- Haskell-Cafe - Haskell.org

ICFP09 Final Call for Participation
by Matthew Fluet (ICFP Publicity Chair) 13 Jul '09

13 Jul '09

===================================================================== Final Call for Participation The 14th ACM SIGPLAN International Conference on Functional Programming (ICFP 2009) http://www.cs.nott.ac.uk/~gmh/icfp09.html Edinburgh, Scotland, 31 August - 2 September 2009 ===================================================================== ***** Accommodation Deadline: July 20, 2009 ***** Due to the overlap with Edinburgh Festival, accommodations may be difficult to secure after the deadline. Reserve now! http://www.haskell.org/haskellwiki/ICFP_2009_Local_Arrangements ***** Early Registration Deadline: July 30, 2009 ***** http://www.regmaster.com/conf/icfp2009.html ICFP 2009 provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. Preliminary program: * Abstracts: + http://web.cecs.pdx.edu/~apt/icfp09_accepted_papers/accepted.html * Schedule: + http://web.cecs.pdx.edu/~apt/icfp09_preliminary_program.pdf * Invited speakers: + Guy Steele -- Organizing Functional Code for Parallel Execution: or, foldl and foldr Considered Slightly Harmful + Benjamin Pierce -- Lambda, the Ultimate TA: Using a Proof Assistant to Teach Programming Language Foundations + Dan Piponi -- Commutative Monads, Diagrams and Knots Schedule including related workshops: * Aug 30: ACM SIGPLAN Workshop on ML * Aug 30: ACM SIGPLAN Workshop on Generic Programming * Aug 31-Sep 2: ICFP09 * Sep 3: ACM SIGPLAN Haskell Symposium * Sep 3: ACM SIGPLAN Developer Tracks on Functional Programming * Sep 4: Commercial Users of Functional Programming * Sep 4: ACM SIGPLAN Workshop on Mechanizing Metatheory * Sep 4: ACM SIGPLAN Workshop on Approaches and Applications of Inductive Programming * Sep 5: ACM SIGPLAN Erlang Workshop * Sep 5: ACM SIGPLAN Developer Tracks on Functional Programming * Sep 5: ACM SIGPLAN Haskell Implementors Workshop Conference organizers: * General Chair: Graham Hutton (University of Nottingham) * Program Chair: Andrew Tolmach (Portland State University) * Local Arrangements Chairs: Philip Wadler (University of Edinburgh), Kevin Hammond (University of St Andrews), and Gregory Michaelson (Heriot-Watt University) * Workshop Co-Chairs: Christopher Stone (Harvey Mudd College), and Michael Sperber (DeinProgramm) * Programming Contest Chair: Andrew Gill (University of Kansas) * Publicity Chair: Matthew Fluet (Toyota Technological Institute at Chicago)

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Haskell Weekly News: Issue 125 - July 12, 2009
by Brent Yorgey 12 Jul '09

