > for > > syntactic sugaring of HList: > > http://www.haskell.org/pipermail/haskell-cafe/2011-April/090986.html > > Exposing newbies to HList seems like a recipe for disaster for me =). > > -- > Felipe. >

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Re: [Haskell-cafe] ANNOUNCE: HaTeX 3 - First release
by Daniel Díaz Casanueva 06 Oct '11

06 Oct '11

It seems there is an error in the hackage URL's. Working URL's: http://hackage.haskell.org/package/HaTeX http://hackage.haskell.org/package/HaTeX-meta Sorry!

1 0

ANNOUNCE: HaTeX 3 - First release
by Daniel Díaz Casanueva 06 Oct '11

06 Oct '11

Hello, folks! I have just released HaTeX 3 to Hackage: http://hackage.haskell.org/HaTeX It is a first version, not well documented yet, but now you can test it with a simple "cabal install hatex". What is HaTeX? HaTeX consists in a set of combinators which allow you to build LaTeX code, following the LaTeX syntax in a type-safe manner. This allows you to build programs which generates LaTeX code automatically for any purpose you can figure out. Further reading about HaTeX 3: * http://deltadiaz.blogspot.com/2011/09/hatex-3-previous-notes.html * http://deltadiaz.blogspot.com/2011/10/hatex-3-two-styles.html I wanted to make the release before end the proper documentation because: 1) This allow people to test the library (and hence, help me to build the documentation if any feedback comes). 2) I am too busy currently and I can't expend to HaTeX the time I'd like. The library may need some improvements, but it worked well for my own tests. With HaTeX, I uploaded the HaTeX-meta package. This package is needed if you want to develope a subset of the HaTeX modules, because they are automatically generated by this application. Anyway, there are notes in the API reference warning this. HaTeX-meta in Hackage: http://hackage.haskell.org/HaTeX-meta A continously updated version of the source code from these packages can be found at: http://dhelta.net/hprojects/HaTeX/code http://dhelta.net/hprojects/HaTeX-meta/code Thanks for reading, Daniel Díaz.

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How to install haddock with ghc-7.2.1?
by Paulo Pocinho 06 Oct '11

06 Oct '11

Hi list. Installing haddock fails with ghc-7.2.1. How do I get to install it along with all dependencies? C:\>cabal install haddock Resolving dependencies... Downloading ghc-paths-0.1.0.8... [1 of 1] Compiling Main ( C:\Users\user\AppData\Local\Temp\ghc-paths-0.1.0.86336\ghc-pths-0.1.0.8\Setup.hs, C:\Users\user\AppData\Local\Temp\ghc-paths-0.1.0.86336\ghc-paths-0.1.0.8\dis \setup\Main.o ) Linking C:\Users\user\AppData\Local\Temp\ghc-paths-0.1.0.86336\ghc-paths-0.1.0.8\dist\setup\setup.exe ... Configuring ghc-paths-0.1.0.8... Building ghc-paths-0.1.0.8... Preprocessing library ghc-paths-0.1.0.8... [1 of 1] Compiling GHC.Paths ( GHC\Paths.hs, dist\build\GHC\Paths.o ) Registering ghc-paths-0.1.0.8... Installing library in C:\Users\user\AppData\Roaming\cabal\ghc-paths-0.1.0.8\ghc-7.2.1 Registering ghc-paths-0.1.0.8... Downloading xhtml-3000.2.0.4... Configuring xhtml-3000.2.0.4... Preprocessing library xhtml-3000.2.0.4... Building xhtml-3000.2.0.4... [ 1 of 15] Compiling Text.XHtml.BlockTable ( Text\XHtml\BlockTable.hs, dist\build\Text\XHtml\BlockTable.o ) Implicit import declaration: base:Prelude can't be safely imported! The package (base) the module resides in isn't trusted. cabal: Error: some packages failed to install: haddock-2.9.4 depends on xhtml-3000.2.0.4 which failed to install. xhtml-3000.2.0.4 failed during the building phase. The exception was: ExitFailure 1

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Science, FRP and Haskell
by Tom Nielsen 06 Oct '11

