- Haskell-Cafe - Haskell.org

Hoogle index completeness
by Radical 21 Dec '12

21 Dec '12

Searching Hoogle for symbols like `rstrip` or `lstrip` produces "No results found" for me, even though they exist in the MissingH library. To wit: http://hackage.haskell.org/packages/archive/MissingH/1.2.0.0/doc/html/Data-… Is this behavior intentional, or a regression of some sort? I could have sworn the above symbols showed up at some point. Thanks! Alvaro

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GHC shows wrong line number in error?
by Niklas Hambüchen 20 Dec '12

20 Dec '12

Hello, I have some code like this (the contents don't really matter): 42 data TestChain next = ChainEntry (forall b . TestG b) next 43 | ChainDescribe String (Free TestChain... 44 deriving (Functor) 45 46 -- deriving instance Show a => Show (TestChain a) 47 48 it :: (SomeT typ, Partition t typ) => String -> t -> ... 49 it desc test = liftF (ChainEntry (mkTestG test) ()) And get the error: Clean.hs:49:43: Could not deduce (t ~ b) from the context (SomeT typ, Partition t typ) bound by the type signature for it :: (SomeT typ, Partition t typ) => String -> t -> Free TestChain () at Clean.hs:49:1-51 `t' is a rigid type variable bound by the type signature for it :: (SomeT typ, Partition t typ) => String -> t -> Free TestChain () at Clean.hs:49:1 `b' is a rigid type variable bound by a type expected by the context: TestG b at Clean.hs:49:23 [...] In that last error line, should that not be "Clean.hs:42:..." (as it references the 'b', which I only really have there), or is that intended, and if yes, why? I'm using GHC 7.4.2. Thanks Niklas

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An assembly DSL example.
by Tom Hawkins 20 Dec '12

20 Dec '12

A few folks have asked me about building EDSLs in Haskell for assembly programming, so I've posted an example of the approach we have had success using at BAE Systems. It's a bit rough, so if anyone's motivated to polish it up, by all means. https://github.com/tomahawkins/asm-dsl-example/ -Tom

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Observer pattern in haskell FRP
by Nathan Hüsken 19 Dec '12

19 Dec '12

Hey, When writing games in other (imperative) languages, I like to separate the game logic from the rendering. For this I use something similar to the observer pattern. With rendering I mean anything only related to how objects are drawn to the screen. Animation state for example. On my journey of exploring game programming with haskell (and FRP), I wonder what a good way of archiving something similar would be. If the rendering has no internal state, one can just write a function draw :: GameLogicState -> IO () But when the rendering of an object has an internal state (e.g. animation frame) than this is not possible. Now I could write a Wire/Signal (whatever FRP implementation I use) that translates to a RenderingState: render :: Signal GameLogicState RenderingState draw :: RenderingState -> IO () which is fine, except when my game is made of many objects. Than I need to associate the state of one object in GameLogicState with a sub-signal in render. That could be done by giving every object an ID and letting GameLogicState contain a map from IDs to ObjectLogicState. I fear that when I really have a lot of objects, assembling and decomposing the map could become a major bottleneck. So I am wondering: Is there (or can someone think of) a different pattern by which this could be archived? Or asked different: How would you do it? Thanks! Nathan

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containers license issue
by Dmitry Kulagin 19 Dec '12

19 Dec '12

Hi Cafe, I am faced with unpleasant problem. The lawyer of my company checked sources of containers package and found out that it refers to some GPL-library. Here is quote: "The algorithm is derived from Jorg Arndt's FXT library" in file Data/IntMap/Base.hs The problem is that FXT library is GPL and thus containers package can not be considered as BSD3. And it means that it can not be used in my case (closed source software). Is this logic actually correct and containers should be considered as GPL? The package is widely used by other packages and the only way I see right now is to fix sources to reimplement this functionality, which is not good option. Thank you!

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Re: [Haskell-cafe] Tail recursive
by J.W. Krol 19 Dec '12

19 Dec '12

From: jkrzzz(a)live.com To: haskell-cafe(a)haskell.org Subject: Tail recursive Date: Wed, 19 Dec 2012 17:07:14 +0100 Hello, I need a non tail recursive version of scanl, to produce a large enough list of >100K vectors (vectors defined as a list) I have following code: scanl'::(a -> a -> a) -> a -> [a] -> [a]scanl' f q ls = scanl'' ls (q:[]) where scanl'' (x:xs) ys = let h = head ys x' = f x h ys' = x':ys in h `seq` x' `seq` ys' `seq` scanl'' xs ys' scanl'' [] ys = ys If I call this function as below I still got stack-overflow error: head (scanl' (zipWith (+)) ([0,0]) (take 100000 (repeat [0,1]))) What do I wrong. I am not an experienced Haskell programmer, but find the behavior quite unexplainable. Thank for your answer. RegardsJacq

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A weird bug of regex-pcre
by Magicloud Magiclouds 19 Dec '12

19 Dec '12

Attachment is the test text file. And I tested my regexp as this: Prelude> :m + Text.Regex.PCRE Prelude Text.Regex.PCRE> z <- readFile "test.html" Prelude Text.Regex.PCRE> let (b, m ,a, ss) = z =~ "<a href=\"(.*?)\">.*?<img class=\"article-image\"" :: (String, String, String, [String]) Prelude Text.Regex.PCRE> b ... n of the Triumvirate</td>\r\n <td class=\"small\">David Rapoza</td>\r\n <td class=\"small\">\r\n <i>Return to Ravnica</i>\r\n </td>\r\n <td class=\"small\">10/31/2012</td>\r\n </tr><tr>\r\n <td class=\"small\"><" Prelude Text.Regex.PCRE> m "a href=\"/magic/magazine/article.aspx?x=mtg/daily/activity/1088\"><img class=\"article-image\" " >From the value of b and m, it was weird that the matching was moved forward by 1 char ( the ss (sub matching) was even worse, 2 chars ). Rematch to a and so on gave correct results. It was only the first matching that was broken. Tested with regex-posix (with modified regexp), everything is OK. $ ghc-pkg describe regex-pcre name: regex-pcre version: 0.94.4 id: regex-pcre-0.94.4-d45e00c9e113c7c9352d0785497e1dca license: BSD3 copyright: Copyright (c) 2006, Christopher Kuklewicz maintainer: TextRegexLazy(a)personal.mightyreason.com stability: Seems to work, passes a few tests homepage: http://hackage.haskell.org/package/regex-pcre package-url: http://code.haskell.org/regex-pcre/ synopsis: Replaces/Enhances Text.Regex description: The PCRE backend to accompany regex-base, see www.pcre.org category: Text author: Christopher Kuklewicz exposed: True exposed-modules: Text.Regex.PCRE Text.Regex.PCRE.Wrap Text.Regex.PCRE.String Text.Regex.PCRE.Sequence Text.Regex.PCRE.ByteString Text.Regex.PCRE.ByteString.Lazy hidden-modules: trusted: False import-dirs: /home/magicloud/.cabal/lib/regex-pcre-0.94.4/ghc-7.6.1 library-dirs: /home/magicloud/.cabal/lib/regex-pcre-0.94.4/ghc-7.6.1 hs-libraries: HSregex-pcre-0.94.4 extra-libraries: pcre extra-ghci-libraries: include-dirs: includes: depends: array-0.4.0.1-cbe8814e07792e8f0d66cac77a2c0b6b base-4.6.0.0-9108e251636b0c8499261c52a7809ea1 bytestring-0.10.0.1-11d4f52c4f4ed9833f768577b77050c5 containers-0.5.2.1-b183418bc7f43ce98b6916ef296c2669 regex-base-0.93.2-1ee07f806ad6b0c911226883d15b64f2 hugs-options: cc-options: ld-options: framework-dirs: frameworks: haddock-interfaces: /home/magicloud/.cabal/share/doc/regex-pcre-0.94.4/html/regex-pcre.haddock haddock-html: /home/magicloud/.cabal/share/doc/regex-pcre-0.94.4/html pkgroot: "/home/magicloud/.ghc/x86_64-linux-7.6.1" -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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efficient data structure: column with repeating values
by Christopher Howard 19 Dec '12