12 Jul '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090712 Issue 125 - July 12, 2009 --------------------------------------------------------------------------- Welcome to issue 125 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Announcing the new Haskell Prime process, and Haskell 2010. Simon Marlow [2]announced the new and improved [3]Haskell Prime process, and a list of proposals which are currently under consideration to be accepted into the next (2010) revision of the standard. AspectAG 0.1.1. Marcos Viera [4]announced the release of [5]AspectAG, a library of strongly typed Attribute Grammars implemented using type-level programming. Colour tutorial. Russell O'Connor [6]announced a [7]tutorial wiki page for the [8]colour library. Haskell Hack Day, Edinburgh, 30 August 2009. Eric Kow [9]announced a [10]Haskell Hack Day to be held in Edinburgh, on Sunday 30 August, before ICFP. The Hack Day will be held at the ICFP conference venue, the Royal College of Physicians. sendfile-0.3. Matthew Elder [11]announced the release of [12]sendfile-0.3, which includes a more general interface, updated documentation, and more. Gtk2hsGenerics. Michael Dever [13]announced the release of [14]Gtk2hsGenerics, a package which contains utility functions for extracting from and adding to stores. darcs 2.3 beta 2. Petr Rockai [15]announced that darcs 2.3 beta 2 is available for testing. It can be installed using the [16]darcs-beta package on Hackage (be sure to 'cabal update' first). The new beta release adds index upgrade functionality to hashed-storage, and now uses an architecture-independent index format. hsparql, a SPARQL query generator/DSL and client. Jeff Wheeler [17]announced the first version of [18]hsparql, which makes it easy to query SPARQL-compliant servers using a relatively intuitive DSL and very simple client. SPARQL is a SQL-ish query language for stores of RDF data. Hayoo! beta 0.4. Timo B. [19]announced the next beta version 0.4 of [20]Hayoo!, the Haskell API search engine with find-as-you-type and suggestions. This release includes some major changes to the web interface, including the ability to display the full description of a function, and some example queries featured on the start page, as well as an updated search index. texmath 0.1.0.1 - conversion of LaTeX math to MathML. John MacFarlane [21]announced an early release of [22]texmath, a Haskell library for converting LaTeX math formulas to MathML. The package includes a standalone test program, testTeXMathML, and a CGI script, texmath-cgi, that can be used in web apps. You can see demos of the script [23]here and [24]here. BostonHaskell: Next meeting - July 16th at MIT CSAIL Reading Room (32-G882). Ravi Nanavati [25]announced the July meeting of the [26]Boston Area Haskell Users' Group, to be held Thursday, July 16th from 6:30pm - 8:30pm in the MIT CSAIL Reading Room. Scheduled talks include "An Introduction to GHC Hacking" by Alec Heller, and "Haskell on the iPhone" by Ryan Trinkle. There are still openings for Lightning Talks (5-minute talk, 2-minute Q&A). See the announcement for more details. AC-Vector, AC-Colour and AC-EasyRaster-GTK. Andrew Coppin [27]announced the release of three packages: [28]AC-Vector, which provides unboxed vectors of Doubles with arithmetic, dot product and cross product, and a few other useful items; [29]AC-Colour, which provides two simple RGB color types, optimized for simplicity and speed; and [30]AC-EasyRaster-GTK, a layer over Gtk2hs which provides easy pixel-twiddling functionality. Google Summer of Code Progress updates from participants in the 2008 [31]Google Summer of Code. Haddock improvements. Isaac Dupree has had a [32]good deal of success getting cross-package documentation to work, with a few remaining loose ends. space profiling. Gergely Patai's profile graphing application [33]is now capable of connecting to a profile relay server that can broadcast the heap profile of its associated process on the fly, so it is now possible to attach an observer to a program that was started earlier. He now plans to shift to working on a history manager. fast darcs. Petr Rockai has produced [34]another darcs 2.3 beta release, and has done more work on [35]designing an efficient storage system. Discussion Leaner Haskell.org frontpage. haskell [36]proposed a new design for the haskell.org front page. Bikeshedding ensues, news at 11. exercise - a completely lazy sorting algorithm. Petr Pudlak [37]asked whether it is possible to write a lazy sorting algorithm which allows access to the kth item of the sorted output in linear time, for all k. An interesting discussion and mind-expanding code followed. Implementing Las Vegas algorithms in Haskell. Matthias Görgens began a [38]discussion on implementing Las Vegas algorithms, which use a source of randomness but have results which are still deterministic. What should the type of such functions be? Is it OK to use unsafePerformIO in their definition? Blog noise [39]Haskell news from the [40]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Petr Rockai: [41]Designing Storage for darcs. * >>> haskelladdict: [42]Having fun sorting with Haskell. * Jeff Heard: [43]C2HS example: To save other people frustration. * JP Moresmau: [44]Mazes of Monad: 1.0.5 and (maybe) final version uploaded. * Sean Leather: [45]Fun and generic things to do with EMGM at the London HUG. * Greg Bacon: [46]Gyrigrams. * Magnus Therning: [47]Making a choice from a list in Haskell, Vty (part 2). * Jeff Heard: [48]A quick buster example. * Jeff Heard: [49]An ugly force-directed layout implementation. * Petr Rockai: [50]darcs 2.3 beta 2. * Isaac Dupree: [51]Cross-package documentation going well!. * >>> dasuxullebt: [52]I (don't) like Haskell. * Holumbus: [53]Some Hayoo! Improvements. * David Amos: [54]Conjugacy classes, part 2. * Michael Snoyman: [55]hack-handler-webkit. * Dan Piponi (sigfpe): [56]A Monad for Combinatorial Search with Heuristics. * Greg Bacon: [57]A programmable semicolon explained. * Holumbus: [58]Another Hayoo! Update. * Gergely Patai: [59]Remote profiling at your fingertips. * The GHC Team: [60]Visualising the Haskell package dependency graph. * Bryan O'Sullivan: [61]First steps with Haskell text API improvement. * Paul Hudak: [62]Haskell and the Arts. * >>> John Gunderman: [63]Haskell Cabal in Ubuntu. * Sebastian Sylvan: [64]Ray tracing signed distance functions. Quotes of the Week * SimonPJ: [re: class constraints on data declarations] In GHC's source code these contexts are consistently called stupid_theta. * JN: this tutorial just introduced _|_ and called it bottom. I assume that's because it looks like an ass? * uman: so you can pass functions around as objects... this sounds like JavaScript * Twey: Mr. Wing, sir, if you are listening: you have the I.Q. of a semolina pudding. And your homepage is full of <font> tags. I'm not sure which is the greater insult, but at least one is objectively true. * jmcarthur: #haskell: Overwhelmingly helpful. * lilac: class Monad m where / return and Kleisli compose / must form a monoid <lilac> that's my new monad tutorial haiku <jmcarthur> i think that is the best monad tutorial i have ever read * dcoutts: make sure happy is on your path and it'll all be ok * ski: my answer to that is : don't deduce, denote! * roco: guys i have problem, does anybody know programming ? * dhjdhj: All new features added to C++ are intended to fix previously new features added to C++ About the Haskell Weekly News New editions are posted to [65]the Haskell mailing list as well as to [66]the Haskell Sequence and [67]Planet Haskell. [68]RSS is also available, and headlines appear on [69]haskell.org. To help create new editions of this newsletter, please see the information on [70]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [71]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/haskell/2009-July/021472.html 3. http://hackage.haskell.org/trac/haskell-prime/wiki/Process 4. http://www.haskell.org//pipermail/haskell/2009-July/021475.html 5. http://hackage.haskell.org/package/AspectAG 6. http://www.haskell.org//pipermail/haskell-cafe/2009-July/064065.html 7. http://www.haskell.org/haskellwiki/Colour 8. http://hackage.haskell.org/package/colour 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17345 10. http://www.haskell.org/haskellwiki/Hac7 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17343 12. http://hackage.haskell.org/package/sendfile 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17336 14. http://hackage.haskell.org/package/Gtk2hsGenerics 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61071 16. http://hackage.haskell.org/package/darcs%2Dbeta 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61062 18. http://hackage.haskell.org/package/hsparql 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61060 20. http://holumbus.fh-wedel.de/hayoo 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61049 22. http://hackage.haskell.org/package/texmath 23. http://johnmacfarlane.net/texmath.xhtml 24. http://johnmacfarlane.net/texmath.html 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61003 26. http://www.haskell.org/haskellwiki/Boston_Area_Haskell_Users%27_Group 27. http://article.gmane.org/gmane.comp.lang.haskell.cafe/60944 28. http://hackage.haskell.org/package/AC-Vector 29. http://hackage.haskell.org/package/AC-Colour 30. http://hackage.haskell.org/package/AC-EasyRaster-GTK 31. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 32. http://haddock2009.wordpress.com/2009/07/09/cross-package-documentation-goi… 33. http://just-bottom.blogspot.com/2009/07/remote-profiling-at-your-fingertips… 34. http://web.mornfall.net/blog/darcs_2.3_beta_2.html 35. http://web.mornfall.net/blog/designing_storage_for_darcs.html 36. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61082 37. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60981 38. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60998 39. http://planet.haskell.org/ 40. http://haskell.org/haskellwiki/Blog_articles 41. http://web.mornfall.net/blog/designing_storage_for_darcs.html 42. http://haskelladdict.wordpress.com/2009/07/11/having-fun-sorting-with-haske… 43. http://vis.renci.org/jeff/2009/07/10/c2hs-example-to-save-other-people-frus… 44. http://jpmoresmau.blogspot.com/2009/07/mazes-of-monad-105-and-maybe-final.h… 45. http://feedproxy.google.com/~r/splonderzoek/~3/qnt94yp_Er4/fun-and-generic-… 46. http://feedproxy.google.com/~r/gbacon/~3/QKPjQ0Fxl_4/gyrigrams.html 47. http://therning.org/magnus/archives/689 48. http://vis.renci.org/jeff/2009/07/09/a-quick-buster-example/ 49. http://vis.renci.org/jeff/2009/07/09/an-ugly-force-directed-layout-implemen… 50. http://web.mornfall.net/blog/darcs_2.3_beta_2.html 51. http://haddock2009.wordpress.com/2009/07/09/cross-package-documentation-goi… 52. http://uxul.wordpress.com/2009/07/09/i-dont-like-haskell/ 53. http://holumbus.fh-wedel.de/blog/?p=24 54. http://haskellformaths.blogspot.com/2009/07/conjugacy-classes-part-2.html 55. http://blog.snoyman.com/2009/07/08/hack-handler-webkit/ 56. http://blog.sigfpe.com/2009/07/monad-for-combinatorial-search-with.html 57. http://feedproxy.google.com/~r/gbacon/~3/UGYWKr-huLY/programmable-semicolon… 58. http://holumbus.fh-wedel.de/blog/?p=23 59. http://just-bottom.blogspot.com/2009/07/remote-profiling-at-your-fingertips… 60. http://ghcmutterings.wordpress.com/2009/07/06/visualising-the-haskell-packa… 61. http://www.serpentine.com/blog/2009/07/06/first-steps-with-haskell-text-api… 62. http://www.infoq.com/presentations/haskell-arts-paul-hudak 63. http://sporkcode.wordpress.com/2009/07/05/haskell-cabal-in-ubuntu/ 64. http://sebastiansylvan.spaces.live.com/blog/cns!4469F26E93033B8C!173.entry 65. http://www.haskell.org/mailman/listinfo/haskell 66. http://sequence.complete.org/ 67. http://planet.haskell.org/ 68. http://sequence.complete.org/node/feed 69. http://haskell.org/ 70. http://haskell.org/haskellwiki/HWN 71. http://code.haskell.org/~byorgey/code/hwn/

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Haskell used for data analysis - OLAP?
by Günther Schmidt 12 Jul '09

12 Jul '09

Hi, I've developed this commercial app in Haskell with all of the business logic coded in SQL with the help of haskelldb. Some of the intermediate results (of queries) I had to manifest in extra tables because the initial query was expensive, the intermediate result would be the "source" data of other queries and queries can't themselves be indexed for further efficient querying. Since the amount of data could become rather large, in-memory processing wasn't an option and I thus chose SQL (Sqlite). The solution works, but still I'm not quite happy about it, since I consider the tables that hold intermediate results fixes. I understand that the financial industry employs Haskell to analyse large data set under complex schemes. I wonder what techniques are employed for that, do they use Haskell to create some sort of OLAPish tools? Do they use Haskell to run complex queries against SQL data warehouses? Günther

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Text.JSON, Speed and Bytestrings
by Max Cantor 11 Jul '09

11 Jul '09

Hi Cafe, I am using the Text.JSON library to [un]marshall messages passed over the network and was wondering if the speed would be significantly improved by either changing the code or adding a module to implement the same functionality using Bytestrings instead of classical strings? If the consensus is that it would be worth it, I'll look into adding that code.. Thanks, Max

