August 2011 - Haskell-Cafe

Compilation error in Chapter 5 of "Real World Haskell"
by Paul Reiners 18 Aug '11

18 Aug '11

I'm trying to do the following from Chapter 5 of "Real World Haskell": Our choice of naming for the source file and function is deliberate. To create an executable, *ghc* expects a module named Main that contains a function named main. The main function is the one that will be called when we run the program once we've built it. 6 comments<comments:%20show%20/%20hide> *ghc -o simple Main.hs SimpleJSON.o* ---from http://book.realworldhaskell.org/read/writing-a-library-working-with-json-d… When I do that, I get this error: C:\ch05>ghc -o simple Main.hs SimpleJSON.o [2 of 2] Compiling Main ( Main.hs, Main.o ) Linking simple.exe ... SimpleJSON.o:fake:(.data+0x0): multiple definition of `SimpleJSON_getArray_closure' .\SimpleJSON.o:fake:(.data+0x0): first defined here SimpleJSON.o:fake:(.text+0x54): multiple definition of `SimpleJSON_getArray_info' .\SimpleJSON.o:fake:(.text+0x54): first defined here SimpleJSON.o:fake:(.data+0x4): multiple definition of `SimpleJSON_getObject_closure' .\SimpleJSON.o:fake:(.data+0x4): first defined here What's going wrong here?

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writing a good Setup.hs is *hard* (was: broken on install encoding 0.6.6)
by Rogan Creswick 18 Aug '11

18 Aug '11

On Wed, Aug 17, 2011 at 6:29 AM, Gang <yugang.bj(a)gmail.com> wrote: > > sorry to bother > Not in the least! I was trying to figure out what problem you were encountering, and I learned a lot. I never did recreate your problem, but in the process I ran into a whole host of other annoyances and strange situations relating to slightly broken cabal files and cabal build intricacies that confound our existing tools. I don't mean to pick on encoding or HaXml - I've seen the same kinds of issues in many, many Haskell packages (including most of the packages I've written myself) these are just the ones I ran into today. (and I am quite serious when I say that writing a good Setup.hs is very, very, difficult). My objective is just to motivate a minor change to cabal that would help to make this easier. On to the build problems: encoding-0.6.6 uses it's own source during the build process! It actually has to be partially compiled before the build tool can build encoding (!). I'm *amazed* that this actually works at all (impressed, too), and indeed, it doesn't work with cabal-dev, which is unfortunate: cabal-dev would have identified one of the dependency issues in HaXml 1.19 (pretty is an undocumented dependency, although I am unsure if that was true at the time 1.19 and encoding-0.6.6 were released. I haven't tracked the pretty dependency down to the exact source - it could be a transitive dep introduced by a recent version of something HaXml 1.19 needs. Per my subject line - this is amazingly difficult - keep in mind that we're not talking about building *packages*, this is just to build the *build tool*, and none of this is documented. How long before it is effectively impossible to recreate the circumstances that result in a successful encoding-0.6.6 build?). cabal-dev can't build encoding-0.6.6 because the encoding source that is referenced from encoding's Setup.hs depends on (at least) HaXml >= 1.19 && < 1.22.5; this is OK with cabal, since cabal-install installs a necessary HaXml into your user package db prior to building encoding's Setup.hs (modulo the missing upper version bound). Therefore HaXml "just happens" to be visible when cabal builds Setup.hs -- a build step that cabal-dev isn't capable of controlling (and probably never will be -- we could intercept system calls, but that's difficult when only supporting one OS, much less three). You can, of course, manually install HaXml to your local package db and then cabal-dev install encodings, but that's somewhat of a hack (and you'll find that the current HaXml won't work for the current encodings release, as Gang Yu did). Adding a section to the cabal file that specifies build dependencies for Setup.hs would make some very important parts of this process obvious - and it would *greatly* assist in debugging failing builds, not to mention allowing us to make use of the extensive libraries on hackage to write more expressive build systems. (We could actually use openshake in Setup.hs, once it's released, for example!) I've floated the idea past a number of Haskell programmers with positive responses, and there is a GSOC ticket that provides some more details (the project was not selected - no students proposed to work on it): http://hackage.haskell.org/trac/summer-of-code/ticket/1602 --Rogan > Regards > Gang > On 08/17/2011 04:59 PM, Gang Yu wrote: > > hi, cafe: > > I go through to the encoding package installation problem again. > > cabal install encoding > Resolving dependencies... > /tmp/encoding-0.6.68093/encoding-0.6.6/dist/setup/setup: 4: Syntax error: > ";" unexpected > > when I have a check on the this issue, I get: > > file > ~/.cabal/packages/hackage.haskell.org/encoding/0.6.6/encoding-0.6.6/dist/setup/setup > /home/gang/.cabal/packages/hackage.haskell.org/encoding/0.6.6/encoding-0.6.6/dist/setup/setup: > ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked > (uses shared libs), for GNU/Linux 2.6.27, not stripped > > My machine is: > > gang@gang-laptop:~$ uname -a > Linux gang-laptop 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:50 UTC > 2011 i686 i686 i386 GNU/Linux > > is this related? How can I manually install the package from scratch? Any > helps or directs will be appreciated > > Thanks > Gang > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Gaussian elimination
by Tobias Nipkow 18 Aug '11