19 Dec '12

Is there some good data type out there that basically provides a simple table, but with optimization for repeating values on one column? Something like: Data Table a b ...where it assumes that 'a' values will usually be unique, while 'b' values will usually be repeated from a small set? (But not needing to be fixed beforehand.) Like in... client | patron --------------- Bob | Tom Sarah | Tom Dick | Tom George | Harry Moe | Harry -- frigidcode.com

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Incremental regular expressions - article and library
by Eugene Kirpichov 19 Dec '12

19 Dec '12

Hi Haskellers, I just published an article that can be interesting to lovers of functional programming, even though it's not directly relevant to Haskell per se. http://jkff.info/articles/ire/ It's based on Dan Piponi's blogpost "Fast incremental regular expression matching with monoids" http://blog.sigfpe.com/2009/01/fast-incremental-regular-expression.html but expands Dan's ideas to include *locating* matches, matching multiple regular expressions at once, using a more compact datastructure than fingertrees, etc. And - sorry - the implementation is in Java, for reasons explained in the article :) -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov

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HTF & Quickcheck
by graham＠fatlazycat.com 18 Dec '12

18 Dec '12

Getting test failures, I believe, due to using conditional properties within quickckeck etc [TEST] RecFunSpec:map (test/RecFunSpec.hs:48) Gave up! Passed only 20 tests. *** Failed! (2ms) Is there a way for HTF/Quickcheck to specify these are not failed i.e. only fail if there is an actual error ? Thanks

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Race Condition in threads
by mukesh tiwari 18 Dec '12

18 Dec '12

Hello All I have two questions. 1. I wrote this code to create 10 simultaneous threads. Could some one please tell me if this is correct or not ? incr_count :: MVar () -> MVar Int -> IO () incr_count m n = ( forM_ [ 1..10000 ] $ \_ -> modifyMVar_ n ( return . ( + 10 ) ) ) >> putMVar m () main :: IO() main = do count <- newMVar 0 list <- forM [1..10] $ \_ -> newEmptyMVar forM_ list $ \var -> forkIO . incr_count var $ count forM_ list $ \var -> takeMVar var val <- takeMVar count print val 2. I am trying to create race condition which makes the variable in inconsistent state. Could some one please tell me how to achieve this ? I look at IORef but it does not have function like modifyMVar_. Mukesh Tiwari

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TAP 2013: 2nd Call for Papers
by Achim D. Brucker 18 Dec '12

18 Dec '12

(Apologies if you receive this announcement multiple times.) **************************************************** *** *** *** TAP 2013 *** *** *** *** Abstract submission: January 25, 2013 *** *** Paper submission: February 1, 2013 *** **************************************************** *** TAP 2013 solicits both full papers and *** *** (industrial) experience/tool papers *** *** in combining proofs and (security) testing *** **************************************************** CALL FOR PAPERS 7th INTERNATIONAL CONFERENCE ON TESTS AND PROOFS (TAP 2013) http://www.spacios.eu/TAP2013 Budapest, Hungary, June 18-19, 2013 The TAP conference is devoted to the synergy of proofs and tests, to the application of techniques from both sides and their combination for the advancement of software quality. Testing and proving seem to be contradictory techniques: once you have proved your program to be correct then additional testing seems pointless; on the other hand, when such a proof in not feasible, then testing the program seems to be the only option. This view has dominated the research community since the dawn of computer science, and has resulted in distinct communities pursuing the seemingly orthogonal research areas. However, the development of both approaches has lead to the discovery of common issues and to the realization of potential synergy. Perhaps, use of model checking in testing was one of the first signs that a counterexample to a proof may be interpreted as a test case. Recent breakthroughs in deductive techniques such as satisfiability modulo theories, abstract interpretation, and interactive theorem proving, have paved the way for new and practically effective methods of powering testing techniques. Moreover, since formal, proof-based verification is costly, testing invariants and background theories can be helpful to detect errors early and to improve cost effectiveness. Summing up, in the past few years an increasing number of research efforts have encountered the need for combining proofs and tests, dropping earlier dogmatic views of incompatibility and taking instead the best of what each of these software engineering domains has to offer. The TAP conference aims to bring together researchers and practitioners working in the converging fields of testing and proving, and will offer a generous allocation of papers, panels and informal discussions. Topics of interest cover theory definitions, tool constructions and experimentations, and include (other topics related to TAP are welcome): - Bridging the gap between concrete and symbolic techniques, e.g. using proof search in satisfiability modulo theories solvers to enhance various testing techniques - Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing - Program proving with the aid of testing techniques - New problematics in automated reasoning emerging from specificities of test generation - Verification and testing techniques combining proofs and tests - Generation of test data, oracles, or preambles by deductive techniques such as: theorem proving, model checking, symbolic execution, constraint logic programming - Model-based testing and verification - Generation of specifications by deduction - Automatic bug finding - Debugging of programs combining static and dynamic analysis - Formal frameworks - Tool descriptions and experience reports - Case studies combining tests and proofs - Domain specific applications of testing and proving to new application domains such as validating security protocols, vulnerability detection of programs, security Important Dates: ================ Abstract submission: January 25, 2013 Paper submission: February 1, 2013 Notification: March 3, 2013 Camera ready version: April 5, 2013 TAP conference: June 17-21, 2013 Program Chairs: =============== Margus Veanes (Microsoft Research, USA) Luca Vigano` (University of Verona, Italy) Program Committee: ================== Paul Ammann Dirk Beyer Achim D. Brucker Robert Clarisò Marco Comini Catherine Dubois Gordon Fraser Angelo Gargantini Christoph Gladisch Martin Gogolla Arnaud Gotlieb Wolfgang Grieskamp Reiner Hähnle Bart Jacobs Thiérry Jeron Jacques Julliand Gregory Kapfhammer Nikolai Kosmatov Victor Kuliamin Michael Leuschel Karl Meinke Alexandre Petrenko Holger Schlingloff T.H. Tse Margus Veanes (co-chair) Luca Viganò (co-chair) Burkhart Wolff Fatiha Zaidi Submission: =========== Please submit your papers via easychair https://www.easychair.org/conferences/?conf=tap2013 TAP 2013 will accept two types of papers: - Research papers: full papers with at most 16 pages in LNCS format (pdf), which have to be original, unpublished and not submitted elsewhere. - Short contributions: work in progress, (industrial) experience reports or tool demonstrations, position statements; an extended abstract with at most 6 pages in LNCS format (pdf) is expected. Subject to final approval by Springer, accepted papers will be published in the Springer LNCS series and will be available at the conference. The contents of previous TAP proceedings is available at: http://www.informatik.uni-trier.de/~ley/db/conf/tap/ -- Dr. Achim D. Brucker, SAP AG, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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haskell-like scripting language
by Christopher Howard 17 Dec '12

17 Dec '12

I gave Shelly a try. Pretty cool - using it for some of the scripts on my system. Has me wondering though: is anyone working on creating a actual Haskell-like scripting language and engine? Shelly is cool, as I said, but I imagine it would be more valuable to have another language that is actually separate from Haskell, with an interpreter that is more lightweight and changes much less frequently (than GHC). Something that could be nearly as portable as Bash or Perl. -- frigidcode.com

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Navigating 'Strategic' programming babel
by Ravi Sahni 17 Dec '12

17 Dec '12

Clearly Haskell has great possibilities in the field of language-processing. And the nuisances associated with little actual computation buried under much data-structure navigation are well addressed by 'strategic-programming' systems. But now comes the rub -- there seem to be a lot of very similar systems. Any guidance on which/what/how to choose? My own current sketchy-patchy knowledge is as below. I would appreciate links/pointers to more substansive literature. First there was Meertens and his folks working on generic haskell Did that later become template haskell? That branched out into strafunski, stratego/xt. SYB is ___ not sure here: some literature suggests that its identical to strafunski. Some suggests that it is strafunski done more within the haskell language rather than in libraries. Then there's uniplate. How does it compare to SYB? Or is that a confused comparison?