06 Oct '11

Dear cafe, I am please to announce that our paper on using FRP (and Haskell!) in physiological experimentation and analysis has now been published in Journal of the Royal Society Interface. Since most people on this list probably don't read that journal on a regular basis, I thought a quick note to the list would be appropriate. The full PDF is available here: http://rsif.royalsocietypublishing.org/cgi/reprint/rsif.2011.0616?ijkey=vFo… If this link doesn't work, just email me for a copy. I'll provide a brief summary here: -The main point of the paper is that there is a large conceptual overlap between FRP and the observation and analysis of physiological data. We show that many different quantities map on to Signals or Events (we introduce a third type scheme, Durations, similar to Events but with a start and an end time). These types can be used not only for data but also for meta-data. In fact, we make no distinction between the two. -We have developed a non-embedded functional programming language with first-class signals and events for executing experiments defined as functional reactive programs. A key innovation is that differential equations can be introduced by pattern matching on the derivative of a signal. One back-end supports the execution of some programs under a real-time (LXRT) environment. -We have carried out two non-trivial neurophysiology experiments to demonstrate the generality, concision and practical usefulness of our approach. -All code used for data acquisition, stimulus generation and presentation, analysis and producing figures was written in Haskell or in programming languages implemented in Haskell. I stress that the paper is about ideas and does not describe a mature software package. I am reimplementing a lot of those ideas and integrating them with Bayesian inference, such that we can analyse data in terms of physical, probabilistic models. This more mature language is called BAYSIG. here is the code described in the paper, in all it's messiness: https://github.com/glutamate/bugpan The repository for BAYSIG is here https://github.com/glutamate/baysig Both of these packages depend on a couple of unreleased cabal packages that can be found in my github account. Regards, Tom

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How to mark a package obsolete on Hackage?
by Geoffrey Mainland 06 Oct '11

06 Oct '11

How does one mark a package obsolete on Hackage? For example, monads-fd no longer appears on the main web page listing available packages, but it still exists on Hackage (but is marked obsolete [1]). I have several package that are similarly obsolete due to mtl2 and would like to flag them appropriately. Thanks, Geoff [1] http://hackage.haskell.org/package/monads-fd

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Haskell Weekly News: Issue 202
by Daniel Santa Cruz 06 Oct '11

