- Haskell-Cafe - Haskell.org

workarounds for Codec.Compression.Zlib errors in darcs
by Eric Kow 01 Dec '08

01 Dec '08

Hi everybody, This advisory is for people who have installed darcs 2.1.2 via the Cabal build method. As you may have noticed, the cabalised darcs sometimes fails with errors like Codec.Compression.Zlib: incorrect data check Why this happens ---------------- Older versions of darcs can to produce gzipped files with broken CRCs. We never noticed this because our homegrown wrapper around the C libz library does not pick up these errors. Lately, we have been working on adopting the Haskell zlib wrapper, which is made available by default in darcs.cabal. This new wrapper is more stringent and fails when it encounters these files. Workaround 1 : use C libz instead of Haskell zlib ------------------------------------------------- So how can you work around these errors? If you are building darcs on any Unix-y operating system (e.g. Linux or MacOS X), you can cabal configure darcs to use the old C libz binding: cabal configure -f external-zlib This will restore our homegrown wrapper which ignores the broken CRCs (note that the darcs head no longer *produces* these broken files, thanks to debugging by Matthias Andree and to a bugfix by David Roundy; http://bugs.darcs.net/issue844 for details). In principle, the same advice applies for Windows users, with more details hopefully to follow on how the C libz in a GHC-accesible location. Details to follow. In the meantime, you can either build darcs using the old configure and make technique (assuming you have MSYS and related tools), or use a binary that does not use the Haskell zlib wrapper (for example, by downgrading to http://www.haskell.org/~simonmar/darcs-2.0.2+75.zip ) Workaround 2 : fix your broken gzipped files -------------------------------------------- If you have control over the repositories with broken gzipped files, it should be possible to repair these files by gunzipping them and then redo-ing the gzip. We think that the attached script should help. Please report back if this is not the case. How we will fix this problem in the long term --------------------------------------------- I'm very sorry for the grief this has caused. To begin with, we will ensure that the 2.2 release gets more testing before releasing it in January. It will also handle these broken CRCs more gracefully. Our plan is to - either extend darcs repair or provide a Haskell script to fix these broken files - detect the broken files and advise users to run darcs repair (or the script) as needed - somewhere in the future, disallow broken CRC files whilst still advising users on how to fix their files. Many thanks! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Problem with directory-1.0.0.0
by Bartosz Wójcik 01 Dec '08

01 Dec '08

Hi Folks, I'm facing problem after I've reinstalled directory-1.0.0.0 (setup configure/build/install). Since then I cannot complie anything that needs this library. It fails with following messages: Preprocessing library haddock-2.4.0... Preprocessing executables for haddock-2.4.0... Building haddock-2.4.0... /usr/bin/ar: creating dist/build/libHShaddock-2.4.0.a Linking dist/build/haddock/haddock ... /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `scXR_info': (.text+0x17b7c): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_lvl29_closure' /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `s8xU_info': (.text+0x1792c): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_a43_info' /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `r7aC_closure': (.data+0xd18): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_a43_closure' collect2: ld returned 1 exit status Situation is following: Old directory-1.0.0.0 resides in /usr/local/lib/ghc-6.8.2/lib/directory-1.0.0.0. New one in /usr/local/lib/directory-1.0.0.0. Why new one doesn't work? How to force linker to use old one? Best, Bartek

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getLine and ^C on Windows
by Lyle Kopnicky 01 Dec '08

01 Dec '08

Hi folks, I'm using System.IO.getLine to read input in my program. I've compiled it on Windows Vista with ghc-6.10.1. I've noticed that if I press Ctrl+C while the program is waiting for input, it will get stuck. No keypresses can get me out of the program - I have to kill the shell. I've tried cmd.exe, Powershell and cygwin - same behavior. I'm sure editline would work better, but it's overkill. I just want to read lines of text and don't need fancy edit behavior. And I think editline complicates the build process. I also want my program to work right in non-interactive mode, reading redirected input. Am I missing something? Thanks, Lyle