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ANN: Haskell bindings for the igraph C library
by Nils Schweinsberg 17 Dec '12

17 Dec '12

Hi Haskellers, dear igraph community, I am pleased to announce the release of our (inofficial) Haskell bindings to the igraph C library. igraph is a powerfull library for creating and manipulating directed, undirected and weighted graphs. Our package offers a complete coverage of all functions on structural properties of graphs. Compared to the official packages for R and Python it is the first library that offers type level distinction between directed/undirected and weighted graphs. Haskells type system allows to keep track of the types of your nodes and whether or not your graph and its edges are directed or undirected, weighted or unweighted. Haskell graphs may contain any Haskell value as node values, kind of similar to Pythons attributes. igraph on hackage, the official package database for Haskell: http://hackage.haskell.org/package/igraph (requires the current igraph-0.6 C library) The official igraph website: http://igraph.sourceforge.net/index.html Until hackage generates the haddock documentation, the documentation is also available at: http://hs.nils.cc/igraph-0.1/html/index.html Any feedback is appreciated, - Nils Schweinsberg - George Giorgidze

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Cabal: Retrieve directory in which binaries get installed
by Björn Peemöller 17 Dec '12

17 Dec '12

Hello cafe, I'd like to retrieve the path in which binaries get installed when I `cabal install` a package. Normally, this would be ~/.cabal/bin on my Linux, but since I share my home folder on two architectures, in ~/.cabal/config I set the user-install prefix to ~/.cabal/$arch, so binaries get installed to ~/.cabal/$arch/bin. I have the feeling that it should be rather simple to retrieve the FilePath from the Cabal API, does anyone know how to do this? Thanks in advance, Björn

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ANN: JMacro 0.6 and JMacro-RPC, featuring cross-platform json-rpc, easy AJAX applications, evented reactive panels and more
by Gershom Bazerman 17 Dec '12

17 Dec '12

jmacro: http://hackage.haskell.org/package/jmacro jmacro-rpc: http://hackage.haskell.org/package/jmacro-rpc jmacro-rpc-snap: http://hackage.haskell.org/package/jmacro-rpc-snap jmacro-rpc-happstack: http://hackage.haskell.org/package/jmacro-rpc-happstack I'm happy to announce the release of JMacro 0.6 (0.6.3 actually) which brings a host of improvements. JMacro is a quasiquoted library for the programmatic generation of JavaScript code. It lets you write in a superset of JavaScript with haskell-like syntax and compile-time guarantees of syntactic correctness, and is intended to be used for both embedded javascript and compilers and libraries targeting JavaScript--where hygienic naming, proper scoping, and anti-quotation are especially important. Documentation is on the Haskell Wiki: http://www.haskell.org/haskellwiki/Jmacro This release brings a host of improvements to efficiency, driven largely by user-submitted patches and requests. Text is used more uniformly, as opposed to String, and JSON integration is provided via aeson rather than the json package. Additionally, nearly all of JavaScript is supported, including more obscure operators, as well as labels, and labeled break and continue. The big news, however, is JMacro-RPC, which provides multi-server tools for serving JSON-RPC 2.0 [1] remote procedure calls, as well as binding to them from both Haskell and JavaScript. On the Haskell side, rpc server functions and caller stubs are automatically generated by typeclasses. On the JavaScript side, which is less typed, a function is exposed to marshall javascript function arguments directly into RPC calls to a located URL. [1] http://jsonrpc.org/specification On top of these RPCs, JMacro-RPC provides the conversation page abstraction, which allows the easy creation of web-applications with server side state. The user simply provides a function from JavaScript -> Page that splices in some JavaScript, and a list of functions they wish to be accessible from the page. All JavaScript within the page then has access to these functions, which run, transparently, server-side, with access to page-invocation-local shared state. Backends exist for JMacro-RPC for Happstack and Snap, and can be easily written for any other server or framework. JMacro-RPC now also provides the Panels library, which is even higher-level, providing the convenience of continuation-style web development with no scaling hassle. On the server side, Panels are entirely stateless, storing no client-specific state (so requests can be sharded to multiple servers as well without worrying about session replication). Code is written with a set of combinators over Panels, which package up display and behavior simultaneously. Panels, inspired by FRP, can provide Signals, which are sampleable, Events, which are discrete and can trigger updates, and Sinks which can be bound to Signals (behaving similarly to FRP wormholes). Semantics, by virtue of client-server interaction, are necessarily evented rather than continuous. As such, this is properly a system for declarative, event-driven reactivity rather than genuine FRP. Such a system is impoverished compared to the richer continuous-time or fully stream-based discrete-time semantics provided by of real FRP libraries. However, it is more than powerful enough for the sorts of user interfaces Panels are designed to build, and holds the added advantage of being straightforward, tractable, and a tight fit for client-server interactions. While Signals are applicative and Sinks are contravariant functors, which exposes at least some flavor of FRP, simple Signals and Sinks which don't require server-side processing (and are safely distinguished as such by types) can be bound to one-another purely on the client-side, avoiding unnecessary round-trips. The Panels code combines concepts from FRP [1][2], Self-Adjusting Computation [3], ASP.NET update panels [4], and research in stable naming schemes for bound variables [5]. The JavaScript "rts" driving evented updates is designed to be reasonably efficient and leak-free. Between basic RPCs, Conversations, and Panels, JMacro-RPC is intended to provide the full "power scale" for writing robust AJAX web applications with complex server-side logic. Because it is designed to be usable from any Haskell web framework or server, it is suitable for widespread adaptation. Patches, bug-reports, contributions, feature requests, documentation, etc. all are very welcome. [1] http://www.haskell.org/haskellwiki/Functional_Reactive_Programming [2] http://stackoverflow.com/questions/1028250/what-is-functional-reactive-prog… [3] http://www.umut-acar.org/self-adjusting-computation [4] http://msdn.microsoft.com/en-us/library/bb399001(v=vs.100).aspx [5] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.66.6645 ("I am not a number: I am a free variable", McBride and McKinna. See section four, on hierarchical names.) Cheers, Gershom

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Re: [Haskell-cafe] edge: compile testing
by John Lato 17 Dec '12

17 Dec '12

> > From: Christopher Howard <christopher.howard(a)frigidcode.com> > > On 12/14/2012 07:05 PM, Clark Gaebel wrote: > > Unacceptable argument type in foreign declaration > > Thanks for giving it a try. Could you send off a bug report to the > OpenAL Haskell module maintainer? <sven.panne(a)aedion.de> > > (I might offer to do it, but I tried to e-mail him once about a > different issue and never heard back. Probably didn't make it through > his spam filter.) I think Sven's been incommunicado for a few years now. Most likely somebody else will need to step forward as the OpenAL maintainer, if there's interest in keeping that package current. John L.

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Can cabal be turned into a package manager?
by Janek S. 16 Dec '12

16 Dec '12

In the recent months there was a lot of dicussion about cabal, dependency hell and alike. After reading some of these discussions there is a question I just have to ask: Why not create a package manager (like rpm or apt) for Haskell software? I've been using Linux for years. Software for Linux is mostly written in C and C++. There are thousands of libraries with lots of dependencies and yet: a) Linux distributions manage to have package repositories that are kept in a consistent state b) Linux package managers can avoid dependency hell, automatically update to new packages, etc. Linux people did it! Is there any technical issue that prevents Haskell people from doing exactly the same thing? Or are we just having non-technical problems like lack of money or developers? Janek

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Hackage suggestion: Gather the list of the licenses of all dependencies of a package
by Petr P 16 Dec '12

16 Dec '12

Dear Haskellers, following up the recent discussion about copyleft licenses, I'd suggest a (hopefully minor) improvement of Hackage: For each package, gather the list of the licenses of everything it depends on. I think this would help considerably people who don't want or can't use software licensed under a particular license (most often (L)GPL). In particular, we can have a BSD package that depends on a LGPL package, and this is fine for FOSS developers. But for a commercial developer, this can be a serious issue that is not apparent until one examines *every* transitive dependency. This idea is a bit vague, because a dependency is actually a range of packages, which in theory could have different licenses. But I suppose this will rarely happen in practice, so it'd be safe just to take the last package in the range (or maybe take all licences of the packages in the range). Best regards, Petr

