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Haskell Weekly News: Issue 76 - July 9, 2008
by Brent Yorgey 10 Jul '08

10 Jul '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080709 Issue 76 - July 09, 2008 --------------------------------------------------------------------------- Welcome to issue 76 of HWN, a newsletter covering developments in the [1]Haskell community. The [2]ICFP Programming Contest is this weekend! Go forth and kick some butt, Haskell-style. A big thank you in advance to all those at PSU and U Chicago who are working hard to write and run the contest. Community News Luke Palmer (luqui) is [3]having a great time in Antwerp. John Goerzen's son is [4]so cute, it should be illegal. Announcements Haskell-cafe on lively.com. Edward Kmett has created a [5]Haskell Cafe room on Google's new virtual-world platform [6]Lively (which is unfortunately windows-only at the moment). Uniplate 1.2. Neil Mitchell [7]announced the release of [8]Uniplate 1.2, a library for reducing boilerplate code by performing generic traversals. Version 1.2 features some bug fixes, a compatibility layer with Compos and SYB, and a 25-50% performance increase over Uniplate 1.0. GHC 6.8.2 stable in Gentoo. Luis Araujo [9]announced that GHC 6.8.2, and its accompanying libraries, have now been marked as stable in the official Gentoo portage tree. The Monad.Reader (11) - Call for Copy. Wouter Swierstra [10]announced a call for copy for Issue 11 of [11]the Monad.Reader. The submission deadline is August 1, although you should let Wouter know as soon as possible if you plan to submit something. hCsound. John Lato [12]announced the initial public release of [13]hCsound, a Haskell binding to the Csound audio processing language API. Portland and OSCon. John Goerzen [14]inquired whether any Haskellers in Portland would be interested in getting together during OSCon July 23 or 24. Faster graph SCCs. Iavor Diatchki [15]announced that he has implemented Tarjan's algorithm for computing the strongly connected components of a graph, which is considerably faster than the containers package for larger graphs. Iavor's implementation is available in the [16]GraphSCC package. parallel map/reduce. jinjing [17]exhibited some code for doing parallel map/reduce computations. Disciplined Disciple Compiler. Ben Lippmeier [18]announced version 1.1 of the [19]Disciplined Disciple Compiler (DDC), an explicitly lazy dialect of Haskell, with support for first class destructive update of arbitrary data, computational effects without the need for state monads, and type directed field projections. Version 1.1 includes a number of new features and more example code. darcs 2.0.2. David Roundy [20]announced the release of [21]darcs 2.0.1 and 2.0.2. These releases fix quite a few bugs, and users of darcs 2 are strongly recommended to upgrade. Google Summer of Code Progress updates from participants in the 2008 [22]Google Summer of Code. GHC plugins. Max Bolingbroke is working on dynamically loaded plugins for GHC. Over the [23]past two weeks, he has implemented type safe dynamic loading, an annotations system, and some sample plugins. Hoogle 4. Neil Mitchell (ndm) is working on [24]Hoogle 4. [25]This week, he has been working on type searching, using a much more efficient algorithm than type search in previous versions of Hoogle. Next week, he plans to finish off type search and work on the build system. DPH physics engine. Roman Cheplyaka (Feuerbach) is working on a [26]physics engine using [27]Data Parallel Haskell. He spent most of [28]this week fixing bugs and improving existing simulation code. And he now has something to [29]show for it! Language.C. Benedikt Huber (visq) is [30]working on Language.C, a standalone parser/pretty printer library for C99. [31]This week, he created a semantic representation for declarations and types, and a way to convert between an AST representation and a semantic representation. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a [32]make-like dependency analysis framework for Cabal. Generic tries. Jamie Brandon is working on a library for efficient maps using generalized tries. GHC API. Thomas Schilling (nominolo) is working on [33]improvements to the GHC API. Libraries Proposals and extensions to the [34]standard libraries. Extensible exceptions. Ian Lynagh sent out a [35]proposal to replace the current exception mechanism in the base library with extensible exceptions, a la Simon Marlow's [36]extensible extensions paper. Deadline for discussion is 25th July. Discussion Qualified import syntax badly designed (?). Neil Mitchell began a [37]discussion about Haskell syntax for qualified module imports (and module imports in general). Trouble with zip12. Michael Feathers is [38]having trouble with the zip12 function and some weird SQL-related errors... Santana on my evil ways. John D. Ramsdell [39]set off a spate of Haskell song and poetry. Alternatives to convoluted record syntax. Dougal Stanton [40]asked about alternatives to convoluted record update syntax, eliciting a number of interesting responses. Jobs Lectureship in Functional Programming, Nottingham. Graham Hutton [41]announced an opening for a Lecturer in the Functional Programming Lab in Nottingham, a recently formed research group that comprises Thorsten Altenkirch, Graham Hutton, Henrik Nilsson, four research fellows, and eleven PhD students. Applications from the Haskell community are encouraged! The closing date for applications is Friday 15th August 2008. Blog noise [42]Haskell news from the [43]blogosphere. * Edward Kmett: [44]A Lively Haskell Cafe. A Haskell Cafe room on lively.com! * Sterling Clover: [45]Comonads in everyday life. A neat post on using a zipper comonad to render a website menu hierarchy without lots of duplicated effort. * Chung-chieh Shan: [46]Differentiating regions. * Real-World Haskell: [47]Real World Haskell, The Book, Available for Pre-Order. * Benedikt Huber: [48]An analysis-friendly representation. An update on Benedikt's Google Summer of Code project, Language.C. * Chris Okasaki: [49]Breadth-First Numbering: An Algorithm in Pictures. Algorithms without words! * >>> Jeremy Frens: [50]PE Problem #2 in All Languages (Part I). Jeremy explores solutions to Project Euler problem #2 in a variety of languages. * Roman Cheplyaka: [51]Double buffering & demo. A demo of Roman's Google Summer of Code physics simulator! * Roman Cheplyaka: [52]QuickCheck puzzle: the answer. Why Roman's QuickCheck test involving nonzero vectors didn't terminate. Sneaky. * Roman Cheplyaka: [53]Status report: week 6. A status report on Roman's Google Summer of Code project. * >>> JP Moresmau: [54]Handling errors in JSON to Haskell deserialization. JP adds error handling with an Either monad to his JSON deserialization code. * Braden Shepherdson: [55]Fixed Point Datatypes. Braden explains the concept of recursive data types as fixed points of functors. * Luis Araujo: [56]GHC 6.8.2 stable! (Himerge 0.21.9 too!). * Matthew Sackman: [57]Anglo Haskell 2008. * Neil Mitchell: [58]GSoC Hoogle: Week 6. * >>> James Hague: [59]Functional Programming Went Mainstream Years Ago. * Max Bolingbroke: [60]Compiler Plugins For GHC: Weeks Three and Four. An update on Max's Google Summer of Code project. * >>> David Overton: [61]A Haskell Sudoku Solver using Finite Domain Constraints. David shows how to use his Haskell constraint solver to solve Sudoku puzzles. Pretty neat! * >>> Lorenz Pretterhofer: [62]Haskell Does Concurrency. * Edward Kmett: [63]Anamorphism. The newest installment in Edward's [64]field guide to recursion schemes. * John Goerzen (Real World Haskell): [65]Last Call for Comments on Most Chapters. Real World Haskell is going to press soon! Get your final comments in ASAP! * Edward Kmett: [66]MSFP 2008. * Lennart Augustsson: [67]Lost and Found. A very slick Haskell library for tracing how expressions are actually evaluated, including the ability to explicitly see the sharing involved! * Tom Nielsen (FP Lunch): [68]braincurry. A domain specific language to define and execute experiments and simulations related to cellular neuroscience. * Alex McLean: [69]Visualisation of a triangular mesh. * Paul R Brown: [70]Beust Sequence Ruminations. Thoughts on solving an interesting puzzle in Haskell. * >>> David Overton: [71]Constraint Programming in Haskell. David is working on a constraint logic programming system in Haskell. * >>> chaource: [72]Why functional programming is almost dead (and has always been). Interesting argument? Flawed premises? Both? None of the above? You decide! Quotes of the Week * jfredett: I'd code but I'm so drugged up I could only write effective code in perl. * SamB: [SamB] @let forkbomb n = forkbomb (2*n) `par` forkbomb (2*n+1) [lambdabot] Defined. [SamB] > forkbomb 1 -!- lambdabot [n=lambdabo@72.249.126.23] has quit [Remote closed the connection] * vinicius: haskell is macgyver with bananas, barbed wired and envelopes * dons: huh, amazon recommends Neal Stephenson + RWH * Pseudonym: trapped in the IO monad: The lesser known R. Kelly opera * byorgey: Extreme Anger Programming: you are paired with a really dumb partner and after twenty minutes of agony you rip the keyboard from their hands, delete everything they typed, and do it yourself About the Haskell Weekly News New editions are posted to [73]the Haskell mailing list as well as to [74]the Haskell Sequence and [75]Planet Haskell. [76]RSS is also available, and headlines appear on [77]haskell.org. Headlines are available as [78]PDF. To help create new editions of this newsletter, please see the information on [79]how to contribute. Send stories to byorgey at seas dot upenn dot edu. The darcs repository is available at darcs get [80]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.icfpcontest.org/ 3. http://luqui.org/blog/archives/2008/07/05/fun-with-flemish/ 4. http://changelog.complete.org/posts/728-guid.html 5. http://www.lively.com/dr?rid=-4485567674160322075 6. http://www.lively.com/ 7. http://article.gmane.org/gmane.comp.lang.haskell.general/16292 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniplate 9. http://araujoluis.blogspot.com/2008/07/ghc-682-stable-himerge-0219-too.html 10. http://article.gmane.org/gmane.comp.lang.haskell.general/16290 11. http://www.haskell.org/haskellwiki/The_Monad.Reader 12. http://article.gmane.org/gmane.comp.lang.haskell.general/16288 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hCsound 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42047 15. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9470 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GraphSCC 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41944 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41941 19. http://www.haskell.org/haskellwiki/DDC 20. http://lists.osuosl.org/pipermail/darcs-users/2008-June/012480.html 21. http://darcs.net/ 22. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 23. http://blog.omega-prime.co.uk/2008/07/05/compiler-plugins-for-ghc-weeks-thr… 24. http://code.haskell.org/hoogle/ 25. http://neilmitchell.blogspot.com/2008/07/gsoc-hoogle-week-6.html 26. http://haskell.org/haskellwiki/Hpysics 27. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 28. http://physics-dph.blogspot.com/2008/07/status-report-week-6.html 29. http://physics-dph.blogspot.com/2008/07/double-buffering-demo.html 30. http://www.sivity.net/projects/language.c/wiki/ 31. http://hsbene.blogspot.com/2008/07/analysis-friendly-representation.html 32. http://code.haskell.org/~Saizan/cabal 33. http://hackage.haskell.org/trac/ghc/wiki/GhcApiStatus 34. http://haskell.org/haskellwiki/Library_submissions 35. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9481 36. http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf 37. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/42080 38. http://www.haskell.org//pipermail/haskell-cafe/2008-July/044956.html 39. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41975 40. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41936 41. http://article.gmane.org/gmane.comp.lang.haskell.general/16285 42. http://planet.haskell.org/ 43. http://haskell.org/haskellwiki/Blog_articles 44. http://comonad.com/reader/2008/haskell-cafe/ 45. http://fmapfixreturn.wordpress.com/2008/07/09/comonads-in-everyday-life/ 46. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Differentiation/ 47. http://www.realworldhaskell.org/blog/2008/07/08/real-world-haskell-the-book… 48. http://hsbene.blogspot.com/2008/07/analysis-friendly-representation.html 49. http://okasaki.blogspot.com/2008/07/breadth-first-numbering-algorithm-in.ht… 50. http://jdfrens.blogspot.com/2008/07/pe-problem-2-in-all-languages-part-i.ht… 51. http://physics-dph.blogspot.com/2008/07/double-buffering-demo.html 52. http://physics-dph.blogspot.com/2008/07/quickcheck-puzzle-answer.html 53. http://physics-dph.blogspot.com/2008/07/status-report-week-6.html 54. http://jpmoresmau.blogspot.com/2008/07/handling-errors-in-json-to-haskell.h… 55. http://braincrater.wordpress.com/2008/07/07/fixed-point-datatypes/ 56. http://araujoluis.blogspot.com/2008/07/ghc-682-stable-himerge-0219-too.html 57. http://www.wellquite.org/anglo_haskell_2008.html 58. http://neilmitchell.blogspot.com/2008/07/gsoc-hoogle-week-6.html 59. http://prog21.dadgum.com/31.html 60. http://blog.omega-prime.co.uk/2008/07/05/compiler-plugins-for-ghc-weeks-thr… 61. http://overtond.blogspot.com/2008/07/haskell-sudoku-solver-using-finite.html 62. http://krysole.net/2008/07/04/haskell-does-concurrency/ 63. http://comonad.com/reader/2008/anamorphism/ 64. http://comonad.com/reader/2008/recursion-schemes/ 65. http://www.realworldhaskell.org/blog/2008/07/03/last-call-for-comments-on-m… 66. http://comonad.com/reader/2008/msfp/ 67. http://augustss.blogspot.com/2008/07/lost-and-found-if-i-write-108-in.html 68. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=105 69. http://doc.gold.ac.uk/~ma503am/alex/visualisation-of-a-mesh/ 70. http://mult.ifario.us/p/beust-sequence-ruminations 71. http://overtond.blogspot.com/2008/07/pre.html 72. http://chaource.livejournal.com/34530.html 73. http://www.haskell.org/mailman/listinfo/haskell 74. http://sequence.complete.org/ 75. http://planet.haskell.org/ 76. http://sequence.complete.org/node/feed 77. http://haskell.org/ 78. http://code.haskell.org/~byorgey/code/hwn/archives/20080709.pdf 79. http://haskell.org/haskellwiki/HWN 80. http://code.haskell.org/~byorgey/code/hwn/

