- Haskell-Cafe - Haskell.org

haskell.org is so fragile
by Takayuki Muranushi 13 Jul '12

13 Jul '12

Today I have observed that hackage.haskell.org/ timeout twice (in the noon and in the evening.) What is the problem? Is it that haskell users have increased so that haskell.org is overloaded? That's very good news. I am eager to donate some money if it requires server reinforcement. Sincerely, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Monads with "The" contexts?
by oleg＠okmij.org 13 Jul '12

13 Jul '12

Tillmann Rendel has correctly noted that the source of the problem is the correlation among the random variables. Specifically, our measurement of Sun's mass and of Mars mass used the same rather than independently drawn samples of the Earth mass. Sharing (which supports what Functional-Logic programming calls ``call-time choice'') is indeed the solution. Sharing has very clear physical intuition: it corresponds to the collapse of the wave function. Incidentally, a better reference than our ICFP09 paper is the greatly expanded JFP paper http://okmij.org/ftp/Computation/monads.html#lazy-sharing-nondet You would also need a generalization of sharing -- memoization -- to build stochastic functions. The emphasis is on _function_: when applied to a value, the function may give an arbitrary sample from a distribution. However, when applied to the same value again, the function should return the same sample. The general memo combinator is implemented in Hansei and is used all the time. The following article talks about stochastic functions (and correlated variables): http://okmij.org/ftp/kakuritu/index.html#all-different and the following two articles show just two examples of using memo: http://okmij.org/ftp/kakuritu/index.html#noisy-or http://okmij.org/ftp/kakuritu/index.html#colored-balls The noisy-or example is quite close to your problem. It deals with the inference in causal graphs (DAG): finding out the distribution of conclusions from the distribution of causes (perhaps given measurements of some other conclusions). Since a cause may contribute to several conclusions, we have to mind sharing. Since the code works by back-propagation (so we don't have to sample causes that don't contribute to the conclusions of interest), we have to use memoization (actually, memoization of recursively defined functions).

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Another topic of ghc --make/ghci differences
by Andrey Yankin 12 Jul '12

12 Jul '12

1. I wrote this: -----8<----- test.hs --- module Main where import qualified Graphics.UI.GLFW as GLFW main = do True <- GLFW.initialize print =<< GLFW.openGLProfile print =<< GLFW.getGlfwVersion print =<< GLFW.getGlVersion print "Trying to open the window" result <- GLFW.openWindow GLFW.defaultDisplayOptions print result -----8<----- 2. Compiled like this: ghc --make -o test test.hs 3. Executed: ./test DefaultProfile Version {versionBranch = [2,7,2], versionTags = []} Version {versionBranch = [0,0,0], versionTags = []} "Trying to open the window" True 4. Window appeared. 2'. Executed in GHCI like this: ghci -fno-ghci-sandbox Prelude> :l test.hs Prelude> main 3'. Get (random numbers as GLVersion): Loading package GLFW-b-0.1.0.2 ... linking ... done. DefaultProfile Version {versionBranch = [2,7,2], versionTags = []} Version {versionBranch = *[34972080,-1323041296,-1334802519]*, versionTags = []} "Trying to open the window" False 4'. Window didn't appear. What should I do to get GLFW window in ghci too? What do I do wrong?

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Bad interface problem.
by Magicloud Magiclouds 12 Jul '12

12 Jul '12

Hi, This had never happened to me. And I have no idea what to do. $ cabal --upgrade-dependencies --enable-documentation --force-reinstalls --solver=topdown QuickCheck-2.5 Test/QuickCheck/All.hs:15:1: Bad interface file: /home/magicloud/.cabal/lib/template-haskell-2.6.0.0/ghc-7.4.2/Language/Haskell/TH.hi Something is amiss; requested module template-haskell-2.6.0.0:Language.Haskell.TH differs from name found in the interface file template-haskell:Language.Haskell.TH $ cabal --version cabal-install version 0.15.0 using version 1.15.0 of the Cabal library -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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What's wrong with these foreign calls?
by Андрей Янкин 12 Jul '12

12 Jul '12

Hi all! I want to use GLFW-b ( http://hackage.haskell.org/packages/archive/GLFW-b/0.1.0.2/doc/html/Graphic…) and I wrote this: initWindow = do True <- GLFW.initialize print =<< GLFW.openGLProfile print =<< GLFW.getGlfwVersion print =<< *GLFW.getGlVersion* print "Trying to open the window" result <- GLFW.openWindow GLFW.defaultDisplayOptions print result And got this: DefaultProfile Version {versionBranch = [2,7,2], versionTags = []} Version {versionBranch = [*-1867951663,39789305,39789305*], versionTags = []} "Trying to open the window" False Why do I always get random numbers as GLVersion? How can I detect incompatibility preventing me from opening a window? Thanks in advance

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Hackage is down?
by Edward Amsden 12 Jul '12

12 Jul '12

http://www.downforeveryoneorjustme.com/hackage.haskell.org -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Monad with limited backtracking
by Darryn Reid 12 Jul '12

12 Jul '12

Kind Sirs/Madams, Thanks in advance for your patience; the solution will no doubt be obvious to those with greater experience than me. I have formulated a monad to provide limited backtracking for implementing a rewrite system. The success case works fine, but my intention is that on failure the result should contain the list of input symbols consumed up to the failure, and it is this part that I would like some advice about. Code follows: ------------------------------------------------------------------------------- import Control.Monad import Debug.Trace newtype Rewrite a b = Rewrite { run :: Input a -> Result a b } data Input a = Input [a] data Result a b = Fail [a] | Ok b [a] deriving (Eq, Show) instance (Show a) => Monad (Rewrite a) where return y = Rewrite $ \(Input xs) -> Ok y xs p >>= f = Rewrite $ \inp -> case run p inp of Fail xs -> trace ("1.xs=" ++ show xs) $ Fail xs Ok y xs -> case run (f y) (Input xs) of Fail xs' -> trace ("2.xs=" ++ show xs) $ Fail xs okay -> okay instance (Show a) => MonadPlus (Rewrite a) where mzero = Rewrite $ \inp -> Fail [] p `mplus` q = Rewrite $ \inp -> case run p inp of Fail _ -> run q inp okay -> okay (>>=?) ::(Show a) => Rewrite a b -> (b -> Bool) -> Rewrite a b p >>=? f = p >>= \y -> guard (f y) >> return y next :: Rewrite a a next = Rewrite $ \(Input xs) -> case xs of [] -> Fail [] (x:xs') -> Ok x xs' exactly :: (Show a, Eq a) => [a] -> Rewrite a [a] exactly = mapM $ \i -> next >>=? (==i) ------------------------------------------------------------------------------- For example, using ghci: *Main> run (exactly [1,2]) (Input [1,2,3,4]) Ok [1,2] [3,4] which is what I intend. However, while the thing correctly reports failure, I cannot get it to return the list of symbols up to the point of failure: *Main> run (exactly [1,2,3]) (Input [1,2,7,4]) 1.xs=[] 2.xs=[4] 1.xs=[4] 1.xs=[4] 2.xs=[7,4] 1.xs=[7,4] 2.xs=[2,7,4] Fail [2,7,4] *I would like Fail [1,2] here instead* I thank you in advance for any guidance you might offer. Dr Darryn J Reid.

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Solutions for multi-platform compilation?
by Ivan Perez 12 Jul '12

12 Jul '12

Hi, I work developing multi-platform applications in Haskell. This poses the following problem: I cannot compile binaries for windows from linux (AFAIK). I "solved" this problem with the following sledgehammer: I installed windows in a VM, I installed GHC, I installed all the C/C++ headers & binaries of the libraries that I use (Gtk, OpenGL, SDL, OpenCV, etc.) their Haskell counterparts, and I created several scripts that connect to the VM using SSH, push the changes to the repo, cabal clean & cabal install all my packages in sequence without me having to even login into the windows machine. I did this because I was unable to get GHC to run properly in Wine at that time (over 2 years ago). This solution is still unsatisfactory because: 1) It's slow, even though Windows itself works fine (well, as well as windows can work, but it runs at a decent spped, I can play games and all). 2) When I update a library with lots of dependencies, or GHC itself, I have to rebuild almost everything. This is particularly painful with big packages like Gtk, for instance. Because I have to tell cabal where headers and libraries are located, updating a package is almost never an automatic process. I haven't always been able to make GHC "just pick them up" properly with pkg-config. 3) When I make a change in a library with lots of dependencies, recompiling all the packages can take several hours. I don't think it's a problem with my machine: I'm giving a fair amount of resources to windows, and I use a 3Ghz quadcore with 8GB of RAM. Another relevant fact is: I use this for commercial purposes. I have customers, each requiring a completely different program, they do not have infinite budgets and, if there's a problem in the compilation process and something requires my attention and manual intervention too often, my salary per hour can easily drop to a ridiculous amount. If I'm going to redo this, I'd rather just redo it once. Any suggestions? How do you solve this kind of problem in your work environment? Cheers, Ivan

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Control defaulting in ghci
by Roman Cheplyaka 12 Jul '12

12 Jul '12

Is there a way to control defaulting in ghci? (Like the "default" declaration in source files does.) ghci 7.4.1 doesn't accept "default" declarations. I tried loading a module with a "default" declaration, but that also didn't affect the ghci session. It would be ironic if this is possible in source files but not in ghci, since defaulting is much more useful in an interactive session than in modules. -- Roman I. Cheplyaka :: http://ro-che.info/

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GHC users on PowerPC and *BSD/Darwin?
by Erik de Castro Lopo 12 Jul '12

12 Jul '12

Hi all, Users of GHC on PowerPC would probably be aware of this bug: http://hackage.haskell.org/trac/ghc/ticket/2972 which causes GHCi to segfault as soon as it was started. Ben M. Collins has come up with a fix for this I am preparing it for submission to the GHC git tree. I have have tested it on linux-powerpc, but would also like to test it on *BSD/Darwin. Anybody out there interested? You will need: * A PowerPC machine running *BSD or Darwin. * A working GHC compiler. If you're interested email me at erikd AT mega-nerd dot com. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Haskell Weekly News: Issue 235
by Daniel Santa Cruz 12 Jul '12

