- Haskell-Cafe - Haskell.org

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 18 Aug '11

18 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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broken on install encoding 0.6.6
by Gang Yu 17 Aug '11

17 Aug '11

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

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Getting segmentation fault when profiling, despite `-K100M'.
by David Banas 16 Aug '11

16 Aug '11

Hi all, I'm trying to profile a mixed language program, in which Haskell is NOT the top layer and does not contain the `main' function. (C is/does.) Is this effort doomed to fail? I'm getting a segmentation fault, as soon as the first Haskell function is called, despite having included `-K100M' in my list of +RTS args: 19 int argc = 5; 20 char* argv[] = {"ghcDll", 21 "+RTS", 22 "-hc", 23 "-p", 24 "-K100M", 25 NULL}; // argv must end with NULL 26 27 // Initialize Haskell runtime 28 char** args = argv; 29 hs_init(&argc, &args); Using gdb to trace through the C-code, up until the first call to Haskell, I get to here: 64 // Call the Haskell function. 65 res = amiInit( 66 impulse_matrix, 67 row_size, Here's the beginning of `amiInit': 41 -- Our Haskell implementation of `AMI_Init'. 42 amiInit :: Ptr CDouble -> CInt -> CInt -> CDouble -> CDouble -> 43 CString -> Ptr CString -> Ptr (StablePtr AmiModel) -> Ptr CString -> IO Int 44 amiInit impulse_matrix row_size aggressors sample_interval bit_time 45 ami_parameters_in ami_parameters_out ami_memory_handle msgHndl 46 | impulse_matrix == nullPtr = return 0 47 | otherwise = do 48 putStrLn "I'm here." I never see "I'm here." printed to my console, before I get the segmentation fault. Any thoughts? Thanks, -db

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Hiding "growing" state using existentials.
by Tom Schouten 16 Aug '11

16 Aug '11

Dear Cafe, I'm building an abstraction for representing sequences as difference equations, storing initial values and update equation. I have something that resembles a Monad, but has an extra state parameter s that "grows" on _join or _bind, so I can't simply create an instance Monad (Sig s). > {-# LANGUAGE ExistentialQuantification #-} > data Sig s a = Sig s (s -> (a, s)) > _join :: (Sig s1 (Sig s2 a)) -> Sig (s1,s2) a > _join (Sig i1 u1) = Sig (i1, i2) u12 where > ((Sig i2 _), _) = u1 i1 > u12 (s1, s2) = (a, (s1', s2')) where > ((Sig _ u2), s1') = u1 s1 > (a, s2') = u2 s2 > _fmap :: (a -> b) -> Sig s a -> Sig s b > _fmap f (Sig s0 u) = Sig s0 u' where > u' s = (f a, s') where > (a, s') = u s > _return x = Sig () $ \() -> (x, ()) > _bind :: (Sig s1 a) -> (a -> Sig s2 b) -> (Sig (s1,s2) b) > m `_bind` g = _join ((_fmap g) m) I try to hide the state parameter using existential quantification. > data Signal a = forall s. Signal (Sig s a) This approach works for defining Functor and Applicative instances, but I can't seem to find a way to obtain an unwrapped version of f that can be passed to _bind to implement the Monad instance's (>>=). The following returns: Couldn't match type `t' with `Sig s1 b' `t' is a rigid type variable bound by the inferred type of f' :: a -> t at /home/tom/meta/ssm/SigHC.lhs:52:7 In the expression: mb In a case alternative: Signal mb -> mb In the expression: case (f a) of { Signal mb -> mb } > instance Monad Signal where > return = Signal . _return > (>>=) (Signal ma) f = Signal $ ma `_bind` f' where > f' a = case (f a) of > Signal mb -> mb Any tips on how to work around this? Cheers, Tom

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