18 Aug '11

Hi, I came up with the following extremely compact version of Gaussian elimination for matrices represented as functions. I searched the web but found nothing resembling it, probably because of its inefficiency. Has anybody seen something like it before? Thanks Tobias gauss :: Int -> (Int -> Int -> Rational) -> Maybe (Int -> Int -> Rational) gauss 0 a = Just a gauss (n+1) a = case dropWhile (\i -> a i n == 0) [0..n] of [] -> Nothing (k:_) -> let ak' j = a k j / a k n a' i = if i == k then ak' else (\j -> a i j - a i n * ak' j) in gauss n (swap k n a') swap :: Int -> Int -> (Int -> a) -> (Int -> a) swap k n f = g where g x | x == k = f n | x == n = f k | otherwise = f x

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conditionally exposing modules in cabal
by Rogan Creswick 18 Aug '11

18 Aug '11

I would like to conditionally expose a number of internal modules in a library depending on a cabal flag - the idea is that new features could be implemented externally to the library without contaminating the source with undesirable changes. However, I've been unable to find a way to structure a cabal file that does not require repeating the list of modules. For example, this works: exposed-modules: Public.Stable.Module if flag(exposeInternals) exposed-modules: <long list of internal modules> else other-modules: <long list of internal modules, identical to above> But I don't want to maintain two identical lists of modules in the cabal file. I've tried putting the section headers in the if's, but this doesn't appear to work (cabal doesn't complain, but the modules to be exposed are not available as expected): exposed-modules: Public.Stable.Module if flag(exposeInternals) { exposed-modules: } else { other-modules: } <long list of internal modules> Does anyone have suggestions? Thanks! Rogan

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FFI question - FunPtrs to top-level functions
by Antoine Latter 18 Aug '11

18 Aug '11

Hi Haskell, I have a question about the Haskell FFI and creating FunPtrs to Haskell functions. Does anyone have any recommendations for when I have a top-level function that I would like to pass to a C function as a function pointer (that is called via a foreign import)? I know that the FFI provides the "wrapper" foreign import I can use to wrap Haskell functions, but then I would need to jump through hoops to manage the liefetime of of the FunPtr wrapper. If I were closing over interesting state I would want this - but the function (side-effectfully) operates only on its arguments (and the Haskell RTS, of course). Is it "okay" to place an "unsafePerformIO $ mkWrapper myFunc" as a top-level declaration, or am I journeying into uncharted lands? Is there something clever I can do with foreign exports and foreign imports, or is this just making things too complex? Thanks, Antoine

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Haskell Weekly News: Issue 195
by Daniel Santa Cruz 17 Aug '11