12 Jul '12

Welcome to issue 235 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 1 to 7, 2012. Quotes of the Week * Kukkua: They say there are two things, once mastered, you will harness boundless strength from the cosmos itself, continuations and monads. * startling: saying "monad" within earshot of a muggle is pretty funny * maybefbi: Haskell is forcing me to think a lot * Wadler says: So, the essence of XML is this: the problem it solves is not hard, and it does not solve the problem well. Top Reddit Stories * This is what my first attempt at writing code in Haskell went like Domain: self.haskell, Score: 82, Comments: 32 On Reddit: [1] http://goo.gl/uq3Tt Original: [2] http://goo.gl/uq3Tt * The Architecture of Open Source Applications: The Glasgow Haskell Compiler Domain: aosabook.org, Score: 59, Comments: 5 On Reddit: [3] http://goo.gl/5v906 Original: [4] http://goo.gl/yYqUJ * Slides for 7 lectures on Parallel and Concurrent Haskell (including GPU programming with Accelerate) Domain: community.haskell.org, Score: 51, Comments: 10 On Reddit: [5] http://goo.gl/pmGsr Original: [6] http://goo.gl/wumiT * How pythoners view Haskell compared with Python Domain: wiki.python.org, Score: 40, Comments: 70 On Reddit: [7] http://goo.gl/Xe3AF Original: [8] http://goo.gl/ydpyj * How I made the switch to functional programming. Domain: self.haskell, Score: 38, Comments: 14 On Reddit: [9] http://goo.gl/m6IXs Original: [10] http://goo.gl/m6IXs * Why Functional Matters: Your white board will never be the same Domain: blog.jessitron.com, Score: 35, Comments: 5 On Reddit: [11] http://goo.gl/b71zi Original: [12] http://goo.gl/SkJaA * Building a better AES package Domain: tab.snarc.org, Score: 33, Comments: 19 On Reddit: [13] http://goo.gl/A0hwE Original: [14] http://goo.gl/0p9PE * FP Complete is hiring Haskell programmers Domain: haskellers.com, Score: 32, Comments: 4 On Reddit: [15] http://goo.gl/L8Xuh Original: [16] http://goo.gl/6AxT7 * Parallel and Concurrent Programming in Haskell (Lab exercises by Simon Marlow) Domain: community.haskell.org, Score: 31, Comments: On Reddit: [17] http://goo.gl/cjBJ0 Original: [18] http://goo.gl/olFva * pipes 2.1 released with prettier Frame, using indexed and restricted monads Domain: haskellforall.com, Score: 30, Comments: 31 On Reddit: [19] http://goo.gl/Trajz Original: [20] http://goo.gl/vqluc * conduit 0.5.0 released, now with extensive tutorial Domain: hackage.haskell.org, Score: 27, Comments: 2 On Reddit: [21] http://goo.gl/s5Wmh Original: [22] http://goo.gl/RUrpw * A (experimental) treat for Emacs interactive-mode users Domain: youtube.com, Score: 26, Comments: 2 On Reddit: [23] http://goo.gl/13wD9 Original: [24] http://goo.gl/C54V5 * Exploring CPU design using Haskell - Introduction to the architecture Domain: random.axman6.com, Score: 25, Comments: 17 On Reddit: [25] http://goo.gl/f1v2M Original: [26] http://goo.gl/xvSUF * Haskell is dead! (pun intended, not a flame :D) Domain: i.imgur.com, Score: 22, Comments: 37 On Reddit: [27] http://goo.gl/jPRu9 Original: [28] http://goo.gl/2DclX * Announcing lens-family-core, lens-family, and lens-family-th Domain: self.haskell, Score: 22, Comments: 8 On Reddit: [29] http://goo.gl/Axyxs Original: [30] http://goo.gl/Axyxs * Happstack Irregular News Issue #2 Domain: happstack.com, Score: 21, Comments: 3 On Reddit: [31] http://goo.gl/n7a0Z Original: [32] http://goo.gl/kU7OV * "Tying the Knot" using the RWS Monad Domain: mergeconflict.com, Score: 21, Comments: 16 On Reddit: [33] http://goo.gl/FavKj Original: [34] http://goo.gl/TESiO * LiveScript: a more functional JavaScript Domain: gkz.github.com, Score: 18, Comments: 9 On Reddit: [35] http://goo.gl/NQecG Original: [36] http://goo.gl/1hsTo Top StackOverflow Questions * Functional design patterns votes: 36, answers: 5 Read on SO: [37] http://goo.gl/hjzwm * Using return vs. not using return in the list monad votes: 22, answers: 6 Read on SO: [38] http://goo.gl/zkQH1 * Compute as much of a list as possible in a fixed time votes: 19, answers: 3 Read on SO: [39] http://goo.gl/BpBFe * Why is `logBase 10 x` slower than `log x / log 10`, even when specialized? votes: 16, answers: 2 Read on SO: [40] http://goo.gl/4rkTi * How has Haskell changed? votes: 16, answers: 2 Read on SO: [41] http://goo.gl/Rkz41 * What are the similarities and differences among Haskell indentation modes for Emacs? votes: 16, answers: 0 Read on SO: [42] http://goo.gl/xHn7L * Arrow equivalent of mapM? votes: 9, answers: 1 Read on SO: [43] http://goo.gl/XlQWR * Can I pass a pattern into a function? votes: 9, answers: 2 Read on SO: [44] http://goo.gl/J6N3w * Referential transparency and mmap in Haskell votes: 8, answers: 1 Read on SO: [45] http://goo.gl/iJGp5 * Is EC2 Ubuntu 12.04 different, if compiling Haskell locally? votes: 7, answers: 1 Read on SO: [46] http://goo.gl/Qivdq Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/w3k4v/this_is_what_my_first_attemp… 2. http://www.reddit.com/r/haskell/comments/w3k4v/this_is_what_my_first_attemp… 3. http://www.aosabook.org/en/ghc.html 4. http://www.reddit.com/r/haskell/comments/w2skx/the_architecture_of_open_sou… 5. http://community.haskell.org/~simonmar/slides/cadarache2012/ 6. http://www.reddit.com/r/haskell/comments/vyzj8/slides_for_7_lectures_on_par… 7. http://wiki.python.org/moin/PythonVsHaskell 8. http://www.reddit.com/r/haskell/comments/w2jiv/how_pythoners_view_haskell_c… 9. http://www.reddit.com/r/haskell/comments/w77xy/how_i_made_the_switch_to_fun… 10. http://www.reddit.com/r/haskell/comments/w77xy/how_i_made_the_switch_to_fun… 11. http://blog.jessitron.com/2012/06/why-functional-matters-your-white-board.h… 12. http://www.reddit.com/r/haskell/comments/w1453/why_functional_matters_your_… 13. http://tab.snarc.org/posts/haskell/2012-07-07-building-a-better-haskell-aes… 14. http://www.reddit.com/r/haskell/comments/w6btf/building_a_better_aes_packag… 15. http://www.haskellers.com/jobs/14 16. http://www.reddit.com/r/haskell/comments/w4xyc/fp_complete_is_hiring_haskel… 17. http://community.haskell.org/~simonmar/lab-exercises-cadarache.pdf 18. http://www.reddit.com/r/haskell/comments/vzpfy/parallel_and_concurrent_prog… 19. http://www.haskellforall.com/2012/07/pipes-21-and-index-core-10-indexed.html 20. http://www.reddit.com/r/haskell/comments/vx1dt/pipes_21_released_with_prett… 21. http://hackage.haskell.org/packages/archive/conduit/0.5.0/doc/html/Data-Con… 22. http://www.reddit.com/r/haskell/comments/vwge4/conduit_050_released_now_wit… 23. http://www.youtube.com/watch?v=pu9AGSOySlE 24. http://www.reddit.com/r/haskell/comments/w1k7s/a_experimental_treat_for_ema… 25. http://random.axman6.com/blog/?p=327 26. http://www.reddit.com/r/haskell/comments/w4aan/exploring_cpu_design_using_h… 27. http://i.imgur.com/zzQrx.png 28. http://www.reddit.com/r/haskell/comments/vvkkm/haskell_is_dead_pun_intended… 29. http://www.reddit.com/r/haskell/comments/w5ine/announcing_lensfamilycore_le… 30. http://www.reddit.com/r/haskell/comments/w5ine/announcing_lensfamilycore_le… 31. http://www.happstack.com/C/ViewPage/12 32. http://www.reddit.com/r/haskell/comments/vxi3c/happstack_irregular_news_iss… 33. http://mergeconflict.com/archives/57 34. http://www.reddit.com/r/haskell/comments/w0haf/tying_the_knot_using_the_rws… 35. http://gkz.github.com/LiveScript/ 36. http://www.reddit.com/r/haskell/comments/w39a3/livescript_a_more_functional… 37. http://stackoverflow.com/questions/11339637/functional-design-patterns 38. http://stackoverflow.com/questions/11323300/using-return-vs-not-using-retur… 39. http://stackoverflow.com/questions/11320180/compute-as-much-of-a-list-as-po… 40. http://stackoverflow.com/questions/11290929/why-is-logbase-10-x-slower-than… 41. http://stackoverflow.com/questions/11296157/how-has-haskell-changed 42. http://stackoverflow.com/questions/11365815/what-are-the-similarities-and-d… 43. http://stackoverflow.com/questions/11303642/arrow-equivalent-of-mapm 44. http://stackoverflow.com/questions/11336687/can-i-pass-a-pattern-into-a-fun… 45. http://stackoverflow.com/questions/11283975/referential-transparency-and-mm… 46. http://stackoverflow.com/questions/11278733/is-ec2-ubuntu-12-04-different-i…

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wondering about a MonadIO instance for a heap data type
by Qi Qi 12 Jul '12

12 Jul '12

Hi, I was wondering about creating an instance of MonadIO for a heap data. Any hints? data Heap a = E | T Int a (Heap a) (Heap a) deriving (Eq, Ord, Read, Show) The reason is that I want to use liftIO during a heapsort to print out intermediate results. Thanks. Qi Qi

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There is some article mentioning that FP is mainly good for mathematical and list programming.
by KC 11 Jul '12

11 Jul '12

There is some article mentioning that functional programming is mainly good for mathematical programming and list programming because otherwise it is to cryptic for business and database applications. Being semi-intelligent, I thought I could reduce what others think of as cryptic but maybe instead of 18 wheels I have only 17 wheels. :D -- -- Regards, KC

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DPH matrix product
by mblanco 11 Jul '12

11 Jul '12

Hi, I'm trying to implement a matrix product example using DPH. This is the code: ------------------------------------------------------------------------------------------------------------------- type MMultType = Double type Matrix = [:[:MMultType:]:] type MVector = [:MMultType:] type Matrix_wrapper = PArray (PArray MMultType) {-# NOINLINE matMult_wrapper #-} matMult_wrapper :: Matrix_wrapper -> Matrix_wrapper -> Matrix_wrapper matMult_wrapper mA mB = toPArrayP (mapP toPArrayP (matMult (fromNestedPArrayP mA) (fromNestedPArrayP mB))) matMult :: Matrix -> Matrix -> Matrix matMult mA mB = mapP (\row -> mapP (\col -> dotp row col) (transposeP mB)) mA dotp :: MVector -> MVector -> MMultType dotp row col = D.sumP (zipWithP (D.*) row col) transposeP :: Matrix -> Matrix transposeP m = let h = lengthP m w = lengthP (m !: 0) rh = I.enumFromToP 0 (h I.- 1) rw = I.enumFromToP 0 (w I.- 1) in if h I.== 0 then [: :] else mapP (\y -> mapP (\x -> m !: x !: y) rh) rw ------------------------------------------------------------------------------------------------------------------- My problem is at execution time, on matrices of size 300*300 the program does finish (although it is very slow), but on 700*700 it consumes GBs of RAM until the process is aborted. In the paper "Work Efficient Higher-Order Vectorisation" it is explained that a work complexity problem (wich involved unnecesary array replication) was recently treated. So at first I thought the code implementation related to the paper had not been uploaded to hackage. But as I understand it must have been, as that seems to be the motive of the "dph-lifted-vseg" package. Does anybody notice the problem with the example or if the problem is related to the subject treated in the paper? Thanks in advance!

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ANN: unification-fd 0.8.0
by wren ng thornton 11 Jul '12