06 Oct '11

Welcome to issue 202 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of September 25 to October 1, 2011. New and Updated Projects * ParserFunction (Enzo Fabrizio; 0.0.5) Provides utilities for parsing and evaluating mathematical expressions. [1] http://goo.gl/xNOe1 * flexiwrap (Iain Alexander; 0.1.0) Also updates to related packages: data-type, function-combine, and flexiwrap-smallcheck. [2] http://goo.gl/dN0ew * crypto-api-tests (Thomas DuBuisson) These tests have been split out into their own package. Tests now use the test-framework package. [3] http://goo.gl/7kUIV * hledger (Simon Michael; 0.16) Stability/bugfix/polish release. [4] http://goo.gl/hW085 * GA (Kenneth Hoste; 1.0) Major version bump to this genetic algorithms package. [5] http://goo.gl/NoAl4 Quotes of the Week * glguy: lazy evaluation is best 50% of time because there are 2 cases, the cases where it is the best and the cases where it is not the best * Cale: Computing lengths is admitting defeat * syntaxfree: Many people think the core ideas behind Haskell [snip] In the Deep Complex Universe some Gnomes were having Tangling problems. Wadler & friends solved their problems and as a reward the Tangled Gnomes gave them the Secret of Haskell * monochrom: How do I extract the IO out of IO String? Top Reddit Stories * GHC-specific alias analysis for LLVM Domain: blog.omega-prime.co.uk, Score: 74, Comments: 4 On Reddit: [6] http://goo.gl/kNXp9 Original: [7] http://goo.gl/851qj * GHC Projects for All Domain: dterei.blogspot.com, Score: 45, Comments: 2 On Reddit: [8] http://goo.gl/X3jpJ Original: [9] http://goo.gl/FpnUP * The Evolution of a Haskell Programmer Domain: willamette.edu, Score: 41, Comments: 17 On Reddit: [10] http://goo.gl/4WQyK Original: [11] http://goo.gl/gbNsH * Stanford CS240h, Functional Systems in Haskell course materials available on GitHub Domain: github.com, Score: 29, Comments: 3 On Reddit: [12] http://goo.gl/5aL1X Original: [13] http://goo.gl/EEflc * Introduction to programming with shift and reset (Tutorial by Oleg) Domain: okmij.org, Score: 29, Comments: 1 On Reddit: [14] http://goo.gl/Ogsjo Original: [15] http://goo.gl/xvVmQ * A few Haskell notes from the Strange Loop conference Domain: serpentine.com, Score: 27, Comments: 2 On Reddit: [16] http://goo.gl/qx4qn Original: [17] http://goo.gl/nplIs * Par Monad Slides from CUFP Domain: community.haskell.org, Score: 27, Comments: 0 On Reddit: [18] http://goo.gl/pFkgw Original: [19] http://goo.gl/Ax4bW * Slides from the CUFP 2011 Snap Framework Tutorial Domain: gregorycollins.net, Score: 23, Comments: 0 On Reddit: [20] http://goo.gl/uVrlv Original: [21] http://goo.gl/myRYa * Haskell Job Opportunity at Sensor Sense Domain: self.haskell, Score: 22, Comments: 1 On Reddit: [22] http://goo.gl/PpSdW Original: [23] http://goo.gl/PpSdW * Slides from my Haskell Implementors Workshop talk Domain: blog.johantibell.com, Score: 20, Comments: 3 On Reddit: [24] http://goo.gl/sgmzC Original: [25] http://goo.gl/BT4H0 Top StackOverflow Questions * Learning Haskell with a view to learning Scala votes: 23, answers: 4 Read on SO: [26] http://goo.gl/8BnAv * Why are the functions making Vector an instance of Functor, Monad, Applicative, Alternative, Foldable and Traversable slow? votes: 11, answers: 2 Read on SO: [27] http://goo.gl/F8sph * Strategies in Scala votes: 10, answers: 1 Read on SO: [28] http://goo.gl/NzUiU * What's the smoothest way to update haskell platform to latest? votes: 8, answers: 2 Read on SO: [29] http://goo.gl/EnG7p * Does Haskell have an equivalent to Sage? votes: 8, answers: 3 Read on SO: [30] http://goo.gl/v7oE8 * Haskell syntax for 'or' in case expressions votes: 7, answers: 3 Read on SO: [31] http://goo.gl/Akl9D * Is operator && strict in Haskell? votes: 6, answers: 3 Read on SO: [32] http://goo.gl/TLe86 Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18942 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18943 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92700 4. http://permalink.gmane.org/gmane.comp.finance.ledger.hledger/521 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92746 6. http://blog.omega-prime.co.uk/?p=135 7. http://www.reddit.com/r/haskell/comments/kwaee/ghcspecific_alias_analysis_f… 8. http://dterei.blogspot.com/2011/09/ghc-project-for-all.html 9. http://www.reddit.com/r/haskell/comments/kwm0g/ghc_projects_for_all/ 10. http://www.willamette.edu/~fruehr/haskell/evolution.html 11. http://www.reddit.com/r/haskell/comments/kv610/the_evolution_of_a_haskell_p… 12. https://github.com/bos/stanford-cs240h 13. http://www.reddit.com/r/haskell/comments/kvjda/stanford_cs240h_functional_s… 14. http://okmij.org/ftp/continuations/index.html#tutorial 15. http://www.reddit.com/r/haskell/comments/kxg3o/introduction_to_programming_… 16. http://www.serpentine.com/blog/2011/09/27/the-strange-loop-conference-was-a… 17. http://www.reddit.com/r/haskell/comments/ktscd/a_few_haskell_notes_from_the… 18. http://community.haskell.org/~simonmar/slides/CUFP.pdf 19. http://www.reddit.com/r/haskell/comments/ky3cw/par_monad_slides_from_cufp/ 20. http://gregorycollins.net/posts/2011/10/01/cufp-tutorial-slides 21. http://www.reddit.com/r/haskell/comments/kxkex/slides_from_the_cufp_2011_sn… 22. http://www.reddit.com/r/haskell/comments/kt5g4/haskell_job_opportunity_at_s… 23. http://www.reddit.com/r/haskell/comments/kt5g4/haskell_job_opportunity_at_s… 24. http://blog.johantibell.com/2011/09/slides-from-my-haskell-implementors.html 25. http://www.reddit.com/r/haskell/comments/kti89/slides_from_my_haskell_imple… 26. http://stackoverflow.com/questions/7594337/learning-haskell-with-a-view-to-… 27. http://stackoverflow.com/questions/7583403/why-are-the-functions-making-vec… 28. http://stackoverflow.com/questions/7574937/strategies-in-scala 29. http://stackoverflow.com/questions/7546949/whats-the-smoothest-way-to-updat… 30. http://stackoverflow.com/questions/7556053/does-haskell-have-an-equivalent-… 31. http://stackoverflow.com/questions/7603509/haskell-syntax-for-or-in-case-ex… 32. http://stackoverflow.com/questions/7612580/is-operator-strict-in-haskell