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About the return type
by xu zhang 11 Jul '09

11 Jul '09

I have trouble in returning a list of Figures. I want return a type of m (Maybe [Figure IO]), but the type of dv_findFigure is :: a -> Point -> s (Maybe (Figure s)). How can change the code below to get a s (Maybe [Figure s])? Thank you in advance! dv_findFigure :: a -> Point -> s (Maybe (Figures)) fig_contains :: fig -> Point -> m Bool anc :: Point do fs <- dv_getSelFigs dv fs' <- filterM (`fig_contains` anc) fs f <- case fs' of [] -> dv_findFigure dv anc fig : _ -> return $ Just fig case f of Just f' -> tool_dragtrack self f' Nothing -> dv_clearSel dv >> tool_areatrack self Couldn't match expected type `Figure m' against inferred type `[Figure IO]' Expected type: m (Maybe (Figure m)) Inferred type: m (Maybe [Figure IO]) In the expression: return $ Just fs In a case alternative: fig : _ -> return $ Just fs

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following up on space leak
by Uwe Hollerbach 11 Jul '09

11 Jul '09

Good evening, all, following up on my question regarding space leaks, I seem to have stumbled across something very promising. I said I was using this tiny function "lastOrNil" to get the last value in a list, or the empty (scheme) list if the haskell list was empty. The uses of it were all of the form lastOrNil (mapM <something> <some list>) so I wrote a different function mapML to do this directly: > mapML fn lst = mapMLA (List []) fn lst > where mapMLA r _ [] = return r > mapMLA ro fn (x:xs) = > do rn <- fn x > mapMLA rn fn xs This isn't an accumulator, it's a replacer (or, if you like, the "accumulation" is "drop the old one on the floor"), it starts out with the scheme empty list that I want as the default, and it never even builds the list which it'll just dump an instant later. Shazam! Memory usage dropped by roughly an order of magnitude in my little Collatz benchmark, and incidentally runtime improved by 25% or so as well. The horror! :-) Having tasted blood, I will of course be continuing to benchmark... but not tonight. Uwe

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get cabal info for self?
by Keith Sheppard 11 Jul '09

11 Jul '09

Is there a way for a cabalized program to get its own info. I'm specifically interested in version info. Thanks Keith -- keithsheppard.name

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Flipping *->*->* kinds, or monadic finally-tagless madness
by Kim-Ee Yeoh 11 Jul '09

11 Jul '09

I'm trying to write HOAS Show instances for the finally-tagless type-classes using actual State monads. The original code: http://okmij.org/ftp/Computation/FLOLAC/EvalTaglessF.hs Two type variables are needed: one to vary over the Symantics class (but only as a phantom type) and another to vary over the Monad class. Hence, the use of 2-variable type constructors. > type VarCount = int > newtype Y b a = Y {unY :: VarCount -> (b, VarCount)} Not knowing of a type-level 'flip', I resort to newtype isomorphisms: > newtype Z a b = Z {unZ :: Y b a} > instance Monad (Z a) where > return x = Z $ Y $ \c -> (x,c) > (Z (Y m)) >>= f = Z $ Y $ \c0 -> let (x,c1) = m c0 in (unY . unZ) (f > x) c1 -- Pace, too-strict puritans > instance MonadState String (Z a) where > get = Z $ Y $ \c -> (show c, c) > put x = Z $ Y $ \_ -> ((), read x) So far so good. Now for the Symantics instances (abridged). > class Symantics repr where > int :: Int -> repr Int -- int literal > add :: repr Int -> repr Int -> repr Int > lam :: (repr a -> repr b) -> repr (a->b) > instance Symantics (Y String) where > int = unZ . return . show > add x y = unZ $ do > sx <- Z x > sy <- Z y > return $ "(" ++ sx ++ " + " ++ sy ++ ")" The add function illustrates the kind of do-sugaring we know and love that I want to use for Symantics. > lam f = unZ $ do > show_c0 <- get > let > vname = "v" ++ show_c0 > c0 = read show_c0 :: VarCount > c1 = succ c0 > fz :: Z a String -> Z b String > fz = Z . f . unZ > put (show c1) > s <- (fz . return) vname > return $ "(\\" ++ vname ++ " -> " ++ s ++ ")" Now with lam, I get this cryptic error message (under 6.8.2): Occurs check: cannot construct the infinite type: b = a -> b When trying to generalise the type inferred for `lam' Signature type: forall a1 b1. (Y String a1 -> Y String b1) -> Y String (a1 -> b1) Type to generalise: forall a1 b1. (Y String a1 -> Y String b1) -> Y String (a1 -> b1) In the instance declaration for `Symantics (Y String)' Both the two types in the error message are identical, which suggests no generalization is needed. I'm puzzled why ghc sees an infinite type. Any ideas on how to proceed? -- View this message in context: http://www.nabble.com/Flipping-*-%3E*-%3E*-kinds%2C-or-monadic-finally-tagl… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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[Haskell Cafe] Parsec: buildExpressionParser and parens typecheck problem
by Paul Sujkov 11 Jul '09

11 Jul '09

Hi haskellers, I'm trying to use buildExpressionParser parser generator from ParsecExpr module of Parsec ( http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#ParsecExpr) It works well, except for the "parens" token parser ( http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#parens) This code (sample from Expressions part of the manual) typechecks fine: expr :: Parser Integer expr = buildExpressionParser table factor <?> "expression" table = [[op "*" (*) AssocLeft, op "/" div AssocLeft] ,[op "+" (+) AssocLeft, op "-" (-) AssocLeft] ] where op s f assoc = Infix (do{ string s; return f}) assoc factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <?> "simple expression" but if I try to use parens: factor = parens expr <|> number <?> "simple expression" it fails to typecheck: Couldn't match expected type `GenTokenParser s u m' against inferred type `Parser Integer' In the first argument of `parens', namely `expr' In the first argument of `(<|>)', namely `parens expr' In the first argument of `(<?>)', namely `parens expr <|> number' Failed, modules loaded: none. the type of expr infers to GenParser Char () Integer, and the expected type for the parens is GenTokenParser s u m (however, manual introduces it with the type CharParser st a expected) It seems pretty weird for me, as there are numerous examples of using parens with the buildExpressionParser (e.g. http://blog.moertel.com/articles/2005/08/27/power-parsing-with-haskell-and-…) and nobody comments such an error. I know I'm missing something very simple here, maybe someone could help me with it? Thanks in advace I'm using GHC 6.10.1 and Parsec 3.0.0 -- Regards, Paul Sujkov

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Laziness enhances composability: an example
by Cristiano Paris 11 Jul '09

11 Jul '09

Hi, I'm wondering what a good example of why laziness enhances composability would be. I'm specifically looking for something that can't implemented in Python with iterators (at least not elegantly), but can actually be implemented in Haskell. Thanks, Cristiano

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first project. code reviews?
by Jared Jennings 10 Jul '09

10 Jul '09

I've written a parser for OFX 1.x (these are the files you can download from most banks detailing recent account activity). I'm not sure I've written it well. This is my first serious thing in Haskell. (Kudos to the writers of the guide for how to make a new Hackage project! It's brilliant.) http://patch-tag.com/r/ofx1/pullrepo

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Leaner Haskell.org frontpage
by Jeff Wheeler 10 Jul '09