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Hackage down, again?
by Magnus Therning 16 Dec '12

16 Dec '12

It looks like hackage is down again. Is it planned or unplanned downtime this time? There doesn't happen to be some mirror of the packages that is a bit more reliable than the original? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Most software today is very much like an Egyptian pyramid with millions of bricks piled on top of each other, with no structural integrity, but just done by brute force and thousands of slaves. -- Alan Kay

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A new FFMPEG library?
by Kyle Hanson 15 Dec '12

15 Dec '12

The current hs-ffmpeg library is labeled as "old" on github: https://github.com/anders-/hs-ffmpeg is there a newer one or perhaps an alternative to play media files in haskell? -- Kyle Hanson

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Template Haskell Splicing
by satvik chauhan 15 Dec '12

15 Dec '12

Is there any way to splice declarations inside where? If not, then what is the reason for not supporting this? -Satvik

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ANNOUNCE: timeout-with-results
by Petr P 15 Dec '12

15 Dec '12

A small library that extends System.Timeout.timeout. It adds the possibility of saving partial results. Useful for AI-like algorithms that should return the best result found within a time limit. It comes in two variants: (1) Simple, which only allows computations to save partial results, not retrieve what has been written already. If a computation times out, the last saved partial result is returned. It comes in two flavours: One that converts saved values to WHNF, the other to NF. (This is required so that the producing thread performs the computations, not the consuming thread.) (2) Based on MonadWriter. The types of partial results have to be monoids. Saving a partial result combines it with the saved value using `mappend`. It also adds the ability to run a contained computation within another one, without disturbing its output. See https://github.com/ppetr/timeout-with-results#examples for examples. Repo: https://github.com/ppetr/timeout-with-results Hackage: http://hackage.haskell.org/package/timeout-with-results (I apologize for not being able to come up with a better package/module names. Suggestions welcomed.) Petr Pudlak

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Is it possible to have constant-space JSON decoding?
by oleg＠okmij.org 15 Dec '12

15 Dec '12

I am doing, for several months, constant-space processing of large XML files using iteratees. The file contains many XML elements (which are a bit complex than a number). An element can be processed independently. After the parser finished with one element, and dumped the related data, the processing of the next element starts anew, so to speak. No significant state is accumulated for the overall parsing sans the counters of processed and bad elements, for statistics. XML is somewhat like JSON, only more complex: an XML parser has to deal with namespaces, parsed entities, CDATA sections and the other interesting stuff. Therefore, I'm quite sure there should not be fundamental problems in constant-space parsing of JSON. BTW, the parser itself is described there http://okmij.org/ftp/Streams.html#xml

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edge: compile testing
by Christopher Howard 15 Dec '12

15 Dec '12

Hey guys, to teach myself Haskell I wrote a little arcade game called The Edge, built on gloss. It is in hackage under the package name "edge". Are there a few kind souls who would be willing to compile it on their machines and let me know if there are any problems at the compiling level? In the past, I've had issues with Haskell code compiling fine on my development system but not on others (due to dependency-related issues). I want to double check this before I try to make any distro-specific packages. I developed with GHC 7.4 and cabal-install 1.16.0.2 on a Gentoo system. Requires OpenGL and OpenAL (for sound). cabal update && cabal install edge -- frigidcode.com

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LGPL and Haskell (Was: Re: ANNOUNCE: tie-knot library)
by Petr P 14 Dec '12

14 Dec '12

2012/12/12 David Thomas <davidleothomas(a)gmail.com> > IANAL, but reviewing what others have written, it sounds like it may be > possible to maintain *some* distinction between LGPL and GPL in Haskell, > but it's a different distinction than with an LGPL shared library, so even > if applicable it's certainly worth being aware of. > > It sounds (and I'd very much like clarification) like providing source of > an application to users *without* license to redistribute, which can be > built using an LGPL library, would be compliant - whereas if the library > was GPL this would not be the case? > > As I understand it, this is one of the main differences: Using GPL-ed library forces the entire Combined Work to be GPL-ed (requiring to release the source codes), while using LGPL-ed library forces only the possibility to re-link with another version of it - you can distribute your own sources to users with whatever license you like (unless it's a modification of the library, of course). This could be an interesting question for FSF. Thanks to all for valuable comments. I didn't want to start a discussion about (L)GPL vs others, but since that already happened, let me give my opinion: First, LGPL has been discussed already: http://www.haskell.org/pipermail/haskell-cafe/2007-March/023158.html The problem with LGPL is that it is apparently tailored to languages that strongly rely on dynamic linking. TBH I wasn't aware of this before. One solution (taken by wxWidgets for example) is to modify LGPL so that they explicitly allow distributing the software without the re-linking requirement (which grants a little less rights to end users). I tried asking that on StackExchange: "Is there a modified LGPL license that allows static linking?" <http://programmers.stackexchange.com/q/179084/61231> Anther solution would be to focus on improvements of dynamic linking. Some links I found: http://hackage.haskell.org/trac/ghc/wiki/SharedLibraries/PlatformSupport Haskell Static vs Dynamic Linking in Deployment < http://stackoverflow.com/q/7854614/1333025> Small Haskell program compiled with GHC into huge binary < http://stackoverflow.com/q/6115459> Yet another solution would be what David Thomas suggest: To provide the source code to your users, but don't allow them to use the code for anything but relinking the program with a different version of the library (no distribution, no modification etc.). <http://www.haskell.org/pipermail/haskell-cafe/2007-March/023158.html> Second, it's my wish to use (L)GPL for FOSS programs I write. My opinion has varied for many years, but in recent years I strongly incline to GPL-like licenses. We all know that GPL-like licenses make life more complicated for us programmers. Programmers want to use libraries with as little restrictions as possible. The main reason for me for choosing (L)GPL despite the complications is the protection and rights it gives to the *users* of a program. Sadly, no users are actually aware of this and what they could gain by choosing GPL-ed programs (let me know if you find a non-programmer who is aware of GPL's benefits). Just imagine if Facebook had to use AGPL, or if your mobile phone software would be all GPL. (Knowing what a program does with your data is becoming more important, like the US intelligence report suggests: http://reut.rs/UxbLtvsection "Technology .."). The other reason for me that GPL-like licenses give an advantage to FOSS developers over private companies. I feel this is fair - anybody who develops something that can be used by everyone deserves an advantage. Another thing to consider is the aspect of adoption/marketing strategy. Using a more permissive license can make adoption of a piece of software much faster. This can be desirable in some cases, like if the OS community wants to prevent the adoption of proprietary standards (like Vorbis VS MP3). Or in the case of Haskell, to motivate private companies to use it. Richard Stallman himself recommends this in some cases http://lwn.net/2001/0301/a/rms-ov-license.php3 But if a project is strong enough, using (L)GPL can actually strengthen it, like in the case of Linux VS NetBSD: http://www.dwheeler.com/blog/2006/09/01/#gpl-bsd To quote: "So while the BSDs have lost energy every time a company gets involved, the GPL'ed programs gain every time a company gets involved." Of course I respect other opinions in that matter, every author is free to use whatever licence (s)he likes. After all, FOSS is about freedom. The important thing to know is what advantages and drawbacks each license has and what are the consequences of using them. Best regards, Petr

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Re: [Haskell-cafe] ANNOUNCE: tie-knot library
by Niklas Larsson 14 Dec '12