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catting to cat gets stuck at > 135K
by Jason Dusek 01 Dec '08

01 Dec '08

I've put together a simple test case for a rather annoying problem. I've got a program that drives other programs. For example, it can drive `cat`: :; Simple cat a-file When the file is a little bit greater than 135060 bytes, this program fails to produce any output at all -- I need to use ^C to get my console back. I have a similar program that encounters the same issue a little after 139192 bytes. The inconsistency is one baffling feature of this bug. If I remove the `hClose`, the example program just hangs, no matter the size of the input. It is entirely possible that I have overlooked some subtle feature of pipe semantics -- indeed, I hope that is the case. -- _jsn import Data.ByteString.Lazy import System.Process import System.Environment import System.IO (hClose) import Prelude hiding (writeFile, readFile) main = do exe:file:_ <- getArgs bytes <- readFile file foo <- simple exe bytes [] writeFile (file ++ ".foo") foo -- Manufactures a simple stream handler from a command line utility. simple :: String -> ByteString -> [String] -> IO ByteString simple exe bytes args = do (i, o, e, p) <- runInteractiveProcess exe args Nothing Nothing hPut i bytes s <- hGetContents o hClose i return s

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Combining licences
by Emil Axelsson 01 Dec '08

01 Dec '08

Hello, I know very little about licensing, so I'm hoping to get some advice from you guys. In the next release of Wired, I plan to include an open-source standard cell library (or rather data derived from it). This cell library has a custom license (found at the bottom of http://www.nangate.com/openlibrary) It appears to be very permissive, but it states that the license text "must be included in all copies of the Library, in whole or in part, and all derivative works of the Library". Wired itself is BSD3-licensed. How do I arrange this in the package? Can I still have a BSD3 license for Wired? Where do I put the respective license texts? Any other things I should have in mind? Or perhaps it's better to put the cell library in its own package? I'm a bit reluctant to do this, because it means that Wired will be essentially useless on its own. Thanks, / Emil

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Re: [Haskell-cafe] Cabal
by Henning Thielemann 01 Dec '08

01 Dec '08

On Sun, 30 Nov 2008, Don Stewart wrote: > lemming: >> >> Maybe you like to add a pointer in cabal-install.cabal/Homepage field to >> this page. > > Good idea. Duncan? After I finished that article, I also found: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall

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The Knight's Tour: solutions please
by oleg＠okmij.org 01 Dec '08

01 Dec '08

It seems the following pure functional (except for the final printout) version of the search has almost the same performance as the Dan Doel's latest version with the unboxed arrays and callCC. For the board of size 40, Dan Doel's version takes 0.047s on my computer; the version below takes 0.048s. For smaller boards, the difference is imperceptible. Interestingly, the file sizes of the compiled executables (ghc -O2, ghc 6.8.2) are similar too: 606093 bytes for Dan Doel's version, and 605938 bytes for the version below. The version below is essentially Dan Doel's earlier version. Since the problem involves only pure search (rather than committed choice), I took the liberty of substituting FBackTrack (efficient MonadPlus) for LogicT. FBackTrack can too be made the instance of LogicT; there has not been any demand for that though. import Data.List import Data.Ord import qualified Data.IntMap as Map import System.Environment import FBackTrack import Control.Monad -- Emulate the 2-dimensional map as a nested 1-dimensional map initmap n = Map.fromList $ (1,Map.singleton 1 1):[ (k,Map.empty) | k <- [2..n] ] notMember (i,j) m = Map.notMember j $ Map.findWithDefault undefined i m insrt (i,j) v m = Map.update (Just . Map.insert j v) i m lkup (i,j) m = Map.findWithDefault undefined j $ Map.findWithDefault undefined i m successors n b = sortWith (length . succs) . succs where sortWith f = map fst . sortBy (comparing snd) . map (\x -> (x, f x)) succs (i,j) = [ (i', j') | (dx,dy) <- [(1,2),(2,1)] , i' <- [i+dx,i-dx] , j' <- [j+dy, j-dy] , i' >= 1, j' >= 1, i' <= n, j' <= n , notMember (i',j') b ] tour n k s b | k > n*n = return b | otherwise = do next <- foldl1 mplus.map return $ successors n b s tour n (k+1) next $ insrt next k b showBoard n b = unlines . map (\i -> unwords . map (\j -> pad $ lkup (i,j) b) $ [1..n]) $ [1..n] where k = length . show $ n*n + 1 pad i = let s = show i in replicate (k-length s) ' ' ++ s main = do (n:_) <- map read `fmap` getArgs let (b:_) = runM Nothing . tour n 2 (1,1) $ initmap n putStrLn $ showBoard n b