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ANN: hit a reimplementation of git storage in haskell.
by Vincent Hanquez 05 Oct '11

05 Oct '11

Hi Haskellers, I just want to announce the hit project [1], which is a reimplementation of low level git operations to read *AND* write to a git repository. It support reading from anything i threw at it (loose objects, packed objects, deltas), a subset of revisions specifier (man gitrevisions), and writing new objects (blob, tree, commit, tag). I don't necessarily want to re-implement git itself (although patches welcome if someone want to go in this direction), and as such the project is a bit of a toy to investigate git storage (for another project of mine) and superseeding my own libgit project (for yet another project). Yet it should be completely functional and have good performance. A few word of the implementation: it's very IO based at the moment; The way things are done by git, doesn't necessarily cope with pure and nice stuff if performance need to follow. That said it should still be easier to understand than reading the git source :-) Any comments welcome, [1] http://hackage.haskell.org/package/hit/ -- Vincent

3 4

Cabal rebuilding all of the C++ code for wxHaskell
by DukeDave 05 Oct '11

05 Oct '11

Hi all, I've been trying to resolve a compile time issue[1] with wxHaskell, and I thought I'd throw it open to see if anyone on cafe can help. Here's the crux of the issue: The Setup.hs for wxcore (the major component of wxHaskell) uses "simpleUserHooks", overriding only "confHook". However there is also "cleanHook", which is defined by simpleUserHooks to be: cleanHook = \p _ _ f -> clean p f, If you consult the source for clean[2] you'll see that it tries to "remove the whole dist/ directory rather than tracking exactly what files we created in there". I presume that's why we have to do a full re-build every time? To try and circumvent this I modified the definition of "main" in Setup.hs to this: main = defaultMainWithHooks simpleUserHooks { confHook = myConfHook, cleanHook = (\_ _ _ _ -> return ())} Unfortunately it still seems to re-build all the C++ on each 'install' from cabal. Not sure why? Dave, [1] http://sourceforge.net/mailarchive/message.php?msg_id=28099997 [2] Taken from http://www.haskell.org/ghc/docs/6.10.4/html/libraries/Cabal/src/Distributio… -- Cleaning clean :: PackageDescription -> CleanFlags -> IO () clean pkg_descr flags = do let distPref = fromFlag $ cleanDistPref flags notice verbosity "cleaning..." maybeConfig <- if fromFlag (cleanSaveConf flags) then maybeGetPersistBuildConfig distPref else return Nothing -- remove the whole dist/ directory rather than tracking exactly what files -- we created in there. chattyTry "removing dist/" $ do exists <- doesDirectoryExist distPref when exists (removeDirectoryRecursive distPref) -- these live in the top level dir so must be removed separately removeRegScripts -- Any extra files the user wants to remove mapM_ removeFileOrDirectory (extraTmpFiles pkg_descr) -- If the user wanted to save the config, write it back maybe (return ()) (writePersistBuildConfig distPref) maybeConfig where removeFileOrDirectory :: FilePath -> IO () removeFileOrDirectory fname = do isDir <- doesDirectoryExist fname isFile <- doesFileExist fname if isDir then removeDirectoryRecursive fname else if isFile then removeFile fname else return () verbosity = fromFlag (cleanVerbosity flags)

