- Haskell-Cafe - Haskell.org

Extracting exposed modules from an installed library
by Corentin Dupont 21 Mar '13

21 Mar '13

Hi Cafe! I'm looking for how to extract the exposed modules (as a list of strings) from an installed library, giving the library name. I can see some structures in Cabal (InstalledPackageInfo) and some functions in ghc-pkg.hs in GHC, but nothing readily useable... Thanks, Corenti

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ANN: bytestring-lexing 0.4.2
by wren ng thornton 21 Mar '13

21 Mar '13

-------------------------------------------- -- bytestring-lexing 0.4.2 -------------------------------------------- The bytestring-lexing package offers efficient reading and packing of common types like Double and Integral types. -------------------------------------------- -- Changes (since 0.4.0) -------------------------------------------- * Faster implementation of packDecimal. Inspired by [1] I applied the same optimizations to the ByteString implementation of packDecimal. According to the benchmarks I have on hand the new version is about 1.3x faster than the old version. So the difference isn't nearly as impressive; but it's not trivial either. * Added buffer overflow check for asHexadecimal. This only matters if you try converting a ByteString longer than (maxBound `quot` 2 ::Int). I forget if the old version segfaulted or what; but now you get an error message about trying to allocate a buffer that's too large. [1] <http://www.serpentine.com/blog/2013/03/20/whats-good-for-c-is-good-for-hask…> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-lexing Darcs: http://community.haskell.org/~wren/bytestring-lexing Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-lexing/dist/doc/html/bytestri… -- Live well, ~wren

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Haskell Weekly News: Issue 263
by Daniel Santa Cruz 21 Mar '13