10 Jul '09

On Thu, Jul 9, 2009 at 6:12 PM, Derek Elkins<derek.a.elkins(a)gmail.com> wrote: > I'm not a newbie and I don't use the front page terribly often, but I > do like most of the links that are on it. The Ruby page is certainly > prettier, but the layout of the Haskell page is fine in my opinion; > the difference is mainly eye-candy. On another topic, I know people > have expressed that they have liked the fact that the entire Haskell > site is a wiki; this expressing openness and community involvement. > I personally don't find the Haskell front page too cluttered and I > think most of issue in that vein could be resolved by simply making > sure the most important/newbie-oriented links are "above the fold" and > appropriately emphasized/categorized as is partially done already. I strongly feel that the homepage should be made more newbie friendly, and I think the Ruby page has done this well, disregarding the news section. I suspect most people who like the Ruby page see the "Ruby is..." section as especially effective at introducing the language, and the random snippet is a simple way to show off a bit of code before they dive into a tutorial. Furthermore, the "Download" link is useful, but since GHC can be complicated (and varies by platform), we probably want to include pretty well thought-out instructions behind the link if we include a similar feature. I also quite like the "Participate" box on the Ruby page, which is very inviting. Regarding the current Haskell homepage, I feel the events are given far too high a place on the homepage. Almost no newbies will be interested in these, and most experienced users will know of the events via the mailing lists. The headlines below that deserve significantly more attention, and perhaps should be updated with greater frequency (and dated, and have RSS). The updated package list is fine, I think. The navigation is a bit tricky, in my opinion. To a beginner (that doesn't know what GHC is), the two download links may be confusing, although I suspect most would correctly assume that "Download Haskell" was correct option. The "Find A Library" is a good link, but the Search that follows it is awkward. There are three large search choices for beginners: 1) the search at the top, which confusingly has two submit buttons (with ambiguous differences to a beginner); 2) the Search link near the top of the navigation (which links to an almost empty page that might as well be included at the link's location); and 3) the Search link underneath the About header, which doesn't seem to belong at all. Jeff Wheeler (Sorry, sent this to just Derek at first.)

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Re: [Haskell-cafe] Laziness enhances composability: an example
by Marcin Kosiba 10 Jul '09

10 Jul '09

On Thursday 09 July 2009, you wrote: > 2009/7/9 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > > I thought I'll go the "smart" way and rely on the Python yield construct > > to do a CPS transformation of my code. > > While this worked to a certain extent, composability was a > > problem, because any sub-procedure which used yield (and thus was an > > iterator) required being called in a rather inelegant way. > > i confess to not understanding the inelegance. > can you provide an example where an iterator inside CPS requires > special support? Hi, To illustrate what I meant I'm attaching two examples. In example_1.py I've written code the way I think would be elegant (but it doesn't work). In example_2.py I've written code so that it works, but it isn't elegant. I know I'm abusing Python iterators here. Also, I'm not sure the way to compose iterators shown in example_2.py is the only option. Actually I'd love to see a better solution, because it would remove a lot of bloat from my code ;) Thanks! Marcin Kosiba

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Re: [Haskell-cafe] Leaner Haskell.org frontpage
by haskell＠kudling.de 10 Jul '09

10 Jul '09

> I've seen book providing a "chapters at a glance" part, just before the real table of content. Such an inverted pyramid is exactly the consequence Nielson draw from the "F shape pattern" (http://www.useit.com/alertbox/reading_pattern.html) And that's my critque: i don't see the most important things there are to be sayed about Haskell in the top left corner. > For the hompage we're talking about, glancing is even simpler since everything is on the same page and you can scroll it quite easily. I don't agree that "everything on one page" makes comprehension easier. > I'm not sure hiding a level of the hierarchy of information behind a few clicks make things easier. That depends on which task we are talking about: - getting an overview of all available information, or - finding exactly what you are looking for I think we should optimize for the latter, where "What is Haskell?" being the most improtant question.

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iteratee enumHandle
by Paolino 10 Jul '09

10 Jul '09

I'm testing iteratee. This is the possible bug I've found import Data.Iteratee.IO import Data.Iteratee.Base import Data.Iteratee.Char import System.IO import Control.Exception main = do h <- openFile "mamma23" ReadWriteMode hPutStrLn h "ciao" hSeek h AbsoluteSeek 0 l <- (enumHandle h stream2list :: IO (Iteratee IO String)) >>= run print $ assert (l == "ciao") () assertion failed This aside, I'd like to know if there is a way to use parsec library to make an Iteratee, so if it's possible to make parsec spit out a continuation on EOF, or maybe as in delcont language to capture it. thanks paolino

2 1

c2hs confusion with a simple function
by Jeff Heard 09 Jul '09

09 Jul '09

The c2hs documentation at http://www.cse.unsw.edu.au/~chak/haskell/c2hs/docu/implementing.html#id3149… gives me an example to follow for this case: {#fun notebook_query_tab_label_packing as ^ `(NotebookClass nb, WidgetClass cld)' => {notebook `nb' , widget `cld' , alloca- `Bool' peekBool*, alloca- `Bool' peekBool*, alloca- `PackType' peekEnum*} -> `()'#} so I took the original C binding: void SHPGetInfo(SHPHandle, int*, int*, double*, double*) And wrote a c2hs binding: {#fun SHPGetInfo as getInfo { fromSHPHandle `SHPHandle' , alloca- `Int' , alloca- `Int' , alloca- `Double' , alloca- `Double' } -> `()' #} The error I get: Internal.chs:85:2: Couldn't match expected type `Int' against inferred type `(Ptr b -> IO c) -> IO c' Expected type: IO (Int, Int, Double, Double) Inferred type: IO ((Ptr b -> IO c) -> IO c, (Ptr b1 -> IO c1) -> IO c1, (Ptr b2 -> IO c2) -> IO c2, (Ptr b3 -> IO c3) -> IO c3) In the expression: alloca $ \ a2' -> alloca $ \ a3' -> alloca $ \ a4' -> ... In the expression: let a1' = fromSHPHandle a1 in alloca $ \ a2' -> alloca $ \ a3' -> alloca $ ... I've tried ending it with withCIntConv and withCFloatConv to no avail. The original c2hs file is attached. I'm sure there are many more problems, but this is the one I'm working on at the moemnt... -- Jeff

1 0

[Haskell-cafe] ANN: hsparql, a SPARQL query generator/DSL and client
by Jeff Wheeler 09 Jul '09

09 Jul '09

I'm excited to announce the first version of hsparql. HSparql makes it easy to query SPARQL-compliant servers using a relatively intuitive DSL and very simple client. SPARQL is a SQL-ish query language for stores of RDF data. So, SPARQL lets you search the structured data in several big databases. I wrote it so that I could search DBpedia [1], a neat site that pulls structured data from Wikipedia's infoboxes, and also ties it to remote data services like MusicBrainz and US Census information. HSparql isn't tied to DBpedia in any way, though, and can search any server. The DSL looks something like this: > simple :: Query [Variable] > simple = do > resource <- prefix (iriRef "http://dbpedia.org/resource/") > dbpprop <- prefix (iriRef "http://dbpedia.org/property/") > foaf <- prefix (iriRef "http://xmlns.com/foaf/0.1/") > > x <- var > name <- var > page <- var > > triple x (dbpprop .:. "genre") (resource .:. "Web_browser") > > triple x (foaf .:. "name") name > triple x (foaf .:. "page") page > > return [name, page] It's currently available on Hackage [2], and at the moment the source is only available from the tarball hosted there. I'd greatly appreciate any feedback on this package. Thanks! Jeff Wheeler [1] http://dbpedia.org/ [2] http://hackage.haskell.org/package/hsparql

2 2

Re: [Haskell-cafe] Leaner Haskell.org frontpage
by haskell＠kudling.de 09 Jul '09

09 Jul '09

> I find it very to the point and not overwhelming at all : it's easy to glance over it and find quickly what I want. Thanks for your feedback. Most people feel overwhelmed when confronted with more than 7+-2 items: http://www.smashingmagazine.com/2007/10/09/30-usability-issues-to-be-aware-… On the current Haskell frontpage there are over 60 links competing for attention. I am not sure whether we should design interfaces solely with few people having exceptional abilities in mind. This could be understood as a statement about who Haskell is made for in itself. Bye, Lenny