11 Jul '12

-------------------------------------------- -- unification-fd 0.8.0 -------------------------------------------- The unification-fd package offers generic functions for single-sorted first-order structural unification (think of programming in Prolog, or of the metavariables in type inference)[1][2]. The library *is* sufficient for implementing higher-rank type systems a la [Peyton Jones, Vytiniotis, Weirich, Shields], but bear in mind that unification variables are the metavariables of type inference--- not the type-variables. As of this version, the library is also sufficient for implementing (non-recursive) feature structure unification. An effort has been made to make the package as portable as possible. However, because it uses the ST monad and the mtl-2 package it can't be H98 nor H2010. However, it only uses the following common extensions which should be well supported[3]: Rank2Types MultiParamTypeClasses FunctionalDependencies -- Alas, necessary for type inference FlexibleContexts -- Necessary for practical use of MPTCs FlexibleInstances -- Necessary for practical use of MPTCs UndecidableInstances -- For Show instances due to two-level types -------------------------------------------- -- Changes (since 0.7.0) -------------------------------------------- This release is another API breaking release, though hopefully minor. In particular, the type of the zipMatch method for Unifiable has changed from: zipMatch :: t a -> t b -> Maybe (t (a, b)) to: zipMatch :: t a -> t a -> Maybe (t (Either a (a, a))) With the new type each of the unification subproblems can be declared as fully resolved (Left xy) or as still pending solution (Right (x,y))--- whereas the previous type only allowed the latter. This extension is necessary for implementing unification of feature structures (e.g., t ~ Map k). Given two feature structures which do not fully overlap, we can now declare that the non-intersecting parts have been unified and only require further unification for the parts which intersect--- whereas previously we would've been forced to declare the non-intersecting parts as requiring unification with themselves, which will always succeed and introduces extra work. This feature is one I had available in earlier unpublished versions of the code, but errantly removed it when simplifying things for publishing on Hackage. To fix extant Unifiable instances, just wrap each tuple with Right. For those who care about such things, I have not been able to discern any noticeable difference in performance between the two versions. If you can provide benchmarks demonstrating otherwise, I'd be pleased to look at them. In addition, the Show instance for Fix has been adjusted to use showsPrec in lieu of show, correcting some infelicities with the output. -------------------------------------------- -- Description -------------------------------------------- The unification API is generic in the type of the structures being unified and in the implementation of unification variables, following the two-level types pearl of Sheard (2001). This style mixes well with Swierstra (2008), though an implementation of the latter is not included in this package. That is, all you have to do is define the functor whose fixed-point is the recursive type you're interested in: -- The non-recursive structure of terms data S a = ... -- The recursive term type type Term = Fix S And then provide an instance for Unifiable, where zipMatch performs one level of equality testing for terms and returns the one-level spine filled with pairs of subterms to be recursively checked, or Nothing if this level doesn't match. Each subterm can be separately marked as being resolved, Left xy, or as requiring further unification, Right(x,y). class (Traversable t) => Unifiable t where zipMatch :: t a -> t a -> Maybe (t (Either a (a, a))) The choice of which variable implementation to use is defined by similarly simple classes Variable and BindingMonad. We store the variable bindings in a monad, for obvious reasons. In case it's not obvious, see Dijkstra et al. (2008) for benchmarks demonstrating the cost of naively applying bindings eagerly. There are currently two implementations of variables provided: one based on STRefs, and another based on a state monad carrying an IntMap. The former has the benefit of O(1) access time, but the latter is plenty fast and has the benefit of supporting backtracking. Backtracking itself is provided by the logict package and is described in Kiselyov et al. (2005). In addition to this modularity, unification-fd implements a number of optimizations over the algorithm presented in Sheard (2001)--- which is also the algorithm presented in Cardelli (1987). * Their implementation uses path compression, which we retain. Though we modify the compression algorithm in order to make sharing observable. * In addition, we perform aggressive opportunistic observable sharing, a potentially novel method of introducing even more sharing than is provided by the monadic bindings. Basically, we make it so that we can use the observable sharing provided by the modified path compression as much as possible (without introducing any new variables). * And we remove the notoriously expensive occurs-check, replacing it with visited-sets (which detect cyclic terms more lazily and without the asymptotic overhead of the occurs-check). A variant of unification which retains the occurs-check is also provided, in case you really need to fail fast. * Finally, a highly experimental branch of the API performs *weighted* path compression, which is asymptotically optimal. Unfortunately, the current implementation is quite a bit uglier than the unweighted version, and I haven't had a chance to perform benchmarks to see how the constant factors compare. Hence moving it to an experimental branch. These optimizations pass a test suite for detecting obvious errors. If you find any bugs, do be sure to let me know. Also, if you happen to have a test suite or benchmark suite for unification on hand, I'd love to get a copy. -------------------------------------------- -- Notes and limitations -------------------------------------------- [1] At present the library does not appear amenable for implementing higher-rank unification itself; i.e., for higher-ranked metavariables, or higher-ranked logic programming. To be fully general we'd have to abstract over which structural positions are co/contravariant, whether the unification variables should be predicative or impredicative, as well as the isomorphisms of moving quantifiers around. It's on my todo list, but it's certainly non-trivial. If you have any suggestions, feel free to contact me. [2] At present it is only suitable for single-sorted (aka untyped) unification, a la Prolog. In the future I aim to support multi-sorted (aka typed) unification, however doing so is complicated by the fact that it can lead to the loss of MGUs; so it will likely be offered as an alternative to the single-sorted variant, similar to how the weighted path-compression is currently offered as an alternative. [3] With the exception of fundeps which are notoriously difficult to implement. However, they are supported by Hugs and GHC 6.6, so I don't feel bad about requiring them. Once the API stabilizes a bit more I plan to release a unification-tf package which uses type families instead, for those who feel type families are easier to implement or use. There have been a couple requests for unification-tf, so I've bumped it up on my todo list. -------------------------------------------- -- References -------------------------------------------- Luca Cardelli (1987) /Basic polymorphic typechecking/. Science of Computer Programming, 8(2):147--172. Atze Dijkstra, Arie Middelkoop, S. Doaitse Swierstra (2008) /Efficient Functional Unification and Substitution/, Technical Report UU-CS-2008-027, Utrecht University. <http://www.cs.uu.nl/research/techreps/repo/CS-2008/2008-027.pdf> Simon Peyton Jones, Dimitrios Vytiniotis, Stephanie Weirich, Mark Shields /Practical type inference for arbitrary-rank types/, to appear in the Journal of Functional Programming. (Draft of 31 July 2007.) Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, and Amr Sabry (2005) /Backtracking, Interleaving, and/ /Terminating Monad Transformers/, ICFP. <http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf> Tim Sheard (2001) /Generic Unification via Two-Level Types/ /and Paramterized Modules/, Functional Pearl, ICFP. <http://web.cecs.pdx.edu/~sheard/papers/generic.ps> Tim Sheard & Emir Pasalic (2004) /Two-Level Types and/ /Parameterized Modules/. JFP 14(5): 547--587. This is an expanded version of Sheard (2001) with new examples. <http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps> Wouter Swierstra (2008) /Data types a la carte/, Functional Pearl. JFP 18: 423--436. <http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unification-fd Darcs: http://community.haskell.org/~wren/unification-fd Haddock (Darcs version): http://community.haskell.org/~wren/unification-fd/dist/doc/html/unification… -- Live well, ~wren

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Any good tool to write Haskell documents including tests?
by Takayuki Muranushi 10 Jul '12

10 Jul '12

Hello, I have been a forgetful person, and lots of things I have only pretended to understand. I want to change this. So, to educate myself, I'd like to write documented tests for many libraries I meet, and also publish them onto the web so that others may find them useful or find mistakes for me. OK, blog articles are good, but they have no (forced) tests. Maybe some of you have practiced this or developping such tools. I see some candidate tools, too. What is your suggestion for this? I have tried doctest, because of its read–eval–print loop (REPL) style I liked. https://github.com/nushio3/practice/tree/master/control-monad-loop It produces html as attached to this mail. It's pretty, but I'd like to have more control on HTML. Maybe Gitit + Doctest in Pandoc is a good alternative? I'd also like to know what is a good way to publish a small executable examples with automated dependency install capability. Can you point out any problems with following Makefile + cabal ? What are better ways to do this? https://github.com/nushio3/practice/blob/master/control-monad-loop/Loop.cab… https://github.com/nushio3/practice/blob/master/control-monad-loop/Makefile Thanks in advance, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Memory corruption issues when using newAlignedPinnedByteArray, GC kicking in?
by Nicolas Trangez 10 Jul '12

10 Jul '12

All, While working on my vector-simd library, I noticed somehow memory I'm using gets corrupted/overwritten. I reworked this into a test case, and would love to get some help on how to fix this. Previously I used some custom FFI calls to C to allocate aligned memory, which yields correct results, but this has a significant (+- 10x) performance impact on my benchmarks. Later on I discovered the newAlignedPinnedByteArray# function, and wrote some code using this. Here's what I did in the test case: I created an MVector instance, with the exact same implementation as vector's Data.Vector.Storable.Mutable.MVector instance, except for basicUnsafeNew where I pass one more argument to mallocVector [1]. I also use 3 different versions of mallocVector (depending on compile-time flags): mallocVectorOrig [2]: This is the upstream version, discarding the integer argument I added. Then here's my first attempt, very similar to the implementation of mallocPlainForeignPtrBytes [3] at [4] using GHC.* libraries. Finally there's something similar at [5] which uses the 'primitive' library. The test case creates vectors of increasing size, then checks whether they contain the expected values. For the default implementation this works correctly. For both others it fails at some random size, and the values stored in the vector are not exactly what they should be. I don't understand what's going on here. I suspect I lack a reference (or something along those lines) so GC kicks in, or maybe the buffer gets relocated, whilst it shouldn't. Basically I'd need something like GHC.ForeignPtr.mallocPlainAlignedForeignPtrBytes :: Int -> Int -> IO (ForeignPtr a) Thanks, Nicolas [1] https://gist.github.com/3084806#LC37 [2] https://gist.github.com/3084806#LC119 [3] http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/GHC-Fo… [4] https://gist.github.com/3084806#LC100 [5] https://gist.github.com/3084806#LC81

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hiring haskell programmer.
by Vagif Verdi 10 Jul '12

10 Jul '12

Hello. My company is looking to hire a haskell developer. South California (San Dimas), full time job, local only (no telecommute) We use yesod (haskell web framework) for internal web application and web services, and compojure (clojure web framework) for customer facing web site. All development will be ether with haskell (if libraries permit) or clojure on JVM. If interested please contact me. Regards, Vagif Verdi

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Connecting Travis CI and hackage
by Dmitry Malikov 10 Jul '12

10 Jul '12

*tldr*: is there a way to upload package to hackage from Travis automatically and safely (hiding password to hackage account)? Several days ago I discovered Travis ( http://travis-ci.org/ ) - nice open source build-service, easy to start and monitoring. I've configured some github projects and add build status to it's readme. It's usable to know how is your haskell package building on separate fresh machine. Following git-flow developing model there is a develop branch for snapshots and master branch for release. That why every commit in master branch should produce new release build ⇒ every commit in master branch should produce new hackage build upload. And also it will be nice to have that routine process done automatically. It could be done using cabal-upload tool and --username and --password options. But it means that .travis.yml file should contain my hackage-password explicitly. How can I avoid that? -- Best regards, dmitry malikov !

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Combining Network Descriptions in Reactive.Banana
by Alexander Foremny 10 Jul '12

10 Jul '12

Hello list, I am currently experimenting with Reactive.Banana and I am trying to write a program that simply displays a String which is supplied by the user. Thus in Main.hs I'd like to create a Behavior t String from some arbitrary sources and pass this to a library function performing the printing. However, I am not able to get the desired behavior of my program. A sample Main.hs is available as Gist [1]. The cause of error appears to be that the type t over which NetworkDescription and Behavior are parameterized are distinct! I tried several variations of the code in [1] using ExistentialQuantification over t and/or giving explicit type signatures. The error is slightly different in each case, but always boils down to the same reason. In case explicit error messages are required I can post them to the list. Any insight into this would be of great help. And additionally I'd be interested if this kind of program is possible with Reactive.Banana at all. [1] https://gist.github.com/3004430

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Linking against Sqlite3 on Windows problem
by Eugene Dzhurinsky 10 Jul '12

10 Jul '12

Hi all! I created simple application, which uses sqlite3 as it's datastore back-end. I faced no problems when building and running it on Linux, but after I tried to build it on Windows, I see weird linking error: Linking dist\build\hnotes\hnotes.exe ... C:\Documents and Settings\Admin\Application Data\cabal\sqlite-0.5.2.2\ghc-7.0.4/libHSsqlite-0.5.2.2. a(sqlite3-local.o):sqlite3-local.c:(.text+0x21): undefined reference to `sqlite3_temp_directory' C:\Documents and Settings\Admin\Application Data\cabal\sqlite-0.5.2.2\ghc-7.0.4/libHSsqlite-0.5.2.2. a(sqlite3-local.o):sqlite3-local.c:(.text+0x40): undefined reference to `sqlite3_temp_directory' collect2: v ld 1 cabal.EXE: Error: some packages failed to install: hnotes-0.1 failed during the building phase. The exception was: ExitFailure 1 What may be wrong there? I suspect that qalite3.dll has to be added to linking stage, but have no idea how to do that. Adding --extra-lib-dirs=path-to-sqlite-dll doesn't help either (perhaps because I need to update my cabal file somehow, to support this?). Thank you for the help! -- Eugene N Dzhurinsky

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ANNOUNCE: control-monad-loop 0.1
by Joey Adams 09 Jul '12

09 Jul '12

This package provides imperative-style loops supporting "continue" and "break". For example: import Control.Monad import Control.Monad.IO.Class import Control.Monad.Trans.Loop import Control.Monad.Trans.Class main :: IO () main = foreach [0..] $ \i -> foreach [0..] $ \j -> do when (j == 0) $ continue -- skip to next iteration when (j >= 5) $ exit -- exit the loop when (i >= 5) $ lift exit -- exit the outer loop by calling 'exit' in the parent monad liftIO $ print (i, j) It works by having the loop body run under the LoopT monad transformer, which provides early exit functions 'continue' and 'exit'. Functions like 'foreach' and 'while' run a LoopT callback, passing it continuations defining 'continue' and 'exit'. http://hackage.haskell.org/package/control-monad-loop

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ANNOUNCE: Sifflet visual programming language, release 2.0.0.0
by gdweber＠iue.edu 09 Jul '12