14 Dec '12

Yes, if you link statically, if you want to distribute without giving away source (or relinkable object files) you can link dynamically or use integer-simple in your GHC. Niklas ----- Ursprungligt meddelande ----- Från: "Herbert Valerio Riedel" <hvr(a)gnu.org> Skickat: ‎2012-‎12-‎14 11:55 Till: "Brandon Allbery" <allbery.b(a)gmail.com> Kopia: "Ramana Kumar" <Ramana.Kumar(a)cl.cam.ac.uk>; "haskell-cafe" <haskell-cafe(a)haskell.org> Ämne: Re: [Haskell-cafe] ANNOUNCE: tie-knot library Brandon Allbery <allbery.b(a)gmail.com> writes: > Haskell libraries are mostly BSD licensed, as is GHC itself. (Oddly > enough, GPL is not the only open source license.) btw, what about GHC's reliance on the LGPLed GMP library? Doesn't that already taint the whole GHC eco-system? Quoting [1]: | GMP is licensed under the GNU Lesser General Public License (LGPL), a | kind of "copyleft" license. According to the terms of the LGPL, | paragraph 5, you may distribute a program that is designed to be | compiled and dynamically linked with the library under the terms of | your choice (i.e., commercially) but if your program incorporates | portions of the library, if it is linked statically, then your program | is a "derivative"--a "work based on the library"--and according to | paragraph 2, section c, you "must cause the whole of the work to be | licensed" under the terms of the LGPL (including for free). | | The LGPL licensing for GMP is a problem for the overall licensing of | binary programs compiled with GHC because most distributions (and | builds) of GHC use static libraries. (Dynamic libraries are currently | distributed only for OS X.) The LGPL licensing situation may be worse: | even though The Glasgow Haskell Compiler License is essentially a | "free software" license (BSD3), according to paragraph 2 of the LGPL, | GHC must be distributed under the terms of the LGPL! [1]: http://hackage.haskell.org/trac/ghc/wiki/ReplacingGMPNotes#ReasonsforReplac… cheers, hvr _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

ANNOUNCE: tie-knot library
by Petr P 14 Dec '12

14 Dec '12

Dear Haskellers, I'd like to announce a small library "tie-knot": "Ties the knot" on a given set of structures that reference each other by keys - replaces the keys with their respective values. Takes Map k (v k) and converts into Map k v' where v' is the fixed point of v. Motivation: I needed to assemble a finite-state machine from an external description where each node was described by some identifier. I needed a simple library that would replace all key referencess with the actual values. See https://github.com/ppetr/tie-knot for examples. Hackage: http://hackage.haskell.org/package/tie-knot - Petr Pudlak

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How to start with GHC development?
by Janek S. 14 Dec '12

14 Dec '12

Dear list, I would like to learn about internals of GHC and contribute to its development in the future. I read a couple of papers that give a very general overview of GHC (chapter from AoS, papers about inliner and multicore support) and I'm thinking what direction should I pursue now. I was thinking about: a) getting GHC sources, reading the commentary on the GHC wiki and hacking the code b) reading SPJs "Implementing Functional Languages: a tutorial" c) reading SPJs "The Implementation Of Functional Languages" book I would definitely go for a), but I don't know if I'll manage to understand the code without knowing how functional languages are compiled. I have a rather basic knowledge of compilers in general - I took Stanford's online course on Compilers at Coursera. Should I get more theoretical knowledge? Any advice is greatly appreciated. Janek

5 8

Second Call for Copy: Monad.Reader #21
by Edward Z. Yang 13 Dec '12

13 Dec '12

Second Call for Copy: The Monad.Reader - Issue 21 ------------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 21 will be: **Tuesday, January 1** Less than half a month away, but that's what Christmas break is for, right? :-) The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

1 0

ANNOUNCE: hoodle : Pen note-taking program written in haskell
by Ian-Woo Kim 13 Dec '12

13 Dec '12

Hi, Haskellers! I am happy to announce hoodle : pen note-taking program written in haskell. Hoodle is a continuation of the development of hxournal. I renamed it to hoodle, apparently meaning Haskell + doodle. It has undergone many changes and bug fixes. Hopefully, this program is now useful to users. See how it works in action : http://www.youtube.com/watch?v=Z2wzpyxsVSU Hoodle has been uploaded on hackage and is installable using cabal. After the installation of gtk2hs, you can install it by simply typing > cabal install hoodle As a default, pdf annotation is turned off, but if you have installed poppler, then you can install hoodle with pdf-annotation function by typing > cabal install -fpoppler hoodle-render hoodle-core hoodle Hoodle Homepage is http://ianwookim.org/hoodle Have fun with hoodle! Thanks. best, Ian-Woo Kim

1 0

Kernel Loops in Accelerate
by Clark Gaebel 13 Dec '12

13 Dec '12

Hi Trevor (and cafe), I've been playing more and more with accelerate, and I find it quite annoying that there are no loops. It makes implementing many algorithms much harder than it should be. For example, I would love to submit a patch to fix issue #52 [0] on github by implementing MWC64X [1], but it's very hard to port the OpenCL code on that page when it's impossible to write kernel expressions with loops. Also, that means there are no high-level combinators I'm used to for my sequential code (such as map and fold) that would work on an accelerate CUDA kernel. As a nice strawman example, how would one implement the following kernel in accelerate, assuming 'rand_next', 'rand_get', and 'rand_skip' can all be implemented cheaply? : typedef uint64_t rand_state; __device__ rand_state rand_next(rand_state s); __device__ uint32_t rand_get(rand_state s); __device__ rand_state rand_skip(rand_state s, uint64_t distance); __device__ uint32_t round_to_next_pow2(uint32_t n); // Fills an array with random numbers given a random seed, // a maximum random number to generate, and an output // array to put the result in. The output will be in the range // [0, rand_max). __kernel__ void fill_random(rand_state start_state, uint32_t rand_max, uint32_t* out) { rand_state current_state = start_state; int i = blockDim.x*blockIdx.x + threadIdx.x; // assumes we skip less than 1 million times per element... current_state = rand_skip(current_state, i*1e6); uint32_t mask = round_to_next_pow2(rand_max) - 1; uint32_t result; do { result = rand_get(current_state); current_state = rand_next(current_state); } while(result & mask >= rand_max); out[i] = result; } // note: code was neither debugged, run, nor compiled. Thanks, - Clark [0] https://github.com/AccelerateHS/accelerate/issues/52 [1] http://cas.ee.ic.ac.uk/people/dt10/research/rngs-gpu-mwc64x.html

2 2

Control.bimap?
by Gregory Guthrie 13 Dec '12

13 Dec '12

I found a nice idiom for a graph algorithm where the pairs of nodes representing links could be merged into node lists by something like: ns = nub $ map fst g --head nodes ne = nub $ map snd g -- tail nodes And found a nicer approach: (ns,ne) = (nub***nub) unzip g Or perhaps: (ns.ne) = bimap nub nub $ unzip g -- from Control.Bifunctor The SO reference I saw described bimap as a way to map a function over a pair, and it seemed like a great match, but I cannot find the bimap function, and cabal reports no package Control.Bifunctor. ?? -------------------------------------------------------

4 5

Yesod double free or corruption
by Andras Gyomrey 12 Dec '12

12 Dec '12

Hi, i got the following error using "yesod devel" it happened after adding the file which seems to have invalid content (not true). What should i do? Andras Gyomrey

2 1

reviving HaNS (haskell network stack)
by Carter Schonwald 12 Dec '12

12 Dec '12

Hey All, I noticed that the really cool HaNS work (a mostly pure HS + bits of C networking stack) seems to have languished for quite some time, and the absence of a publicly visible repo certainly doesnt help! accordingly, i've taken the most recent code snapshot from the galois archive repo and created a copy on github! https://github.com/cartazio/HaNS I'll be having a go at trying to get things to build on ghc 7.6 + seeing about doing travisCI integration over the next few days, but if anyone else might be interested in helping out, that'd be welcome! cheers -Carter