3 2

About return type
by ZhangXu 09 Jul '09

09 Jul '09

_________________________________________________________________ Messenger安全保护中心，免费修复系统漏洞，保护Messenger安全！ http://im.live.cn/safe/

1 0

Leaner Haskell.org frontpage
by haskell＠kudling.de 09 Jul '09

09 Jul '09

Hi, i find the current www.haskell.org frontpage quite overwhelming. Compare it for example with the home pages of other programming languages : http://caml.inria.fr/ http://factorcode.org/ http://sbcl.sourceforge.net/ http://www.ruby-lang.org/en/ http://www.falconpl.org/ Here is my sketch of a leaner, more structured Haskell front page: http://www.haskell.org/haskellwiki/User:Lenny222/Haskell Comments? Bye, Lenny

3 2

ANN: darcs 2.3 beta 2
by Petr Rockai 09 Jul '09

09 Jul '09

Hello! Two weeks passed and it is about time to release darcs 2.3 beta 2 (it's already a day late, sorry about that). As with beta 1, there is only a single installation package for this release of darcs: cabalised source. (Please note that the final version with also come with the legacy autoconf-based buildsystem, for the last time.) You can either download a tarball from <http://repos.mornfall.net/darcs/darcs-2.2.98.2.tar.gz> and build manually (see the build instructions in README inside the tarball), or, alternatively, you can use cabal-install to obtain a copy (the beta release is now available on Hackage): $ cabal update $ cabal install darcs-beta This, if successful, will give you a darcs binary in `~/.cabal/bin` -- you should probably add that to your PATH. What's new since beta 1 ----------------------- There are basically two relatively intrusive changes since beta 1. First, I have added index upgrade functionality to hashed-storage and this is now used by darcs, so that any bad or incompatible indexes are recreated transparently by darcs. Moreover, the index format is now architecture-independent, meaning that repositories shared across multiple architectures should not suffer from excessive index rebuilding. Second, Ganesh has done further work on his gzip CRC correction code. There is a very slight risk of regressions, so if you have around any repositories with broken CRCs in them, please test this functionality. Moreover, there is now an option to limit the repair to current repository, avoiding changes in related branches or caches. Trent, Thorkil and Eric have done cleanup work and a number of minor fixes for this release, improving overall quality and polish. One bug in the new indexing code has been found and reported by Eric and Guillaume independently, and is fixed in hashed-storage 0.3.4 (which required by this beta). You can track the release plan and progress at our wiki: <http://wiki.darcs.net/development>. The question of GHC 6.8 ----------------------- Using GHC 6.10.3 or newer is *strongly recommended*. You may compile darcs with GHC 6.8, but there are several caveats. If you are using 6.8.2 or older, please disable mmap support (pass -f-mmap to cabal install or runghc Setup configure below). Note that the GHC 6.8.2 that ships with Debian Lenny is not affected and it should be safe to keep mmap enabled. It is also recommended to disable use of Hackage zlib when compiling with GHC 6.8.2 (including the Debian Lenny version): pass -f-zlib to cabal. When using zlib, we have seen occasional crashes with error messages like "openBinaryFile: file locked" -- this is a known GHC 6.8.2 bug (and is fixed in GHC 6.8.3). Last, if you are using a 64-bit system, darcs may hang when you exit a pager when compiled with GHC older than 6.10.3. Although this is harmless, it is quite inconvenient. Overall, the status of GHC 6.8 is "semi-supported": for many cases, things will work just fine, especially if you take a little extra caution with compilation flags. Yours, Petr. -- Petr Ročkai | http://web.mornfall.net A physicist is an atom's way of knowing about atoms. (George Wald)

1 0

ANN: Hayoo! beta 0.4
by Timo B. Hübel 09 Jul '09

09 Jul '09

Hello! We are pleased to announce the next beta version 0.4 of Hayoo!, the Haskell API search engine with find-as-you-type and suggestions. Visit Hayoo! here: http://holumbus.fh-wedel.de/hayoo This release includes some major changes to the web interface: - The full description of a function can now be displayed upon request, using the small "+" symbol in front of the description text - The most common root modules for all functions found are shown as list, providing a shortcut for further restricting the current query - Some example queries on the start page for easier use of Hayoo! Additionally, we have again updated the search index. It contains all packages from Hackage as well as gtk2hs as of 06.07.2009, a total of 111.946 function and type definitions. Please let us know about any problems, hints and suggestions. Happy hacking, Timo

2 1

Re: [Haskell-cafe] Where can I get GHC for Solaris?
by John Ky 09 Jul '09

09 Jul '09

Hi Aycan, This is the version of make I am using: joky@sun05:~ $ make -v GNU Make 3.81 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. This program built for sparc-sun-solaris2.9 joky@sun05:~ $ uname -a SunOS sun05 5.9 Generic_118558-30 sun4u sparc SUNW,Sun-Fire-280R Thanks, -John On Wed, Jul 8, 2009 at 4:48 PM, Aycan iRiCAN <aycan.irican(a)core.gen.tr>wrote: > Çar, 2009-07-08 tarihinde 09:55 +1000 saatinde, John Ky yazdı: > > Hi, > > > > I had a go with the binary dist, and got this error message on the > > make install: > > > > /home/a-m/joky/.tools/ghc-6.8.2/lib/ghc-6.8.2"' > > -DPKG_DATADIR='"/export/home/a-m/joky/.tools/ghc-6.8.2/share/ghc-6.8.2"' > package.conf.in \ > > | grep -v '^#pragma GCC' \ > > | sed -e 's/""//g' -e 's/:[ ]*,/: /g' \ > > > > | /export/home/a-m/joky/ghc-6.8.2/utils/ghc-pkg/ghc-pkg.bin > > --global-conf > /export/home/a-m/joky/.tools/ghc-6.8.2/lib/ghc-6.8.2/package.conf update - > --force > > ld.so.1: ghc-pkg.bin: fatal: libm.so.2: version `SUNW_1.2' not found > > (required by > > file /export/home/a-m/joky/ghc-6.8.2/utils/ghc-pkg/ghc-pkg.bin) > > ld.so.1: ghc-pkg.bin: fatal: libm.so.2: open failed: No such file or > > directory > > Killed > > make[1]: *** [install] Error 137 > > make[1]: Leaving directory `/export/home/a-m/joky/ghc-6.8.2/rts' > > make: *** [install] Error 2 > > Are you using gmake? > > -- > aycan > >

2 1

Haskell Platform on Ubuntu
by Rafael Gustavo da Cunha Pereira Pinto 08 Jul '09

08 Jul '09

Is there anyone working on a Haskell Platform package for Ubuntu? GHC in Ubuntu is a pain! Regards Rafael Gustavo da Cunha Pereira Pinto

12 25

Re: [Haskell] Haskell Hack Day, Edinburgh, 30 August 2009
by Johan Tibell 08 Jul '09

08 Jul '09

On Wed, Jul 8, 2009 at 12:12 PM, Eric Kow <kowey(a)darcs.net> wrote: > Dear Haskellers, > > I am pleased to report that we will be having a Haskell Hack Day in > Edinburgh, on Sunday 30 August (before the ICFP begins). The day will > be held at the ICFP conference venue, the Royal College of Physicians. > > The Hack Day will be free and open to all. > Come join us; it'll be a lot of fun! My flight arrives too late for actually taking part in the hackathon but I would be up for some beers after if anyone is interested. -- Johan

1 0

ANN: texmath 0.1.0.1 - conversion of LaTeX math to MathML
by John MacFarlane 08 Jul '09

08 Jul '09

I'm pleased to announce an early release of texmath, a Haskell library for converting LaTeX math formulas to MathML. Hackage: http://hackage.haskell.org/package/texmath-0.1.0.1 Github: http://github.com/jgm/texmath/tree/master The package includes a standalone test program, testTeXMathML, and a CGI script, texmath-cgi, that can be used in web apps. This script is used in the demos at http://johnmacfarlane.net/texmath.xhtml http://johnmacfarlane.net/texmath.html texmath supports most of basic LaTeX and AMS extensions, but I'm sure some things have been left out, so let me know if you try a formula that should work but doesn't. John

1 0

simple state monad exercises? (besides labeling trees)
by Thomas Hartman 08 Jul '09

08 Jul '09

Can someone give some simple common scenarios where the state monad is useful, besides labeling trees? References to puzzles like those in project Euler or similar would be nice. Thanks!