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Haskell Weekly News: Issue 95 - November 30, 2008
by Brent Yorgey 01 Dec '08

01 Dec '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20081130 Issue 95 - November 30, 2008 --------------------------------------------------------------------------- Welcome to issue 95 of HWN, a newsletter covering developments in the [1]Haskell community. Real World Haskell is [2]finally here! [3]Read it online, and/or [4]get your own dead tree copy. Better yet, get two copies, one for yourself and one for a friend. The fifteenth [5]Haskell Communities and Activities Report is also here---check out all the exciting stuff being worked on in the Haskell world! Announcements HCAR. Janis Voigtlaender [6]announced the 15th edition of the [7]Haskell Communities and Activities Report (HCAR) is now available! Not quite another Haskell tutorial, but .... Janis Voigtlaender [8]announced that he submitted his [9]Habilitation thesis last week. The first few chapters of it try to give an introduction to Haskell with emphasis on types and reasoning principles. hledger 0.2. Simon Michael [10]announced version 0.2 of [http://joyful.com/hledger hledger, a minimal haskell clone of John Wiegley's [11]"ledger" text-based accounting tool. darcs zlib error workaround. Eric Kow [12]outlined workarounds and future plans for a darcs bug relating to broken CRCs in gzipped patch files. You should read this if you have installed darcs 2.1.2 via the Cabal build method. Turbinado 0.2. Alson Kemp [13]announced version 0.2 of [14]Turbinado, an easy-to-use, fast web application framework. Fun with type functions. Simon Peyton-Jones [15]requests examples of compelling use cases for type functions: "can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise." Jobs PhD Positions in Language-based Security at Chalmers. Andrei Sabelfeld [16]announced the availability of [17]PhD student positions in programming language-based security in the Department of Computer Science and Engineering at Chalmers University of Technology, Sweden. The application deadline is January 30, 2009. FP Jobs. Julien Sylvestre [18]announced several new permanent positions, based in Paris, with MLstate -- an IT company whose functional programming approach to SaaS and cloud computing has been recently recognized by the French Ministry of Research Innovation Award. Blog noise [19]Haskell news from the [20]blogosphere. * Dan Piponi (sigfpe): [21]An Approach to Algorithm Parallelisation. Dan illustrates a clever, general framework for parallelizing some not-so-obviously-parallelizable algorithms, based on linear operations over rings. * Real-World Haskell: [22]First European Orders Now Landing. * Braden Shepherdson: [23]Pimp Your XMonad #3: Prompt. Braden continues his series on getting the most out of your xmonad configuration with a post describing the Prompt family of extensions. * Real-World Haskell: [24]Amazon orders now arriving. * Real-World Haskell: [25]Orders Now Arriving on the US West Coast. * Luke Palmer: [26]Relative time FRP. A new, more elegant semantics for FRP -- relative time instead of absolute time? * JP Moresmau: [27]Predictable random for testing. * Osfameron: [28]London Perl Workshop tomorrow!. * >>> Phil Ratzsch: [29]Initial Haskell Impressions. * Chung-chieh Shan: [30]Metalinguistics. * Alex McLean: [31]Babble. A simple vocable synthesiser that runs in a web browser, written in HaXe. * Jonathan Tang: [32]Sum types vs. typeclasses. * Magnus Therning: [33]Re: Redesigning GHC's build system. Magnus's response to a recent post about redoing GHC's build system. * Real-World Haskell: [34]Real World Haskell is shipping!. * Twan van Laarhoven: [35]Knight in n, part 1: moves. Twan explores knight-move problems in Haskell. * Conal Elliott: [36]Semantic editor combinators. An elegant explanation of semantic editor combinators -- a principled framework for building up ways to "edit" components of structured values. * David