1 1

Large, numerical calculations in Haskell
by Emil Hedevang 12 Dec '12

12 Dec '12

Hi Haskell Cafe, I need to perform very large numerical computations requiring tens of GB of memory. The computations consist essentially of generation of random numbers and discrete convolutions of large arrays of random numbers with somewhat smaller kernel arrays. Should I use Haskell and call some C libraries when necessary? Should I just write everything in C? Or should I use e.g. Chapel (chapel.cray.com)? To be more specific, a very large array X is filled with random numbers drawn independently from some distribution. Then, the discrete convolution Y = K * X of X with a somewhat smaller kernel array K is calculated. Typical sizes of X and K could be 1000 x 1000 x 1000 and 100 x 100 x 100, respectively. The storage of X alone requires 8 GB of memory. Luckily I have recently been blessed with a well-equipped workstation with 128 GB RAM and two 8-core Xeon E5-2650 running at 2.0 GHz. I see immediate opportunities for parallelism. Firstly, parallel random number generators could be employed to populate the array X (e.g. SPRNG). Secondly, since the size of the kernel K is much smaller than the size of the array X, the discrete convolution can split into several smaller convolutions (see e.g. Kim and Strintzis (1980)). Finally, the discrete convolutions can be computed efficiently using Fourier methods (using e.g. FFTW). It seems fairly straightforward to implement it all in C, perhaps with some OpenMP pragmas scattered around in the code, since the heavy numerics would be performed in preexisting libraries (SPRNG and FFTW), I would like to use Haskell, since I like Haskell and find it tremendously fascinating, but I will not use it for the sake of just using it. I am willing to pay a small price in terms of performance, say, twice the run time, if I in turn will get more convenience and less suffering while coding. Therefore, would you claim it to be feasible to use Haskell? I suppose it very much depends on the quality of the Haskell FFI bindings to parallel random number generators and fast Fourier transforms. Unfortunately, I find it difficult to assess these qualities. Best regards, Emil Hedevang

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How is default method signatures supposed to be used together with Generics
by Johan Tibell 12 Dec '12

12 Dec '12

Hi, I noticed that you're not required to export the types mentioned in the default method signature. For example, you could have: default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int hashWithSalt salt = ghashWithSalt salt . from and not export the GHashable class. However, if users try to define an instance of Hashable but forget to derive Generic: data Foo a = Foo a String deriving (Eq) -- Oops, forgot Generic here instance (Hashable a) => Hashable (Foo a) they get a pretty bad error message: Test.hs:10:10: Could not deduce (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) arising from a use of `Data.Hashable.Class.$gdmhashWithSalt' from the context (Hashable a) bound by the instance declaration at Test.hs:10:10-41 Possible fix: add (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) to the context of the instance declaration or add instance declarations for (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) In the expression: (Data.Hashable.Class.$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt) In the instance declaration for `Hashable (Foo a)' Exporting GHashable would help a little bit in that users would at least know what this GHashable class that the error talks about is. What's best practice? -- Johan

3 4

Profiling slow multiplication of elements read from vector
by Richard Janis Beckert 12 Dec '12

12 Dec '12

Hello everybody! For testing purposes, I punched down a small program which... + puts 2^n elements into an unmutable vector (fromList); + generates a random index in the vector (using random-mersenne); + reads the value at the index i and at i+{-2,-1,1,2} and makes product of these values (making sure the indices stay within the boundary of the vector using `mod 2^n'); + makes a new vector with the product replacing the element at the index; + repeats the whole process 10^7 times starting from the random index. Now, after enforcing a minimum of strictness and profiling the program, I find that for some odd reason the program spends most of its time calculating the product of the read numbers! This is completely contrary to what I expected; namely, that it would spend most of the time either a) generating random numbers, b) reading the vector, or c) making a new vector. But it turns out, that reading the vector or indeed making the new one takes only a fraction of the actual time. I have attached the program and the profile to this email. Could it be that profiling assigns the Cost Centres to the wrong parts of the program? Did I overlook something crucial? Most likely the program could be implemented in a smarter way, but it's pretty close to what my actual program looks like and it shows similar behaviour. (My code, additionally, has a Boltzmann-weighed probability condition on whether the new value gets written to the vector or not.) I would be very happy for some input on this, because I am pretty new to Haskell and I don't really know how to do proper profiling. Cheers Janis (This being my first email to this mailing list, I hope I adhere to all rules of conduct out there; please tell me if I should have written this email differently or more concise.)

2 2

Predicates in data types
by Navid Hallajian 12 Dec '12

12 Dec '12

Hello, I'm a beginner in Haskell, so forgive me if this is a basic question, but I'd like to know if it's possible to have a predicate as part of a data type, so that when the data type is created, it can only be done if it satisfies the predicate else a type error is thrown. For instance, a matrix with integer elements could be modelled as [[Int]], given the restrictions that - it must have at least one column and one row (so there must be at least one list), and - every list must have the same length so that when a matrix is created, the type system wont allow it if the predicates aren't met. Thanks, Navid

3 2

Haskell Advent Calendar in Japan
by Kazu Yamamoto 12 Dec '12

12 Dec '12

Hello, This is just for your information. Haskell Advent Calendar is going on in Japanese Haskell Community: http://partake.in/events/45a01d39-af5e-42f1-91c7-e8fcc91db244 One article is evaluated lazily but other articles are in time. :-) --Kazu

1 0

ANN: diagrams-0.6
by Brent Yorgey 12 Dec '12

12 Dec '12

I am pleased to announce the release of version 0.6 of diagrams [1], a full-featured framework and embedded domain-specific language for declarative drawing. Check out the gallery [2] for examples of what it can do! [3,4] [1] http://projects.haskell.org/diagrams [2] http://projects.haskell.org/diagrams/gallery.html [3] http://mathlesstraveled.com/2012/11/05/more-factorization-diagrams/ [4] http://projects.haskell.org/diagrams/gallery/PythagoreanTree.html Highlights of this release include: - Diagrams now comes with a native-Haskell SVG backend [5] by default. If you were holding off on trying diagrams because you couldn't install cairo, you no longer have an excuse! - Proper support for subdiagrams: previous versions of diagrams-core had a mechanism for associating names with a pair of a location and an envelope. Now, names are associated with actual subdiagrams (including their location and envelope, along with all the other information stored by a diagram). This enables cool techniques like constructing a diagram in order to position its subelements and then taking it apart again, or constructing animations via keyframing. - Traces: in addition to an envelope, each diagram now stores a "trace", which is like an embedded raytracer: given any ray (represented by a base point and a vector), the trace computes the closest point of intersection with the diagram along the ray. This is useful for determining points on the boundary of a diagram, e.g. when drawing arrows between diagrams. - The core data structure underlying diagrams has been completely refactored and split out into its own separate package, dual-tree [6]. - Support for GHC 7.6. - Many more new features, bug fixes, and improvements! See the release notes [7] for complete details, and the diagrams wiki [8] for help migrating from 0.5 to 0.6. [5] http://hackage.haskell.org/package/diagrams%2Dsvg [6] http://hackage.haskell.org/package/dual%2Dtree [7] http://projects.haskell.org/diagrams/releases.html Try it out ---------- For the truly impatient: cabal install diagrams Diagrams is supported under GHC 7.0 through 7.6, with the exception that the cairo and gtk backends do not build under GHC 7.0 (but the SVG backend does), and the gtk backend does not build under GHC 7.6. To get started with diagrams, read the quick tutorial [9], which will introduce you to the fundamentals of the framework. For those who are less impatient and want to really dig in and use the power features, read the user manual [10]. [8] http://www.haskell.org/haskellwiki/Diagrams/Migrate0.6 [9] http://projects.haskell.org/diagrams/tutorial/DiagramsTutorial.html [10] http://projects.haskell.org/manual/diagrams-manual.html Get involved ------------ Subscribe to the project mailing list [11], and/or come hang out in the #diagrams IRC channel on freenode.org for help and discussion. Make some diagrams. Fix some bugs [12]. Submit your cool examples for inclusion in the gallery [13] or your cool code for inclusion in the diagrams-contrib [14] package! [11] http://groups.google.com/group/diagrams-discuss [12] http://github.com/diagrams/ [13] http://projects.haskell.org/diagrams/gallery.html [14] http://hackage.haskell.org/package/diagrams%2Dcontrib Happy diagramming! Brought to you by the diagrams team: - Michael Sloan - Ryan Yates - Brent Yorgey with contributions from: - Sam Griffin - Niklas Haas - Peter Hall - Claude Heiland-Allen - Deepak Jois - John Lato - Felipe Lessa - Chris Mears - Ian Ross - Vilhelm Sjöberg - Jim Snavely - Luite Stegeman - Kanchalai Suveepattananont - Michael Thompson - Scott Walck