5 4

lazy data structure for best-first search
by Martin Hofmann 08 Jul '09

08 Jul '09

I am looking for a good (preferably lazy) way to implement some kind of best-first search. The problem is, the expansion of the 'best' node in the search space forces other 'inferior' nodes to expand too. So my function expand :: Node -> ([Node],(Node -> [Node])) does not only return some successor nodes, but also a function to expand other Nodes with certain characteristics. So in fact, after one expansion, I need to fold over my complete search space, maybe updating other nodes and recompute the value of the cost function for the affected nodes. At the moment I am using StateT monad transformers, and retrieving the affected Nodes from a map (to avoid going through whole space), but this is not very elegant and I doubt whether it is efficient, too. I have already read through Spivey's paper "Algebras for combinatorial search". Unfortunately, he only proposes the use of heuristics and best-first search in combination with his framework for future work. Does anybody now some further papers on this topic? Thanks a lot in advance, Martin

4 5

Adaptive transactions
by Ryan Ingram 07 Jul '09

07 Jul '09

It seems to me that the Adaptive framework[1][2] is well-suited to STM transactions. Consider this snippet: > expensive_transaction :: STM Int -- does a bunch of work > > example :: TVar Int -> STM Int > example t = (+) <$> readTVar t <*> expensive_transaction If the TVar passed to example is rapidly changing (perhaps it represents the current time), then this transaction will starve; it never gets a chance to complete, even if the values used by expensive_transaction aren't changing at all, we still re-evaluate the entire action each time we "retry" this transaction. But STM has a wonderful property: what we need to commit a transaction is just the result value and a transaction log; both of these depend only on the values of TVars read by that transaction. So, if "expensive_transaction" doesn't refer to the TVar "t", then even if "t" is rapidly changing, the transaction log generated by "expensive_transaction" must not change, and we can just re-generate the part of the transaction log for "readTVar t" and recompute the (+). This gets even better when considering big processes that are blocked by an explicit "retry"; if some change in input is just going to cause the transaction to retry again (and go back to sleep), we'd like that to happen as quickly as possible. Has anyone looked down this path? It seems quite promising to me. -- ryan [1] http://hackage.haskell.org/package/Adaptive [2] Acar et al, Adaptive Functional Programming, POPL 2002 http://citeseer.ist.psu.edu/old/752721.html

1 0

Re: what about moving the record system to an addendum?
by John Meacham 07 Jul '09

07 Jul '09

Well, without a replacement, it seems odd to remove it. Also, Haskell currently doesn't _have_ a record syntax (I think it was always a misnomer to call it that) it has 'labeled fields'. None of the proposed record syntaxes fit the same niche as labeled fields so I don't see them going away even if a record syntax is added to haskell in the future. I would like to see the simple modifications to the record syntax listed on this page though http://hackage.haskell.org/trac/haskell-prime/wiki/ExistingRecords and a reworking of the standard to not refer to the current system as a 'record syntax' but rather a 'labeled fields' syntax. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

4 4

[Haskell] ANNOUNCE: ChristmasTree 0.1 (excuses for second attempt)
by S.Doaitse Swierstra 07 Jul '09

07 Jul '09

We are pleased to announce the availability of the package "ChristmasTree", which contains the code associated with our paper at the last Haskell symposium: @inproceedings{1411296, author = {Marcos Viera and S. Doaitse Swierstra and Eelco Lempsink}, title = {Haskell, do you read me?: constructing and composing efficient top-down parsers at runtime}, booktitle = {Haskell '08: Proceedings of the first ACM SIGPLAN symposium on Haskell}, year = {2008}, isbn = {978-1-60558-064-7}, pages = {63--74}, location = {Victoria, BC, Canada}, doi = {http://doi.acm.org/10.1145/1411286.1411296}, publisher = {ACM}, address = {New York, NY, USA}, } The name of the package stands for: "Changing Haskell's Read Implementation Such That by Manipulating Abstract Syntax Trees it Reads Expressions Efficiently" which, given the time of year, seems appropriate. Feel free to download and unpack this "present" at what for the Dutch is called "Sinterklaasavond" (http://en.wikipedia.org/wiki/Sinterklaas) Arthur Baars Marcos Viera Eelco Lempsink Doaitse Swierstra PS: the package uses our library supporting transformation of typed abstract syntax, which we placed in a separate package TTTAS

2 1

Implementing Las Vegas algorithms in Haskell
by Matthias Görgens 07 Jul '09

07 Jul '09

A Las Vegas algorithm, like randomized quicksort, uses a source of randomness to make certain decisions. However its output is unaffected by the randomness. So a function > f :: RandomGen g => g -> a -> b implementing a Las-Vegas-Algorithm 'looks' like a pure function, ignoring its first argument and depending solely on its second argument. What is an idiomatic way to implement such a function? I believe, Monads are too linear and strict.

8 13

Optimizing Compiler for the ICFP 09 contest's VM
by Matthias Görgens 07 Jul '09

07 Jul '09

The byte code for the virtual machine of this years ICFP specified a language with single assignment per simulation step. Interestingly most memory locations get overwritten each simulation step before they are read. That means, those locations don't have to be remember between steps. Also locations that never get overwritten (e.g. location associated with Noops), are constant. Thus the variables state of the simulation is orders of magnitude smaller than the naive 2^16 * 32 bit + 1 bit. I wrote a small program that analyses the dataflow of a byte code program (and initial memory setup) for the VM. After analyzing my program emits Haskell code to run the given byte code. If anyboby is interested, I can document my program and put it online somewhere. I also made pretty graphs of the dataflow with graphviz.

3 3

ANN: bloxorz clone
by Patai Gergely 07 Jul '09

07 Jul '09

Hello all, This post is not about my own creation, it's just a little fun program written by a student of mine. You can install the bloxorz package to try it out, and read more about its background on my blog: http://just-bottom.blogspot.com/2009/07/playing-and-learning.html Gergely -- http://www.fastmail.fm - Access all of your messages and folders wherever you are

3 6

DHUG: meeting tomorrow (July 7th) in Utrecht
by Chris Eidhof 07 Jul '09

07 Jul '09

Hi everyone, The Dutch HUG [1] will meet again tomorrow. This time we'll meet in Utrecht again, at the Stairway To Heaven. The meeting will start at 19h. The venue is a sometimes a bit noisy, but there's beer and nice big tables. If you want to hear everything about the Dutch HUG straight from the horse's mouth: make sure you're at the meetings and subscribed to the mailing list [2]. We already have more than 40 members after two meetings! See you tomorrow! -chris [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug

1 0

How to read safely?
by Magicloud Magiclouds 07 Jul '09

07 Jul '09

Hi, Read often throws runtime errors, which breaks the robust of the problem. How to deal with it? Without lost too much proformance (so reads is a no). At least, if its error could be catched, that'd be better. -- 竹密岂妨流水过山高哪阻野云飞

8 9

Could FFI support pass-by-value of structs?
by Maurício 07 Jul '09

07 Jul '09

It's not usual, but it is allowed to have values of structs passed between functions directly instead of using pointers: /*****/ struct ex { int x; int y; int z; }; ex example_functions (ex p) { (...) } /*****/ Would it be possible to allow that in Haskell FFI by, say, allowing any instance of Storable to be used in a 'foreign' declaration? Like: -- data Ex = (...) instance Storable Ex where sizeOf _ = ... alignment = sizeOf (...) foreign import ccall "example_functions" exampleFunction :: Ex -> IO Ex -- Thanks, Maurício