17 Aug '11

Welcome to issue 195 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of August 7 to 13, 2011. [1] http://goo.gl/8hDku You can find a HTML rendition of this newsletter at: http://contemplatecode.blogspot.com/2011/08/haskell-weekly-news-issue-195.h… Announcements Luite Stegeman created a nifty tool called hdiff, which shows code changes between different versions of hackage packages. [2] http://goo.gl/uqh97 New and Updated Projects * Netwire (New - Ertugrul Soeylemez) Arrowised FRP library. [3] http://goo.gl/tVlsH * Snap Riak Extensions (New - Edward Tate). [4] http://goo.gl/O0C2A * time (Update - Ashley Yakeley) Adds "instance RealFrac DiffTime". [5] http://goo.gl/Wm3UY * file-location (Update - Greg Weber) Error and debug information for haskell that gives file locations. [6] http://goo.gl/fTG4P * HTTP (Update - Ganesh Sittampalam) Bumps base dependency to work with GHC 4.2.1 [7] http://goo.gl/bydXi * hslogger, convertible, HDBC, HDBC-odbc, HDBC-postgresql, HDBC-sqlite3 (Update - John Goerzen) Update license to BSD3. [8] http://goo.gl/sD0Qg * taffybar (New - Tristian Ravitch) Slightly fancier status bar for xmonad. [9] http://goo.gl/SlNgi Quotes of the Week * Brend: Whoever chose the title "A Gentle Introduction to Haskell" is obviously accustomed to wrestling bears in piranha pits or something. * kmc: compare "trifecta" "parsec" --> GT * elliott: ... demonstrating C++ templates' wide range of expressivity in applications from stupid compile-time hacks to _really_ stupid compile-time hacks * solrize: "The last good thing written in C was Schubert's Ninth Symphony." * cmccann: [I] learned Haskell mostly by typing a lot of really dumb things into an editor window, then feeding them to GHC Top Reddit Stories * Simon Marlow: “[…] I think I finally understand how to track call stacks properly in a lazy functional language. If this pans out, we'll get backtraces in GHCi and more accurate profiling” Domain: plus.google.com, Score: 121, Comments: 15 On Reddit: [10] http://goo.gl/0DTba Original: [11] http://goo.gl/yIhnI * A Very General Method of Computing Shortest Paths (the Gauss-Jordan-Floyd-Warshall- McNaughton-Yamada algorithm) Domain: r6.ca, Score: 41, Comments: 8 On Reddit: [12] http://goo.gl/dgW4s Original: [13] http://goo.gl/e8ZBF * I'm reading http://www.cs.ru.nl/~wouters/Publications/ThePowerOfPi.pdf What is this syntax called and where can I learn it? Domain: imgur.com, Score: 36, Comments: 15 On Reddit: [14] http://goo.gl/FpFIu Original: [15] http://goo.gl/CE9Dz * Simon Marlow: "If you're using GHC 7.2.1 and find a performance loss relative to an earlier GHC, trying playing around with the new stack option" Domain: plus.google.com, Score: 25, Comments: 8 On Reddit: [16] http://goo.gl/2FTsH Original: [17] http://goo.gl/CJZsR * In your haskell-dreams, how does a shell look? Domain: self.haskell, Score: 23, Comments: 43 On Reddit: [18] http://goo.gl/jGyqH Original: [19] http://goo.gl/jGyqH * SO: Converting floating point between Word and Float/Double (my poor, unanswered question!) Domain: stackoverflow.com, Score: 21, Comments: 3 On Reddit: [20] http://goo.gl/csKRb Original: [21] http://goo.gl/JFDn2 * Current status of gtk2hs? Domain: self.haskell, Score: 18, Comments: 10 On Reddit: [22] http://goo.gl/HFwCR Original: [23] http://goo.gl/HFwCR * (Live from CamHac) ANN: fclabels-1.0: First class accessor labels. Domain: hackage.haskell.org, Score: 13, Comments: 19 On Reddit: [24] http://goo.gl/FXdY6 Original: [25] http://goo.gl/Ijl4H * PhD Position availalble at Strathclyde Domain: lists.seas.upenn.edu, Score: 12, Comments: 2 On Reddit: [26] http://goo.gl/FbdNs Original: [27] http://goo.gl/0R8Vi * [GSoC] Text/UTF-8: Studying memory usage of a simple web server Domain: jaspervdj.be, Score: 9, Comments: 1 On Reddit: [28] http://goo.gl/HkcJL Original: [29] http://goo.gl/EQn9h * Arrow = Category + Applicative? (Part IIa) Domain: cdsmith.wordpress.com, Score: 9, Comments: 21 On Reddit: [30] http://goo.gl/hB6YY Original: [31] http://goo.gl/dBSuA Top StackOverflow Questions * Lazy Evaluation vs Macros votes: 19, answers: 5 Read on SO: [32] http://goo.gl/28jcJ * Comparison of Priority Queue implementations in Haskell votes: 16, answers: 1 Read on SO: [33] http://goo.gl/MyMp4 * Converting IEEE 754 floating point in Haskell Word32/64 to and from Haskell Float/Double votes: 16, answers: 4 Read on SO: [34] http://goo.gl/2YaVC * Applicatives compose, monads don't votes: 15, answers: 5 Read on SO: [35] http://goo.gl/Ncdij * How can I speed up my Haskell program (to the level of Python) votes: 13, answers: 3 Read on SO: [36] http://goo.gl/jOXnL * How does ArrowLoop work? Also, mfix? votes: 12, answers: 1 Read on SO: [37] http://goo.gl/JrFRX * How could I remove the "if … then … else …" keywords in Haskell (GHC)? votes: 9, answers: 3 Read on SO: [38] http://goo.gl/gjNrm * how to use hoogle locally (like ctags)? votes: 8, answers: 1 Read on SO: [39] http://goo.gl/IGVEW * Would a type class "between" Category and Arrow make sense? votes: 8, answers: 2 Read on SO: [40] http://goo.gl/zgb7w * Can I avoid "rightward drift" in Haskell? votes: 8, answers: 4 Read on SO: [41] http://goo.gl/pc0Bn * Haskell Polyvariadic Function With IO votes: 8, answers: 4 Read on SO: [42] http://goo.gl/dfUVv * Why don't Haskell compilers facilitate deterministic memory management? votes: 8, answers: 4 Read on SO: [43] http://goo.gl/uhqw6 * Defining a function a -> String, which works for types without Show? votes: 7, answers: 3 Read on SO: [44] http://goo.gl/Mjcfs * Haskell: List Comprehension to Combinatory votes: 7, answers: 3 Read on SO: [45] http://goo.gl/tLryK * Applicative without a functor votes: 7, answers: 3 Read on SO: [46] http://goo.gl/UcXZ5 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91658 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91525 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91576 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18880 6. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91590 7. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18883 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91621 9. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91672 10. https://plus.google.com/u/0/107890464054636586545/posts/XE4T6hHm3tK 11. http://www.reddit.com/r/haskell/comments/jdj2r/simon_marlow_i_think_i_final… 12. http://r6.ca/blog/20110808T035622Z.html 13. http://www.reddit.com/r/haskell/comments/jc7pb/a_very_general_method_of_com… 14. http://imgur.com/pPfjs 15. http://www.reddit.com/r/haskell/comments/jbfdc/im_reading/ 16. https://plus.google.com/107890464054636586545/posts/LqgXK77FgfV 17. http://www.reddit.com/r/haskell/comments/jfskp/simon_marlow_if_youre_using_… 18. http://www.reddit.com/r/haskell/comments/jce8q/in_your_haskelldreams_how_do… 19. http://www.reddit.com/r/haskell/comments/jce8q/in_your_haskelldreams_how_do… 20. http://stackoverflow.com/q/6976684/204932 21. http://www.reddit.com/r/haskell/comments/jd6wq/so_converting_floating_point… 22. http://www.reddit.com/r/haskell/comments/jgzcf/current_status_of_gtk2hs/ 23. http://www.reddit.com/r/haskell/comments/jgzcf/current_status_of_gtk2hs/ 24. http://hackage.haskell.org/package/fclabels-1.0 25. http://www.reddit.com/r/haskell/comments/jhoay/live_from_camhac_ann_fclabel… 26. http://lists.seas.upenn.edu/pipermail/types-announce/2011/002643.html 27. http://www.reddit.com/r/haskell/comments/jdjms/phd_position_availalble_at_s… 28. http://jaspervdj.be/posts/2011-08-09-text-utf8-memory-usage.html 29. http://www.reddit.com/r/haskell/comments/jdk6z/gsoc_textutf8_studying_memor… 30. http://cdsmith.wordpress.com/2011/08/13/arrow-category-applicative-part-iia/ 31. http://www.reddit.com/r/haskell/comments/ji8si/arrow_category_applicative_p… 32. http://stackoverflow.com/questions/7046950/lazy-evaluation-vs-macros 33. http://stackoverflow.com/questions/6976559/comparison-of-priority-queue-imp… 34. http://stackoverflow.com/questions/6976684/converting-ieee-754-floating-poi… 35. http://stackoverflow.com/questions/7040844/applicatives-compose-monads-dont 36. http://stackoverflow.com/questions/7003572/how-can-i-speed-up-my-haskell-pr… 37. http://stackoverflow.com/questions/6976944/how-does-arrowloop-work-also-mfix 38. http://stackoverflow.com/questions/6996286/how-could-i-remove-the-if-then-e… 39. http://stackoverflow.com/questions/6989668/how-to-use-hoogle-locally-like-c… 40. http://stackoverflow.com/questions/6997517/would-a-type-class-between-categ… 41. http://stackoverflow.com/questions/7008493/can-i-avoid-rightward-drift-in-h… 42. http://stackoverflow.com/questions/7030476/haskell-polyvariadic-function-wi… 43. http://stackoverflow.com/questions/7038811/why-dont-haskell-compilers-facil… 44. http://stackoverflow.com/questions/7005628/defining-a-function-a-string-whi… 45. http://stackoverflow.com/questions/7007222/haskell-list-comprehension-to-co… 46. http://stackoverflow.com/questions/7024788/applicative-without-a-functor