21 Mar '13

Welcome to issue 263 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of March 10 to 16, 2013. Quotes of the Week * shachaf: Did someone give edwardk a committee bit? Top Reddit Stories * Restored: the Haskell mailing list archives: 1990-2000. The lost books of Haskell Domain: code.haskell.org, Score: 71, Comments: 9 On Reddit: [1] http://goo.gl/wYs4E Original: [2] http://goo.gl/1Hw4G * Dependently Typed Functional Programming with Idris [video] Domain: vimeo.com, Score: 49, Comments: 20 On Reddit: [3] http://goo.gl/RDII5 Original: [4] http://goo.gl/AmmPJ * Snake game in Haskell Domain: self.haskell, Score: 48, Comments: 19 On Reddit: [5] http://goo.gl/HssiG Original: [6] http://goo.gl/HssiG * A simple graphical notation for lambda calculus Domain: binarylambda.blogspot.com, Score: 46, Comments: 35 On Reddit: [7] http://goo.gl/I3xqp Original: [8] http://goo.gl/sY6gl * Simon Peyton Jones on Adventures with types (Functional Programming eXchange 2013) Domain: skillsmatter.com, Score: 46, Comments: 27 On Reddit: [9] http://goo.gl/wUvon Original: [10] http://goo.gl/XCWsE * Guide to GHC Extensions (just some simple ones for now, will expand over time) - School of Haskell Domain: fpcomplete.com, Score: 42, Comments: 25 On Reddit: [11] http://goo.gl/XhFK5 Original: [12] http://goo.gl/Wa0K3 * layers-0.1: Modular type class machinery for monad transformer stacks. Domain: hackage.haskell.org, Score: 37, Comments: 30 On Reddit: [13] http://goo.gl/nkimL Original: [14] http://goo.gl/HFBxI * Type Families and Pokemon: School of Haskell Pick of the Week Domain: fpcomplete.com, Score: 35, Comments: 3 On Reddit: [15] http://goo.gl/mITH3 Original: [16] http://goo.gl/oymkn * Gale: an IDE for creating games in Haskell Domain: youtube.com, Score: 31, Comments: 4 On Reddit: [17] http://goo.gl/2k28x Original: [18] http://goo.gl/9SxFS * What is the best way to make a simple web service in Haskell? Domain: self.haskell, Score: 31, Comments: 34 On Reddit: [19] http://goo.gl/sVAe7 Original: [20] http://goo.gl/sVAe7 * Wadler's Blog: Informatics 1: Functional Programming Domain: wadler.blogspot.ro, Score: 25, Comments: 3 On Reddit: [21] http://goo.gl/70Ocg Original: [22] http://goo.gl/z4b38 * Constructions on Typeclasses, Part 1: F-Algebras Domain: lukepalmer.wordpress.com, Score: 24, Comments: 0 On Reddit: [23] http://goo.gl/qGsVj Original: [24] http://goo.gl/s947R * cabal-dev updated on hackage Domain: haskell.org, Score: 24, Comments: 37 On Reddit: [25] http://goo.gl/LUOQZ Original: [26] http://goo.gl/nMut4 * monoid-subclasses: ListLike-like, only built on sound theoretical foundations Domain: hackage.haskell.org, Score: 22, Comments: 24 On Reddit: [27] http://goo.gl/gA46h Original: [28] http://goo.gl/4QOKw * Introduction to Concurrent Haskell - Simon Marlow [FP Days Cambridge 2012] Domain: infoq.com, Score: 22, Comments: 1 On Reddit: [29] http://goo.gl/CS0xV Original: [30] http://goo.gl/TrVo9 Top StackOverflow Questions * Proposal for local data declarations / instances votes: 15, answers: 1 Read on SO: [31] http://goo.gl/VCd4K * What is the equivalent Haskell type for C99 bool when using FFI? votes: 14, answers: 2 Read on SO: [32] http://goo.gl/mm3hC * Disadvantages of using INLINABLE pragma votes: 13, answers: 1 Read on SO: [33] http://goo.gl/zxnBu * How to compare two lists in Haskell? votes: 11, answers: 4 Read on SO: [34] http://goo.gl/kTXqL * Haskell space usage compile time restrictions votes: 11, answers: 2 Read on SO: [35] http://goo.gl/vy7wO * haskell - Average floating point error using QuickCheck votes: 10, answers: 1 Read on SO: [36] http://goo.gl/MNsnL * Trying to understand function application operator in Haskell votes: 9, answers: 1 Read on SO: [37] http://goo.gl/ymFv4 * Haskell equivalent to Scala's groupBy votes: 9, answers: 4 Read on SO: [38] http://goo.gl/0uceW * (Num a) vs Integer type inference votes: 9, answers: 3 Read on SO: [39] http://goo.gl/N3CWn Until next time, +Daniel Santa Cruz References 1. http://code.haskell.org/~dons/haskell-1990-2000/ 2. http://www.reddit.com/r/haskell/comments/1aescj/restored_the_haskell_mailin… 3. http://vimeo.com/61576198 4. http://www.reddit.com/r/haskell/comments/1a7rbk/dependently_typed_functiona… 5. http://www.reddit.com/r/haskell/comments/1a12vg/snake_game_in_haskell/ 6. http://www.reddit.com/r/haskell/comments/1a12vg/snake_game_in_haskell/ 7. http://binarylambda.blogspot.com/2013/03/lambda-diagrams-lambda-diagrams-fe… 8. http://www.reddit.com/r/haskell/comments/1a47ox/a_simple_graphical_notation… 9. http://skillsmatter.com/podcast/java-jee/keynote-3842 10. http://www.reddit.com/r/haskell/comments/1aedqt/simon_peyton_jones_on_adven… 11. https://www.fpcomplete.com/user/PthariensFlame/guide-to-ghc-extensions ? 12. http://www.reddit.com/r/haskell/comments/1a3ye8/guide_to_ghc_extensions_jus… 13. http://hackage.haskell.org/package/layers 14. http://www.reddit.com/r/haskell/comments/1aazok/layers01_modular_type_class… 15. https://www.fpcomplete.com/user/nubis/type-families-and-pokemon 16. http://www.reddit.com/r/haskell/comments/1adukd/type_families_and_pokemon_s… 17. http://www.youtube.com/watch?v=dKBFJa9N3zY 18. http://www.reddit.com/r/haskell/comments/1a3k64/gale_an_ide_for_creating_ga… 19. http://www.reddit.com/r/haskell/comments/1acfbx/what_is_the_best_way_to_mak… 20. http://www.reddit.com/r/haskell/comments/1acfbx/what_is_the_best_way_to_mak… 21. http://wadler.blogspot.ro/2013/03/informatics-1-functional-programming.html 22. http://www.reddit.com/r/haskell/comments/1a5uio/wadlers_blog_informatics_1_… 23. http://lukepalmer.wordpress.com/2013/03/12/constructions-on-typeclasses-par… 24. http://www.reddit.com/r/haskell/comments/1a5dtt/constructions_on_typeclasse… 25. http://www.haskell.org/pipermail/haskell-cafe/2013-February/106719.html 26. http://www.reddit.com/r/haskell/comments/1a6t0j/cabaldev_updated_on_hackage/ 27. http://hackage.haskell.org/package/monoid-subclasses 28. http://www.reddit.com/r/haskell/comments/1a33sb/monoidsubclasses_listlikeli… 29. http://www.infoq.com/presentations/Concurrent-Haskell 30. http://www.reddit.com/r/haskell/comments/1a4pvi/introduction_to_concurrent_… 31. http://stackoverflow.com/questions/15320391/proposal-for-local-data-declara… 32. http://stackoverflow.com/questions/15368658/what-is-the-equivalent-haskell-… 33. http://stackoverflow.com/questions/15440804/disadvantages-of-using-inlinabl… 34. http://stackoverflow.com/questions/15319136/how-to-compare-two-lists-in-has… 35. http://stackoverflow.com/questions/15374829/haskell-space-usage-compile-tim… 36. http://stackoverflow.com/questions/15364518/haskell-average-floating-point-… 37. http://stackoverflow.com/questions/15362990/trying-to-understand-function-a… 38. http://stackoverflow.com/questions/15412027/haskell-equivalent-to-scalas-gr… 39. http://stackoverflow.com/questions/15448351/num-a-vs-integer-type-inference 40. https://plus.google.com/105107667630152149014/about