4 4

what happens to ()'s from Core?
by John Lato 05 Oct '11

05 Oct '11

Hello, I'm working on a small EDSL, and I think I've finally managed to get GHC to compile it to good core. Basically, it allows for the creation of expressions like: > g = 0.5*x + 0.1*y which is then compiled to a tuple (related work: CCA, stream fusion) > exists s. (s, s -> Double -> (s,Double)) I also have a function 'mapAccumL :: (V.Unbox a, V.Unbox b) => (s -> a -> (s,b)) -> s -> V.Vector a -> V.Vector b'. Basic usage would be similar to: > import qualified Data.Vector.Unboxed as V > > main = do > let (gs, gf) = $(compile [] g) > ys = mapAccumL gf gs $ V.enumFromTo (1::Double) 5 > print ys For 'g' as above, I currently get 's :: (((), ()), Double)', which is expected. GHC produces the following core for the inner loop, which looks pretty good to me: letrec { $s$wa_s2OL [Occ=LoopBreaker] :: () -> () -> GHC.Prim.Double# -> GHC.Prim.Int# -> GHC.Prim.State# (Control.Monad.Primitive.R:PrimStateST s_a1Y9) -> (# GHC.Prim.State# s_a1Y9, () #) [LclId, Arity=5, Str=DmdType LLLLL] $s$wa_s2OL = \ _ _ (sc2_s2Oq :: GHC.Prim.Double#) (sc3_s2Or :: GHC.Prim.Int#) (sc4_s2Os :: GHC.Prim.State# (Control.Monad.Primitive.R:PrimStateST s_a1Y9)) -> case GHC.Prim.<# sc3_s2Or rb1_a2EV of _ { GHC.Types.False -> (# sc4_s2Os, GHC.Unit.() #); GHC.Types.True -> let { x#_a2aI [Dmd=Just L] :: GHC.Prim.Double# [LclId, Str=DmdType] x#_a2aI = GHC.Prim.+## (GHC.Prim.*## (GHC.Prim.indexDoubleArray# rb2_a2EW (GHC.Prim.+# rb_a2EU sc3_s2Or)) 0.5) (GHC.Prim.*## sc2_s2Oq 0.1) } in $s$wa_s2OL GHC.Unit.() GHC.Unit.() x#_a2aI (GHC.Prim.+# sc3_s2Or 1) ((GHC.Prim.writeDoubleArray# @ (Control.Monad.Primitive.PrimState (GHC.ST.ST s_a1Y9)) arr#_a29n sc3_s2Or x#_a2aI (sc4_s2Os `cast` (GHC.Prim.State# (Sym (Control.Monad.Primitive.TFCo:R:PrimStateST <s_a1Y9>)) :: GHC.Prim.State# (Control.Monad.Primitive.R:PrimStateST s_a1Y9) ~ GHC.Prim.State# (Control.Monad.Primitive.PrimState (GHC.ST.ST s_a1Y9))))) `cast` (GHC.Prim.State# (Control.Monad.Primitive.TFCo:R:PrimStateST <s_a1Y9>) :: GHC.Prim.State# (Control.Monad.Primitive.PrimState (GHC.ST.ST s_a1Y9)) ~ GHC.Prim.State# (Control.Monad.Primitive.R:PrimStateST s_a1Y9))) }; } in So my question is, what happens to the ()'s after this stage? Since they're not used, and also expressed as literals in core (both in the recursive case and the original call site of $s$wa_s2OL, is the backend smart enough to get rid of them completely? Thanks for any advice, John L.

1 0

Really Simple explanation of Continuations Needed
by Mark Spezzano 05 Oct '11

05 Oct '11

Hi, Can someone please give me a _lucid_ and _simple_ explanation of exactly how continuations can be used in Haskell? I've already had a look at most of the tutorials and explanations on the web, but I'm still confused. Continuations and CPS have me baffled. (I have most of the Haskell textbooks and even these are sketchy on Continuations) I don't understand the notion of the Cont monad and how it can be used for multitasking, backtracking and interrupting computations. I understand that functions take in a (continuation) function that represents the work remaining to do, but al of the explanations on the web and in technical papers seems to trip over themselves in explaining the fundamentals to a CPS-newbie. If anyone could explain such concepts to me in unambiguous, clear English then this would be very helpful. Thanks in advance for your help, Mark

6 7

categories in Haskell and module names
by Roman Beslik 05 Oct '11

05 Oct '11

Hello. IMHO there are 2 ways to define categories in Haskell: W0) the category of types and functions ("base.Data.Functor" and company belongs to it); W1) the class "base.Control.Category.Category". (Defining a category where the class of objects is a type seems impossible in Haskell.) But these ways are mingled in "base". E.g. "Control.Applicative" (W0) and "Control.Category" (W1) are neighbors. Oddly, "Data.Functor" (W0) is under "Data". I believe that the whole library "category-extras" belongs to W1. I suggest to split these ways under distinct names, e.g.: W0) Category.Function (@hom == (->)@); W1) Category.Hom ("hom" is an abstract type constructor). Maybe both should be under "Mathematics".