1 0

How to do a special kind of comment with the TokenParser
by John Ky 09 Jul '08

09 Jul '08

Hi, TokenParser supports two kinds of comments, the multi-line comments (ie. {- -}) and the single line comments (ie. -- \n). The language I am trying to parse, however, has comments which are neither. The -- acts like a single line comment which extends to the end of the line usually, but can also be truncated to before the end of the line by another --. For example: noncomment -- comment comment noncomment -- comment comment -- noncomment noncomment -- comment -- noncomment noncomment I haven't been able to get the TokenParser to work with this style of comment. The best I could do was copy the whole Token module and modify the code: data LanguageDef st = LanguageDef { {- snip -} *, commentLine :: String* {- snip -} } {- snip -} makeTokenParser languageDef = TokenParser{ {- snip -} } where {- snip -} whiteSpace | noLine && noMulti = skipMany (simpleSpace *<|> customComment* <?> "") | noLine = skipMany (simpleSpace *<|> customComment* <|> multiLineComment <?> "") | noMulti = skipMany (simpleSpace *<|> customComment* <|> oneLineComment <?> "") | otherwise = skipMany (simpleSpace *<|> customComment* <|> oneLineComment <|> multiLineComment <?> "") where noLine = null (commentLine languageDef) noMulti = null (commentStart languageDef) *customComment = do{commentCustom languageDef ;return() }* Then I put my specialised comment parser in the customComment field: languageDef = TOKEN.LanguageDef { {- snip -} , TOKEN.commentCustom = customComment {- snip -} } where customComment = do string "--" untilLineCommentEnd return () untilLineCommentEnd = do c <- manyTill anyChar (string "\n" <|> try (string "--")) return () Anyone know of a way I could reuse the TokenParser code rather than copy and tweaking it? Thanks -John

2 1

Interesting feature
by fero 09 Jul '08

09 Jul '08

Hi I have read in one tutorial (I can't find it again, but it was probably either one on ibm, gentle introduction or yaht), that it is possible to define relationships between free variables and the same program can be used to calculate either first variable when second is set or second when first is set. I have understood this as if I set first free variable, run program and write name of second variable and I get result, and vice versa. I don't know if I understood it well. It looks really interesting but I can't figure out how to do it. Is this really possible? (I doubt.) If yes show example please. Thanks Fero -- View this message in context: http://www.nabble.com/Interesting-feature-tp18311432p18311432.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

9 13

QuickCheck: outdated manual
by Roman Cheplyaka 09 Jul '08

09 Jul '08

Online QC manual[1] says[2] that 'vector' takes number of elements and generator, while in QuickCheck-1.1.0.0 it takes only number and generates vector of arbitrary's. Please fix that. By the way, I find the old version as useful as the new one. Although both are trivially implemented, I don't see any reason of why one is included and the other is not. Does anyone? 1. http://www.cs.chalmers.se/~rjmh/QuickCheck/manual.html 2. http://www.cs.chalmers.se/~rjmh/QuickCheck/manual_body.html#17 -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

1 0

What is a rigid type variable?
by Xiao-Yong Jin 09 Jul '08

09 Jul '08

Hi all, I'm writing a short function as follows, but I'm not able to find a suitable type signature for `go'. It uses Numeric.LinearAlgebra from hmatrix. -- | Map each element in a vector to vectors and thus form a matrix -- | row by row mapVecToMat :: (Element a, Element b) => (a -> Vector b) -> Vector a -> Matrix b mapVecToMat f v = fromRows $ go (d - 1) [] where d = dim v go :: Element b => Int -> [Vector b] -> [Vector b] go 0 vs = f (v @> 0) : vs go !j !vs = go (j - 1) (f (v @> j) : vs) If I give the type signature to go as this, I got the following error Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the type signature for `go' at test.hs:36:20 `b' is a rigid type variable bound by the type signature for `mapVecToMat' at test.hs:31:35 Expected type: Vector b1 Inferred type: Vector b In the first argument of `(:)', namely `f (v @> 0)' In the expression: f (v @> 0) : vs So what is this rigid type variable all about and what is correct type of the function `go'? Thanks in advance, X-Y -- c/* __o/* <\ * (__ */\ <

5 7

more database issues
by Galchin, Vasili 09 Jul '08

09 Jul '08

vigalchin@ubuntu:~/Desktop/hsql-1.7$ runhaskell Setup.lhs clean cleaning... vigalchin@ubuntu:~/Desktop/hsql-1.7$ runhaskell Setup.lhs configure --user --prefix=$HOME Configuring hsql-1.7... Warning: No license-file field. vigalchin@ubuntu:~/Desktop/hsql-1.7$ runhaskell Setup.lhs build Preprocessing library hsql-1.7... Building hsql-1.7... Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden vigalchin@ubuntu:~/Desktop/hsql-1.7$ I have a global ghc installation plus a local one off my user directory. In any case, when I do "ghc-pkg list" .... old-time-1.0.0.0 is not hidden. ???? Bottom line is how do I install DB libraries with least pain?? Regards, Vasili

3 3

data Color3 a = Color3 !a !a !a
by Daryoush Mehrtash 09 Jul '08

09 Jul '08

Can some one explain what the !a does in this: data Color3<http://cvs.haskell.org/Hugs/pages/libraries/OpenGL/Graphics-Rendering-OpenG…>a = Color3<http://cvs.haskell.org/Hugs/pages/libraries/OpenGL/Graphics-Rendering-OpenG…>!a !a !a http://cvs.haskell.org/Hugs/pages/libraries/OpenGL/Graphics-Rendering-OpenG… Thanks, Daryoush

4 3

Swapping Monads
by Dominic Steinitz 08 Jul '08

08 Jul '08

I have a solution so this is for interest only. It is not normally the case that two monads compose to give another monad. Monad transformers capture when this is possible. However, when there is a "swap" function satisfying some commutative diagrams then it can be proved that the monads compose to produce a monad. Is there such a swap function in a library? I had a look in the comprehensive category-extra package but couldn't find anything. Here's two possible implementations. Big caveat: I haven't proved that these satisfy the commutative diagrams but I am confident that they will. Option 1 define swap :: (Functor m, Monad m) => Either String (m a) -> m (Either String a) swap (Left s) = return (Left s) swap (Right x) = fmap Right x Option 2 use Traversable (EvilTerran's suggestion) instance F.Foldable (Either String) where foldMap f (Left s) = mempty foldMap f (Right x) = f x instance T.Traversable (Either String) where traverse f (Left s) = pure (Left s) traverse f (Right x) = Right <$> f x and now you can use T.sequence to swap the monads around. For further information see Composing Monads by Mark Jones and Luc Duponcheel and Toposes, Triples and Theories by Barr & Wells (Section 9.2). Dominic.