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Maintaining lambdabot
by Jan Stolarek 21 Mar '13

21 Mar '13

Hi all, as some of you may have noticed Lambdabot doesn't build on GHC 7.6.1 due to OldException being removed (and a few other changes). I updated the code so that it builds on latest GHC release. The updated code is available here: https://github.com/killy9999/lambdabot/tree/upstream It builds on Linux, but I am not 100% certain that it will build on Windows. If it doesn't I will be grateful for reporting this (and any other) issues on the bug tracker: https://github.com/killy9999/lambdabot/issues The changes required to make the code run weren't as small as I initially thought they will be. In some places this resulted in a bit ugly code, but aside from that I think that everything should work correctly. Now I would like to do a bit of maintanance on the project, since the code is a bit of a mess and it could use some cleanup. My plans are: - clean up the package namespace by placing existing modules in Lambdabot module - integrate lambdabot-utils into lambdabot (already done) - reformat the code so that it's easier to read ans if time allows: - cleanup some messy code - update the package dependencies so that lambdabot relies on maintained libraries (e.g. I thought about replacing readline with haskeline). This changes would be quite invasive and code wouldn't be compatible with the lambdabot repo on haskell.org. So before I start making any of them I would like to hear from the community if such changes in the source code of lambdabot would be considered helpful and acceptable. Janek

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package show needs QuickCheck<2.6?
by Johannes Waldmann 21 Mar '13

21 Mar '13

Hi, I noticed that compilation of mueval (recent: 0.8.2) breaks because show (0.5) cannot be built: it seems the type of Failure changed in QuickCheck (from 2.5 to 2.6). The build succeeds with --constraint 'QuickCheck<2.6' .

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Does GHC 7.8 make targeting bare metal ARM any easier?
by Jeremy Shaw 20 Mar '13

20 Mar '13

There have been at least a couple projects, such as hOp and HaLVM which attempt to run GHC on the bare metal or something similar. Both these projects required a substantial set of patches against GHC to remove dependencies things like POSIX/libc. Due to the highly invasive nature, they are also highly prone to bitrot. With GHC 7.8, I believe we will be able to cross-compile to the Raspberry Pi platform. But, what really appeals to me is going that extra step and avoiding the OS entirely and running on the bare metal. Obviously, you give up a lot -- such as drivers, network stacks, etc. But, there is also a lot of potential to do neat things, and not have to worry about properly shutting down an embedded linux box. Also, since the raspberry pi is a very limited, uniform platform, (compared to general purpose PCs) it is feasible to create network drivers, etc, because only one chipset needs to be supported. (Ignoring issues regarding binary blobs, undocumented chipsets, usb WIFI, etc). I'm wondering if things are any easier with cross-compilation support improving. My thought is that less of GHC needs to be tweaked? - jeremy

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ANN: LearnByHaking.org
by Deian Stefan 20 Mar '13

20 Mar '13

Hi cafe, I've been delaying the announcement of LearnByHacking.org as I wish to add more features, but some of you may be interested in it as is. The web app is inspired by and, to some degree, a lighter-weight clone of School of Haskell. I am currently using it as glorified blog that provides a means for collaborating with others on posts and include active code. For those unfamiliar with SoH, that is, code that users can execute with a single click and modify. There are lots of things on the TODO list, so if anybody wants to hack, please join in. Cheers, Deian

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Streaming bytes and performance
by Konstantin Litvinenko 20 Mar '13

20 Mar '13

Hi All! I tune my toy project for performance and hit the wall on simple, in imperative world, task. Here is the code that model what I'm trying to achieve import qualified Data.ByteString.Lazy as L import Data.Word8(isSpace) import Data.Word import Control.Monad.State type Stream = State L.ByteString get_byte :: Stream (Maybe Word8) get_byte = do s <- get case L.uncons s of Nothing -> return Nothing Just (x, xs) -> put xs >> return (Just x) main = do f <- L.readFile "test.txt" let r = evalState count_spaces f print r where count_spaces = go 0 where go a = do x <- get_byte case x of Just x' -> if isSpace x' then go (a + 1) else go a Nothing -> return a It takes the file and count spaces, in imperative way, consuming bytes one by one. The problem is: How to rewrite this to get rid of constant allocation of state but still working with stream of bytes? I can rewrite this as one-liner L.foldl, but that doesn't help me in any way to optimize my toy project where all algorithms build upon consuming stream of bytes. PS. My main lang is C++ over 10 years and I only learn Haskell :)