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Munich Haskell Meeting
by Heinrich Hördegen 05 Oct '11

05 Oct '11

Dear all, last Thursday's get-together for Haskell users in Munich was a success. About 12 people meet at Cafe Puck where we spent a nice evening. The overall opinion was, that gatherings of this kind should be held on a more regular basis. Therefore, I'd like to announce the following page: http://www.haskell-munich.de If you can make it to Munich, please feel invited to join us at one of the given dates. We would highly appreciate your company. If you have suggestions about the page or anything else, please contact me. Have a nice day, Heinrich -- -- kontakt(a)funktional.info www.funktional.info --

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Is it possible to represent such polymorphism?
by Du Xi 05 Oct '11

05 Oct '11

--I tried to write such polymorphic function: expand (x,y,z) = (x,y,z) expand (x,y) = (x,y,1) --And it didn't compile. Then I added a type signature: expand::a->b expand (x,y,z) = (x,y,z) expand (x,y) = (x,y,1) --It still didn't compile. I think the reason is that the following is disallowed: f::a->b f x = x --Is it possible to get around this and write the "expand" function? Of course, x and y may be of different types

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Haddocok in Haskell Platform 2011.2.0.1?
by . 05 Oct '11

05 Oct '11

Hi, I just installed a new computer, downloaded ghc 7.0.3 and HP 2011.2.0.1 from http://hackage.haskell.org/platform/linux.html as proposed there, and I can not install haddock. It says it requires ghc >=7.2 and <=7.4. Is that intended? Cheers, Christian

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DSL for data definition (e.g. compiling Haskell type defs into Google's protocol buffers type defs)
by Karel Gardas 05 Oct '11

05 Oct '11

Hello, I'm trying to find out if it's possible to use Haskell data type definition capability to define types and compile defined types into other languages, for example into Google's protocol buffers data definition language. So basically speaking I'm thinking about using Haskell sub-set as a data-definition DSL together with some functions which will generate some code based on supplied defined data types. My idea is: data Person = Person { id :: Int , name :: String , email :: Maybe String } deriving (Show, Data, Typeable) emit_proto Person 1 where emit_proto is function which will translate Person data type definition into Google's proto language (the 1 is index from which start to index type's fields) by traversing data type definition and translating all its children plus do some header/footer generation etc: message Person { required int32 id = 1; required string name = 2; optional string email = 3; } I've looked for something like that and found SYB papers which works on top of data instance (i.e. actual data, not data type). I also found JSON lib which again works on top of data and not data type. I've tried to look into Data.Typetable etc, but have not found function which will print data type's field name and field type name (although JSON lib seems to use field name for JSON generation so I'll need to investigate this more). I've tested `typeOf' function and it's quite useful, but its limitation is that it's not working on ADT name: data Color = RED|GREEN|BLUE *Main> typeOf Color <interactive>:1:8: Not in scope: data constructor `Color' *Main> typeOf RED Main.Color and I would need that in order to translate Color defined above into enum like: enum Color { RED = 0; GREEN = 1; BLUE = 2; } My question is: do you think I'm looking into good direction (i.e. Data/Typeable) or do you think I'll need to use something different for data definition DSL (Template Haskell?, or impossible in Haskell so write my own language with full parser? etc?) Thanks for any idea or opinion on this! Karel

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ANN: hledger 0.16
by Simon Michael 05 Oct '11