2 1

extensible data types in Haskell?
by David Walker 08 Jul '08

08 Jul '08

Hi all, SML conveniently contains the type "exn" which is an instance of an "extensible data type". In other words, unlike normal data types that are "closed" (can't admit new constructors once defined), SML's exn type is "open," allowing programmers to keep adding new alternatives as often as they choose. Like normal datatypes, the elimination form for an extensible data type is a case statement (or match function). Friends have told me that Haskell doesn't have extensible data types. However, it seems fairly straightforward to code them up using type classes....though the solution I'm thinking of has a little bit of boilerplate I'd like to scrap (you have to define a new type declaration *and* an instance of a type class with a "match" method) and matching occurs through a string comparison (which can lead to silly programmer errors if there is accidentally a typo in the string). Anyway, it's possible with some thought I could come up with a better solution, but before worrying about it, I figured I'd ask if anybody else already has a package that does this. It seems like a pretty natural feature to want to have. Thanks in advance, Dave

4 4

Hoogle often updates ?
by L.Guo 08 Jul '08

08 Jul '08

Hi Haskellers: I have just been asked for usage of timer in haskell. Which I did not remember clearly. So I ask the search engine. In Hoogle: timer In Google: haskell timer After I tried these, I wonder, when and how often the hoogle update its database? And, could hoogle search range cover the hackages? I like hoogle engine. Bcuz it is a very usful tool which helps me learn haskell. Wish it better. Regards -------------- L.Guo 2008-07-08

2 1

Avoiding lazy evaluation in an ErrorT computation
by Tim Bauer 08 Jul '08

08 Jul '08

The file below models a problem I have been trying to figure out. This file simplifies my original code, while still illustrating the problem. > import Prelude hiding (catch) > import Control.Monad.Reader > import Control.Monad.Error > import Control.Exception > import System.IO(readFile) > import Data.Either(either) Our monad transformer is an ErrorT which wraps the IO monad. ErrorT allows us to use throwError, but we won't use it in this example. > type T a = ErrorT String IO a The following runs a (T a) in the context of the IO monad. We wrap runErrorT in try so as to catch things like division by zero and what not. > doTinIO :: T a -> IO (Either String a) > doTinIO ta = do > exesa <- try (runErrorT ta) -- IO (Either Exception (Either String a)) > return $ case exesa of > Left x -> Left ("EX: "++(show x)) > Right esa -> esa > > boom = 1 `div` 0 > b1 = return boom :: T Int > bad = doTinIO b1 The above, bad, results in: Right *** Exception: divide by zero My hope was to get Left "EX: divide by zero" I cannot understand why the `try' does not get a chance at the erroneous calculation. That is, I want the try to catch the runtime exception. Indeed, if the IO computation is strictly computed, I get the proper result. > g1 = boom `seq` (return boom :: T Int) > good = doTinIO g1 Stuff that raises exceptions in IO actions does not work either. > g2 = doTinIO (return boom) Results in: ``Right *** Exception: divide by zero'' However, other actions that do raise errors work correctly. It appears the value of the computation must be used as the next two examples show. > g3 = doTinIO (liftIO (readFile "nonexistent")) > g4 = doTinIO (liftIO (print boom)) My problem is that I control `doTinIO', but someone else provides the computation (T a). I cannot force callers to strictly evaluate their computations. I've tried three other variants (given below) that are all nearly equivalent. > handler :: Exception -> IO (Either String a) > handler = return . Left . ("EX: "++) . show > > doTinIO2 :: T a -> IO (Either String a) > doTinIO2 ta = catch (runErrorT ta >>= evaluate) handler > > doTinIO3 :: T a -> IO (Either String a) > doTinIO3 ta = do > esa <- catch (runErrorT ta) handler > case esa of > Right a -> catch (evaluate (return a)) handler > l -> return l > > doTinIO4 :: T a -> IO (Either String a) > doTinIO4 ta = catch (runErrorT ta) handler *Main> doTinIO2 b1 Right *** Exception: divide by zero *Main> doTinIO3 b1 Right *** Exception: divide by zero *Main> doTinIO4 b1 Right *** Exception: divide by zero Any suggestions? Thanks all.

4 3

A Monad for on-demand file generation?
by Joachim Breitner 08 Jul '08

08 Jul '08

Hi, for an application such as a image gallery generator, that works on a bunch of input files (that are assumed to be constant during one run of the program) and generates or updates a bunch of output files, I often had the problem of manually tracking what input files a certain output file depends on, to check the timestamps if it is necessary to re-create the file. I thought a while how to do this with a monad that does the bookkeeping for me. Assuming it’s called ODIO (On demand IO), I’d like a piece of code like this: do file1 <- readFileOD "someInput" file2 <- readFileOD "someOtherInput" writeFileOD "someOutput" (someComplexFunction file1 file2) only actually read "someInput" and "someOtherInput", do the calculation and write the output if these have newer time stamps than the output. The problem I stumbled over was that considering the type of >>= (>>=): Monad m => m a -> (a -> m b) -> m b means that I can not „look ahead“ what files would be written without actually reading the requested file. Of course this is not always possible, although I expect this code to be the exception: do file1 <- readFileOD "someInput" file2 <- readFileOD "someOtherInput" let filename = decideFileNamenameBasedOn file2 writeFileOD filename (someComplexFunction file1 file2) But assuming that the input does not change during one run of the program, it should be safe to use "unsafeInterleaveIO" to only open and read the input when used. Then, the readFileOD could put the timestamp of the read file in a Monad-local state and the writeFileOD could, if the output is newer then all inputs listed in the state, skip the writing and thus the unsafeInterleaveIO’ed file reads are skipped as well, if they were not required for deciding the flow of the program. One nice thing is that the implementation of (>>) knows that files read in the first action will not affect files written in the second, so in contrast to MonadState, we can forget about them, which I hope leads to quite good guesses as to what files are relevant for a certain writeFileOD operation. Also, a function cacheResultOD :: (Read a, Show a) => FilePath -> a -> ODIO a can be used to write an (expensive) intermediate result, such as the extracted exif information from a file, to disk, so that it can be used without actually re-reading the large image file. Is that a sane idea? I’m also considering to use this example for a talk about monads at the GPN¹ next weekend. Greetings, Joachim ¹ http://entropia.de/wiki/GPN7 -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

10 19

type classes
by Cotton Seed 08 Jul '08

08 Jul '08

Hi everyone, I'm working on a computational algebra program and I've run into a problem. In my program, I have types for instances of algebraic objects, e.g. ZModN for modular integers, and types for the objects themselves, e.g. ZModNTy for the ring of modular integers. Now, I want to subclass ZModNTy from something like class RingTy a b where order :: a -> Integer units :: a -> [b] where `a' represents algebraic object, and `b' the type of instances of that object. I want an instance instance RingTy ZModNTy ZModN where ... but then code that only uses order fails with errors like No instance for (RingTy ZModNTy b) arising from a use of `order' at Test2.hs:16:8-15 since there is no constraint on the second type variable. I think what I really want is class RingTy a where order :: a b -> Integer units :: a b -> [b] but this doesn't work either since ZModNTy is not parametric in its type like, say, `Polynomial a' is. Is this a common problem? Is there a standard way to handle it? Thank you for your attention, Cotton

5 7

parsec linking problem
by naruto canada 08 Jul '08

08 Jul '08

hi I've just managed to get ghc-6.8.3 installed and tried to run an example from "http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours/Parsing" I run into linking error with parsec: Here is the code: module Main where import System.Environment import Text.ParserCombinators.Parsec hiding (spaces) symbol :: Parser Char symbol = oneOf "!#$%&|*+-/:<=>?@^_~" main :: IO () main = do -- args <- getArgs -- putStrLn ("Hello, " ++ (args!!0) ++ " " ++ (args!!1) ) -- putStrLn ( show( read(args!!0) + read(args!!1) ) ) putStrLn ( "give me a name:" ) name <- getLine putStrLn ( name ) ghc -o /tmp/a.out accu.hs accu.o: In function `so8_info': : undefined reference to `parseczm2zi1zi0zi1_TextziParserCombinatorsziParsecziChar_oneOf_closure' accu.o: In function `spP_info': : undefined reference to `__stginit_parseczm2zi1zi0zi1_TextziParserCombinatorsziParsec_' accu.o: In function `so8_closure': : undefined reference to `parseczm2zi1zi0zi1_TextziParserCombinatorsziParsecziChar_oneOf_closure' collect2: ld returned 1 exit status Thanks my installation steps (just in case you need to know): WGET_UNPACK http://ftp.gnu.org/gnu/readline/readline-4.3.tar.gz ./configure --prefix=/usr make clean ; make || INTERVENTION find . -name libhistory.so* find . -name libreadline.so* cp shlib/libhistory.so.4.3 /lib cp shlib/libreadline.so.4.3 /lib ln -sfvn libhistory.so.4.3 /lib/libhistory.so.4 ln -sfvn libreadline.so.4.3 /lib/libreadline.so.4 WGET_UNPACK http://haskell.org/ghc/dist/6.8.2/ghc-6.8.2-i386-unknown-linux.tar.bz2 cd ghc-6.8.2 autoreconf ./configure --prefix=/usr make install echo 'main = putStrLn "hello"' > t.hs ghc t.hs ./a.out | grep hello || INTERVENTION rm -rf /build/ghc-6.8.2 WGET_UNPACK http://haskell.org/ghc/dist/6.8.3/ghc-6.8.3-src-extralibs.tar.bz2 WGET_UNPACK http://haskell.org/ghc/dist/6.8.3/ghc-6.8.3-src.tar.bz2 cd ghc-6.8.3 autoreconf ./configure --prefix=/usr make clean ; make || INTERVENTION make || INTERVENTION make install echo 'main = putStrLn "hello"' > t.hs ghc t.hs ./a.out | grep hello || INTERVENTION ghc --version | grep 6.8.3 || INTERVENTION

2 2

Re: [Gtk2hs-users] ANN: Gtk2Hs 0.9.13 released
by Peter Gavin 08 Jul '08

08 Jul '08

Peter Hercek wrote: > Thanks for the package. > I experienced some bugs with it on windows. When showing a dynamically > built context menu, I'm getting errors like these: > > _cairo_win32_surface_show_glyphs(ExtTextOutW failed): The operation completed successfully. > _flush_glyphs: The operation completed successfully. > > ... and the text of some menu items is not shown (otherwise the menu > items do work - actions are called when they are clicked). > > When I try to run the app under ghci I get error like this: > > Loading package glib-0.9.13 ... <interactive>: Unknown PEi386 section name `.reloc' (while processing: > C:/tools/Gtk2Hs/HSglib.o) > ghc.exe: panic! (the 'impossible' happened) > (GHC version 6.8.3 for i386-unknown-mingw32): > loadObj: failed > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug > > Using older Gtk (2.10.13) helps for the first one. > Using my own compile of Gtk2Hs (I compiled older version 0.9.12.1 > myself) helps for the second one. > > Peter. Hi Peter, The first problem looks like a problem in cairo. I'm pretty sure the functions named are cairo internal API, you may want to report a bug to their list. I suppose it's possible those functions always print the error message (even on success), but behave correctly otherwise. It looks like the second problem is a GHC bug, since there isn't anything special about our library (that I can think of) that might cause that to happen. Pete > > Peter Gavin wrote: >> Hello everyone, >> >> Gtk2Hs version 0.9.13 is now available. [1] >> >> New features: >> >> * bindings for Gnome VFS and GStreamer >> * a new Gtk+ tutorial has been adapted by Hans van Thiel >> * cairo image stride support >> * many new demos >> * compiles with GHC 6.8.3 >> * lots of bug fixes >> >> This release has been tested on a variety of platforms with different >> versions of Gtk+ and GHC, so you should have no trouble compiling it if >> you're using an older version of Gtk+. >> >> Note that the binaries for Win32 for this release are only provided for >> GHC 6.8.3 and Gtk+ 2.12. As with older releases, all the C libraries >> needed are included in the installer, so you don't need to download >> anything else to get up an running. I've also created zip files >> containing only the C libraries that can be used for redistribution. >> The sources for these binaries are available at [2] >> >> Links: >> [1] >> http://sourceforge.net/project/showfiles.php?group_id=49207&package_id=42440 >> [2] http://ftp.gnome.org/pub/GNOME/sources/ >> >> >> -- >> Pete >> >> ------------------------------------------------------------------------- >> Check out the new SourceForge.net Marketplace. >> It's the best place to buy or sell services for >> just about anything Open Source. >> http://sourceforge.net/services/buy/index.php > > > > ------------------------------------------------------------------------- > Sponsored by: SourceForge.net Community Choice Awards: VOTE NOW! > Studies have shown that voting for your favorite open source project, > along with a healthy diet, reduces your potential for chronic lameness > and boredom. Vote Now at http://www.sourceforge.net/community/cca08 > _______________________________________________ > Gtk2hs-users mailing list > Gtk2hs-users(a)lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/gtk2hs-users