09 Jul '12

I don't usually get this excited about a release, but after nearly a year of not being able to do any work on Sifflet, I am now *Extremely Happy* to announce -- Sifflet and sifflet-lib 2.0.0.0, now available on Hackage! This version introduces a type checker and partial support for higher order functions in Sifflet, the visual, functional programming language and support system for students learning about recursion. Sifflet programmers define functions by drawing diagrams, and the Sifflet interpreter uses the diagrams to show how function calls are evaluated. Sifflet-lib is the library containing many modules of the Sifflet application. What's New ---------- July 5, 2012, Version 2.0.0.0: * Partial support for higher order functions, like map and filter. See Lesson 10 in the Sifflet Tutorial. * Sifflet now provides type checking and type inference. This should make it possible to provide exporters to languages, like Java, that require type declarations. (Unfortunately, the error messages for incorrectly typed functions are not yet friendly for novice programmers.) * Added a menu command (File / Save image ...) to save vector graphic images of functions in the Sifflet Workspace or Edit function windows. Images can be saved in SVG, Postscript, and PDF formats. * Reorganized hierarchical modules in the library to conform to the recommended practice. See the RELEASE-NOTES for details. * Several bugs are fixed, including one which crashed Sifflet when applying a function definition with an incomplete `if` tree. See ISSUES for details. * There is a new file format for saving function definitions to support higher-order function types. The new format, siffml 2.0, has a RELAX NG schema; siffml 1.0 files can still be opened in Sifflet 2.0. About Sifflet ------------- Sifflet is a visual, functional programming language intended as an aid for learning about recursion. * A picture explains Sifflet better than words: please see the screenshot showing how to evaluate 3!: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Features: - Visual editor. - Visual tracer/debugger which shows how recursive and other function calls are evaluated. To support active learning and avoid screen clutter, Sifflet displays only as much of the computation as the user requests. - Carefully crafted tutorial with 44 pictures, about 26 pages if printed. - Number, string, and list data types. - Small collection of primitive functions. - Runnable examples of compound functions. - Sifflet functions can be exported to Scheme, Python 3, and Haskell. References ---------- * Download: http://hackage.haskell.org/package/sifflet-lib http://hackage.haskell.org/package/sifflet * Home page: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Sifflet Tutorial: http://mypage.iu.edu/~gdweber/software/sifflet/doc/tutorial.html * RELEASE-NOTES: http://mypage.iu.edu/~gdweber/software/sifflet/RELEASE-NOTES * ISSUES: http://mypage.iu.edu/~gdweber/software/sifflet/ISSUES ------------------------------------------------------------------------ -- Gregory D. Weber, Ph. D. : Associate Professor of Informatics / \ Indiana University East 0 : Tel. (765) 973-8420; FAX (765) 973-8550 / \ http://mypage.iu.edu/~gdweber/ 1 []

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combining predicates, noob question
by Sebastián Krynski 09 Jul '12

09 Jul '12

As I was using predicates (a -> bool) , it appeared the need for combining them with a boolean operator (bool -> bool -> bool) in order to get a new predicate combining the previous two. So I wrote my function combinerPred (see code below). While I think this is JUST ok, i'm feeling a monad in the air. So.. where is the monad? combinerPred :: (a -> Bool) -> (a -> Bool) -> (Bool -> Bool -> Bool) -> (a -> Bool) combinerPred pred1 pred2 op = \x -> op (pred1 x) (pred2 x) thanks Sebastián

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Best way to build a GHC backend?
by Jonathan Geddes 09 Jul '12

09 Jul '12

Venerable Haskell Hackers, I love Haskell and think it should run everywhere. Now supposing I would like to build another backend for GHC, perhaps for Java Bytecode, .Net CIL, or JavaScript, What would be the best way to approach that? I can think of a few options: 1. Produce External Core with -fext-core and compile that with a completely separate compiler 2. Use the GHC apis to build a compiler that reuses a load of GHC's code, but has it's own backend 3. Add a new backend directly into GHC Any other options? While I'm on the subject, why has Haskell not been ported to the likes of the JVM, .NET CLR, or JavaScript? Are Haskell's non-strict semantics just too different from the semantics of these other platforms? SPJ is known for saying that Haskell's plan for world domination is support for many parallelism/concurrency idioms. I believe running on many platforms is just as important. From my point of view, languages that cannot run on one of the 3 aforementioned platforms will become irrelevant. (with the exception of C, of course). Thoughts? --J Arthur

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Haskell Implementors' Workshop talk proposals due in one week!
by Johan Tibell 09 Jul '12

09 Jul '12

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ Important dates Proposal Deadline: 10th July 2012 Notification: 27th July 2012 Workshop: 14th September 2012 The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2012. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to: johan.tibell(a)gmail.com We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Lennart Augustsson (Standard Chartered Bank) * Manuel M T Chakravarty (University of New South Wales) * Gregory Collins - co-chair (Google) * Simon Marlow (Microsoft Research) * David Terei (Stanford University) * Johan Tibell - co-chair (Google)

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Re: [Haskell-cafe] sample terms and interpreting program output from Tc Monad
by Andres Löh 09 Jul '12

09 Jul '12

[Sorry, I forgot to reply to the list.] Hi. > I understand these to be Rank 0 terms: > > (\(x::Int) . x) (0 :: Int) :: (forall. Int) -- value > > (\(x::Int). x) :: (forall. Int -> Int) Yes. > (\(x::a). x) :: (forall. a -> a) > > Although the program prints forall, the absence of a type variable > indicates Rank 0, correct? It's a bit unclear what you mean, and the prototype implementation you seem to be using deviates from Haskell conventions here. The prototype checker sees "a" as a type constant here, so it plays the same role as "Int" before. In particular, "a" isn't a type variable, and there's no quantification. > I understand these to be Rank 1 terms: > > (\x. x) :: (forall a. a -> a) -- This is not the same as the third > example above, right? This one identifies the type variable a, the one > above does not. Also, there's no explicit annotation, it's inferred. The implementation indeed considers them to be different. Before "a" was a concrete type, and the function was the monomorphic identity function on that type. Now, "a" is a type variable, and the function is the polymorphic identity function. [...] > I understand these to be Rank 2 terms: > > (\(x::(forall a. a)). 0) :: (forall. (forall a. a) -> Int) > > The explicit forall annotation on the bound and binding variable x > causes the program to infer a Rank 2 polytype as indicated by the "-> > Int" following the (forall a. a), while noting the absence of a type > variable following the left-most forall printed by the program, correct? Your description here is a bit strange. It's a rank-2 type because there's a rank-1 type occurring to the left of the function arrow. > > (\(x::(forall a. a -> a)). x) :: (forall b. (forall a. a -> a) -> b -> > b) > > Also Rank 2, only one arrow to the right of (forall a. a -> a) counts. It's rank 2, yes. I'm not sure what you mean here by saying that only one arrow counts. > The universal quantifier on type variable b ranges over the type > variable a, correct? No. The universal quantifier ranges over all (mono)types. > I understand this to be a Rank 3 term: > > (\(f::(forall a. a -> a)). \(x::(forall b. b)). f (f x)) :: (forall c. > (forall a. a -> a) -> (forall b. b) -> c) No, this is still rank 2. It uses two rank 1 types as function arguments. For it to be rank 3, it'd have to use a rank 2 type in the argument position of a function type. Note that the function arrow associates to the right: forall c. (forall a. a -> a) -> ((forall b. b) -> c) -- rank 2 (forall c. ((forall a. a -> a) -> (forall b. b)) -> c) -- rank 3 > The arrows to the right of the universally quantified a and b > expressions qualify this as Rank 3. No, you seem to be applying a wrong definition of rank. The correct definition is given in Section 3.1 of the paper. Here's how you can derive the type above as a sigma_2: sigma_2 ~> forall c. sigma_2 ~> forall c. sigma_1 -> sigma_2 ~> forall c. sigma_1 -> sigma_1 -> sigma_2 ~> forall c. sigma_1 -> sigma_1 -> sigma_1 ~> forall c. sigma_1 -> sigma_1 -> sigma_0 ~> forall c. sigma_1 -> sigma_1 -> c ~> forall c. (forall a. sigma_1) -> sigma_1 -> c ~> forall c. (forall a. sigma_0) -> sigma_1 -> c ~> forall c. (forall a. tau -> tau) -> sigma_1 -> c ~> forall c. (forall a. a -> a) -> sigma_1 -> c ~> forall c. (forall a. a -> a) -> (forall b. sigma_1) -> c ~> forall c. (forall a. a -> a) -> (forall b. sigma_0) -> c ~> forall c. (forall a. a -> a) -> (forall b. b) -> c > Type variable c ranges over type > variables a and b, correct? No. Each of the type variables ranges over all (mono)types. (Or do you mean what the scope of "c" is? If so, then the scope of c is the complete type signature.) Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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Interest in typed relational algebra library?
by Paul Visschers 09 Jul '12

09 Jul '12

Hello, I've been out of the Haskell game for a bit, but now I'm back. A couple of years ago I made a small library that implements relational algebra with types so that malformed queries and other operations are caught at compile time. It is heavily based off of the internals of HaskellDB (see http://hackage.haskell.org/packages/archive/haskelldb/2.1.1/doc/html/Databa…) but types so that it can actually be used directly instead of having to use HaskellDB's query monad. Besides the joy of using relational algebra directly in your code, this also means that you can make query-optimizing code in a type-safe way, you can subquery results returned by the database directly without accessing the database again and you have more options when converting from relation algebra to SQL or another query language. The library isn't quite ready for release, but I might want to work on it a bit and then release it. Is anyone interested in such a library? Paul Visschers

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Haskell User Group Hamburg (Germany) - Meeting Thursday July 12th 2012
by Stefan Schmidt 08 Jul '12

08 Jul '12

Hello cafe, there's an upcomming meeting of the Haskell user group Hamburg next Thursday (July 12th). We'll start 19:00 at the Xing AG in Hamburg (Dammtorstraße 35, 20354 Hamburg ). Jan Ahrens will give a short talk about web development with Yesod ( http://www.yesodweb.com/) Although this announcement is written in english the talk will be given in german. If you are interested to join us, have a look at our doodle (also in german) http://www.doodle.com/5ekivcacuam9czxs Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh.

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vector-simd: some code available, and some questions
by Nicolas Trangez 08 Jul '12

08 Jul '12

All, After my message of yesterday [1] I got down to it and implemented something along those lines. I created a playground repository containing the code at [2]. Initial benchmark results at [3]. More about the benchmark at the end of this email. First some questions and requests for help: - I'm stuck with a typing issue related to 'sizeOf' calculation at [4]. I tried a couple of things, but wasn't able to figure out how to fix it. - I'm using unsafePerformIO at [5], yet I'm not certain it's OK to do so. Are there better (safer/performant/...) ways to get this working? - Currently Alignment phantom types (e.g. A8 and A16) are not related to each other: a function (like Data.Vector.SIMD.Algorithms.unsafeXorSSE42) can have this signature: unsafeXorSSE42 :: Storable a => SV.Vector SV.A16 a -> SV.Vector SV.A16 a -> SV.Vector SV.A16 a Yet, imaging I'd have an "SV.Vector SV.A32 Word8" vector at hand, the function should accept it as well (a 32-byte aligned vector is also 16-byte aligned). Is there any way to encode this at the type level? That's about it :-) As of now, I only implemented a couple of the vector API functions (the ones required to execute my benchmark). Adding the others should be trivial. The benchmark works with Data.Vector.{Unboxed|Storable}.Vector (UV and SV) vectors of Word8 values, as well as my custom Data.Vector.SIMD.Vector type (MV) using 16-byte alignment (MV.Vector MV.A16 Word8). benchUV, benchSV and benchMV all take 2 pre-calculated Word8 vectors of given size (1024 and 4096) and xor them pairwise into the result using "zipWith xor". benchMVA takes 2 suitable MV vectors and xor's them into a third using a rather simple and unoptimized C implementation using SSE4.2 intrinsics [6]. This could be enhanced quite a bit (I guess using the prim calling convention, FFI overhead can be reduced as well). Currently, only vectors of a multiple of 32 bytes are supported (mostly because of laziness on my part). As you can see, the zipWith Data.Vector.SIMD implementation is slightly slower than the Data.Vector.Storable based one. I didn't perform much profiling yet, but I suspect allocation and ForeignPtr creation is to blame, this seems to be highly optimized in GHC.ForeignPtr.mallocPlainForeignPtrBytes as used by Data.Vector.Storable. Thanks for any input, Nicolas [1] http://www.haskell.org/pipermail/haskell-cafe/2012-July/102167.html [2] https://github.com/NicolasT/vector-simd/ [3] http://linode2.nicolast.be/files/vector-simd-xor1.html [4] https://github.com/NicolasT/vector-simd/blob/master/src/Data/Vector/SIMD/Al… [5] https://github.com/NicolasT/vector-simd/blob/master/src/Data/Vector/SIMD/Al… [6] https://github.com/NicolasT/vector-simd/blob/master/cbits/vector-simd.c#L47

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Which mailing list for GHC hacking discussion?
by Erik de Castro Lopo 08 Jul '12

08 Jul '12

Hi all, For GHC development questions (ie hacking on GHC, not with GHC) which is the appropriate mailing list ghc-cvs or ghc-users? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

1 0

sample terms and interpreting program output from Tc Monad
by rickmurphy 07 Jul '12