Sankel: [37]Why is the Reactive Behavior tutorial taking so long? splitB. * Chung-chieh Shan: [38]Bowling balls. A nice solution to the bowling-ball problem in Haskell. * >>> Cory: [39]Developing in "impractical" languages. * Chris Double: [40]Dynamic Compilation and Loading of Modules in Haskell. A mini-tutorial on using the GHC API. * Ashish Hanwadikar: [41]More on Haskell DSL. A Haskell DSL for replacing 'make'. * Luke Palmer: [42]Screw laziness (w.r.t. Fran semantics). Quotes of the Week * quicksilver: I ACCIDENTALLY THE WHOLE VERB * dons: [on ghc's new code generation] <byorgey> so how's the new code gen better? <dons> it's got 98% less dumbs. * adu: source code is transient, dreams are forever. * monochrom: n is the nth English letter. * nomeata: Ah, it seems I'm creating a tuple with more than 62 elements somewhere... * dons: we had 15 years building ivory towers - time to throw rocks from the top! About the Haskell Weekly News New editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. To help create new editions of this newsletter, please see the information on [48]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [49]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.realworldhaskell.org/blog/2008/11/25/real-world-haskell-is-shipp… 3. http://book.realworldhaskell.org/read/ 4. http://book.realworldhaskell.org/#buy 5. http://www.haskell.org/communities/ 6. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051112.html 7. http://www.haskell.org/communities/ 8. http://article.gmane.org/gmane.comp.lang.haskell.general/16640 9. http://wwwtcs.inf.tu-dresden.de/~voigt/habil.pdf 10. http://www.haskell.org//pipermail/haskell-cafe/2008-November/050920.html] 11. http://newartisans.com/software/ledger.html 12. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051021.html 13. http://www.haskell.org//pipermail/haskell/2008-November/020836.html 14. http://www.turbinado.org/ 15. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051069.html 16. http://article.gmane.org/gmane.comp.lang.haskell.general/16637 17. http://www.chalmers.se/cse/EN/news/vacancies/positions/ph-d-positions-in 18. http://article.gmane.org/gmane.comp.lang.haskell.general/16636 19. http://planet.haskell.org/ 20. http://haskell.org/haskellwiki/Blog_articles 21. http://sigfpe.blogspot.com/2008/11/approach-to-algorithm-parallelisation.ht… 22. http://www.realworldhaskell.org/blog/2008/11/29/first-european-orders-now-l… 23. http://braincrater.wordpress.com/2008/11/29/pimp-your-xmonad-3-prompt/ 24. http://www.realworldhaskell.org/blog/2008/11/26/amazon-orders-now-arriving/ 25. http://www.realworldhaskell.org/blog/2008/11/28/orders-now-arriving-on-the-… 26. http://lukepalmer.wordpress.com/2008/11/28/relative-time-frp/ 27. http://jpmoresmau.blogspot.com/2008/11/predictable-random-for-testing.html 28. http://greenokapi.net/blog/2008/11/28/london-perl-workshop-tomorrow/ 29. http://www.geeklair.net/~pratzsch/blog/2008/11/initial-haskell-impressions.… 30. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Metalinguistics/ 31. http://yaxu.org/babble/ 32. http://eve-language.blogspot.com/2008/11/sum-types-vs-typeclasses.html 33. http://therning.org/magnus/archives/417 34. http://www.realworldhaskell.org/blog/2008/11/25/real-world-haskell-is-shipp… 35. http://twan.home.fmf.nl/blog/haskell/Knight1.details 36. http://conal.net/blog/posts/semantic-editor-combinators/ 37. http://netsuperbrain.com/blog/posts/why-is-the-reactive-behavior-tutorial-t… 38. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Bowling_balls/ 39. http://onag.blogspot.com/2008/11/developing-in-impractical-languages.html 40. http://www.bluishcoder.co.nz/2008/11/dynamic-compilation-and-loading-of.html 41. http://feedproxy.google.com/~r/typepad/ahanwadi/ashishs_niti/~3/n41w6S_82SE… 42. http://lukepalmer.wordpress.com/2008/11/23/screw-laziness/ 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://haskell.org/haskellwiki/HWN 49. http://code.haskell.org/~byorgey/code/hwn/