1 0

ANN: Gtk2Hs 0.9.13 released
by Peter Gavin 08 Jul '08

08 Jul '08

Hello everyone, Gtk2Hs version 0.9.13 is now available. [1] New features: * bindings for Gnome VFS and GStreamer * a new Gtk+ tutorial has been adapted by Hans van Thiel * cairo image stride support * many new demos * compiles with GHC 6.8.3 * lots of bug fixes This release has been tested on a variety of platforms with different versions of Gtk+ and GHC, so you should have no trouble compiling it if you're using an older version of Gtk+. Note that the binaries for Win32 for this release are only provided for GHC 6.8.3 and Gtk+ 2.12. As with older releases, all the C libraries needed are included in the installer, so you don't need to download anything else to get up an running. I've also created zip files containing only the C libraries that can be used for redistribution. The sources for these binaries are available at [2] Links: [1] http://sourceforge.net/project/showfiles.php?group_id=49207&package_id=42440 [2] http://ftp.gnome.org/pub/GNOME/sources/ -- Pete

2 2

Re: [Haskell-cafe] Is there a nicer way to do this?
by Don Stewart 08 Jul '08

08 Jul '08

mfeathers: > Don Stewart wrote: > >mfeathers: > >> > >>segment :: Int -> [a] -> [[a]] > >>segment 0 _ = [] > >>segment _ [] = [] > >>segment n x = (take n x) : segment n (drop n x) > > > >The first set of parens can go, > > > > segment n x = take n x : segment n (drop n x) > > > >>I did a version of this which used splitAt but I wasn't sure whether it > >>was going to buy me anything re performance that would justify its > >>ugliness. > > > >Besides, > > > > splitAt n xs = (take n xs, drop n xs) > > Thanks. That is odd, though. It makes me wonder what to expect re > optimization. Would the compiler/runtime know that splitAt could be > done in a single pass? Not with that definition. It would require some moderately unusual fusion combining the take and drop into a single fold with the (,) on the inside, rather than on the outside. However, GHC actually implements splitAt as: splitAt (I n) ls | n < 0 = ([], ls) | otherwise = splitAt' n ls where splitAt' :: Int -> [a] -> ([a], [a]) splitAt' 0 xs = ([], xs) splitAt' _ xs@[] = (xs, xs) splitAt' m (x:xs) = (x:xs', xs'') where (xs', xs'') = splitAt' (m - 1) xs So there may be some benefit. -- Don

6 7

Compiling GHC 6.8.3 on OS X with readline support
by Chris Eidhof 08 Jul '08

08 Jul '08

Hey all, When compiling GHC 6.8.3 on OS X, I ended up with a GHCi without readline support. That makes interacting quite hard, especially because the Backspace-key didn't even work. With some help on irc from Baughn and by reading a blog post from Paul Brown, I managed to get readline working after all. After installing readline from fink, I had to pass the following options to configure: ./configure --with-readline-includes=/sw/include/readline --with- readline-libraries=/sw/lib After that make and sudo make install worked just fine, and I had readline again! I wrote this down in case anybody else has had the same problems. -chris

3 2

Re: [Haskell-cafe] OpenSSL.Digest linking error: EVP_mdc2
by Donn Cave 07 Jul '08

07 Jul '08

Quoth "Ken Takusagawa" <ken.takusagawa.2(a)gmail.com>: | $ ghc --make test-hopenssl.hs -lcrypto | [1 of 1] Compiling Main ( test-hopenssl.hs, test-hopenssl.o ) | Linking test-hopenssl ... | /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In | function `s2O8_info': | (.text+0xf3a): undefined reference to `EVP_mdc2' | /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In | function `s2TQ_info': | (.text+0x1435): undefined reference to `EVP_mdc2' | collect2: ld returned 1 exit status I see the following in the Configure file for openssl-0.9.8h: my $default_depflags = "-DOPENSSL_NO_CAMELLIA -DOPENSSL_NO_CMS -DOPENSSL_NO_GMP -DOPENSSL_NO_MDC2 -DOPENSSL_NO_RC5 -DOPENSSL_NO_RFC3779 -DOPENSSL_NO_SEED -DOPENSSL_NO_TLSEXT ";my $default_depflags = "-DOPENSSL_NO_CAMELLIA -DOPENSSL_NO_CMS -DOPENSSL_NO_GMP I don't know why this would be, but I infer that the function hopenssl was looking for is commonly missing in SSL builds. Donn Cave, donn(a)avvanta.com

2 1

Definition of hidden instance members (bug in GHC or Hugs+Yhc)
by Neil Mitchell 07 Jul '08

07 Jul '08

Hi, This is either a GHC bug, or a Yhc+Hugs bug - I'm not sure which, but the compilers disagree: import Prelude hiding ((==)) data Foo = Foo instance Eq Foo where (==) a b = True GHC says: Temp.hs:14:4: `==' is not a (visible) method of class `Eq' Yhc and Hugs both successfully compile the module. Does anyone know which compiler(s) are in the wrong, and need bugs filing? Thanks Neil

5 5

Santana on my evil ways
by John D. Ramsdell 07 Jul '08

07 Jul '08

My son's nickname is Rama, so let me adopt it. I am a functional programmer, even when I use languages such as C. Scheme facilitated my development into a functional programmer, however, I appreciate the benefits of pure function programming at times. Yet when I use Haskell, I hear reminders of my Scheme past cast in the music of Santana. The words I hear are set to "Eval Ways": You've got to change your evil ways... Rama Before I stop respecting you. You've got to change... Rama And every word that I say, it's true. You use strange syntax and typing And offset rules You don't mutate locations You use strange do's This can't go on... Lord knows you got to change. John

3 2

Type families versus functional dependencies question
by Alexey Rodriguez 07 Jul '08

07 Jul '08

Hi guys, We are having trouble with the following program that uses type families: > class Blah f a where > blah :: a -> T f f a > class A f where > type T f :: (* -> *) -> * -> * the following function does not type: > wrapper :: forall a f . Blah f a => a -> T f f a > wrapper x = blah x GHC gives the error: Couldn't match expected type `T f1 f1 a' against inferred type `T f f a' In the expression: blah x In the definition of `wrapper': wrapper x = blah x Maybe it is a problem with ambiguous types, namely "f" appears only in applications of "T". But that is not the case, there is a "naked" f appearing as the argument of "T f". But perhaps the type checker does not want to unify those two f's precisely because they are the arguments of "T f". I have tried to encode the above program using FunDeps, because this procedure helps me understand type errors. But in this case I get no type error! > class A' (f :: * -> *) (g :: (* -> *) -> * -> *) | f -> g where > class Blah' f a where > blah' :: A' f g => a -> g f a > wrapper' :: forall a f g . (Blah' f a,A' f g) => a -> g f a > wrapper' x = blah' x So my question is whether my encoding is correct. And if it is, why isn't the type families version working? Thanks! Alexey

3 7

not for me, guv!
by Evan Laforge 07 Jul '08

07 Jul '08

So I wrote a big email about this mysterious message I was getting, but then after some research and poking and debugging tracked it down to a dangling pointer in C that was causing a callback to point to an invalid address, which somehow worked anyway 99% of the time. That all said, I'm not totally convinced that "freeHaskellFunctionPtr: not for me, guv! 0x23143f0" is the clearest error message ever. Now, if it were in a more contemporary American idiom such as "wtf this ptr ain't from MY house lol!!!" I probably would have found the bug in half the time. Anyway, I fixed my bug and I'm happy about that, but I didn't want to waste a chance to send an email with this title...

1 0

The state of database libraries
by Chris Eidhof 07 Jul '08

07 Jul '08

Hey everyone, I'm figuring out how to do databases in Haskell (on OS X). So far, I've tried the following approaches: 1. hdbc. I'd like to connect to MySQL, so I need the ODBC backend. I couldn't get this to work under OS X, while I installed myodbc, which seems to be broken. 2. hsql. The packages on hackage don't compile, so I grabbed the darcs version. hqsl itself installed perfectly, but when I try to compile hsql-mysql, it does not recognize that hsql is already installed and tries to recompile. Compiling fails, so I generated a .tar.gz of hsql using cabal sdist, put it in the .cabal/packages directory in the right place, and finally, it does compile. Now hsql-mysql starts compiling, but it finally fails with: MySQL.hsc:270:0: error: ‘MYSQL_NO_DATA’ undeclared (first use in this function) MySQL.hsc:270:0: error: (Each undeclared identifier is reported only once MySQL.hsc:270:0: error: for each function it appears in.) So what is the state of hsql? And haskelldb? Are they actively maintained? Thanks, -chris

5 4

OpenSSL.Digest linking error: EVP_mdc2
by Ken Takusagawa 07 Jul '08

07 Jul '08

I'm using hopenssl 1.0 from hackage. What am I doing wrong here? $ cat test-hopenssl.hs import OpenSSL.Digest main=undefined $ ghc --make test-hopenssl.hs -lcrypto [1 of 1] Compiling Main ( test-hopenssl.hs, test-hopenssl.o ) Linking test-hopenssl ... /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In function `s2O8_info': (.text+0xf3a): undefined reference to `EVP_mdc2' /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In function `s2TQ_info': (.text+0x1435): undefined reference to `EVP_mdc2' collect2: ld returned 1 exit status $ dpkg-query -S /usr/lib/libcrypto.a libssl-dev: /usr/lib/libcrypto.a $ dpkg-query -l libssl-dev Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad) ||/ Name Version Description +++-======================-======================-============================================================ ii libssl-dev 0.9.8g-10.1 SSL development libraries, header files and documentation $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 $ uname -a Linux localhost 2.6.24-1-686 #1 SMP Thu May 8 02:16:39 UTC 2008 i686 GNU/Linux

1 0

Is there a nicer way to do this?
by Michael Feathers 07 Jul '08

07 Jul '08

segment :: Int -> [a] -> [[a]] segment 0 _ = [] segment _ [] = [] segment n x = (take n x) : segment n (drop n x) I did a version of this which used splitAt but I wasn't sure whether it was going to buy me anything re performance that would justify its ugliness. Michael

4 3

[WARN] Bug fix release of pureMD5 (Was: pureMD5)
by Thomas M. DuBuisson 07 Jul '08