07 Jul '12

Hi All: I'm still working through the following paper [1] and I wondered whether you could help me confirm my understanding of some sample terms and program output from the Tc Monad. For those interested, the language is specified in Parser.lhs available in the protoype here [2]. I understand these to be Rank 0 terms: (\(x::Int) . x) (0 :: Int) :: (forall. Int) -- value (\(x::Int). x) :: (forall. Int -> Int) (\(x::a). x) :: (forall. a -> a) Although the program prints forall, the absence of a type variable indicates Rank 0, correct? I understand these to be Rank 1 terms: (\x. x) :: (forall a. a -> a) -- This is not the same as the third example above, right? This one identifies the type variable a, the one above does not. Also, there's no explicit annotation, it's inferred. (\x. \y. x) :: (forall a b. b -> a -> b) -- Still rank 1. Although there's no explicit annotation, the program infers the type variables and prints the forall and the appropriate type variables for the Rank 1 polytypes. I understand these to be Rank 2 terms: (\(x::(forall a. a)). 0) :: (forall. (forall a. a) -> Int) The explicit forall annotation on the bound and binding variable x causes the program to infer a Rank 2 polytype as indicated by the "-> Int" following the (forall a. a), while noting the absence of a type variable following the left-most forall printed by the program, correct? (\(x::(forall a. a -> a)). x) :: (forall b. (forall a. a -> a) -> b -> b) Also Rank 2, only one arrow to the right of (forall a. a -> a) counts. The universal quantifier on type variable b ranges over the type variable a, correct? I understand this to be a Rank 3 term: (\(f::(forall a. a -> a)). \(x::(forall b. b)). f (f x)) :: (forall c. (forall a. a -> a) -> (forall b. b) -> c) The arrows to the right of the universally quantified a and b expressions qualify this as Rank 3. Type variable c ranges over type variables a and b, correct? Thanks for your help in better understanding this information. I'm home schooling myself on Haskell and community support is a big help. 1. Practical Type Inference for Arbitrary-Rank Types. 2.http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-rank/ -- Rick

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Odd HDBC connection bug
by William Shackleton 07 Jul '12

07 Jul '12

Hi I'm having issues with HDBC when connecting to a remote MySQL server - certain queries cause the DB connection to be lost. The following program demonstrates this: import Database.HDBC import Database.HDBC.ODBC main = do conn <- connectODBC "DSN=owlro" putStrLn "Connected" quickQuery' conn "SELECT time, power FROM Power LIMIT 100;" [] putStrLn "Finished query 1" quickQuery' conn "SELECT time, power FROM Power ORDER BY time;" [] putStrLn "Finished query 2" disconnect conn The DSN points to a remote 32-bit Ubuntu 12.04 Server. The connection and queries work using mysql and isql on the command line, and the table in question contains about 1.3 million rows and only the columns id, time, power. When I compile and run this on a 64-bit Ubuntu 12.04 PC using HDBC-2.3.1.1 and HDBC-odbc-2.3.1.0 using GHC, I get the following (unwanted) output: $ ghc Test.hs $ ./Test Connected Finished query 1 Test: SqlError {seState = "[\"08S01\"]", seNativeError = -1, seErrorMsg = "execute execute: [\"2013: [MySQL][ODBC 5.1 Driver][mysqld-5.5.24-0ubuntu0.12.04.1]Lost connection to MySQL server during query\"]"} This program finishes (and fails) in less than half a second. When I run it using runhaskell however, the slow second query completes: $ runhaskell Test.hs Connected Finished query 1 Finished query 2 I get the same result from ghci; this method takes about 10 seconds as expected. Basically, simple (short) queries complete, yet long ones crash the connection. I also noticed similar results with HDBC-mysql. Any ideas on what is causing this? Thanks, Will Shackleton digitalsquid.co.uk

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ANNOUNCE: lens-family-th 0.1.0.0
by Dan Burton 07 Jul '12

07 Jul '12

Following the announcement of lens-family, I'm pleased to announce lens-family-th 0.1.0.0, a Template Haskell library supplying macros to generate lens-family lenses for fields of data types declared with record syntax. Be warned that currently, type signatures are *not* generated alongside the lens definitions. Type inference should correctly determine the type of the generated lenses, but I have structured the library code so that in the future, type signatures can also be generated. Patches welcome! http://hackage.haskell.org/package/lens-family-th -- Dan Burton

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Re: [Haskell-cafe] translation between two flavors of lexically-scoped type variables
by oleg＠okmij.org 07 Jul '12

07 Jul '12

Kangyuan Niu wrote: > Aren't both Haskell and SML translatable into System F, from which > type-lambda is directly taken? The fact that both Haskell and SML are translatable to System F does not imply that Haskell and SML are just as expressive as System F. Although SML (and now OCaml) does have what looks like a type-lambda, the occurrences of that type lambda are greatly restricted. It may only come at the beginning of a polymorphic definition (it cannot occur in an argument, for example). > data Ap = forall a. Ap [a] ([a] -> Int) > Why isn't it possible to write something like: > > fun 'a revap (Ap (xs : 'a list) f) = f ys > where > ys :: 'a list > ys = reverse xs > > in SML? This looks like a polymorphic function: an expression of the form /\a.<body> has the type forall a. <type>. However, the Haskell function > revap :: Ap -> Int > revap (Ap (xs :: [a]) f) = f ys > where > ys :: [a] > ys = reverse xs you meant to emulate is not polymorphic. Both Ap and Int are concrete types. Therefore, your SML code cannot correspond to the Haskell code. That does not mean we can't use SML-style type variables (which must be forall-bound) with existentials. We have to write the elimination principle for existentials explicitly {-# LANGUAGE ExistentialQuantification, RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} data Ap = forall a. Ap [a] ([a] -> Int) -- Elimination principle deconAp :: Ap -> (forall a. [a] -> ([a] -> Int) -> w) -> w deconAp (Ap xs f) k = k xs f revap :: Ap -> Int revap ap = deconAp ap revap' revap' :: forall a. [a] -> ([a] -> Int) -> Int revap' xs f = f ys where ys :: [a] ys = reverse xs Incidentally, GHC now uses SML-like design for type variables. However, there is a special exception for existentials. Please see 7.8.7.4. Pattern type signatures of the GHC user manual. The entire section 7.8.7 is worth reading.

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Parameterized constraints
by Emil Axelsson 07 Jul '12

07 Jul '12

Hi! The `constraints` package provides ways to manipulate objects of kind `Constraint`. I need the same kind of manipulation, except that I need to work with objects of kind `* -> Constraint`. I.e. I need parameterized constraints that can be applied to different types. BTW, is there a standard term for things of kind `* -> Constraint`? I have a type family type family Constr (f :: * -> *) :: * -> Constraint which returns a parameterized constraint with the property that any value of type `f a` fulfills the constraint `Constr f a`. Since the constraint can be stronger than needed, I need something similar to `:-` from `constraints`, except it should operate on parameterized constraints. I have implemented the stuff I need (see below), but my question is if it's possible to do this with the `constraints` package directly (e.g. using `Forall`). I'm afraid I can't see how. Here is what I've come up with so far: -- Instead of (c1,c2) class (c1 a, c2 a) => (c1 :/\: c2) a instance (c1 a, c2 a) => (c1 :/\: c2) a -- Instead of (:-) type sub :< sup = forall a . Dict (sup a) -> Dict (sub a) -- Instead of weaken1 weak1 :: c1 :< (c1 :/\: c2) weak1 Dict = Dict weak2 :: c2 :< (c1 :/\: c2) weak2 Dict = Dict Thanks! -- / Emil

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ANNOUNCE: husk-scheme, release 3.5.6
by Justin Ethier 07 Jul '12

07 Jul '12

I would like to announce the release of husk-scheme 3.5.6, a Scheme extension language and stand-alone interpreter/compiler that I have been developing. husk implements most of the Scheme R5RS standard<http://www.schemers.org/Documents/Standards/R5RS/HTML/>including advanced features such as continuations, hygienic macros, and a full numeric tower. husk has been an ongoing project for over a year now, but this is the first announcement on the mailing list so feedback is very welcome. Hackage: http://hackage.haskell.org/package/husk-scheme Homepage: http://justinethier.github.com/husk-scheme Change Log: https://github.com/justinethier/husk-scheme/blob/master/ChangeLog.markdown Issues and Enhancement Requests: https://github.com/justinethier/husk-scheme/issues Thanks, Justin

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vector, alignment and SIMD through FFI
by Nicolas Trangez 07 Jul '12

07 Jul '12

Hello Cafe, Recently I've been playing with the implementation of an algorithm, for which we already have highly-optimized implementations available (in plain C/C++ as well as OCaml with calls to C through FFI). The algorithm works on buffers/arrays/vectors/whatever you want to call it, which needs to be combined in certain ways. This can be highly optimized by using SIMD instructions (like the ones provides by several SSE versions). I'd like to get a to Haskell version which is comparable in efficiency as the existing versions, whilst remaining as 'functional' as possible. I don't mind jumping into some low-level C glue and FFI (using ccall or custom primops), but this should be limited. Currently I have something working (still highly unoptimized) using (unboxed) vectors from the vector package, using mutable versions within a well-contained ST environment in some places. One hot zone of the current version is combining several vectors, and the performance of this operation could be greatly improved by using SIMD instructions. There's one catch though: when using these, memory should be aligned on certain boundaries (16 byte in this case). First and foremost, to be able to pass my vectors to some C functions, I should change my code into using Storable vectors (which should be fine, I guess I can expect similar performance characteristics?). I couldn't find any information on alignment guarantees of these vectors though... Which is how I get to my question: are there any such guarantees? If not, are there any pointers to how to proceed with this? I guess tracking alignment at the type level should be the goal, you can find some code trying to explain my reasoning up to now at the end of this email. I have some issues with this: - I'd need to re-implement almost all vector operations, which seems stupid. - It doesn't actually work right now ;-) - It'd be nice to be able to encode 'compatible' alignment: as an example, a 16 byte aligned buffer is also 8 byte aligned... I hope the above explains somewhat my goal. Any thoughts & help on this would be very welcome! Thanks, Nicolas module Data.Vector.SIMD ( -- ... ) where import qualified Data.Vector.Storable as SV import Foreign.Storable (Storable, sizeOf) import Foreign.Ptr (Ptr, FunPtr) import Foreign.ForeignPtr (ForeignPtr, newForeignPtr) import System.IO.Unsafe (unsafePerformIO) class Alignment a where alignment :: a -> Int data A8Byte instance Alignment A8Byte where alignment _ = 8 data A16Byte instance Alignment A16Byte where alignment _ = 16 newtype Alignment a => SIMDVector a b = V (SV.Vector b) replicate :: (Alignment a, Storable b) => a -> Int -> b -> SIMDVector a b replicate a n b = V v where ptr = unsafePerformIO $ do v <- _mm_malloc n (alignment a) -- memset etc return v v = SV.unsafeFromForeignPtr0 ptr n -- These are 2 _stub versions of the procedures since xmminstr.h (or mm_malloc.h -- when using GCC) contains them as inline procedures which are not available -- as-is in a library. There should be some C module which exports -- _mm_malloc_stub and _mm_free_stub, which simply includes xmminstr.h and calls -- the underlying procedures. foreign import ccall "_mm_malloc_stub" _mm_malloc_stub :: Int -> Int -> IO (Ptr a) foreign import ccall "_mm_free_stub" _mm_free_stub :: FunPtr (Ptr a -> IO ()) _mm_malloc :: Storable a => Int -> Int -> IO (ForeignPtr a) _mm_malloc s l = do -- This fails: -- Ambiguous type variable `a0' in the constraint: -- (Storable a0) arising from a use of `sizeOf' -- v <- c_mm_malloc (s * sizeOf (undefined :: a)) l newForeignPtr _mm_free_stub undefined -- This allocates a 16 byte aligned output buffer, takes 2 existing ones and -- calls some FFI function to perform some magic. -- The implementation could run inside ST, if the FFI import (which e.g. works -- on a mutable buffer and returns IO ()) is lifted into ST using unsafeIOToST mySIMDFun :: SIMDVector A16Byte a -> SIMDVector A16Byte a -> SIMDVector A16Byte a mySIMDFun a b = undefined

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cabal equivalent to "make -k"?
by Omari Norman 06 Jul '12

06 Jul '12

When using make (or, at least, GNU make) the "-k" option keeps going as far as possible after a compilation error. It's handy during developing--for instance, "I know half of my code is busted, but I just want to see if this file compiles." Is there a similar way to do this with cabal? Thanks. --Omari

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ANNOUNCE: lens-family 0.0.0
by roconnor＠theorem.ca 06 Jul '12

06 Jul '12

I'm pleased to announce the first release of lens-family-core and lens-family. This package provide first class(†) functional references. In addition to the usual operations of getting, setting and composition, plus integration with the state monad, lens families provide some unique features: * Polymorphic updating * Cast projection functions to read-only lenses * Cast semantic editor combinators to modify-only lenses (†) For optimal first-class support use the lens-family package with rank 2 / rank N polymorphism. Lens.Family.Clone allows for first-class support of lenses for those who require Haskell 98. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Posting jobs on Haskellers.com
by Bartosz Milewski 06 Jul '12

06 Jul '12

There are no job postings on Haskellers.com and I'm wondering whether it's because you have to wait for the "verified" status before you can post (and that's after you have successfully verified your email). Posting the job on the CUFP site, on the other hand, was painless: http://cufp.org/jobs/haskell-systems-administrator .