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Instances that shouldn't overlap
by Paul Johnson 30 Nov '08

30 Nov '08

Hi, I'm trying to set up some operators for applicative versions of prelude types. For instance: -- | Applicative Equality. class (Eq a) => AppEq f a where (.==.), (./=.) :: f a -> f a -> f Bool instance (Applicative f, Eq a) => AppEq f a where (.==.) = liftA2 (==) (./=.) = liftA2 (/=) Hopefully the intention is fairly straightforward: if "f" is an instance of Applicative then the lifted implementation of the underlying type. Otherwise I can just give my own instance, which is useful for things that "wrap" prelude types but where "fmap" doesn't work. For instance: data (Ord a) => Interval a = Interval a a instance (Ord a) => AppEq Interval a where i1@(Interval _ u1) .==. i2@(Interval _ u2) | isSingleton i1 && isSingleton i2 && u1 == u2 = Interval True True | has i1 u2 || has i2 u1 = Interval False True | otherwise = Interval False False i1 ./=. i2 = let Interval b1 b2 = (i1 .==. i2) in Interval (not b2) (not b1) isSingleton :: (Ord a) => Interval a -> Bool isSingleton (Interval lower upper) = lower == upper has :: (Ord a) => Interval a -> a -> Bool has (Interval lower upper) v = v >= lower && v <= upper You can't (easily) define fmap for Interval because the function given as an argument might not be monotonic. So instead you have to write custom implementations for each lifted function, as shown here for (.==.) and (./=.) . The same principle works for AppOrd, AppNum etc, but I'm trying to solve the problem for just AppEq for now. This compiles, but when I try to use it I get this in ghci: *Interval> let i1 = Interval 4 5 *Interval> let i2 = Interval 4 6 *Interval> i1 .==. i2 <interactive>:1:0: Overlapping instances for AppEq Interval Integer arising from a use of `.==.' at <interactive>:1:0-9 Matching instances: instance (Ord a) => AppEq Interval a -- Defined at Interval.hs:(22,0)-(27,78) instance (Control.Applicative.Applicative f, Eq a) => AppEq f a -- Defined at AppPrelude.hs:(32,0)-(34,23) In the expression: i1 .==. i2 In the definition of `it': it = i1 .==. i2 I'm puzzled, because Interval is not an instance of Applicative, so the second instance doesn't apply. Can anyone help me out? I'm using ghc 6.8.3, so its possible that this was a bug fixed in 6.10. Paul.

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Permutations
by Andrew Coppin 30 Nov '08

30 Nov '08

OK, so here's something just for fun: Given a list of items, find all possible *unique* permutations of that list. (E.g., the input list is explicitly _allowed_ to contain duplicates. The output list should not contain any duplicate permutations.) I've found one simple way to do this, but I'm sure there are multiple valid approaches. So let's see what people come up with. ;-)

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