April 2013 - Haskell-Cafe

Haskell / Functional Programmers Group in Madrid (Spain)?
by Alejandro Serrano Mena 04 Sep '13

04 Sep '13

Hi, Is there any Haskell or functional programmers user group in the region of Madrid? If not, I think it could be a great idea to get to know each other, and share experiences. Furthermore, we could try to organize some meetings with presentations, hackatons and so on (my personal ambition is to be able to organize another "ekmett Workshop" as the Japanese one - http://comonad.com/reader/2013/japanese-workshop-1/) I know there is some locations along Madrid dedicated to open source promotion, like http://madridonrails.com/, and which we could use for these meetings. I'm really looking forward to hearing from you! PS: Maybe a bare "Haskell" group is not big enough (at least according to Haskellers.com) but a "functional programmers" one may well be.

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Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
by Henk-Jan van Tuyl 03 Aug '13

03 Aug '13

On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > I am willing to do administrator tasks. > >> 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external >> links' - which is all of the spam. > > This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with "buy" o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, "Nike Air Jordan") Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Monad Transformer Space Leak
by Clark Gaebel 18 Jul '13

18 Jul '13

Hi everyone! For some reason, this leaks thunks: module Main where import Control.Monad import Control.Monad.MC -- from monte-carlo import Control.Monad.ST.Strict go :: Int -> MCT (ST s) () go k = replicateM_ k (return ()) main = print $ runST $ evalMCT (go 100000000) rng where rng = mt19937 0 while this does not: module Main where import Control.Monad import Control.Monad.MC go :: Int -> MC () go k = replicateM_ k (return ()) main = print $ evalMC (go 100000000) rng where rng = mt19937 0 Can anyone help me figure out what's going on here? Thanks, - Clark

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Re: [Haskell-cafe] Optimization flag changing result of code execution
by Azeem -ul-Hasan 18 Jul '13

18 Jul '13

I am using GHC 7.6.1 mwc-random 0.12.0.1 vector 0.9.1 primitive 0.4.1 Azeem On 16.03.2013 13:31, Azeem -ul-Hasan wrote: > Nope that isn't the case either. Even if I make use of defaultSeed > through create the problem still remains. The problem seems to be in the > generation of a vector of (a,a) i.e in the part > > V.generateM ((round $ p*(fromIntegral $ l*z)) `div` 2) (\i-> R.uniformR > ((0,0) , (l-1,l-1)) gen) > > in line 16. Thanks again. > I've tried to run you program and I've got approximately same results regardless of optimization level. Which versions of GHC, mwc-random, vector and primitive do you use?

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Two GHC-related GSoC Proposals
by Patrick Palka 02 Jun '13

02 Jun '13

Hi, I'm interested in participating in the GSoC by improving GHC with one of these two features: 1) Implement native support for compiling modules in parallel (see #910<http://hackage.haskell.org/trac/ghc/ticket/910>). This will involve making the compilation pipeline thread-safe, implementing the logic for building modules in parallel (with an emphasis on keeping compiler output deterministic), and lots of testing and benchmarking. Being able to seamlessly build modules in parallel will shorten the time it takes to recompile a project and will therefore improve the life of every GHC user. 2) Improve existing constant folding, strength reduction and peephole optimizations on arithmetic and logical expressions, and optionally implement a core-to-core pass for optimizing nested comparisons (relevant tickets include #2132 <http://hackage.haskell.org/trac/ghc/ticket/2132>, #5615 <http://hackage.haskell.org/trac/ghc/ticket/5615>,#4101<http://hackage.haskell.org/trac/ghc/ticket/4101>). GHC currently performs some of these simplifications (via its BuiltinRule framework), but there is a lot of room for improvement. For instance, the core for this snippet is essentially identical to the Haskell source: foo :: Int -> Int -> Int -> Int foo a b c = 10*((b+7+a+12+b+9)+4) + 5*(a+7+b+12+a+9) + 7 + b + c Yet the RHS is actually equivalent to 20*a + 26*b + c + 467 And: bar :: Int -> Int -> Int bar a b = a + b - a - b -- the RHS is should be optimized away to 0 Other optimizations include: multiplication and division by powers of two should be converted to shifts; multiple plusAddr calls with constant operands should be coalesced into a single plusAddr call; floating point functions should be constant folded, etc.. GHC should be able to perform all these algebraic simplifications. Of course, emphasis should be placed on the correctness of such transformations. A flag for performing unsafe optimizations like assuming floating point arithmetic is associative and distributive should be added. This proposal will benefit anybody writing or using numerically intensive code. I'm wondering what the community thinks of these projects. Which project is a better fit for GSoC, or are both a good fit? Is a mentor willing to supervise one of these projects? Thanks for your time. Patrick (A little about myself: I'm a Mathematics student in the US, and I've been programming in Haskell for about 3.5 years. Having a keen interest in Haskell and compilers, I began studying the GHC source about 1 year ago and I've since gotten a good understanding of its internals, contributing a few patches along the way.)