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Parallel Haskell Digest 11
by Eric Kow 06 Jul '12

06 Jul '12

Parallel Haskell Digest 11 ========================== HTML version: http://www.well-typed.com/blog/67 Hello Haskellers! It's time for another Parallel Haskell Digest! Unfortunately, this may just be our last one, at least within the context of the Parallel GHC project. That said, we may as a community be at the very beginnings of Haskell as *the* language of choice for your parallel and concurrent needs. Maybe we need to keep something like the Digest going to help our little FP monster through its infancy? Any volunteers in the community? If you're interested in picking up the torch, please give us a shout! Otherwise, if you can't take on a (perhaps rotating) digest commitment, but still want to help, would you be kind enough to fill out a small [survey](http://goo.gl/bP2fn) on the digest? There are just five questions on it, plus a feedback form. Anything you can say will help those of us in the Secret Haskell Propaganda Commitee to fine tune our efforts: http://goo.gl/bP2fn It's been a fantastic year for me, working on the Parallel GHC project, learning about all sorts of neat ideas and technologies (as a basic parallel-naive Haskeller), and trying to reflect them back in a way that hopefully helps the broader community. Thanks to all of you in the parallel Haskell world first for cranking out all this great stuff for us to use, and second for your patience and support. Thanks especially to my follow Well-Typed-ers for all the fun chats, the feedback on drafts, and help getting up to speed. One last thing before signing off as your Parallel Haskell Digester. While the digest may be coming to an end, there will at least be one encore! It turns out we had so much to say in our last word of the month, that we'll have to put in in a follow-up posting. In the meantime, we'll just leave you with a little teaser… News ---------------------------------------------------------------------- * [Announce: Haskell Platform 2012.2.0.0][n1] (3 Jun) The new Haskell Platform is out! If you've been waiting for Haskell Platform before moving on GHC 7.4, now's a great time to upgrade. Of particular interest to parallel Haskellers, this latest GHC offers better profiling flags, multicore profiling, vastly improved DPH, event logging [allows ThreadScope spark profiling], and more convenient RTS flags. * [Introducing FP Complete][n2] (6 Jun) You might have Bartosz Milewski around. If not, have a look at the [Downfall of Imperative Programming][downfall]. Bartosz posted a quick message introducing himself to the community along with the new company FP Complete, which aims to commercialise Haskell. Bartosz believes that “now is the right time for Haskell to become a strong software industry player, especially that functional programming is being widely recognized as the answer to the recent multicore and GPU explosion.” We'll hopefully find out more about FP Complete have in mind as their plans stabilise a bit. * [3 year Bioinformatics R&D position in Granada, Spain][n3] Love Functional Programming and concurrency? If you are a CS/Math/IT graduate without a PhD, and have had no more than 4 years of research experience, and have not lived in Spain for more than 12 months (within the last 3 years), Era7 has a position for you! You'll be hacking Scala and using AWS for everything. So if Akka is the sort of thing you're into, this could be the job for you. Word of the month (teaser!) ---------------------------------------------------------------------- The word of the month series has given us a chance to survey the arsenal of Haskell parallelism and concurrency constructs: * some low level foundations (sparks and threads), * three ways to do parallelism (parallel arrays, strategies, dataflow), * and some concurrency abstractions (locks, transactions, channels) The Haskell approach has been to explicitly recognise the vastness of the parallelism/concurrency space, in other words, to provide a multitude of right tools for a multitude of right jobs. Better still, the tools we have are largely interoperable, should we find ourselves with jobs that don't neatly fit into a single category. The Haskell of 2012 may be in a great place for parallelism and concurrency, but don't think this is the end of the story! What we've seen so far is only a snapshot of the technology as it hurtles through the twenty-tens (How quaint are we, Future Haskeller?). While we can't say what exactly the future will bring, we can look at one of the directions that Haskell might branch into in the coming decade. The series so far has focused on things you might do with a single computer, using parallelism to speed up your software, or using concurrency abstractions to preserve your sanity in the face of non-determinism. But now what if you have more than one computer? Our final word of the month is *actor*. Actors are not specific to distributed programming; they are really more of a low level concurrency abstraction on a par with threads. And they certainly aren't new either. The actor model has been around since the early 70s at least, and has been seriously used for distributed programming since the late 80s with Erlang. Can you guess where this word of the month is going? We have a bit more to say about it shortly, so while this is the last Parallel Haskell Digest, watch this space for the final word of the month :-) Parallel GHC project update ---------------------------------------------------------------------- Our work on the [distributed-process][dist-p] implementation of Cloud Haskell continues apace. We're almost there, having implemented most of the API described in the original [Epstein *et al*][ch-pdf] paper, except for node configuration and initialisation. We are very excited to be getting this out of the door soon and into your hands. In fact, we've even submitted a proposal to present this work at the upcoming [Haskell Implementors Workshop][hiw]; so hopefully you'll be able to join Duncan and Edsko in Copenhagen and catch up on the Cloud Haskell news. As for ThreadScope, we last mentioned that we were working to make use of information from hardware performance counters (specifically, Linux Perf Events). This took a bit more work and trickier GHC patches than we had anticipated, but it does seem to be in order now and we are now in the testing phase for the next release. The next ThreadScope release will also include the use of heap statistics from the (eventual) GHC 7.6 RTS, and some user interface enhancements suggested by our users. Tutorials ---------------------------------------------------------------------- * [Parallel and Concurrent Haskell Course slides][t1] Looks like the recent Summer School was a great time for all! We had expert Haskellers talking parallelism and concurrency, a chateau, good food, and lots of wine. Did you miss out? We can't help with the wine, but if you'd like to get in on some of the parallel action, check out Simon's slides. There are seven lectures in all, and some [lab exercises][lab-exo] to go with them: 1. Basic pure parallelism 2. The Par Monad 3. Basic concurrency 4. Software Transactional Memory 5. Concurrent network servers 6. Distributed programming 7. GPU programming with Accelerate * [Threading and Gtk2Hs][t2] “So you're writing a Gtk2Hs application and you need to do some threading.” Daniel Wagner has just the tutorial for you. The post comes in two parts. First Daniel gives it all away with two keys points (and a simple concrete example): 1. Make all Gtk calls from the main thread. If other threads need to affect the interface, use `postGUIAsync` or `postGUISync` to send your code to the main thread. 2. Link your program with the threaded runtime system by passing GHC the `-threaded` option at link time. For readers who want to learn more, Daniel then goes into much more depth: the things that threading hard both in its own right and when working in Gtk2Hs in particular; the perils of using `unsafe` FFI imports (as opposed to `safe` ones); a peek into the Gtk2Hs guts showing its interaction with Gtk and glib; and finally, some of the possible pitfalls, wrong things Daniel believed when he started and what he now believes instead. Blogs and packages ---------------------------------------------------------------------- * [GHC-7.4.2-Eden - Parallel Haskell on multicore and cluster systems][b0] (17 Jun) There is more than one way to do parallelism and distributed programming in Haskell. Mischa Dieterle announced a new release of [Eden][eden], an extension of Haskell which is “tailored for distributed systems but works equally well on multicore architectures”. These extensions consist of a small number of constructs for working with *processes* (processes work within disjoint address spaces and do not share any data). Eden provides automatic process handling to reduce the amount of low-level detail needed to implement parallel algorithms, but also allows for the explicit control you may need to get good performance. You can either install the full Eden system including the compiler (GHC with the Eden parallel runtime system), libraries and tools; or just install a thread simulation by using a standard GHC to install the libraries and tools off Hackage. * [Forklift - a pattern for performing monadic actions in a worker thread][b1] (7 Jun) Apfelmus recently noticed a recurring applied Haskell puzzle: how do you combine two IO-wrapping monads that don't lend themselves to being combined via monad transformers? This arose in the context of Shae Erisson's Summer of Code Project (Web based GHCi) which uses webserver and interpreter packages providing monads of their own. A typical approach would be to run separate threads and have the two communicate using something like an `MVar`; but each time reinvent this solution, we end up creating some mini communication protocol specific to the task. Apfelmus suggests a more generic variant of this approach, which he calls the “forklift pattern” because it consists in *forking* a worker thread to *lift* arbitrary monadic actions into IO. data ForkLift m = ForkLift { requests :: Chan (m ()) } carry :: MonadIO m => ForkLift m -> m a -> IO a The worker thread maintains a queue of requests. To run an arbitrary action of type `m a`, you “carry” it over into `IO` with a helper function that wraps it up in an `MVar` sandwich, sticks it on the queue, and reads the `MVar` to get the result back. See Apfelmus's posting to see this cute trick in action. * [The Flavours of MVar][b2] (4 Jun) Neil Mitchell finds that that the flexibility of the `MVar` can leave some room for confusion: both taking from and putting to an `MVar` one can block, whereas it is likely that you only expect it to block on one of those. In a quick and practical tutorial, Neil whips through three `MVar` patterns that he tends to use regularly: * lock: guaranteed single-threaded access to some resource * var: thread-safe mutable variables that never block on put * barrier: starts empty, is written to once, then read one or more times. Building on these Neil shows a couple of examples of how one might go one to combine these MVar uses to to get higher level abstractions: an action that can be invoked multiple times but runs at most once, and a queue that collects messages individually and delivers them in bulk. Check his post out, and maybe see why `join $ modifyMVar …` is becoming one of his favourite idioms. * [Being more clever about vectorising nested data parallelism][b3] (4 Jun) Manuel Chakravarty tumbles: Our new draft paper [on Vectorisation Avoidance][chak1] introduces a novel program analysis for nested data parallelism that lets us avoid vectorising purely scalar subcomputations. It includes a set of benchmark kernels that suggest that vectorisation avoidance improves runtimes over merely using array stream fusion. * [Repa 3: more control over array representations with indexed types][b4] Another paper from the UNSW parallel Haskellers: We have got a new draft paper on [Guiding Parallel Array Fusion with Indexed Types][chak2]. It describes the design and use of the 3rd generation Repa API, which uses type indices to give the programmer control over the various parallel array representations. The result are clearer programs that the compiler can more easily optimise. The implementation of Repa 3 is ready for use on Hackage in the repa package. * [Protocol Buffers 2.0.7][b8] (19 May) Chris Kuklewicz is back! In the last digest, some folks in the community were looking for him because they had patches for the protocol buffers package family. Chris has not only resurfaced, but released an update to the packages, making them compile with GHC 7.4.1 and handle missing package names better. * [Generics and Protocol Buffers][b6] (20 May) Nathan Howell thinks the protocol-buffers package is great: full-featured, well-tested, no complaints about performance. However, maintaining `.proto` files is “more than just a chore”. The `hprotoc` tool could help but is trickyp to integrated properly into the build system. Maybe there's another way, one which does not involve separate files or build tools. Have a look at his [GitHub Gist][gproto] for a promising alternative solution using the [type-level][type-level] library. * [SafeSemaphore][b9] (2 Jun) Chris Kuklewicz has a problem, a solution, and a plea for help. * Problem: Control.Concurrent.QSem (and QSemN, SampleVar) are [broken][ghc3160] (they provide no exception safety). * Solution: his [SafeSemaphore][safesema] package, just updated to 0.90, with several safer alternatives. * Plea: Would it be possible to replace parts of GHC with SafeSemaphore, so as to unbreak the Haskell Platform? It looks like Chris' plea has been heard, as Simon Marlow has recently suggested importing the STM version for GHC 7.6.1 * [Paraiso][paraiso] (7 Jun) Kazu Yamamoto [announced][b10] a couple of new parallel libraries from Japan. The first is Paraiso (by Takayuki Muranushi of Monadius fame), a high-level language for implementing explicit partial-differential equations solvers on supercomputers as well as today's advanced personal computers. * [GTALib][gtalib] (7 Jun) Also from the Japan Parallel Haskell workld, is GTALib by Kendo Emoto. It provides core functionalities of the GTA programming framework described in the paper [Generate, Test, and Aggregate A Calculation-based Framework for Systematic Parallel Programming with MapReduce][gta] Mailing lists ---------------------------------------------------------------------- * [How to write Source for TChan working with LC.take?][m1] (20 May) Hiromi Ishii is writing a Data.Conduit `Source` that supplies its values from a `TChan`. He has three versions, one using the raw Pipe constructors directly, one using `sourceState`, and one using `yield`. The `yield' version does not seem to work as expected. At first this seemed like an unfortunate necessity, but after putting some thought into it, Michael Snoyman proposed some [modifications to conduit's await/yield][b5] functions, which should allow Hiromi to write things in the intuitive way. * [Parallel cooperative multithreading?][m2] (22 May) Benjamin Ylvisaker was wondering if it'd be possible to implement something like [Observationally Cooperative Multithreading][ocm-pdf] (OCM) in Haskell. The paper discusses Lua, C, and C++ implementations. Ben thinks that Haskell would be an awesome fit such a framework. The premise behind OCM is that cooperative concurrency can be easier than preemptive concurrency, because you can use reason sequentially between invocations of pause/yield/wait. Historically, it has only worked on single processors, because the blocks of code between the p/y/w calls need to be run atomically. Recent research means we know more about how to efficiently run blocks of code atomically, so maybe cooperative concurrency can make a comeback? Ryan Newton thinks the comeback is indeed happening. He points in the Haskell world to monad-par's use of ConT as an example of a a framework in which tasks cooperatively yield control whenever their desired input data is not yet available. Mario Blažević has also thought about cooperative concurrency in Haskell, particularly in context of his monad-coroutine library; however, he found no speedups when he added support for running multiple co-routines in parallel. Ketil Malde is sceptical that the proposed approach would be better than using STM. He wonders if the paper's critique of STM applies to implementations that keep transactional data are segregated by the type system. * [How to translate Repa 2 program to efficient Repa 3 code?][m3] (26 May) Michael Serra posted a StackOverflow thread asking about the [differences between Repa 2 and 3 APIs][s1]. He has some simple image convolution tests with which run fast enough in Repa 2 with judicious use of `force`. But when translating the tests to Repa 3, he can't quite work out how to get the same kind of performance. See the thread on StackOverflow for more details. In short, `computeP` is the new `force`. * [Is Repa suitable for boxed arrays?...][m4] (3 Jun) Stuart Hungerford needs to build a 2D Array of boxed Haskell values. He's attracted to Repa, but couldn't work out from the documentation if it would work with arbitrary values. Moreover, getting the examples to work. Ben Lippmeier replies that it should work (the array type would be something like `Array V DIM2 Float`). The documentation is out of date (it's for Repa 2 and Repa 3 is different). Until somebody gets a chance to update the documentation, try the [Repa 3 paper][repa3-pdf], Ben just submitted for Haskell Symposium 2012. * [Status and roadmap for Cloud Haskell?][m5] (19 May) Ben Lee is very interested in the our work at Well-Typed on `distributed-process`, the followup implementation of Cloud Haskell. How's progress? As mentioned in the Parallel GHC news above, we're almost there! Edsko de Vries says that so far we've been focusing on two aspects of the new implementation, the design of the transport API, and a robust TCP implementation to sit on top of it. These two parts are nearly done. Meanwhile, we've been laying down [some documentation][dp-docs] on our GitHub project wiki. If you want to help out, we'd love if you could play with the TCP transport, and try to write some transports of your own. * [`anyP' in DPH?][m6] (21 May) Rob Stewart is trying to find the `anyP` function for Data Parallel Haskell. Ben Lippmeier says that DPH is in flux at the moment and that the current user facing API can be found in [dph-lifted-vseg][dph-lv] which provides an `orP` function. * [Everybody should write everything in Go?][m7] (28 May) Ryan Hayes posted a small [snippet of Go][go-snippet] showing how friendly he found it for writing concurrent programs, “No pthread... not stupid crap... just works!”. The program seems to create 4 threads which print out 1 to 100 each. What do Haskellers think? See the comments for some discussion between Haskell people like Simon Marlow, and some folks in the Go community about our respective approaches to the problem. * [Proposal: Control.Concurrent.Async][m8] (8 June) Deep into writing his book on Parallel Haskell, Simon Marlow proposes a higher-level concurrency API for the base package. The proposed [Control.Concurrent.Async][cc-async] would help make sure that exceptions in child threads are dealt with (returned or passed up), and that threads aren't accidentally left running in the background. A few Haskellers commented that they would prefer that base be kept minimal as possible, and have counter-proposed making it a package to be included in the Haskell Platform instead. See the thread for some discussion on the API itself. StackOverflow and Reddit ---------------------------------------------------------------------- * [What are the key differences between the Repa 2 and 3 APIs?][s1] * [Does `par` create another thread?][s2] * [How to improve performence of this Haskell code?][s3] * [Haskell: Why was `par` defined the way it was?][s4] * [Method for capturing monad stack state][s5] * [Killing a thread when MVar is garbage collected][s6] * [Improving simulation performance via concurrency][s7] * [Haskell framework to parallelize non-threadsafe C++ lib][s8] * [Thread-safe state with Warp/WAI][s9] * [How to use multiple cores when compiling with GHC? : haskell][r1] Help and Feedback ---------------------------------------------------------------------- Well, this is the end of the Haskell Parallel Digest, but feedback would still be much appreciated! Get in touch with me, Eric Kow, at <parallel(a)well-typed.com>. Bye for now! [ch-pdf]: http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remot… [dist-p]: https://github.com/haskell-distributed/distributed-process [downfall]: http://fpcomplete.com/the-downfall-of-imperative-programming/ [hiw]: http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop [lab-exo]: http://community.haskell.org/~simonmar/lab-exercises-cadarache.pdf [eden]: http://www.mathematik.uni-marburg.de/~eden [chak1]: http://www.cse.unsw.edu.au/~chak/papers/KCLLP12.html [chak2]: http://www.cse.unsw.edu.au/~chak/papers/LCKP12.html [paraiso]: http://hackage.haskell.org/package/Paraiso [gtalib]: http://hackage.haskell.org/package/GTALib [gta]: http://research.nii.ac.jp/~hu/pub/esop12.pdf [ghc3160]: http://hackage.haskell.org/trac/ghc/ticket/3160 [safesema]: http://hackage.haskell.org/package/SafeSemaphore/ [gproto]: https://gist.github.com/2757253 [type-level]: http://hackage.haskell.org/package/type-level [dp-docs]: https://github.com/haskell-distributed/distributed-process/wiki [dph-lv]: http://hackage.haskell.org/package/dph-lifted-vseg [cc-async]: http://community.haskell.org/~simonmar/async-stm/Control-Concurrent-Async.h… [repa3-pdf]: http://www.cse.unsw.edu.au/~benl/papers/guiding/guiding-Haskell2012-sub.pdf [ocm-pdf]: http://www.cs.hmc.edu/~stone/papers/ocm-unpublished.pdf [go-snippet]: https://gist.github.com/3010649 [n1]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101579.html [n2]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101632.html [n3]: http://functionaljobs.com/jobs/111-3-year-postgraduate-rd-position-at-era7-… [n4]: http://tomschrijvers.blogspot.co.uk/2012/05/ifl-2012-call-for-papers.html [t1]: https://plus.google.com/107890464054636586545/posts/LThYZELANCg [t2]: http://dmwit.com/gtk2hs/ [b0]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101808.html [b1]: http://apfelmus.nfshost.com/blog/2012/06/07-forklift.html [b2]: http://neilmitchell.blogspot.co.uk/2012/06/flavours-of-mvar_04.html [b3]: http://tumblr.justtesting.org/post/24399176080/being-more-clever-about-vect… [b4]: http://tumblr.justtesting.org/post/24398752358/repa-3-more-control-over-arr… [b5]: http://www.yesodweb.com/blog/2012/05/next-conduit-changes [b6]: http://breaks.for.alienz.org/blog/2012/05/20/generics-and-protocol-buffers/ [b8]: http://www.haskell.org/pipermail/haskell-cafe/2012-May/101313.html [b9]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101567.html [b10]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101649.html [b12]: http://fpcomplete.com/asynchronous-api-in-c-and-the-continuation-monad/ [m1]: http://www.haskell.org/pipermail/haskell-cafe/2012-May/101318.html [m2]: http://www.haskell.org/pipermail/haskell-cafe/2012-May/101359.html [m3]: http://www.haskell.org/pipermail/haskell-cafe/2012-May/101454.html [m4]: http://www.haskell.org/pipermail/haskell-cafe/2012-June/101573.html [m5]: https://groups.google.com/d/msg/parallel-haskell/A6mXn1Wv-KY/-63SHoGU31wJ [m6]: https://groups.google.com/d/msg/parallel-haskell/Ykm3QJT12yw/8HpnoP8qsugJ [m7]: https://plus.google.com/app/plus/mp/588/#~loop:view=activity&aid=z13pwzbajp… [m8]: http://www.haskell.org/pipermail/libraries/2012-June/017892.html [s1]: http://stackoverflow.com/questions/10747079/what-are-the-key-differences-be… [s2]: http://stackoverflow.com/questions/10724946/does-par-create-another-thread [s3]: http://stackoverflow.com/questions/10557055/how-to-improve-performence-of-t… [s4]: http://stackoverflow.com/questions/10166640/haskell-why-was-par-defined-the… [s5]: http://stackoverflow.com/questions/11073610/method-for-capturing-monad-stac… [s6]: http://stackoverflow.com/questions/10871303/killing-a-thread-when-mvar-is-g… [s7]: http://stackoverflow.com/questions/10627980/improving-simulation-performanc… [s8]: http://stackoverflow.com/questions/10567223/haskell-framework-to-paralleliz… [s9]: http://stackoverflow.com/questions/10449819/thread-safe-state-with-warp-wai [r1]: http://www.reddit.com/r/haskell/comments/uyy80/how_to_use_multiple_cores_wh… -- Eric Kow <http://erickow.com>