07 Jul '08

Cafe, Daniel Larsson noticed a correctness issue with the pureMD5 package. This issue would affect you if you built the value incrementally via the 'updateMD5' function (vs just using 'md5') and didn't provide 512 bit long bytestrings (an MD5 block of operation). As you can probably tell, I didn't invest enough into the non-performance aspects of pureMD5. Faced with actual users ;-), I have released version 0.2.0 which has the bug fix, a new API (type prevention from re-finalizing a digest), and a reasonable set of quickchecks (covering Show / Binary instances, known answer and incremented hashing). Oh, also the module name has changed to place it inline with 'Crypto' package naming while not colliding. Sorry if this causes anyone headaches. Daniel, Good catch, I hope this didn't consume much of your time. Thanks a bunch! TomMD On Fri, 2008-06-13 at 04:14 +0200, Daniel Larsson wrote: > Hi Thomas, > > > I was fiddling around with your pureMD5 package, and encountered some > problems with using the md5Update/md5Finalize sequence. I tried to > calculate the md5 of some scattered data, but it kept returning the > wrong values. It seems that each md5Update must supply an exact > blockSize number of bits, since the mdLeftOver part isn't taken into > account in subsequent calls to md5Update. > > > I wrote a small patch, and a simple QuickCheck property, to support > calculating md5 of scattered data, attached to this mail. Hopefully I > didn't mess up something... > > > -- > Daniel

8 11

I/O system brokenness with named pipes
by John Goerzen 06 Jul '08

06 Jul '08

So I have a need to write data to a POSIX named pipe (aka FIFO). Long story involving a command that doesn't have an option to read data from stdin, but can from a named pipe. I have created the named pipe from Haskell no problem. But I can't use writeFile to write data to it. Worse, it returns: *** Exception: /tmp/bakroller.zD0xHj/fifo: openFile: does not exist (No such device or address) which is completely false, as it *does* exist, and I can cat to it as expected. The call should block until everything is read on the remote end. I thought maybe writeFile is being weird, so I tried: openFile "/tmp/bakroller.zD0xHj/fifo" WriteMode Same thing. There is no logical reason I can see for this behavior. In fact, something must be going to *extra* effort to avoid writing to a named pipe, and I can't work out why. Named pipes are a standard, useful part of a system and shouldn't be ignored like this. Interestingly, readFile works fine on a named pipe. What's going on here? Am I going to have to resort to the System.Posix interface just to be able to write to a named pipe? -- John

10 20

eyes again please .... on my POSIX aio code
by Galchin, Vasili 05 Jul '08

05 Jul '08

Hello, I have modified my aioError and aioReturn to hopefully be more in line with the actual aio_error and aio_return signatures, respectively. In the first implementation I had suffered a serious brain f**t ... oh well ..... The implementations below are still somewhat preliminary. Not seemingly rocket science .... I am still not getting a correct count from aioReturn .. should be 20 but I get 0! I have a C program which behaves properly giving 20 for aio_return! grrrrrr! aioReturn :: AIOCB -> IO (AIOCB, ByteCount) aioReturn aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb -- DO A PEEK HERE!!!!! -- count <- throwErrnoIfMinus1 "aioReturn" (c_aio_return p_aiocb) count <- (c_aio_return p_aiocb) putStrLn ("count -> " ++ (show count)) aiocb <- peek p_aiocb putStrLn "aioReturn" aiocb1 <- peek p_aiocb dumpAIOCB aiocb1 return (aiocb, fromIntegral count) foreign import ccall safe "aio.h aio_return" c_aio_return :: Ptr AIOCB -> IO CInt aioError :: AIOCB -> IO (Errno) aioError aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb --throwErrnoIfMinus1 "aioError" (c_aio_error p_aiocb) errno <- (c_aio_error p_aiocb) putStrLn ("errno -> " ++ (show errno)) putStrLn "aioError" aiocb1 <- peek p_aiocb dumpAIOCB aiocb1 return (Errno errno) foreign import ccall safe "aio.h aio_error" c_aio_error :: Ptr AIOCB -> IO CInt I added an internal helper function dumpAIOCB to print out the aiocb at points for sanity checking!! dumpAIOCB :: AIOCB -> IO () dumpAIOCB (AIOCB aioFd aioLioOpcode aioReqPrio aioOffset aioBuf aioBytes aioSigevent) = do putStrLn "aiocb dump" putStrLn ("fd => " ++ (show aioFd)) putStrLn ("opcode => " ++ (show aioLioOpcode)) putStrLn ("prio => " ++ (show aioReqPrio)) putStrLn ("offset => " ++ (show aioOffset)) -- aioBuf putStrLn ("nbytes => " ++ (show aioBytes) ++ "\n") -- aioSigevent Kind regards, Vasili

1 0

nested parsing?
by Evan Laforge 05 Jul '08

05 Jul '08

Here's something I occasionally want to do with parsec: msg = do header body <- manyTill anyToken footer vals <- sub_parse parse_body (preprocess body) ... By coincidence I notice that perhaps the "manyTill stack overflow" thread could use something like this too. So I wrote: sub_parse :: Parsec.GenParser subtok st a -> [subtok] -> Parsec.GenParser tok st a sub_parse parser toks = do st <- Parsec.getState pos <- Parsec.getPosition case Parsec.runParser (Parsec.setPosition pos >> parser) st "" toks of Left err -> fail (show err) Right val -> return val -- oh yes, and I'd set the state to the final sub parse state too In this particular case, I found another solution that didn't need sub-parsing, so I haven't actually tested this much. In particular parsec doesn't let me throw a "raw" exception so I have to use "fail" and the error msgs look ugly. In addition, if 'preprocess' adds or removes tokens from 'body', I have to do something awkward like pass tokens around as (pos, tok). But come to think of it, I've done this pattern before, where I invoke a sub-monad and (mostly) splice it in with the current one, in one case to allow the submonad to be monomorphic on the monad type while calling monad remains polymorphic, in another case so I can run the submonad from dynamically loaded code which must be monomorphic but splice its results into the polymorphic calling monad... this is similar to the first one I guess. It seems somewhat similar to what catchError is doing.

1 0

parsec manyTill stack overflow
by Badea Daniel 05 Jul '08

05 Jul '08

I'm trying to parse a large file looking for instructions on each line and for a section end marker but Parsec's manyTill function causes stack overflow, as you can see in the following example (I'm using ghci 6.8.3): > parse (many anyChar) "" ['a'|x<-[1..1024*64]] It almost immediately starts printing "aaaaaaaaaaa...." and runs to completion. > parse (manyTill anyChar eof) "" ['a'|x<-[1..1024*1024]] *** Exception: stack overflow I guess this happens because manyTill recursively accumulates output from the first parser and returns only when it hits the 'end' parser. Is it possible to write a version of 'manyTill' that works like 'many' returning output from 'anyChar' as soon as it advances through the list of tokens? Thanks, Daniel

4 5

Question about abstraction
by Bas van Dijk 04 Jul '08

04 Jul '08

Dear list, I have I question about the following code I was playing with: (you can past the following right into your editor) ---------------------------------------------------------------------- import Data.Foldable (Foldable, foldMap) import Data.Monoid (mempty, mappend) import Data.Traversable (Traversable, traverse) import Control.Applicative (pure, (<$>), (<*>)) -- I was playing with the following tree-like datastructure (my plan -- is to make some kind of kd-tree but that's not important now): data T a = L | N C2 a (T a) (T a) (T a) (T a) type C2 = (Float, Float) -- A fold always comes in handy: foldT :: b -> (C2 -> a -> b -> b -> b -> b -> b) -> T a -> b foldT e _ L = e foldT e n (N c x tl tr bl br) = n c x (foldT e n tl) (foldT e n tr) (foldT e n bl) (foldT e n br) instance Functor T where fmap f = foldT L (\p -> N p . f) -- Now I defined the following instances: instance Foldable T where foldMap f = foldT mempty $ \_ x tl tr bl br -> f x `mappend` tl `mappend` tr `mappend` bl `mappend` br instance Traversable T where traverse f = foldT (pure L) $ \p x tl tr bl br -> N p <$> f x <*> tl <*> tr <*> bl <*> br ---------------------------------------------------------------------- -- If you look at the previous two functions you see a similar pattern: -- they both combine an initial value: 'f x' and 'N p <$> f x' respectively -- with the childs using a combining function: 'mappend' and '<*>' respectively. -- My question is: can I abstract from that? -- It looks like I can using a function like: combineWith :: b -> (b -> a -> b) -> a -> a -> a -> a -> b n `combineWith` f = \tl tr bl br -> n `f` tl `f` tr `f` bl `f` br -- Now 'foldMap' becomes: instance Foldable T where foldMap f = foldT mempty $ \_ x -> f x `combineWith` mappend -- But 'traverse' won't typecheck: instance Traversable T where traverse f = foldT (pure L) $ \p x -> (N p <$> f x) `combineWith` (<*>) -- Is it possible to make 'combineWith' more general so that the -- previous typechecks (maybe using arbitrary-rank polymorphism but I -- don't see how)? ---------------------------------------------------------------------- Thanks, Bas van Dijk

2 1

Question concerning datatype "Either"
by phy51km4n 04 Jul '08

04 Jul '08

There is a exercise using datatype "Either" I'm a bit confused about... The following datatypes are given: data Either a b = Left a | Right b data Tuple a b c d = One a | Two a b | Three a b c | Four a b c d Now the exercise: "Based on our definition of Tuple, write a function which takes a Tuple and returns either the value (if it’s a one-tuple), a Haskell-pair (i.e., (’a’,5)) if it’s a two-tuple, a Haskell-triple if it’s a three-tuple or a Haskell-quadruple if it’s a four-tuple. You will need to use the Either type to represent this." Why does that not work? : fromTuple (One a) = a fromTuple (Two a b) = (a, b) fromTuple (Three a b c) = (a, b, c) fromTuple (Four a b c d) = (a, b, c, d) Why is this correct? : fromTuple (One a ) = Left (Left a) fromTuple (Two a b ) = Left (Right (a,b)) fromTuple (Three a b c ) = Right (Left (a,b,c)) fromTuple (Four a b c d) = Right (Right (a,b,c,d)) Why does this combination of Rights and Lefts work and how does it work?? -- View this message in context: http://www.nabble.com/Question-concerning-datatype-%22Either%22-tp18264739p… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 2

Alternatives to convoluted record syntax
by Dougal Stanton 04 Jul '08

04 Jul '08

Hi Haskellers, This is a style question, as my program works fine but looks really ugly and can be confusing to maintain. I take options from the command line using GetOpt and hand back this structure for later use. > data Opts = Opts > { query :: Query > , queryLimit :: Maybe Limit > , disabledOnly :: Bool > } deriving Show Here's a snippet from the parser for one option (others omitted for clarity): > options :: [OptDescr (Opts -> Opts)] > options = > [ Option "b" ["bus"] (ReqArg busNum "NUM") "Bus number" > , ... > ] > where busNum n os = let b = (query os) { queryBusNumber = Just n } > in if isBusId n then os { query = b } else os Variations on that ugliness are repeated four times for other fields. Is there an alternative way to change the value of nested fields? Thanks, Dougal. -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