6 21

Where can I get GHC for Solaris?
by John Ky 07 Jul '09

07 Jul '09

Hi, Anyone know where I can get the GHC compiler and libraries for Solaris?: SunOS sun05 5.9 Generic_118558-30 sun4u sparc SUNW,Sun-Fire-280R I tried to compile GHC myself and got the following error: $ ./configure --enable-hc-boot checking build system type... sparc-sun-solaris2.9 checking host system type... sparc-sun-solaris2.9 checking target system type... sparc-sun-solaris2.9 Canonicalised to: sparc-sun-solaris2 checking for ghc... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/ccs/bin/ld checking for path to top of build tree... ./configure: -v0: command not found ./configure: utils/pwd/pwd: No such file or directory configure: error: cannot determine current directory Thanks -John

2 1

BostonHaskell: Next meeting - July 16th at MIT CSAIL Reading Room (32-G882)
by Ravi Nanavati 07 Jul '09

07 Jul '09

I'm pleased to announce the July meeting of the Boston Area Haskell Users' Group. Based on the feedback from the June meeting and the constraints of our speakers, the July meeting has been scheduled for Thursday, July 16th from 6:30pm - 8:30pm. Like the June meeting, it will be held in the MIT CSAIL Reading Room (32-G882, i.e. a room on the 8th floor of the Gates Tower of the MIT's Stata Center at 32 Vassar St in Cambridge, MA). We have the following two talks scheduled: "An Introduction to GHC Hacking" by Alec Heller "Haskell on the iPhone" by Ryan Trinkle (a more in-depth discussion than his on-the-spot Lightning Talk last month). As always, there will be a break between the talks for discussion and mingling. We have openings for Lightning Talks (5-minute talk, 2-minute Q&A) during that period. We are also open to short, relevant advertisements (like my Hac Phi plug last month). If you're interested in giving a Lightning Talk, have an advertisement you'd like to share or are interested in speaking at a future meeting, please contact me at ravi_n(a)alum.mit.edu. I'd also appreciate it if people who can (and can't) attend this meeting fill out the attendance poll here: http://www.learnmyself.com/poll10828x74C54f13 I apologize for the URL, but Google Groups seems to ban JavaScript and I didn't want to delay this announcement while I tried to come up with something more convenient. Responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, should I manage to secure a sponsor for refreshments. Sponsorship of or other assistance with refreshments is still being eagerly solicited. If you have any questions about the meeting please send them to the BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the July meeting! Thank you, - Ravi Nanavati

1 0

Catering for similar operations with and without state
by Phil 07 Jul '09

07 Jul '09

Hi, I'm trying to think around a problem which is causing me some difficulty in Haskell. I'm representing a stateful computation using a State Transform - which works fine. Problem is in order to add flexibility to my program I want to performs the same operation using different techniques - most of which require no state. My program at the moment is a stack of state monads/transforms. I have a random number generator as a state monad (seed=state), feeding a Box Muller (normal) generator implemented as a state transform (state is 'maybe' normal, so it only spits out 1 normal at a time), which in turn feeds another state machine. This all works fine, but I want to add flexibility so that I can chop and change the Box Muller algorithm with any number of other normal generators. Problem is most of them do not need to carry around state at all. This leaves me with a messy solution of implementing lots of state monads that don't actually have a state, if I want to maintain the current paradigm. This strikes me as really messy - so I'm hoping someone can point me in the direction of a more sensible approach? Currently I have my Box Muller implemented as below - this works: class NormalClass myType where generateNormal :: myType Double type BoxMullerStateT = StateT (Maybe Double) type BoxMullerRandomStateStack = BoxMullerStateT MyRngState instance NormalClass BoxMullerRandomStateStack where generateNormal = StateT $ \s -> case s of Just d -> return (d,Nothing) Nothing -> do qrnBaseList <- nextRand let (norm1,norm2) = boxMuller (head qrnBaseList) (head $ tail qrnBaseList) return (norm1,Just norm2) But say I have another instance of my NormalClass that doesn't need to be stateful, that is generateNormal() is a pure function. How can I represent this without breaking my whole stack? I've pretty much backed myself into a corner here as my main() code expects to evalStateT on my NormalClass: main = do let sumOfPayOffs = evalState normalState (1,[3,5]) -- (ranq1Init 981110) where mcState = execStateT (do replicateM_ iterations mc) 0 normalState = evalStateT mcState Nothing If it wasn't for this I was thinking about implementing the IdentityT transformer to provide a more elegant pass-through. I've never tried designing my own Monad from scratch but this crossed my mind as another possibillity - i.e. a Monad that either has a state of maybe double, or has no state at all? I may be talking rubbish here of course :-) I'm pretty daunted by where to even start - but I want to improve this as the quick and dirty solution of implementing a load of state monads with no state just to cater for the above method strikes me as very ugly, and as I can easily see ways of doing this in C or C++, I figure there must be a better approach in Haskell - I'm just thinking in the right way! Any advice or hints would be great, Cheers, Phil.

3 2

LINQ, SQL and hs-dotnet
by Günther Schmidt 06 Jul '09

06 Jul '09

Hi all, is there anyone who has already tried using LINQ2SQL with hs-dotnet and would care to share the experience? I'm trying to figure out what exactly it would take to access an SQL database via LINQ / hs-dotnet. Ie. whether or not it's necessary to create Entity classes, or if one can just use LINQ to generate SQL. Günther

1 0

Haskell Platform 2009.2.0.1 and GLUT32.DLL on Windows: which version?
by Peter Verswyvelen 06 Jul '09

06 Jul '09

I know GLUT32.DLL is not bundled with the Haskell Platform installer, but which GLUT32.DLL should I use? Every DLL I tried (even building FreeGLUT myself<http://netsuperbrain.com/blog/posts/freeglut-windows-hopengl-hglut/#more-136>) gives the error: *"The procedure entry point glutAddMenuEntry could not be located in the dynamic library glut32.dll"* Any hints as to which glut32 version to use would be highly appreciated. Thanks, Peter

2 2

Monoid wants a (++) equivalent
by Bryan O'Sullivan 06 Jul '09

06 Jul '09

I've thought for a while that it would be very nice indeed if the Monoid class had a more concise operator for infix appending than "a `mappend` b". I wonder if other people are of a similar opinion, and if so, whether this is worth submitting a libraries@ proposal over.