2 3

OpenShift a free PaaS from RedHat
by C K Kashyap 06 Jul '12

06 Jul '12

Hi Folks, I just found out about OpenShift - its a free PaaS from RedHat. It has some interesting offerings. It does not support Haskell out of the box as of now. Please do check it out and if you like it - vote for Haskell support on it here - https://openshift.redhat.com/community/content/support-for-haskell Meanwhile, I am trying to get complied Haskell executable to run on that platform. Regards, Kashyap

4 6

HUnit/cabal integration
by Richard Cobbe 06 Jul '12

06 Jul '12

I'm working on a little library package (purely for my own consumption) that's built with Cabal, and I have a couple of questions about the pragmatics of using HUnit for it. First, I'd like to be able to run my tests via "cabal test" from the shell prompt. I've seen <http://hackage.haskell.org/trac/hackage/wiki/UpgradingTests> and followed that, and it basically works. I'm curious if I'm doing it in the best (or at least most idiomatic) way. I've included my cabal file and test driver below, for specifics. Two questions: First: the web page I cite above describes the interface that the test binary must support to work with cabal, specifically w.r.t. the binary's exit code. "Your test suites likely already fit this model. However, if you are using an old version of QuickCheck or HUnit, your executable may not be returning the correct error code." This seems to me to suggest that recent versions of HUnit automatically take care of generating the exit code, but I've found that I have to examine HUnit's results and synthesize the exit code manually, as in the driver program below. (I'm running HUnit 1.2.4.2, but the interface for 1.2.4.3 doesn't appear to differ on this point.) Am I misinterpreting the wiki page, or am I missing something in HUnit's API that generates the exit code automatically? Second: Am I specifying the Build-Depends correctly for the Test-Suite? Specifically: do I need to state a dependency on the library defined in the same package, or does it pick that up automatically? Further, foo-tests doesn't use parsec directly. Is the transitive dependency automatically provided for me, or do I need to list it explicitly as below? Thanks much, Richard My cabal file: Name: foo Version: 0.0 Cabal-Version: >= 1.2 Author: Richard Cobbe Synopsis: Sample cabal package for HUnit integration Build-Type: Simple Library Exposed-Modules: Foo, Foo.Parser, Foo.Show Build-Depends: base >= 4.3.1.0 && < 5, parsec >= 3.1.2 Test-Suite foo-tests main-is: foo-tests.hs type: exitcode-stdio-1.0 Build-Depends: base >= 4.3.1.0 && < 5, parsec >= 3.1.2, HUnit >= 1.2.4.2 and foo-tests.hs: module Main where import Test.HUnit import qualified Foo.Parser import System.Exit main :: IO () main = do c <- runTestTT ("Foo" ~: Foo.Parser.tests) if (errors c == 0 && failures c == 0) then exitWith ExitSuccess else exitWith (ExitFailure (-1))

2 2

Cabal problem re. haskelldb-hdbc-mysql
by Yves Parès 06 Jul '12

06 Jul '12

Hi, the package http://hackage.haskell.org/package/haskelldb-hdbc-mysql/ the use of HDBC <2.3.0 I'm using cabal-install 0.14, and with a fresh install (no packages already installed), cabal-install tries to install HDBC-2.1.1 instead of, say, HDBC-2.2.7.0. The problem is that HDBC-2.1.1 is old (2009) and does not compile. When I download haskelldb-hdbc-mysql manually, change the dependency and then install, it compiles fine. Is the dependency ill-declared, or is that a cabal problem?