5 5

Integer = infinite precision integer? How?
by leledumbo 04 Jul '08

04 Jul '08

Does anyone have an explanation how Haskell implement this? Or a pointer to a article describing this? -- View this message in context: http://www.nabble.com/Integer-%3D-infinite-precision-integer--How--tp182738… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

Existential Data Types
by Richard G. 04 Jul '08

04 Jul '08

Hello I've been doing some stuff with existential data types and came across a curious pattern. If anyone has thoughts, comments, or feedback, it would be appreciated. --- I'm trying to port and extend the Be messaging scheme to Haskell for use in distributed, heterogenous environments. BeOS was object oriented and pervasively threaded. That, and my background as an OOP programmer, has resulted in a very OOP approach. Because OOP allows for subclassing and the Be API was designed for it, I've used a wrapper approach. class HandlerC h where . . . data Handler = forall h . HandlerC h => Handler h instance HandlerC Handler where . . . data DefaultHandler = DefaultHandler ... instance HandlerC DefaultHandler where . . . newDefaultHandler = Handler (DefaultHandler ...) newHandler h = Handler h The HandlerC defines the interface. When Handler implements the interface, it mostly passes the functions to the inner Handler that actually has the data and the real functions. The advantage is that I can wrap anything that implements the HandlerC interface and have it behave in a known and simple way from the outside. ---- One area where this might have some application is in unit testing to make mock objects easier to use. If I had a Looper ADT that depended on the Handler, I would want test the Looper without having to worry about the complexity of the Handler interfering with the tests. I could do this with the following: data MockHandler = MockHandler ... instance HandlerC MockHandler where . . . h = newHandler (MockHandler ...) where h is passed to the Looper some how. ---- Overall, this seems very similar to data D = D1 | D2 | D3 | ... The big difference is that all the Dn constructors don't need to be known at compile time, which is very useful when writing libraries. Also, the wrapped data can include functions and state data, making it very easy to create applets. This is handy when doing message passing or request handling, such as in a web server, especially when using STM. Richard

1 0

Wouter Swierstra style extensible records!
by Ron Alford 04 Jul '08

04 Jul '08

Having made over-use of Wouter's expression idioms, I decided to hack it into extensible records. It's not documented, it's probably got holes in its usability, but I thought I'd post it so people can play with it (and give suggestions before I rewrite my code to use it!). I know there are other extensible record systems, but I wondered how this stacks up. Notably, it looks like in HList the records are only extensible in one direction (tacking fields onto the end), although they have a much more developed syntax (if you're into that sort of thing). Current problems: * Defining a new field is rather bulky - currently it takes lines. Definitions are bulky in Wouter's extensible expressions, too, but I could probably shorten the get/set convenience functions to something more readable. * Construction of new records is only convenient by using the defaulting system. Maybe that makes sense, maybe it doesn't. A side benefit: Record access functions are reusable between record types (as long as they have the same type). Anyway, thanks again, Wouter! -Ron

1 0

Re: [Haskell-cafe] Access to Oracle database from Haskell
by Henning Thielemann 04 Jul '08

04 Jul '08

On Thu, 26 Jun 2008, Alistair Bayley wrote: >> Try this version of configOracle in Setup.hs: >> >> configOracle verbose buildtools = do >> if not (sqlplusProgram `isElem` buildtools) >> then return Nothing >> else do >> path <- getEnv "ORACLE_HOME" >> info verbose ("Using Oracle: " ++ path) >> makeConfig path libDir "oci/include" >> where libDir = if isWindows then "bin" else "lib" >> >> >> You'll also need to add this import: >> >> import System.Environment (getEnv) > > Did you get a chance to try this? I'm quite keen to fix the > non-Windows parts of the installation process, if I can. I have replaced configOracle by configOracle verbose buildtools = do if not (sqlplusProgram `isElem` buildtools) then return Nothing else do path <- getEnv "ORACLE_HOME" info verbose ("Using Oracle: " ++ path) let (libDir, includeDir) = if isWindows then ("bin", "oci/include") else ("lib", "/usr/include/oracle/10.2.0.4/client") makeConfig path libDir includeDir This is obviously a hack. We should get the Oracle include path from the user, maybe via another environment variable or a custom Cabal option. Now I can start GHCi with the example program you gave me. However I have to start with -lclntsh, otherwise symbol OCIEnvCreate cannot by found. I thought I do not need this option, because the installed Takusen package contains the library name: $ grep clntsh dist/installed-pkg-config extra-libraries: clntsh clntsh I wonder why it appears twice, because in dist/setup-config it exists only once: extraLibs = ["clntsh"] Nevertheless I can start GHCi with the example program and I can see 'hello world'! Great - many thanks for the hints that led me to that state! Next step is to fetch real data from the database. But now I encounter new problems. If I write a real 'select' statement I get at best: Unexpected null in row 1, column 1. *** Exception: (unknown) and at worst: 50001752 *** glibc detected *** /usr/lib/ghc-6.8.2/ghc-6.8.2: free(): invalid pointer: 0x08d10065 *** ======= Backtrace: ========= /lib/libc.so.6[0xb7dba4b6] /lib/libc.so.6(cfree+0x89)[0xb7dbc179] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(lfvclose+0x1c)[0xb5df9c1e] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(SlfMunmap+0x3f)[0xb5e70c3b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(ldiutzd+0x37)[0xb5e49e7b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(kputerm+0x51)[0xb54c96b9] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(OCITerminate+0x1c)[0xb5596888] [0xb6a33801] /usr/lib/ghc-6.8.2/ghc-6.8.2[0x8a600de] ======= Memory map: ======== ... then GHCi quits. > BTW, the location of your header files is still a puzzle. Oracle's docs: > http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14250/ociabdem.htm… > > state that the header files shoulld be in $ORACLE_HOME/rdbms/public. > But perhaps things are different for the Instant Client. I don't know. I have just installed the RPMs and I expect that others will do so as well.

2 7

ANN: The Disciplined Disciple Compiler - alpha 1.1
by Ben Lippmeier 03 Jul '08

03 Jul '08

Hi All, I'm pleased to announce version 1.1 of the Disciplined Disciple Compiler (DDC) Disciple is an explicitly lazy dialect of Haskell which supports: - first class destructive update of arbitrary data. - computational effects without the need for state monads. - type directed field projections. - allied functional goodness. All this and more through the magic of effect typing. New in this version: - support for x86_64 under linux and darwin, thanks to Jared Putnam. - the -make flag now does a full dependency driven build/rebuild. - constructor classes. - irrefutable patterns. - partial support for monadic do notation. - an unboxed Bool# type with true# and false# literals. - field projection punning. - lots more example code. Project page with full release notes and download at: http://www.haskell.org/haskellwiki/DDC DDC: more than lambdas. Ben.

2 1

parallel_map_reduce, in easy one liner mod
by jinjing 03 Jul '08

03 Jul '08

Hi haskellers, So.. the type says it all p_map_reduce :: ([a] -> b) -> (b -> b -> b) -> [a] -> b so the idea is to write your computation extensive funciton as a map_reduce function, simplest example: sum ( can be seen as map id then reduce (+) ) then instead of calling: sum xs just call: p_map_reduce sum (+) xs the function in the background will split the xs into 16 parts, and use all your cores to process the computation. Enless the computation from the map_reduce function is too trivial or the list is too small, it should bump all your cores to 100% usage, and linearly increase overall performance. You can customize the number of parts by calling the helper function p_map_reduce_to I hope this can be useful to someone besides me :) here is the code warning: i'm using a very annoying coding style, by redefining the (.) operater to be reverse application. Please forgive me and metally transform the order, or just pretend you are reading Java / Python / Ruby or whatever :) module Main where import Data.List import Control.Parallel import Prelude hiding ((.)) -- for my poor oo mind (.) :: a -> (a -> b) -> b a . f = f a infixl 9 . (...) :: (b -> c) -> (a -> b) -> a -> c (...) f g x = f (g x) -- helpers reduce = foldl1 in_group_of n [] = [] in_group_of n xs = xs.take(n) : xs.drop(n).in_group_of(n) split_to n xs = xs.in_group_of(size) where size = if xs.length < n then n else xs.length `div` n -- parallel processing p_eval' xs = xs.pseq(xs.reduce(par)) p_reduce' op xs = xs.p_eval'.reduce(op) p_map_reduce_to n m r xs = xs.split_to(n).map(m).p_reduce'(r) p_map_reduce m r xs = p_map_reduce_to 16 m r xs -- test fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) fibs xs = xs.map(fib).sum test_list = replicate 50 30 s_fibs = test_list.fibs p_fibs = test_list.p_map_reduce fibs (+) main = p_fibs.show.putStrLn

1 0

database 101 question
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I have installed the mysql server (mysqld) on my Ubuntu machine. What are the Haskell libraries/components that I will need to write a mysql client?? Kind regards, Vasili

3 3

POSIX AIO (asych I/O) ...
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I am also testing my aio support. The aio_write binding seems to work ok .. as well as aio_error, Aio_return is a problem child. I think I wrote a really simple binding. I always receive nbytes as 0. I have been staring at the code hoping to catch a stupid mistake. I put putStrLn's in the code. .....Here is the code ... aioReturn :: AIOCB -> IO (AIOCB, ByteCount) aioReturn aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb count <- throwErrnoIfMinus1 "aioReturn" (c_aio_return p_aiocb) aiocb <- peek p_aiocb return (aiocb, fromIntegral count) foreign import ccall safe "aio.h aio_return" c_aio_return :: Ptr AIOCB -> IO CInt Maybe someone can spot something that I haven't. Thanks, Vasili

4 10

GHC ARM Hackathon (Re: Haskell on ARM )
by Jeremy Apthorp 03 Jul '08

03 Jul '08

2008/7/3 Greg Fitzgerald <garious(a)gmail.com>: >> Perhaps we should have some sort of GHC on the ARM hackathon >> when 6.10 comes out > > Would anybody be interested in a "GHC on ARM Hackathon" in San Diego this > year? I'd be interested, but I live in Sydney, Australia. San Diego's a bit of a stretch :) > Jeremy Apthorp said: >> it'll also require that I significantly strip down the runtime system, as >> the current RTS won't fit in 4M > > Do you have a plan for how to do this? Maybe we can invite a Simon to teach > us the innards of the RTS and help guide us? I've discussed it briefly with chak (who'll be supervising the project), but as I said, I haven't looked too deeply into the code yet. Jeremy -- - Jem