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Overloading in a sub-class
by Patrick Browne 17 Aug '11

17 Aug '11

Hi, Below are two questions concerning overloading in a sub-class. Thanks, Pat class Numb0 a where (+) :: a -> a -> a negate :: a -> a instance Numb0 Int where x + y = y negate x = x -- Are + and negate part of the signature of Numb1? class Numb0 a => Numb1 a where -- Is it possible to override these operations in instances of Numb1? -- Something like: -- instance Numb1 Float where -- x + y = y -- negate x = x -- Or even using Int as in the super class instance: -- instance Numb1 Int where -- x + y = y -- negate x = x This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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ANNOUNCE: TKYProf
by Mitsutoshi Aoe 17 Aug '11

17 Aug '11

Hi, forks I'm glad to announce the alpha release of TKYProf. http://hackage.haskell.org/package/tkyprof https://github.com/maoe/tkyprof TKYprof is a web-based interacitve visualizer for GHC time and allocation profiling reports. It helps you to find the bottlenecks in your code quickly! Here is a blog post: http://blog.foldr.in/tkyprof-a-web-based-interactive-visualizer-fo It is still alpha and it have some bugs. I'm happy to hear your feedback. Thanks, -- Mitsutoshi Aoe maoe(a)foldr.in

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Haskell transformation of C code
by mukesh tiwari 17 Aug '11

17 Aug '11

Hello all I am trying implement this algorithm [ http://stackoverflow.com/questions/1621364/how-to-find-largest-triangle-in-… ] in Haskell but i am getting wrong answer for some test cases. A c++ implementation which is accepted for this problem [ http://www.spoj.pl/problems/MTRIAREA ] and i rewrote the C++ code in Haskell . Running both programs on some random test cases produce different result . For Haskell implementation , convex hull part is correct . Both codes [ c++ and Haskell ] produced same convex hull so i am skeptical about looPing :: ( Num a , Ord a , Eq a , Floating a ) => Int -> Int -> Int - > Int -> a -> Array Int ( Point a ) -> a and calArea :: ( Num a , Ord a , Floating a ) => Int -> Int -> Int -> Int -> a -> Array Int ( Point a ) -> ( Int , Int , Int , a ) function . Could some one please tell me what is wrong with these functions. I have posted both code on ideone . Haskell code [ http://ideone.com/IlwBv ] and accepted c++ for SPOJ problem [ http://ideone.com/vgNnt ] . For test case 20 886 9383 6915 2777 8335 7793 492 5386 1421 6649 27 2362 59 8690 3926 7763 3426 540 5736 9172 5368 5211 6429 2567 1530 5782 5123 2862 3135 4067 9802 3929 3058 4022 8167 3069 8456 1393 8042 5011 -1 Haskell produced the convex hull [P 27.0 2362.0,P 3426.0 540.0,P 8456.0 1393.0,P 9802.0 3929.0,P 8335.0 7793.0,P 5736.0 9172.0,P 886.0 9383.0,P 59.0 8690.0] and maximum triangle area 31466755.50 while C++ code produced the convex hull 27 2362 3426 540 8456 1393 9802 3929 8335 7793 5736 9172 886 9383 59 8690 and maximum area 33642111.00 . Based on these observation , I think my convex hull part is correct and I am stuck with looPing and calArea function . I tried to write Haskell code many ways but no success so it would be great if some one can tell me what is wrong with these two functions . Thank you

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ICFP Hotel Roommate
by Daniel Winograd-Cort 17 Aug '11

17 Aug '11

Hi, I'll be heading to ICFP in September, and I'm hoping to find someone to share a room with to save on hotel costs. Are there any other students looking for a roommate? Cheers, Dan

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