3 2

arbitrary rank polymorphism and ghc language pragmas
by rickmurphy 05 Jul '12

05 Jul '12

Hi All: I've been working through some details in these papers [1], [2] and noticed a language pragma configuration that I hope you can confirm. When using explicit foralls in a data constructor, it appears that GHC 7.4.2 requires Rank2Types in the Language pragma for what the papers consider rank 1 types. Here's an example: data T = TC (forall a b. a -> b -> a) Am I correct, or is there another extension? The ExplicitForAll does not appear to support rank 1 types in data constructors. 1. Practical Type Inference for Arbitrary-Rank Types. 2. A Direct Algorithm for Type Inference in the Rank 2 Fragment of the Second-Order Lambda Calculus. -- Rick

3 3

Is haskell.org down?
by C K Kashyap 05 Jul '12

05 Jul '12

Regards, Kashyap

2 2

Haskell Weekly News: Issue 234
by Daniel Santa Cruz 05 Jul '12

05 Jul '12

Welcome to issue 234 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 24 to 30, 2012. Quotes of the Week * monochrom: shae ericsson is shae erisson's evil twin who prefers erlang to haskell :) * shapr: If I were to do this in Python, it would be done by now. and it would be full of bugs. Top Reddit Stories * Performance: yes, it’s worth looking at the small stuff Domain: serpentine.com, Score: 61, Comments: 11 On Reddit: [1] http://goo.gl/x2pGf Original: [2] http://goo.gl/SHBNB * Awesome Haskell plugins for Vim! Domain: self.haskell, Score: 61, Comments: 32 On Reddit: [3] http://goo.gl/Bm6wI Original: [4] http://goo.gl/Bm6wI * Galois is Hiring! Domain: haskell.org, Score: 56, Comments: 7 On Reddit: [5] http://goo.gl/c7zj8 Original: [6] http://goo.gl/y7nBd * Haskell Symposium 2012 list of accepted papers Domain: haskell.org, Score: 43, Comments: 12 On Reddit: [7] http://goo.gl/Fq0rD Original: [8] http://goo.gl/CGZV3 * Live coding the game of Sokoban with GUI. Please give me your input for future sessions. Domain: youtu.be, Score: 42, Comments: 38 On Reddit: [9] http://goo.gl/HslD9 Original: [10] http://goo.gl/0ZeMA * Mirrored Lenses Domain: comonad.com, Score: 36, Comments: 30 On Reddit: [11] http://goo.gl/UyGco Original: [12] http://goo.gl/9ltWW * The Game of Distributed Systems Programming. Which Level Are You? Incubaid Research Domain: blog.incubaid.com, Score: 30, Comments: 7 On Reddit: [13] http://goo.gl/EYWj7 Original: [14] http://goo.gl/P0vhZ * SugarHaskell: language extensions in sugar modules[pdf] Domain: informatik.uni-marburg.de, Score: 27, Comments: 7 On Reddit: [15] http://goo.gl/Z4fL5 Original: [16] http://goo.gl/I78Md * parallel cabal-install patches Domain: haskell.org, Score: 26, Comments: 4 On Reddit: [17] http://goo.gl/Yr1JR Original: [18] http://goo.gl/P9WPK * Is anybody using Sublime Text for their Haskell editing? Domain: self.haskell, Score: 19, Comments: 16 On Reddit: [19] http://goo.gl/pmor9 Original: [20] http://goo.gl/pmor9 * Runge–Kutta in Haskell (RK4) Domain: stochastix.wordpress.com, Score: 19, Comments: 7 On Reddit: [21] http://goo.gl/9Bd2b Original: [22] http://goo.gl/H27Fv * How to build an example proxy server using conduit 0.5 Domain: yesodweb.com, Score: 19, Comments: 14 On Reddit: [23] http://goo.gl/BHLRd Original: [24] http://goo.gl/PtOxh * Found myself wanting lambda-case again Domain: self.haskell, Score: 16, Comments: 13 On Reddit: [25] http://goo.gl/LIOKa Original: [26] http://goo.gl/LIOKa * What is the advantage of monad transformers over monad coproducts? Domain: self.haskell, Score: 16, Comments: 28 On Reddit: [27] http://goo.gl/lxX2D Original: [28] http://goo.gl/lxX2D * Elm 0.3.5: JavaScript Integration, Signal Filters, space savings, ... Domain: testblogpleaseignore.com, Score: 15, Comments: 2 On Reddit: [29] http://goo.gl/jTwAz Original: [30] http://goo.gl/UzoZL * Scribd's AI Challenge Now Supports Haskell Domain: groups.google.com, Score: 14, Comments: 6 On Reddit: [31] http://goo.gl/xmuwG Original: [32] http://goo.gl/oQsTG * EclipseFP: integrating stylish-haskell Domain: jpmoresmau.blogspot.fr, Score: 11, Comments: On Reddit: [33] http://goo.gl/reK9T Original: [34] http://goo.gl/2416v Top StackOverflow Questions * Bug in Data.Map implementation? votes: 15, answers: 1 Read on SO: [35] http://goo.gl/xcgC1 * Monads with Join() instead of Bind() votes: 15, answers: 6 Read on SO: [36] http://goo.gl/phgVU * Haskell numbers and type system? votes: 11, answers: 2 Read on SO: [37] http://goo.gl/nv7Be * How to put constraints on the associated data? votes: 10, answers: 2 Read on SO: [38] http://goo.gl/6zOr9 * How to run Snap haskell webapp in production? votes: 9, answers: 3 Read on SO: [39] http://goo.gl/kx2DR * How can I search a hackage package for a function? votes: 8, answers: 2 Read on SO: [40] http://goo.gl/9fm4O * In Haskell, + is a function, (+ 2) is a function, (+ 2 3) is 5. What exactly is going on there? votes: 8, answers: 4 Read on SO: [41] http://goo.gl/PP2X2 * How can I emulate pointers in Haskell? votes: 8, answers: 1 Read on SO: [42] http://goo.gl/Yq01G * Haskell Precedence: Lambda and operator votes: 8, answers: 2 Read on SO: [43] http://goo.gl/X8yvV * Binding name in type signature using DataKind votes: 8, answers: 2 Read on SO: [44] http://goo.gl/rYcS8 * Missing something with Reader monad - passing the damn thing around everywhere votes: 7, answers: 5 Read on SO: [45] http://goo.gl/zQuBx Until next time, Daniel Santa Cruz References 1. http://www.serpentine.com/blog/2012/06/25/yes-its-worth-looking-at-the-smal… 2. http://www.reddit.com/r/haskell/comments/vkp8t/performance_yes_its_worth_lo… 3. http://www.reddit.com/r/haskell/comments/vomw0/awesome_haskell_plugins_for_… 4. http://www.reddit.com/r/haskell/comments/vomw0/awesome_haskell_plugins_for_… 5. http://www.haskell.org/pipermail/haskell-cafe/2012-June/101978.html 6. http://www.reddit.com/r/haskell/comments/vkwa6/galois_is_hiring/ 7. http://www.haskell.org/haskell-symposium/2012/#schedule 8. http://www.reddit.com/r/haskell/comments/vqjj5/haskell_symposium_2012_list_… 9. http://youtu.be/mtvoOIsN-GU 10. http://www.reddit.com/r/haskell/comments/vkkqc/live_coding_the_game_of_soko… 11. http://comonad.com/reader/2012/mirrored-lenses/ 12. http://www.reddit.com/r/haskell/comments/vk7fy/mirrored_lenses/ 13. http://blog.incubaid.com/2012/03/28/the-game-of-distributed-systems-program… 14. http://www.reddit.com/r/haskell/comments/vnvnw/the_game_of_distributed_syst… 15. http://www.informatik.uni-marburg.de/~seba/projects/sugarj/sugarhaskell.pdf 16. http://www.reddit.com/r/haskell/comments/vtie8/sugarhaskell_language_extens… 17. http://www.haskell.org/pipermail/cvs-ghc/2012-June/074131.html 18. http://www.reddit.com/r/haskell/comments/vmni7/parallel_cabalinstall_patche… 19. http://www.reddit.com/r/haskell/comments/vqhke/is_anybody_using_sublime_tex… 20. http://www.reddit.com/r/haskell/comments/vqhke/is_anybody_using_sublime_tex… 21. http://stochastix.wordpress.com/2012/06/29/runge-kutta-in-haskell/ 22. http://www.reddit.com/r/haskell/comments/vtp5l/rungekutta_in_haskell_rk4/ 23. http://www.yesodweb.com/blog/2012/06/conduit-0-5 24. http://www.reddit.com/r/haskell/comments/vu5hf/how_to_build_an_example_prox… 25. http://www.reddit.com/r/haskell/comments/vu0bb/found_myself_wanting_lambdac… 26. http://www.reddit.com/r/haskell/comments/vu0bb/found_myself_wanting_lambdac… 27. http://www.reddit.com/r/haskell/comments/vuty0/what_is_the_advantage_of_mon… 28. http://www.reddit.com/r/haskell/comments/vuty0/what_is_the_advantage_of_mon… 29. http://www.testblogpleaseignore.com/2012/06/29/announcing-elm-0-3-5-javascr… 30. http://www.reddit.com/r/haskell/comments/vu7z7/elm_035_javascript_integrati… 31. https://groups.google.com/forum/?fromgroups#!topic/scribd-fruit-hunt/Uz5hmP… 32. http://www.reddit.com/r/haskell/comments/vpoza/scribds_ai_challenge_now_sup… 33. http://jpmoresmau.blogspot.fr/2012/06/eclipsefp-integrating-stylish-haskell… 34. http://www.reddit.com/r/haskell/comments/vsgog/eclipsefp_integrating_stylis… 35. http://stackoverflow.com/questions/11180546/bug-in-data-map-implementation 36. http://stackoverflow.com/questions/11234632/monads-with-join-instead-of-bind 37. http://stackoverflow.com/questions/11231859/haskell-numbers-and-type-system 38. http://stackoverflow.com/questions/11258536/how-to-put-constraints-on-the-a… 39. http://stackoverflow.com/questions/11214167/how-to-run-snap-haskell-webapp-… 40. http://stackoverflow.com/questions/11199853/how-can-i-search-a-hackage-pack… 41. http://stackoverflow.com/questions/11217874/in-haskell-is-a-function-2-is-a… 42. http://stackoverflow.com/questions/11222991/how-can-i-emulate-pointers-in-h… 43. http://stackoverflow.com/questions/11237076/haskell-precedence-lambda-and-o… 44. http://stackoverflow.com/questions/11243227/binding-name-in-type-signature-… 45. http://stackoverflow.com/questions/11226338/missing-something-with-reader-m…

1 0

Is there a GHC flag that will allow mutable top level state while you are debugging and then ...
by KC 05 Jul '12

05 Jul '12

you can turn the flag off when you are ready to do the computational heavy lifting so that you don't have to modify your code base? That is, GHC can then apply its algebraic transformation optimizations to the "code algebra" of the pure functions. -- -- Regards, KC

2 1

Inferring Safety
by Ivan Lazar Miljenovic 05 Jul '12

05 Jul '12

When it says up the top of Haddock docs that a module is "Safe-Infered" (which should probably Safe-Infer*r*ed), what does that actually mean? My understanding of Safe Haskell is that there's no unsafe* functions, etc. being used unless the module author swears (by using -XSafe) that the module is indeed safe... Yet as I pointed out on Google+ [1], the Data.GraphViz [2] module of my graphviz package is labelled as being safe-infered, despite having this function in there: dotizeGraph :: (Ord cl, Graph gr) => GraphvizParams Node nl el cl l -> gr nl el -> gr (AttributeNode nl) (AttributeEdge el) dotizeGraph params gr = unsafePerformIO $ graphToGraph params' gr where params' = params { fmtCluster = const [] , fmtNode = const [] , fmtEdge = const [] } This function uses `dot -Tcanon` to add in positional (as well as possibly other) information to FGL graphs. Now, as far as I know this function _is_ safe (assuming that you have Graphviz installed), but I certainly haven't proven it enough that for every possible input graph the same positional attributes will be attached back (i.e. that the graph will always be laid out the same), and thus I haven't used -XSafe. So what's going on here? [1]: https://plus.google.com/101302416956767249890/posts/3NUWxd9P6xV [2]: http://hackage.haskell.org/packages/archive/graphviz/2999.13.0.3/doc/html/D… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

2 2