1 0

calling setWMHints (expects a CInt, I have an Int)
by Martin DeMello 03 Jul '08

03 Jul '08

Could someone give me an example of calling setWMHints from Graphics.X11.Xlib.Extras? The signature is setWMHints :: Display -> Window -> WMHints -> IO Status and WMHints is defined as data WMHints = WMHints { wmh_flags :: CLong wmh_input :: Bool wmh_initial_state :: CInt wmh_icon_pixmap :: Pixmap wmh_icon_window :: Window wmh_icon_x :: CInt wmh_icon_y :: CInt wmh_icon_mask :: Pixmap wmh_window_group :: XID } I can't figure out how to convert an Int to a CInt to construct the WMHints martin

2 2

Haskell Weekly News: Issue 75 - July 2, 2008
by Brent Yorgey 03 Jul '08

03 Jul '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080702 Issue 75 - July 02, 2008 --------------------------------------------------------------------------- Welcome to issue 75 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Anglo Haskell 2008. Matthew Sackman [2]announced [3]AngloHaskell 2008, a gathering of all people Haskell-related from beginners, to seasoned hackers to academic giants. All and more are welcomed by large fuzzy green lambdas. The proposed dates and location are Friday the 8th and Saturday the 9th of August, at Imperial College, London. CFP - Special Issue of Fundamenta Informaticae on Dependently Typed Programming. Wouter Swierstra [4]announced a call for papers for a special issue of [5]Fundamenta Informaticae on [6]Dependently Typed Programming. The deadline for submissions is October 1. Gtk2Hs 0.9.13. Peter Gavin [7]announced the release of Gtk2Hs 0.9.13, including bindings for Gnome VFS and GStreamer, a new Gtk+ tutorial adapted by Hans van Thiel, cairo image stride support, and more. Hasim. Jochem Berndsen [8]announced Hasim, a [9]small project to create a library to do discrete event simulation in Haskell, using monads to define a domain-specific language for "actions" of a process. Galois move. Don Stewart [10]announced that Galois has completed the move of its data center. Expect speedier response times for hackage.haskell.org and darcs.haskell.org. Google Summer of Code Progress updates from participants in the 2008 [11]Google Summer of Code. Hoogle 4. Neil Mitchell (ndm) is working on [12]Hoogle 4. [13]This week, Neil worked on better Haddock database generation, lazy name searching, and a snazzy --info flag for Hoogle. Next up: type search! DPH physics engine. Roman Cheplyaka (Feuerbach) is working on a [14]physics engine using [15]Data Parallel Haskell. [16]This week, he worked on implementing Mirtich's V-Clip algorithm for collision detection (and [17]got it to work), cabalized his project and added documentation. He also ran into an interesting QuickCheck puzzle. Generic tries. Jamie Brandon is working on a library for efficient maps using generalized tries. [18]This week, he created a generic framework for automatically running QuickCheck tests at a number of different types. This week he plans to synthesize the many suggestions from the [19]discussion on the libraries list into a stable API design. Language.C. Benedikt Huber (visq) is [20]working on Language.C, a standalone parser/pretty printer library for C99. [21]This week he worked on a better representation for declarators, and abstracted the notion of an InputStream over both String and ByteString, among other accomplishments. GHC plugins. Max Bolingbroke is working on dynamically loaded plugins for GHC. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a [22]make-like dependency analysis framework for Cabal. GHC API. Thomas Schilling (nominolo) is working on [23]improvements to the GHC API. Officials at HWN headquarters have released a statement reversing their previous position regarding the existence of Thomas, citing regrettably faulty information to explain their previous misapprehensions. Expect to hear more from Thomas soon, now that he has finished graduating and moving. Libraries Proposals and extensions to the [24]standard libraries. GetOpt formatting improvements. Duncan Coutts [25]proposed some modifications to make the output of the System.Console.GetOpt library more readable, resulting in quite a bit of discussion. HughesPJ improvements. Benedikt Huber [26]proposed a patch with some bug fixes, performance improvements, and QuickCheck test suite for the Text.PrettyPrint.HughesPJ pretty-printing library. Discussion A Monad for on-demand file generation?. Joachim Breitner [27]asked about a monad for transparently tracking files which may need to be regenerated due to dependencies, leading to an interesting discussion of incremental computation, strict vs. lazy I/O, and other issues. New mailing list proposal: Haskell-Edu. Benjamin L. Russell sent out a message [28]proposing a new mailing list hosted at haskell.org, "Haskell-Edu: The Haskell Educational Mailing List." The new mailing list would be guided by the principle that Haskell is useful not just in research, but also in teaching programming as part of a liberal arts education. Comments and discussion welcomed. Learning GADT types to simulate dependent types. Paul Johnson is trying to use GADTs to simulate aspects of a dependently typed system, and [29]asks for help improving his Oleg rating. Call graph tool?. C.M.Brown [30]asked whether there is a tool for visualizing the call graph for a collection of source files, leading to a discussion of various tools. Jobs Formal methods and automated reasoning at Rockwell Collins. Janis Voigtlaender [31]passed on an opening for a Senior Systems Engineer at [32]Rockwell Collins. The opening is for a computer scientist or engineer to develop and apply automated analysis to computer systems and to pursue research in formal methods and automated reasoning. Contact: rmgatto at rockwellcollins.com. Blog noise [33]Haskell news from the [34]blogosphere. * Roman Cheplyaka: [35]V-Clip seems to work!. * Benedikt Huber: [36]Last week on Language.C (1). An update on Benedikt's Google Summer of Code project. * Jamie Brandon: [37]Week 3 progress. An update on Jamie's Google Summer of Code project. * Philip Wadler: [38]Welcome to Scotland, Neil, Patricia, and Conor!. * >>> codders: [39]Coding style, Haskell. codders likes how Real World Haskell gives some hints about Haskell coding style and culture in addition to teaching the language itself. * >>> zoo: [40]Haskell plug-in for Eclipse. zoo explains how to install the [41]Haskell Eclipse plugin. * Dan Piponi (sigfpe): [42]A blessed man's formula for holey containers. Dan descries an enlightening derivation of the [43]combinatorial form of Faa di Bruno's formula from the perspective of derivatives of types. * Roman Cheplyaka: [44]Status report: week 5. An update on Roman's Google Summer of Code project. * >>> codders: [45]More Haskell fun. * >>> Marco Tulio Gontijo e Silva: [46]Rank 2 Types. Marco describes a practical use for GHC's rank-2 types. * Edward Kmett: [47]Memoizing Context. * >>> JP Moresmau: [48]Deserializing JSON to Haskell Data objects. * >>> codders: [49]Getting started with Haskell... still. codders is learning Haskell by reading the beta version of [50]Real World Haskell. * Neil Mitchell: [51]GSoC Hoogle: Week 5. * Arnar: [52]Parsing JSON with Haskell. A nice example of using [53]Parsec to parse JSON. * Thomas Hartman: [54]HAppS Tutorial. Quotes of the Week * quicksilver: [on what OS sjanssen uses] sjanssen runs haskell programs in his head; much more efficient. * EvilTerran: "We were somewhere around Barstow, on the edge of the desert, when the phantom types began to take hold." * audreyt: o/~ the phantom of the typesystem is here / inside my mind! o/~ * dmwit: No, no, no, ($) isn't right-assoc, it's wrong-assoc. * solrize: this would never happen in haskell: i sent in a search query to a certain python program, but left the query field empty, expecting to get back an error message. instead it found a bunch of books written by the diet doctor Gary Null. * heatsink: We're all inside do-blocks in the IO monad if you think about it. * djsiegel: [upon having a question answered by dons] oh my, I'm talking to the man * mar77a: the first computers were big because they were actually cupboards with fast humans inside About the Haskell Weekly News New editions are posted to [55]the Haskell mailing list as well as to [56]the Haskell Sequence and [57]Planet Haskell. [58]RSS is also available, and headlines appear on [59]haskell.org. Headlines are available as [60]PDF. To help create new editions of this newsletter, please see the information on [61]how to contribute. Send stories to byorgey at seas dot upenn dot edu. The darcs repository is available at darcs get [62]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/16272 3. http://www.haskell.org/haskellwiki/AngloHaskell/2008 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16269 5. http://fi.mimuw.edu.pl/ 6. http://sneezy.cs.nott.ac.uk/darcs/DTP08/journal.html 7. http://www.haskell.org//pipermail/haskell-cafe/2008-June/044805.html 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41881 9. http://huygens.functor.nl/hasim 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41847 11. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 12. http://code.haskell.org/hoogle/ 13. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-5.html 14. http://haskell.org/haskellwiki/Hpysics 15. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 16. http://physics-dph.blogspot.com/2008/06/status-report-week-5.html 17. http://physics-dph.blogspot.com/2008/07/v-clip-seems-to-work.html 18. http://jamiiecb.blogspot.com/2008/06/week-3-progress.html 19. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9259 20. http://www.sivity.net/projects/language.c/wiki/ 21. http://hsbene.blogspot.com/2008/07/last-week-on-languagec-1.html 22. http://code.haskell.org/~Saizan/cabal 23. http://hackage.haskell.org/trac/ghc/wiki/GhcApiStatus 24. http://haskell.org/haskellwiki/Library_submissions 25. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9415 26. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9377 27. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41868 28. http://thread.gmane.org/gmane.comp.lang.haskell.general/16271 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41848 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/41786 31. http://article.gmane.org/gmane.comp.lang.haskell.general/16266 32. http://www.rockwellcollins.com/ 33. http://planet.haskell.org/ 34. http://haskell.org/haskellwiki/Blog_articles 35. http://physics-dph.blogspot.com/2008/07/v-clip-seems-to-work.html 36. http://hsbene.blogspot.com/2008/07/last-week-on-languagec-1.html 37. http://jamiiecb.blogspot.com/2008/06/week-3-progress.html 38. http://wadler.blogspot.com/2008/06/welcome-to-scotland-neil-patricia-and.ht… 39. http://talkingcode.co.uk/2008/06/30/coding-style-haskell/ 40. http://imonad.com/blog/2008/06/haskell-plug-in-for-eclipse/ 41. http://sourceforge.net/projects/eclipsefp/ 42. http://sigfpe.blogspot.com/2008/06/blessed-mans-formula-for-holey.html 43. http://en.wikipedia.org/wiki/Fa%C3%A0_di_Bruno's_formula#Combinatorial_form 44. http://physics-dph.blogspot.com/2008/06/status-report-week-5.html 45. http://talkingcode.co.uk/2008/06/28/more-haskell-fun/ 46. http://marcotmarcot.blogspot.com/2008/06/rank-2-types.html 47. http://comonad.com/reader/2008/memoizing-context/ 48. http://jpmoresmau.blogspot.com/2008/06/deserializing-json-to-haskell-data.h… 49. http://talkingcode.co.uk/2008/06/27/getting-started-with-haskell-still/ 50. http://www.realworldhaskell.org/ 51. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-5.html 52. http://www.hvergi.net/2008/06/parsing-json-with-haskell/ 53. http://legacy.cs.uu.nl/daan/parsec.html 54. http://ramblings.tumblr.com/post/39794244/happs-tutorial 55. http://www.haskell.org/mailman/listinfo/haskell 56. http://sequence.complete.org/ 57. http://planet.haskell.org/ 58. http://sequence.complete.org/node/feed 59. http://haskell.org/ 60. http://code.haskell.org/~byorgey/code/hwn/archives/20080702.pdf 61. http://haskell.org/haskellwiki/HWN 62. http://code.haskell.org/~byorgey/code/hwn/