05 Oct '11

I'm pleased to announce version 0.16 of the hledger packages. This is a stability/bugfix/polish release (which may become the pattern for even-numbered releases in future.) hledger is a library and set of user tools for working with financial data (or anything that can be tracked in a double-entry accounting ledger.) It is a haskell port and friendly fork of John Wiegley's Ledger. hledger provides command-line, curses and web interfaces, and aims to be a reliable, practical tool for daily use. Given a plain text file describing transactions of money or any other commodity, it will print the chart of accounts, account balances, or just the transactions you're interested in. It can also help you record new transactions, or convert CSV data from your bank. Home: http://hledger.org To install: cabal update, cabal install hledger. If you have trouble, let me know or watch the mail list/bug tracker for updates. Optional add-ons: hledger-web hledger-vty hledger-chart hledger-interest. Pre-built platform-specific binaries, which are time-consuming to make and support, are provided on request to anyone who donates. This is a great way to give back and support the project! http://hledger.org/DOWNLOAD.html Release notes for 0.16: * cli: strip the -- when calling add-on commands, so their options work (#64) * cli: hledger ADDON --version now shows add-on command's version * cli: only the add and web commands auto-create the journal file * cli: give a non-confusing error if LEDGER_FILE contains a literal tilde * add: clearer prompts, more validation, use . to end also * add: use unix line endings consistently, avoiding parse error on windows (#51) * add: avoid excess whitespace between transactions (#46) * balance: ledger compatibility fix: don't elide parent accounts with multiple displayed subaccounts * convert: always order converted transactions by date * convert: rename currency -> base-currency, in-field, out-field -> amount-in-field, amount-out-field * convert: give an error, not a zero when date or amount-in-field/amount-out-field parsing fails * register: show more useful range of intervals with --empty and a query pattern * print, web: always show both dates, ignoring --effective (#42) * web: production builds (the default with cabal) have all web content embedded (dev builds use ./static/) (#63) * web: update to yesod 0.9 * web: obey at least some of the general reporting options, like --cost * web: adjust the default base url when a custom port is specified * web: prevent an infinite redirect when custom base url has a trailing slash * web: fix "not:'multi word'" patterns * web: hide old title and search form when adding/editing * web: adjust --help to indicate command-line arguments are not expected * web: don't bother running cli unit tests at startup Stats: - Release contributors: Simon Michael - 30 days, 100 commits, 4 end-user features and 18 end-user bugfixes since last major release - 219 unit & functional tests and 55% unit test coverage (hledger, hledger-lib packages) - 7632 lines of code (all packages) Best, -Simon

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Fwd: LZMA for Haskell?
by Paulo Pocinho 05 Oct '11

05 Oct '11

(Thought it would be better to put this in the haskell-cafe list). Hello list. I'd like to use LZMA to create/extract archives and compare CRC from archived files. What do I need to use LZMA in Haskell? There is a page called "Library/Compression" in the wiki [1] but it is from 2005, using LZMA version 4.06. Besides being old, I can only find the LZMA SDK version 9.20 for download. The only related package I can find on hackage is lzma-enumerator [2]. Trying to install with cabal requires an old header named lzma.h that is no longer available. [1] http://www.haskell.org/haskellwiki/Library/Compression [2] http://hackage.haskell.org/package/lzma-enumerator

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Composing a list of Enumeratees into an Enumerator using ($=)
by Román González 04 Oct '11

04 Oct '11

Hey guys, Right now I'm facing with a type problem that is really nasty, I want to compose a list of enumeratees using the ($=) operator to create a new enumerator. Whenever I'm trying to use the foldx function in conjunction with ($=) I get this error: *> :t foldr ($=)* <interactive>:1:7: Occurs check: cannot construct the infinite type: b0 = Step ao0 m0 b0 Expected type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumeratee ao0 ao0 m0 b0 Actual type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumerator ao0 m0 b0 In the first argument of `foldr', namely `($=)' In the expression: foldr ($=) *> :t Prelude.foldl ($=)* <interactive>:1:15: Occurs check: cannot construct the infinite type: b0 = Step ao0 m0 b0 Expected type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumerator ao0 m0 (Step ao0 m0 b0) Actual type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumerator ao0 m0 b0 In the first argument of `Prelude.foldl', namely `($=)' In the expression: Prelude.foldl ($=) <interactive>:1:15: Occurs check: cannot construct the infinite type: b0 = Step ao0 m0 b0 Expected type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumerator ao0 m0 (Step ao0 m0 b0) Actual type: Enumerator ao0 m0 (Step ao0 m0 b0) -> Enumeratee ao0 ao0 m0 b0 -> Enumerator ao0 m0 b0 In the first argument of `Prelude.foldl', namely `($=)' In the expression: Prelude.foldl ($=) Obviously there is something I don't quite understand about the ($=) (=$) functions, how can one compose a list of enumeratees, is it even possible? Cheers. Roman.-