2 1

C++
by mukesh tiwari 11 Dec '12

11 Dec '12

Hello All I am trying to transform this C++ code in Haskell. In case any one interested this is solution of SPOJ <http://www.spoj.com/problems/DIEHARD/>problem. #include<cstdio> #include<iostream> #include<cstring> using namespace std; int memo[1100][1100] ; int recurse( int h , int a , int cnt , bool flag ) { if ( h <= 0 || a <= 0 ) return cnt ; if ( memo[h][a] ) return memo[h][a] ; if ( flag ) memo[h][a] = recurse ( h + 3 , a + 2 , cnt + 1 , !flag ) ; else memo[h][a] = max ( memo[h][a] , max ( recurse ( h - 5 , a - 10 , cnt + 1 , !flag ) , recurse ( h - 20 , a + 5 , cnt + 1 , !flag ) ) ) ; return memo[h][a]; } int main() { int n , a , b ; scanf( "%d", &n ); for(int i = 0 ; i < n ; i++) { memset ( memo , 0 , sizeof memo ) ; scanf("%d%d", &a , &b ); printf("%d\n" , recurse( a , b , -1 , 1 )); if( i != ( n - 1 ) ) printf("\n"); } } I am stuck with that memo[1100][1100] is global variable so I tried to solve this problem using state monad ( Don't know if its correct approach or not ) but it certainly does not seem correct to me. Till now I came up with code. Could some one please tell me how to solve this kind of problem ( Generally we have a global variable either multi dimensional array or map and we store the best values found so far in the table ). import qualified Data.Map.Strict as SM import Control.Monad.State {-- funsolve_WF :: Int -> Int -> Int -> Int funsolve_WF h a cnt | h <= 0 || a <= 0 = cnt | otherwise = funsolve_Air h a ( cnt + 1 ) funsolve_Air :: Int -> Int -> Int -> Int funsolve_Air h a cnt = max ( funsolve_WF ( h + 3 - 5 ) ( a + 2 - 10 ) cnt' ) ( funsolve_WF ( h + 3 - 20 ) ( a + 2 + 5 ) cnt' ) where cnt' = cnt + 1 --} funSolve :: Int -> Int -> Int -> Bool -> State ( SM.Map ( Int , Int ) Int ) Int funSolve hl am cnt f | hl <= 0 && am <= 0 = return cnt | otherwise = do mp <- get case () of _| SM.member ( hl , am ) mp -> return mp SM.! ( hl , am ) | f -> do --here I have to insert the value return by function funSolve ( hl + 3 ) ( am + 2 ) ( cnt + 1 ) ( not f ) to map whose key is ( hl , am ) let k = evalState ( funSolve ( hl + 3 ) ( am + 2 ) ( cnt + 1 ) ( not f ) ) mp modify ( SM.insert ( hl , am ) k ) | otherwise -> do do let k_1 = evalState ( funSolve ( hl - 5 ) ( am - 10 ) ( cnt + 1 ) ( not f ) ) mp k_2 = evalState ( funSolve ( hl - 20 ) ( am + 5 ) ( cnt + 1 ) ( not f ) ) mp k_3 = mp SM.! ( hl , am ) modify ( SM.insert ( hl , am ) ( maximum [ k_1 , k_2 , k_3 ] ) ) Regards Mukesh Tiwari

3 2

Moscow Haskell Users Group (MskHUG) December meeting.
by Serguey Zefirov 11 Dec '12

11 Dec '12

I would like to announce MskHUG December meeting and invite everyone interested. The meeting will take place December 13th, 20:00 to 23:30 in the nice conference center in centre of Moscow: http://www.nf-conference.ru/ The meeting's agenda is to start more intense discussions. Most probably, there will be a couple of short presentations - I can tell about creating fast Haskell programs to handle large data to start the discussion. I will bring white paper and pencils for everyone to write and draw, there will be projector and screen and also tea, cofee and snacks. If you want to participate, please, email me.

5 4

GHCi + FFI + global C variables
by Nils 11 Dec '12

11 Dec '12

I'm currently working with a C library that needs to use/modify global C variables, for example: igraph_bool_t igraphhaskell_initialized = 0; int igraphhaskell_initialize() { if (igraphhaskell_initialized != 0) { printf("C: Not initializing. igraphhaskell_initialized = %i\n", igraphhaskell_initialized); return 1; } // initialization } If I compile an example programm using this library everything works fine, but if I try to run the same program in GHCi it dies with the message C: Not initializing. igraphhaskell_initialized = -907777 The value (and apparently the adress of the global variable) is completly off, and I have no idea what is causing this or how to solve this issue and make the library GHCi-friendly. Is it possible to run this code in GHCi at all? Also, since it's a foreign library I obviously cannot just change the C code to simply not use any global variables at all. - Nils

5 4

Variable-arity zipWith (re)invented.
by Takayuki Muranushi 11 Dec '12

11 Dec '12

Continued discussion from https://groups.google.com/d/topic/haskell-cafe/-e-xaCEbd-w/discussion https://groups.google.com/d/topic/haskell-cafe/kM_-NvXAcx8/discussion Thank you for all the answeres and thinkings; Here's zipWithN for general Zip functors: [1] . This, together with [2] may constitute a small hackage. A modification from Wren's idea to [1] is the use of fmap instead of repeat. I'm wondering if there are any laws for Zip functors. I first thought that there are similarity between Zips and Applicatives, as [3] states > instance Applicative f => Zip f where > zip = liftA2 (,) However, my intuition is that zipping two arrays should result in an array of size of the same order as two, giving rise to a Zip functor law candidate: zipWith const xs $ zipWith const xs ys == zipWith const xs ys which is violated by the above statement "zip = liftA2 (,)" . [1] https://github.com/nushio3/practice/blob/master/variable-arity/ZipWithN-2.hs [2] https://github.com/nushio3/practice/blob/master/free-objects/zipf-12.hs [3] http://hackage.haskell.org/packages/archive/TypeCompose/0.9.7/doc/html/Data… -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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mtl: Why there is "Monoid w" constraint in the definition of class MonadWriter?
by Petr P 10 Dec '12

10 Dec '12

The class is defined as > class (Monoid w, Monad m) => MonadWriter w m | m -> w where > ... What is the reason for the Monoid constrait? It seems superfluous to me. I recompiled the whole package without it, with no problems. Of course, the Monoid constraint is necessary for most _instances_, like in > instance (Monoid w, Monad m) => MonadWriter w (Lazy.WriterT w m) where > ... but this is a different thing - it depends on how the particular instance is implemented. I encountered the problem when I needed to define an instance where the monoidal structure is fixed (Last) and I didn't want to expose it to the user. I wanted to spare the user of of having to write Last/getLast everywhere. (I have an instance of MonadWriter independent of WriterT, its 'tell' saves values to a MVar. Functions 'listen' and 'pass' create a new temporary MVar. I can post the detail, if anybody is interested.) Would anything break by removing the constraint? I think the type class would get a bit more general this way. Thanks for help, Petr Pudlak

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cabal configure && cabal build && cabal install
by Albert Y. C. Lai 10 Dec '12

10 Dec '12

Among many programmers, and/or users who manually unpack source tarball before installing, this idiom is very common: cabal configure cabal build cabal install This idiom is an urban legend, i.e., a popular error. "cabal install" re-does the "configure" and the "build" steps, among other things. That it has not caused trouble for you so far is only because: 1. "configure" has been idempotent, so far 2. the 2nd "build" has not caused duplicate work because ghc knows when not to re-compile One day, you will run into a surprise, because you will add some flags to "configure", and then... cabal configure --prefix=/nondefault --enable-shared cabal build cabal install Whee! Your laborously entered flags are lost on deaf ears. You get the default prefix, and you get the default disable-shared. Why? Because "cabal install" re-does the "configure" step, but this time, since you give no flags to "install", you give no flags to the 2nd "configure" either. If you begin with "cabal configure", the correct idiom is: cabal configure [flags] cabal build [cabal haddock, if you want] cabal copy cabal register If you find it too long, why not just begin with: cabal install [flags] [--enable-documentation, if you want]

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Fwd: education or experience?
by Eli Frey 10 Dec '12