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GHC ARM Hackathon (Re: Haskell on ARM )
by Greg Fitzgerald 03 Jul '08

03 Jul '08

It may not be long before most of the computing world has gone mobile. CNET suggests the major players will be Qualcomm and Intel, where QC is more power-efficient, but Intel conveniently targets x86. June 29, 2008 7:30 PM PDT Qualcomm vs Intel: You decide<%20http://news.cnet.com/8301-13924_3-9979989-64.html> I'm a software engineer at Qualcomm with a desire to code more Haskell at work. This desire will only get stronger in coming years as multithreaded apps on mobile devices get more popular, and ways to write them reliably in traditional languages continue not to exist. Jeremy Shaw said: > Perhaps we should have some sort of GHC on the ARM hackathon > when 6.10 comes out Would anybody be interested in a "GHC on ARM Hackathon" in San Diego this year? When is 6.10 estimated to come out? Do we really need to wait for it? Jeremy Apthorp said: > it'll also require that I significantly strip down the runtime system, as > the current RTS won't fit in 4M Do you have a plan for how to do this? Maybe we can invite a Simon to teach us the innards of the RTS and help guide us? Thanks, Greg

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Unboxed array of product type -> product type of unboxed arrays
by Scott Dillard 03 Jul '08

03 Jul '08

Hi, I'm trying to extended the standard unboxed array types and array classes to my own product types, for now let's just say (,). So if the proper MArray and IArray instances exist for e and f, then I can make an instance for (e,f). The actual type of that array, something like (UArray i e, UArray i f), would be given by an associated type. This is how the uvector library does it, but that library defines its own array primitives and classes. I'd like to reuse the standard ones if possible. The problem I keep running into is the kind of the array, * -> * -> *, or 'a i e'. The crucial type there is e, which is used to dispatch the instance to the proper associated type, so if e = (a,b) then the array type would be (UArray i a, UArray I b), and if e is (a,b,c) then (UArray i a, UArray i b, UArray i c). If IArray was instead expecting the array type to be 'a e i' I could maybe do something like this: class UArrClass e where data UArr e :: * -> * instance (IArray UArray e, IArray UArray f) => UArrClass (e,f) where data UArr (e,f) i = UArrPair (UArray i e) (UArray i f) But as it stands, I can't do that. The 'i' type parameter has to be bound as a parameter of UArrClass. So instead I tried this. class UArrClass i e where data UArr i e unsafeAt_ :: UArr i e -> Int -> e --mirror all IArray methods instance ( IArray UArray e , IArray UArray f , Ix i --needed for unsafeAt ) => UArrClass i (e,f) where newtype UArr i (e,f) = UArrPair (UArray i e) (UArray i f) unsafeAt_ (UArrPair ea fa) i = (unsafeAt ea i , unsafeAt fa i) and then the instance for IArray could be defined as follows, just a mapping from the methods of that class onto my own: instance ( IArray UArray e , IArray UArray f , UArrClass i (e,f) ) => IArray UArr (e,f) where unsafeAt = unsafeAt_ The problem I get now is from the 'Ix i' context of the IArray methods. The 'i' there is only mentioned in the context of the methods, not the class, so I have no 'handle' onto that 'i' that I can use to explicitly unify it with the 'i' mentioned in UArrClass. The compiler keeps complaining about rigid type variables. It would be great if I could leave that type variable unbound in my class, and only bind it in the methods, as IArray does, but as far as I can tell, I can't. I need to bind 'i' in my class because it's the first type-argument to the array type constructor, rather than the second. I don't care about the 'i', its the 'e' I'm after, but all applications of the associated type constructor need to be saturated. Can anyone see a way to do this? I understand there's about a million other ways to accomplish what I'm trying to do without IArray and MArray, but I'm just wondering if I should abandon those classes altogether, and use my own array classes, using something like uvector or unsafeIO/ForeignPtr. That seems to be trend. Thanks, Scott

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Re: [Haskell-cafe] gcc as a portable assembler: tail calls vs trampoline
by Loup Vaillant 02 Jul '08

02 Jul '08

2008/7/1 Robin Green <greenrd(a)greenrd.org>: > I think you should use the -march flag to gcc. Otherwise you are tying > the compiler to an ancient version of the x86 instruction set (386). OK, let's try... Done. I used -march=nocona. I couldn't do core2 (not available on my gcc 4.1.2 yet). Did not notice any difference... > On Tue, 1 Jul 2008 16:57:38 +0200 > "Loup Vaillant" <loup.vaillant(a)gmail.com> wrote: > >> Hello, everyone, >> I'm not sure this is the good list for this posting: I plan to write >> yet another lazy graph reduction machine, using gcc, mainly for fun. >> My concern right now is about one particular aspect of gcc: the tail >> calls vs trampoline deathmatch. >> >> First some clarifications: >> -> By tail call, I mean a properly optimized one, which doesn't grow >> the stack. Basically a jump. >> -> By trampoline, I mean the infinite loop referred to as a "tiny >> interpreter" in the STG paper [1]. Instead of (tail) calling a >> function, we return its pointer, which will then be used for the >> actual call. >> >> I've read that trampoline calls are about 3 times as expensive as tail >> calls. Instead of a (direct) jump, you have a return, followed by an >> indirect call. However, testing this with gcc, I didn't manage to show >> any measurable difference. What am I missing? I used -O2 on both >> benchmarks, and turned off inlining (using the __noinline__ >> attribute). I'm running the program on a core2Duo in 32 bits mode, >> using GNU/Linux Ubuntu. Should I also make separate modules to prevent >> whole program analysis from gcc? >> >> Anyway, I send you the code below, so you can test and review it. >> Enjoy! >> >> [1]: >> http://research.microsoft.com/Users/simonpj/Papers/spineless-tagless-gmachi… >> >> >> >> /* This program tests the performance difference between trampoline >> * calls and tail calls. (In the STG and related papers, trampoline >> * is referred to as "tiny interpreter") >> * >> * depending on the mode (tail call or trampoline), some code changes: >> * the return type of the code blocks, and the calls to other blocks >> * (direct tail calls or return to trampoline). Hence the macros ENTER >> * and RETURN. >> * >> * To compile it,type one of the two following commands: >> * gcc -O2 sibcall.c -D SIBCALL >> * gcc -O2 sibcall.c >> * >> * The first one will make the program use sibling calls (enabled at >> * O2), and the second one will fall back to trampoline calls >> */ >> >> #ifdef SIBCALL // tail call mode >> >> #define ENTER(f) (f()) >> #define RETURN void >> #define MAIN(start) start() >> >> #else // trampoline call mode >> >> #define ENTER(f) return f >> #define RETURN void* >> #define MAIN(start) do{ void * (*f)(void) = start; \ >> while (1) \ >> f = (*f)(); \ >> } while (0) >> #endif >> >> // Now, we can begin the generic code >> #include <stdlib.h> >> #define LIMIT 1000000000 // one billion >> >> int counter = 0; >> >> RETURN closure0(void); >> RETURN closure1(void); >> >> >> int main(int argc, char ** argv) >> { >> MAIN(closure0); >> return 0; >> } >> >> __attribute__ ((__noinline__)) >> RETURN closure0(void) >> { >> if (counter >= LIMIT) exit(0); >> counter++; >> ENTER(closure1); >> } >> __attribute__ ((__noinline__)) >> RETURN closure1(void) >> { >> if (counter >= LIMIT) exit(0); >> counter++; >> ENTER(closure0); >> } >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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gcc as a portable assembler: tail calls vs trampoline
by Loup Vaillant 01 Jul '08

01 Jul '08

Hello, everyone, I'm not sure this is the good list for this posting: I plan to write yet another lazy graph reduction machine, using gcc, mainly for fun. My concern right now is about one particular aspect of gcc: the tail calls vs trampoline deathmatch. First some clarifications: -> By tail call, I mean a properly optimized one, which doesn't grow the stack. Basically a jump. -> By trampoline, I mean the infinite loop referred to as a "tiny interpreter" in the STG paper [1]. Instead of (tail) calling a function, we return its pointer, which will then be used for the actual call. I've read that trampoline calls are about 3 times as expensive as tail calls. Instead of a (direct) jump, you have a return, followed by an indirect call. However, testing this with gcc, I didn't manage to show any measurable difference. What am I missing? I used -O2 on both benchmarks, and turned off inlining (using the __noinline__ attribute). I'm running the program on a core2Duo in 32 bits mode, using GNU/Linux Ubuntu. Should I also make separate modules to prevent whole program analysis from gcc? Anyway, I send you the code below, so you can test and review it. Enjoy! [1]: http://research.microsoft.com/Users/simonpj/Papers/spineless-tagless-gmachi… /* This program tests the performance difference between trampoline * calls and tail calls. (In the STG and related papers, trampoline * is referred to as "tiny interpreter") * * depending on the mode (tail call or trampoline), some code changes: * the return type of the code blocks, and the calls to other blocks * (direct tail calls or return to trampoline). Hence the macros ENTER * and RETURN. * * To compile it,type one of the two following commands: * gcc -O2 sibcall.c -D SIBCALL * gcc -O2 sibcall.c * * The first one will make the program use sibling calls (enabled at * O2), and the second one will fall back to trampoline calls */ #ifdef SIBCALL // tail call mode #define ENTER(f) (f()) #define RETURN void #define MAIN(start) start() #else // trampoline call mode #define ENTER(f) return f #define RETURN void* #define MAIN(start) do{ void * (*f)(void) = start; \ while (1) \ f = (*f)(); \ } while (0) #endif // Now, we can begin the generic code #include <stdlib.h> #define LIMIT 1000000000 // one billion int counter = 0; RETURN closure0(void); RETURN closure1(void); int main(int argc, char ** argv) { MAIN(closure0); return 0; } __attribute__ ((__noinline__)) RETURN closure0(void) { if (counter >= LIMIT) exit(0); counter++; ENTER(closure1); } __attribute__ ((__noinline__)) RETURN closure1(void) { if (counter >= LIMIT) exit(0); counter++; ENTER(closure0); }

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lhs syntax highlighting in vim
by Eric Stansifer 01 Jul '08

01 Jul '08

Hello, The syntax highlighting file for literate haskell in vim says that its maintainer is haskell-cafe(a)haskell.org, so hopefully one of you will find this relevant. In literate haskell files, vim optionally highlights the non-code text according to TeX markup. The syntax highlighting file looks for key phrases (e.g., "\documentclass") to decide whether to use TeX markup highlighting; but it (erroneously, in my opinion, and at variance with the documentation) will use TeX markup highlighting on any lhs file that contains the '%' character anywhere in it. The bug is in line 74 of version 1.01 of the lhaskell.vim syntax highlighting file. Thanks, Eric

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