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Install wx on Windows XP
by Eric Wong 29 May '13

29 May '13

Hi Haskellers, I'm trying to install wxHaskell on Windows XP. I have already installed Haskell Platform 2012.04.0.0, MinGW and wxWidgets. When I try to install wx, I got the following error when cabal is installing wxcore: $ cabal install wx Resolving dependencies... [1 of 1] Compiling Main ( C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\wxcore- 0.90.0.3-896\wxcore-0.90.0.3\Setup.hs, C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\wxcore -0.90.0.3-896\wxcore-0.90.0.3\dist\setup\Main.o ) Linking C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\wxcore-0.90.0.3-896\wxcore-0.90.0.3\d ist\setup\setup.exe ... Configuring wxcore-0.90.0.3... Generating class type definitions from .h files generating: src/haskell/Graphics/UI/WXCore/WxcClassTypes.hs reading class definitions: parsing: C:\Documents/include/wxc.h wxdirect: C:\Documents/include/wxc.h: openFile: does not exist (No such file or directory) Generating class info definitions generating: src/haskell/Graphics/UI/WXCore/WxcClassInfo.hs reading class definitions: parsing: C:\Documents/include/wxc.h wxdirect: C:\Documents/include/wxc.h: openFile: does not exist (No such file or directory) Generating class method definitions from .h files parsing: Data\cabal\wxc-0.90.0.4\ghc-7.4.2\include/wxc.h wxdirect: Data\cabal\wxc-0.90.0.4\ghc-7.4.2\include/wxc.h: openFile: does not ex ist (No such file or directory) Building wxcore-0.90.0.3... Preprocessing library wxcore-0.90.0.3... src\haskell\Graphics\UI\WXCore\WxcClassTypes.hs:1:1: File name does not match module name: Saw: `Main' Expected: `Graphics.UI.WXCore.WxcClassTypes' cabal.exe: Error: some packages failed to install: wx-0.90.0.1 depends on wxcore-0.90.0.3 which failed to install. wxcore-0.90.0.3 failed during the building phase. The exception was: ExitFailure 1 It looks like it's confused by the spaces in the path and can't find the wxc.h file. I tried to configure cabal to install all user packages into a simpler directory. But cabal always install to the C:\Documents and Settings\Administrator\Application Data\cabal directory no matter how I change the config file there. Anyone knows how to fix this problem or change cabal directory on Windows? Thanks. p.s. I'm using the user Administrator.

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[ANN] Hac Phi 2013
by Daniel Wagner 23 May '13

23 May '13

Hac Phi 2013, a Haskell hackathon/get-together, will be held June 7-9 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac Phi was a lot of fun, drawing more than 30 Haskellers, and many people have already expressed interest in coming back this year. Everyone is welcome - you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac Phi wiki [2]. This year, we are making a public list of attendees' names and nicks - please let us know if you'd prefer not to be listed when you register. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game - a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! - The Hac Phi team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://www.haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://www.haskell.org/haskellwiki/Hac_%CF%86 [3] http://www.haskell.org/haskellwiki/Hac_%CF%86/Talks

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Re: [Haskell-cafe] partially applied data constructor and corresponding type
by Stephen Tetley 18 May '13

18 May '13

What you probably want are type level integers (naturals) Yury Sulsky used them in the message above - basically you can't use literal numbers 1,2,3,... etc as they are values of type Int (or Integer, etc...) instead you have to use type level numbers: data One data Two Work is ongoing for type level numbers in GHC and there are user libraries on Hackage so there is a lot of work to crib from.

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ANN: Robot - Simulate keyboard and mouse events under X11
by Chris Wong 14 May '13

14 May '13

I'm pleased to announce the initial release of Robot! Robot lets you send fake keyboard and mouse events, just like its namesake in Java. Only X11 systems are supported right now (via XTest), but Windows and Mac can be added later if anyone cares. ## Features + Simple API (only 9 functions and 3 types) + Few dependencies + Exception safe -- unlike with other libraries, all keys and buttons are released automatically when the robot terminates. `bracket` and `finally` are used where appropriate. ## Links Hackage: http://hackage.haskell.org/package/robot-1.0 Examples: https://github.com/lfairy/robot/tree/master/examples Happy hacking! Chris -- Chris Wong, fixpoint conjurer e: lambda.fairy(a)gmail.com w: http://lfairy.github.io/

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Set monad
by oleg＠okmij.org 13 May '13

13 May '13

The question of Set monad comes up quite regularly, most recently at http://www.ittc.ku.edu/csdlblog/?p=134 Indeed, we cannot make Data.Set.Set to be the instance of Monad type class -- not immediately, that it. That does not mean that there is no Set Monad, a non-determinism monad that returns the set of answers, rather than a list. I mean genuine *monad*, rather than a restricted, generalized, etc. monad. It is surprising that the question of the Set monad still comes up given how simple it is to implement it. Footnote 4 in the ICFP 2009 paper on ``Purely Functional Lazy Non-deterministic Programming'' described the idea, which is probably folklore. Just in case, here is the elaboration, a Set monad transformer. {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} module SetMonad where import qualified Data.Set as S import Control.Monad.Cont -- Since ContT is a bona fide monad transformer, so is SetMonadT r. type SetMonadT r = ContT (S.Set r) -- These are the only two places the Ord constraint shows up instance (Ord r, Monad m) => MonadPlus (SetMonadT r m) where mzero = ContT $ \k -> return S.empty mplus m1 m2 = ContT $ \k -> liftM2 S.union (runContT m1 k) (runContT m2 k) runSet :: (Monad m, Ord r) => SetMonadT r m r -> m (S.Set r) runSet m = runContT m (return . S.singleton) choose :: MonadPlus m => [a] -> m a choose = msum . map return test1 = print =<< runSet (do n1 <- choose [1..5] n2 <- choose [1..5] let n = n1 + n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] -- Values to choose from might be higher-order or actions test1' = print =<< runSet (do n1 <- choose . map return $ [1..5] n2 <- choose . map return $ [1..5] n <- liftM2 (+) n1 n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] test2 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return (i,j,k)) -- fromList [(3,4,5),(4,3,5),(6,8,10),(8,6,10)] test3 = print =<< runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return k) -- fromList [5,10]

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