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Compiled program using OpenGL fails to trigger GPU switch on Mac, but works in GHCi
by Jesper Särnesjö 20 Mar '13

20 Mar '13

Hi everybody, This started out on haskell-beginners, as a question about poor performance for a Haskell program using OpenGL. Thanks to a few good suggestions there, I've managed to figure out more or less what the underlying problem is, but not its cause. In short, I have two programs, one written in Haskell [1] and one written in C [2], that consist of calls to the same functions, in the same order, to the same C library, but which do not exhibit the same behavior. Further, the Haskell program behaves differently when compiled using GHC, and when run in GHCi. I would like to know why this is, and how to fix it. A bit of background. My original program used the GLFW library to create an OpenGL rendering context, and rendered a 3D model using OpenGL. I noticed that this program had very high CPU usage, when I expected it to do most of its work on the GPU [3]. The reason for the high CPU usage turned out to be that the program was in fact using a software implementation of OpenGL [4]. My machine, a MacBook Pro running Mac OS X 10.8.2, has two GPUs, a discrete (more powerful) and an integrated one (more energy-efficient). The latter implements a larger part of OpenGL in software, and the OS is supposed to switch transparently between the two. In particular, it is supposed to switch to the discrete GPU when a program tries to use OpenGL features not supported by the integrated GPU [5]. I discovered that while this all worked as intended when running the C version of my program, it did not work quite so well for the Haskell program [6], which would get stuck with a rendering context on the integrated GPU, which in practice meant a software implementation and poor performance. I reduced both programs to fairly minimal test cases. Now, each program simply configures and creates a rendering context, checks if it is hardware-accelerated (for the configuration used, on my machine, this implies that the system has switched to the discrete GPU), and then terminates. This is all done using calls to the GLFW C library. The C program succeeds. The Haskell program fails if compiled (with GHC 7.4.2) and run, but succeeds if run in GHCi. Further, by monitoring which GPU is active, using gfxCardStatus [7] and the system console, I've established that the switch happens immediately following the execution of glfwOpenWindow, for the C program and the Haskell program when run in GHCi. For the compiled Haskell program, the switch is delayed by roughly a second. This delay appears to be what causes the program to get stuck on the integrated GPU. Now, there are a lot of moving parts involved, and many places where things could go wrong, making it tricky to even say where the problem is. Still, a Haskell program consisting entirely of foreign function calls in the IO monad, should surely behave the same as a C program consisting of the same calls? Is this caused by lazy evaluation, and if so, of what? Why does it work correctly in GHCi? I've bashed my head against this for some time now, and have run out of good ideas. I would really appreciate any input that lets me solve this, and get back to the fun parts of 3D programming. ;) To reproduce this, you'll need a Mac that has two GPUs (most of them does, these days). I run Mac OS X 10.8.2, and another user reproduced this on 10.7.5 [8]. You'll also need the GLFW library [9], which can be built from source or installed using Homebrew. Please let me know if I can provide any more information. -- Jesper Särnesjö http://jesper.sarnesjo.org/ [1] https://gist.github.com/sarnesjo/5151894#file-glfw_test-hs [2] https://gist.github.com/sarnesjo/5151894#file-glfw_test-c [3] http://www.haskell.org/pipermail/beginners/2013-March/011557.html [4] http://www.haskell.org/pipermail/beginners/2013-March/011560.html [5] http://developer.apple.com/library/mac/#qa/qa1734/_index.html%23//apple_ref… [6] http://www.haskell.org/pipermail/beginners/2013-March/011601.html [7] http://gfx.io [8] http://www.haskell.org/pipermail/beginners/2013-March/011563.html [9] http://www.glfw.org

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Associated types for number coercion
by Christopher Done 20 Mar '13

20 Mar '13

>From the paper Fun with Type Funs, it's said: > One compelling use of such type functions is to make type > coercions implicit, especially in arithmetic. Suppose we want to be able to > write add a b to add two numeric values a and b even if one is an Integer > and the other is a Double (without writing fromIntegral explicitly). And then an Add class is defined which can dispatch at the type-level to appropriate functions which resolve two types into one, with a catch-all case for Num. Has anyone put this into a package, for all common arithmetic operations? I would use it. Doing arithmetic stuff in Haskell always feels labored because of having constantly convert between number types.

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