10 Dec '12

woops, forgot to reply all ---------- Forwarded message ---------- From: Eli Frey <eli.lee.frey(a)gmail.com> Date: Sun, Dec 9, 2012 at 4:18 PM Subject: Re: [Haskell-cafe] education or experience? To: Mike Meyer <mwm(a)mired.org> So, to put my words in perspective: I am a 23 year old who dropped out when they were 16. I only just entered industry, so it's difficult to say where I have gaps in my knowledge or what I could have gotten if I'd been to college. That being said, I can't really say I'd rather to have done it any other way. Jerzy makes a good point that you might not be the best judge of what you should learn. You arn't yet an expert, and having one help guide you could be very valuable. I certainly wish I'd had a mentor through my experience. At the same token, you might just be the most qualified person to say what you should learn, exactly because you will be drawn to what fascinates you. If you're already showing interest in Haskell, there is a great deal of material produced by the community that I'm sure will wet your apatite in the right directions When I entered the industry, I got the attention of many well-respected companies. Sure there were some places that turned me away because I didn't have a degree, but they weren't the kinds of places I'de like to work anyway. At the end of the day, my dream employer said "come back and talk with us when you have more experience," and I landed a job where I enjoy my boss (an under-considered perk, IMHO. This is worth many $ to me), am given freedom/ownership of my work, and am greatly appreciated and nurtured in my growth. I often chuckle to myself at night about my freedom from student loans, too :P. Cheers, Eli On Sun, Dec 9, 2012 at 2:44 PM, Mike Meyer <mwm(a)mired.org> wrote: > > > Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > >Christopher Howard wrote: > >Concerning a university education, there are two approaches> > >1. I want to learn as much as possible > >2. I want to learn just enough to get a high-paying job > > There's actually a third approach ( and probably more): > > 3. I want to learn to do this job as well as possible. > > >On the other hand, approaching university from the second point of view > > > >usually does not justify the cost for the little benefit you obtain > >this > >way. Unfortunately, it seems to me that the tuition costs in the U.S. > >strongly suggest the second approach. To avoid this, I recommend to > >either go abroad or become very good and acquire a scholarship. > > That really depends on the job at in question. When I was looking for > entry level programming jobs, not having a degree meant you never got past > the hr department. Getting a degree (pretty much any degree) was required > to get the high-paying job. I'm willing to believe that's no longer the > case for programmers, because academia has consistently failed to deliver > sufficient quality programmers to meet industry needs. On the other hand > (watching my sons deal with the job market), the litmus test for "you've > got what it takes to survive in the system" is now a masters, not a > bachelors, so maybe you're wrong about that. > > The other thing to consider is what your long-term goals are. Do you want > to be a code monkey all your life? Or do you aspire to more? What are the > requirements for that "more"? Getting a degree now may well avoid doing it > later. > > Finally, with approach #3, you really need a mentor who can tell you > whether or not you're doing a competent job. You're much more likely to > find that in a university environment than trying to learn things by > yourself. Joining an open source project might get it for you. > -- > Sent from my Android tablet with K-9 Mail. Please excuse my swyping. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > On Sun, Dec 9, 2012 at 4:18 PM, Eli Frey <eli.lee.frey(a)gmail.com> wrote: > So, to put my words in perspective: I am a 23 year old who dropped out > when they were 16. I only just entered industry, so it's difficult to say > where I have gaps in my knowledge or what I could have gotten if I'd been > to college. That being said, I can't really say I'd rather to have done it > any other way. > > Jerzy makes a good point that you might not be the best judge of what you > should learn. You arn't yet an expert, and having one help guide you could > be very valuable. I certainly wish I'd had a mentor through my > experience. At the same token, you might just be the most qualified person > to say what you should learn, exactly because you will be drawn to what > fascinates you. If you're already showing interest in Haskell, there is a > great deal of material produced by the community that I'm sure will wet > your apatite in the right directions > > When I entered the industry, I got the attention of many well-respected > companies. Sure there were some places that turned me away because I > didn't have a degree, but they weren't the kinds of places I'de like to > work anyway. At the end of the day, my dream employer said "come back and > talk with us when you have more experience," and I landed a job where I > enjoy my boss (an under-considered perk, IMHO. This is worth many $ to > me), am given freedom/ownership of my work, and am greatly appreciated and > nurtured in my growth. > > I often chuckle to myself at night about my freedom from student loans, > too :P. > > Cheers, > Eli > > > > On Sun, Dec 9, 2012 at 2:44 PM, Mike Meyer <mwm(a)mired.org> wrote: > >> >> >> Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: >> >Christopher Howard wrote: >> >Concerning a university education, there are two approaches> >> >1. I want to learn as much as possible >> >2. I want to learn just enough to get a high-paying job >> >> There's actually a third approach ( and probably more): >> >> 3. I want to learn to do this job as well as possible. >> >> >On the other hand, approaching university from the second point of view >> > >> >usually does not justify the cost for the little benefit you obtain >> >this >> >way. Unfortunately, it seems to me that the tuition costs in the U.S. >> >strongly suggest the second approach. To avoid this, I recommend to >> >either go abroad or become very good and acquire a scholarship. >> >> That really depends on the job at in question. When I was looking for >> entry level programming jobs, not having a degree meant you never got past >> the hr department. Getting a degree (pretty much any degree) was required >> to get the high-paying job. I'm willing to believe that's no longer the >> case for programmers, because academia has consistently failed to deliver >> sufficient quality programmers to meet industry needs. On the other hand >> (watching my sons deal with the job market), the litmus test for "you've >> got what it takes to survive in the system" is now a masters, not a >> bachelors, so maybe you're wrong about that. >> >> The other thing to consider is what your long-term goals are. Do you want >> to be a code monkey all your life? Or do you aspire to more? What are the >> requirements for that "more"? Getting a degree now may well avoid doing it >> later. >> >> Finally, with approach #3, you really need a mentor who can tell you >> whether or not you're doing a competent job. You're much more likely to >> find that in a university environment than trying to learn things by >> yourself. Joining an open source project might get it for you. >> -- >> Sent from my Android tablet with K-9 Mail. Please excuse my swyping. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

> > Lazy evaluation--at the heart of spreadsheets since the beginning. > Never thought of that -- nice! Unfortunately it's not literally true, because spreadsheets push recalculated values to all the variables that depend on them, rather than waiting until the dependent values are needed. But the idea that sequencing is by dependency rather than by algorithmic specification is the same. I remember playing around (before spreadsheets) with the idea for Fortran-like code and concluding that the benefit of not having to specify sequence wasn't worth the effort. What a lack of imagination! > > Pattern matching--native to string processing (e.g. COMIT, SNOBOL). > > Appeared nearly in its present form in COGENT (1965). > Hmm google gives me different cogents/ I guess you are referring to the > Reynolds one? Yes. > > Booleans as first class*--surely this is a joke. Algol 60 had them. > Not sure what you are saying (unless its about the footnote that Boole > treated bools as ints! This is new to me) I was referring to the fact that > C programmers have great difficulty thinking of bools as first class Your observation is interesting. Booleans and their utility were well known when C was designed. But since the language level was deliberately very close to real machine architecture, which rarely had native support for 1-bit quantities, the data type was deemed superfluous. (If C had begun on the IBM 7030 it probably would have have had Booleans.) I don't remember anybody predicting that the choice risked banishing Booleans from mind. The one deviation from real architecture that C did embrace was to define a representation for the result of comparison. That had another unpredicted consequence: computer architects imitated the language in hardware! (C's largely ignored bitfield capability came later and, if anything, demoted Booleans to an artifact of structs.) The joke I saw was that there was no way that Boolean values had ever been denied first-class citizenship as Strachey defined it--even if they didn't have a special syntactic identity. > > Matlab exploits them heavily (though represented as doubles). > Not sure what you are referring to I had in mind Matlab's frequent use of a Boolean array as a characteristic function describing some property of the elements of another array. Such arrays are used to control other array operations. In this usage, the characteristic array may be optimized out of existense, but it fosters higher-level thinking and concise code. In reply to your second note, Malcolm Wallace is right. I did intend to say that FP was late with lazy evaluation. My mistake was to think VisiCalc had appeared before 1970. Similarly, I believed that call by need came into our vocabulary soon after Algol 60 brought call be name and call by value to our attention as somewhat better-behaved models than Fortran's call by reference. Now I suspect that belief is wrong, too. Doug

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