36 63

Documentation design
by Isaac Dupree 06 Jul '09

06 Jul '09

(for reference, here's the blog-post I wrote that inspired me to ask this list for advice. I'll explain everything in this email anyway though. http://haddock2009.wordpress.com/2009/06/23/how-to-navigate-your-code/ ) My challenge: getting to know an existing code-base quickly and easily, so that I can hack on it. Haddock and HsColour are already pretty helpful at this, but they could be more helpful. (haddock with --ignore-all-exports, i.e. cabal haddock --internal, especially) My dream situation: both haddock-pages and hscolour-pages would be super-hyperlinked and super-readable. For example, haddock would list all a module's definitions, not just its exports. In HsColoured source, every identifier would link to its definition or the haddockumentation of its definition. and so forth. It would be so much easier to generate and browse this in HTML, than to get an IDE working, and it would be so much more readable than a mere text-editor (even with syntax hilighting) and quicker clicking on hyperlinks than grepping for everything. Actually I don't have the resources to worry about designing any HsColour stuff right now (its current design is mainly just a lexeme highlighter). But, with your advices, I can design an improvement on Haddock's ignore-exports mode, which currently has a few shortcomings: 1. you can't tell which identifiers in a module are exported 2. it doesn't document a module's re-exports at all 3. it still obeys {-# OPTIONS_HADDOCK hide #-} et al, even when they're not appropriate for reading documentation of internals 4. 2+3 means that some things may be found nowhere in the documentation at all! Not quite satisfying for something invoked by `cabal haddock --internal`. (Here's a haddock-generated page you can look at so you can figure out the formatting-details I'll be describing: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html and its source code http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.ht… ) The ideal haddockumentation-formatting for this purpose isn't quite obvious though. For example, sometimes you want to think about a module in terms of its abstract interface, but sometimes you want to figure out how it's implemented (without reverting completely to text based code browsing). Maybe a compromise of some sort would be good. so... Here's a proposal, for a new mode (`haddock --all-internal`?, to be invoked by `cabal haddock --internal` rather than --internal's current effect of ignore-all-exports) : Files with no explicit export list can be treated as-is anyway. For all files that have an explicit export list, generate the synopsis-of-exports near the top, as usual. But have the index link, generally, to where functions are originally defined (modulo being from a non-internally-documented separate package, where it should link to the appropriate place), and have the fuller documentation below be a compilation of the identifiers *defined* in this module. Actually that would need some revision because the sections and subsections -- *, -- **, etc. defined in the export-list in the .hs, actually are displayed 1. above the table of contents, linking to places in 2. the full list of definitions. Which might be defined in the module in a different order than they're listed in the export list. Why not add the sections into the synopsis? In this case, instead of adding them to the main doc section. But hmm... in ordinary non-internal documentation, would it be nice to *additionally* have the sections marked in the synopsis (in addition to in the Contents and in the main docs section)? Maybe this mode should also abstain from "hiding" any module, because that would cause missing docs. Etc. Questions? Comments? Opinions? Does anyone want this feature, and/or not think it's particularly useful? -Isaac

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How to pretty print code efficiently
by John Ky 06 Jul '09

06 Jul '09

Hi, Currently I'm pretty printing code by building arrays of strings and calling indent. For example: instance JavaPrintableNamed AST.EnumeratedType where javaLinesNamed parentName (AST.EnumeratedType memberDefinitions) = [ "public enum " ++ asJavaId(parentName) , "{" ] ++ memberCodeLines ++ [ "}" , "" ] where memberCodeLines = indent $ javaLines memberDefinitions The indent function takes a list of strings and adds an indent to the beginning of every line. I can imagine this to be very inefficient as it builds many strings and concatenates them. In Ruby, I might do the same thing like this: class EnumeratedType < JavaPrintableNamed def writeTo(writer) writer.print "public enum " writer.puts self.asJavaId writer.puts "{" writer.indent do self.memberDefinitions.writeTo(writer) writer.puts end where above, the writer.indent takes care of the indent, and everything is appended to a stream, which doesn't seem so bad in terms of efficiency. I'm looking for a way to do something similar in Haskell. Anyone can give me a hand? Thanks -John

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Read & Show instance generation for GADTs?
by Conal Elliott 05 Jul '09

05 Jul '09

Are there any tools for generating Show & Read instances for GADTs? - Conal

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ANN: AC-Vector, AC-Colour and AC-EasyRaster-GTK
by Andrew Coppin 04 Jul '09

04 Jul '09

OK, so having released AC-HalfInteger, I got slightly carried away and released three other small packages. These are packages that many programs I write all end up using. I'm forever copying these files, so I made them into actual bonafide packages. http://hackage.haskell.org/package/AC-Vector-1.1.1 This provides two types, Vector2 and Vector3, which are unboxed vectors of Doubles, with arithmetic, dot product and cross product, and a few other useful items. http://hackage.haskell.org/package/AC-Colour-1.1.1 This provides two types, Colour and Colour8. Both implement simple RGB colour types with arithmetic. Colour has unboxed Double fields, and Colour8 has unboxed Word8 fields. My usual workflow is to do all the image generation with Colour, and to convert to Colour8 just before the data hits the I/O channels. You can, however, do arithmetic directly on Colour8. (I haven't extensively tested that it works properly though...) http://hackage.haskell.org/package/AC-EasyRaster-GTK-1.1.1 This is a layer over Gtk2hs. As you all probably know, Gtk2hs provides a Cairo binding that makes vector graphics wonderfully easy. However, *bitmapped* graphics is darned tricky. I basically had to sit in the #haskell channel with Duncan for a few hours trying to figure out how the hell to do it. This knowledge is now codified in the above package. Load it up and you don't need to know a thing about GTK; you can just create an ImageBuffer, write some pixels to it (efficiently!), save it to disk or display it on screen. (But you *can* access the underlying GTK+ resources if you wish...) In other news, it appears that the batch job to generate the documentation just ran, so you can view it all online. :-D Comments, suggestions, random flames, etc... [I'm particularly curios to know what Duncan will make of the GTK thing...]

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Re: [Haskell-cafe] ANN: AC-Vector, AC-Colour and AC-EasyRaster-GTK
by Andrew Coppin 04 Jul '09

04 Jul '09

Paul Johnson wrote: > Andrew Coppin wrote: >> Well, no, because now I'm going to have to spend a few hours trying >> to find out what CIE is before I can even use that library. >> >> I think really it's just aimed at a different problem. It looks like >> it's trying to specify actual real-world colours. [It's news to me >> that this isn't fundamentally impossible...] I'm only trying to >> specify colours on a computer screen. And as we all know, computer >> screens aren't calibrated in any way, and the same RGB value looks >> different on each display. But then, I'm only trying to write a >> fractal generator, so CIE specifications are somewhat overkill here. ;-) > Your display may not be calibrated, but those used for graphic design > certainly are. Indeed. And if you're in any kind of position where you *care* about such things, you should be using color, not AC-Colour. If, however, you just want to throw together pretty pictures, AC-Colour is simpler and easier. Different libraries for slightly different tasks. ;-) > On the package naming front: I appreciate your wish to avoid just > having another "colour" library. But "AC_Colour" doesn't help much. > "Simple_colour" might be better. Mmm, yeah. Naming everything with "AC" precludes name clashes and doesn't require too much thinking. Coming up with a better name requires thinking about what actually makes your package unique. And, of course, if another package comes along, that analysis may change. (E.g., I seem to recall there's a "newbinary" package which has actually been long since superceeded - so not so "new" any more!) If I name my package simple-colour, and then somebody else makes an even simpler one... the name becomes kind of meaningless. (Admitedly there's not too much danger of this happening...) I just like the idea of having definitely unique, distinctive package names. Otherwise I'd have to come up with stuff like geovector and trivicolour and so on... Arguably EasyRaster-GTK should have been a sufficiently unique name by itself though.

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Re: [Haskell-cafe] ANN: HalfInteger-1.1.1
by Andrew Coppin 04 Jul '09

04 Jul '09

Alexander Dunlap wrote: > Couple of suggestions: > > - You should put an (Integer i) => constraint on the halve function so > that it becomes impossible to create invalid HalfIntegers. > Right. Currently you can *make* such a HalfInteger. You just won't be able to *do* anything with it afterwards. It would probably be more sensible to just add a constraint there. > - The documentation for toHalfInteger is truncated. Also, why can't > you make the rounding more predictable? > ...wow. OK, I'm looking at the source, and it seems I must have got distracted at that moment or something, because I just 100% didn't actually finish writing the documentation! o_O That's pretty special. ;-) As I *should* have written, the rounding for something like toHalfInteger 0.25 is kind of unpredictable; but if you do toHalfInteger 0.5, the result is *guaranteed* to be exact. All the functions are carefully tuned to work correctly on things which really are half integers, and to give plausible results otherwise. Just don't let your life depend on 0.25 being mapped to exactly a half or to exactly zero... > Nice work! > Heh, thanks.

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Flow Graphs for Language-independent Code Representation
by Cetin Sert 04 Jul '09

04 Jul '09

Hi *^o^*, I'm working on a source code transformation project for numerical automatic differentiation for Fortran and C. I would love to know the best Haskell way/package available today to represent procedural (non-OO) code in a language-independent manner. Any tips or resource, paper references are most welcome! Regards, Cetin Sert

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