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Re: [Haskell-cafe] Is it possible to represent such polymorphism?
by oleg＠okmij.org 04 Oct '11

04 Oct '11

sdiyazg at sjtu.edu.cn wrote: > >> generalizedFilterMap (\[x,y,z]-> if(x==1&&z==1)then [y*10] else > >> [0]) (3,1) [1,2,3,4,1,2,1,3,1,4,1,5,2] > [0,0,0,0,20,0,30,0,40,0,0] > > Of course, I could have simply used [Int] , (Num a)=>[a] or > (Int,Int,Int), but I'm trying to write code as generic as possible. As I understand, the point of generalizedFilterMap is to permit the filter function to examine several elements of the list within the current window (rather than just the current element). The step argument determines the step of sliding the window. Further, you wish to make the step argument default (setting it to 1 if not given). First of all, if you are interested in stencil computations, there are several good packages and even the whole DSLs: http://research.microsoft.com/en-us/um/people/simonpj/papers/ndp/index.htm http://arxiv.org/abs/1109.0777 http://people.csail.mit.edu/yuantang/ Second, the given code is not as generic as possible. In the above example, the filter function examines values within the window of size 3. That fact has to be specified twice: in the pattern \ [x,y,z] -> ... and as the number 3. Having to specify the same information twice is always less than satisfactory: if we expand the pattern to \ [x,y,z,u] -> we have to remember to update the other argument to the function. There are many solutions to the problem, involving as much type hacking as one may wish (some of the stencil packages above do track the size of the stencil statically, in types). Perhaps one of the simplest solution is the following > generalizedFilterMap3 :: Int -> ([a]->[a]) -> [a] -> [a] > generalizedFilterMap3 step f = concatMap f . decimate step . tails > > -- pick every n-th > decimate :: Int -> [a] -> [a] > decimate 1 lst = lst > decimate _ [] = [] > decimate n lst = head lst : decimate n (drop n lst) > > test1 = generalizedFilterMap3 1 f [1,2,3,4,1,2,1,3,1,4,1,5,2] > where > f (x:y:z:_) = if x==1&&z==1 then [y*10] else [0] > f _ = [] Again there are many, many approaches to default arguments. Perhaps the simplest, involving no typeclasses and no type computation is the following > data GMapArgs a = GMapDflt{step :: Int, > gmapf :: [a] -> [a]} > > dflt = GMapDflt{step = 1, gmapf = id} > > generalizedFilterMap dflt = generalizedFilterMap3 (step dflt) (gmapf dflt) > > test2 = generalizedFilterMap dflt{gmapf=f} [1,2,3,4,1,2,1,3,1,4,1,5,2] > where > f (x:y:z:_) = if x==1&&z==1 then [y*10] else [0] > f _ = [] > > test3 = generalizedFilterMap dflt{gmapf=f,step=2} [1,2,3,4,1,2,1,3,1,4,1,5,2] > where > f (x:y:z:_) = if x==1&&z==1 then [y*10] else [0] > f _ = [] It requires no extensions. Record puns and other Record extensions make the approach nicer. > What I want is some thing like this in C++: > > float f(char x){ return 0.1f; } > int f(double x){ return 1; } One should point out the difficulties of comparing C++ with Haskell. Haskell is designed to be higher-order, making it simple to pass functions as arguments to other functions. Your code for generic maps relies on that behavior. Try passing the overloaded C++ function 'f' defined above to some other function. That is, implement something like the following float g(??? f) { return f('x') + f(1.0); } int main(void) {printf("%g",g(f)); return 0;} One can certainly implement the above outline in C++, but it won't be simple.

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CAL or Frege as a Haskell replacement on JVM?
by Karel Gardas 04 Oct '11

04 Oct '11

Hello, since there is no Haskell compiler/interpreter for JVM yet (considering LambdaVM outdated), then the question for kind of Haskell replacement on this platform is either CAL[1] or Frege[2]. I'd like to ask here those who have used either of those languages for your experience with it/them, what's more similar to Haskell (at least from your point of view) and what you like more as a Haskell replacement on JVM. Thanks! Karel [1]: http://openquark.org/Welcome.html [2]: http://code.google.com/p/frege/

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