March 2013 - Haskell-Cafe

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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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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Categories (cont.)
by Christopher Howard 08 May '13

08 May '13

I've perhaps been trying everyones patiences with my noobish CT questions, but if you'll bear with me a little longer: I happened to notice that there is in fact a Category class in Haskell base, in Control.Category: quote: -------- class Category cat where A class for categories. id and (.) must form a monoid. Methods id :: cat a a the identity morphism (.) :: cat b c -> cat a b -> cat a c morphism composition -------- However, the documentation lists only two instances of Category, functions (->) and Kleisli Monad. For instruction purposes, could someone show me an example or two of how to make instances of this class, perhaps for a few of the common types? My initial thoughts were something like so: code: -------- instance Category Integer where id = 1 (.) = (*) -- and instance Category [a] where id = [] (.) = (++) ------- But these lead to kind mis-matches. -- frigidcode.com

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BayHac '13
by Mark Lentczner 04 May '13

04 May '13

Please join us for a weekend of Haskell hacking: *BayHac '13* *May 17th ~ 19th, 2013* *Hacker Dojo* *Mountain View, CA* Full details on the Haskell Wiki: BayHac '13<http://www.haskell.org/haskellwiki/BayHac2013> - Mark

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Summer school on Applied Functional Programming at Utrecht University; deadline for registration May 15
by Doaitse Swierstra 02 May '13

02 May '13

Again we will teach an "Applied Functional Programming Summer in Haskell" school this year at Utrecht University. In the previous four occasions students were all very happy with the school and we plan to repeat this success this year. Among the intended audience are prospective master students who gained an interest in Functional Programming, e.g. by taking a general course on programming languages, and want to learn more about Haskell and its typical programming patterns. In previous years we have taught an introductory part (advanced bachelor level), an advanced part (beginning master level) and a shared part for both groups. Topics covered are, besides some examples of domain specific languages, also monads, monad transformers, arrows, parser combinators and self-analysing programs, underlying principles, type inferencing, etc. Half of the course time is spent on a larger programming exercise; you can also come with a problem of your own if you want, and get help from the Utrecht University Software Technology group in finding the proper Haskell idioms, tools and libraries, for solving it. Important links: -- our own page where we supply information based on questions asked http://www.cs.uu.nl/wiki/USCS/ -- on this page you can find the poster you can print and hang somewhere (why not your office door): Furthermore we ask for your cooperation to bring this announcement under the attention of potential participants. Best, Doaitse Swierstra, Atze Dijkstra and Johan Jeuring

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ANNOUNCE: monoid-subclasses-0.1.1, incremental-parser-0.2.2, and SCC-0.8
by Mario Blažević 13 Apr '13

13 Apr '13

The new package monoid-subclasses [1] exports a number of classes that sit between monoids and groups: ReductiveMonoid, CancellativeMonoid, GCDMonoid, MonoidNull, and FactorialMonoid among others. The package also comes with class instances for all applicable data types from base, vector, containers, bytestring, and text packages. These classes were initially a part of the incremental-parser package [2] but they've moved, so incremental-parser-0.2.2 now depends on monoid-subclasses which has added more class methods and instances. The parsers constructed using the incremental-parser library generally work on any ReductiveMonoid, FactorialMonoid or TextualMonoid. You can thus construct a generic parser that operates on String, ByteString, or Text input, or even on Product Integer or a Map if you are so inclined. The version 0.8 of Streaming Component Combinators (a.k.a. SCC) has also been re-designed from the ground up. In the earlier versions, as in the pipes and conduit packages, the types of streams and stream-processing components were parametrized by the type of the individual items flowing through. All sources and sinks are now parametrized instead by the type of the monoid they produce or consume. As a consequence, SCC coroutines can now communicate in chunks of ByteString or Text instead of just lists, and these chunks can be of arbitrary size. The boundaries of producer's chunks are invisible to the consumer (excepting some low-level functions), which must specify the amount of data it wants to receive using a ReadRequest. This change at the heart of SCC has had a positive impact on outer layers, and especially on the Splitter component type which has become much simpler. [1] http://hackage.haskell.org/package/monoid-subclasses-0.1.1 [2] http://hackage.haskell.org/package/incremental-parser-0.2.2 [3] http://hackage.haskell.org/package/scc-0.8

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ANN: psqueue-benchmarks - benchmarks of priority queue implementations
by Niklas Hambüchen 13 Apr '13

13 Apr '13

(This is a slightly detailed email. If you are the maintainer of one of the packages benchmarked here, you might want to read it though.) Today I was looking for a Priority Queue that also allows a delete operation (some call this a "Priority Search Queue"). I found http://stackoverflow.com/questions/6976559/comparison-of-priority-queue-imp… and after looking at the 10 queue alternatives, I got to the following conclusions: * Only 3 of them allow to delete entries (are p*s*queues) * Most of them are outdated or at least unmaintained for the last 3-5 years * There was an effort to get a priority queue into containers (see the stackoverflow link), but it was not agreed on * Those efforts were driven by Louis Wasserman, who wrote one of the 3 psqueues (queuelike), but now only maintains a non-ps-queue (pqueue) that seems to be the most popular priority queue implementation PSQueue implementations ----------------------- The three packages that are psqueues are: - PSQueue (http://hackage.haskell.org/package/PSQueue-1.1) * original implementation from paper of Ralf Hinze * last upload 2008 * no test suite (but small commented out QC properties), no benchmarks * no code repository - fingertree-psqueue (http://hackage.haskell.org/package/fingertree-psqueue-0.3) * last upload 2011 * no tests, no benchmarks * no code repository - queuelike (http://hackage.haskell.org/package/queuelike-1.0.9) * last upload 2009 * no tests, no benchmarks * no code repository Benchmarks ---------- Unfortunately, none of them had tests, code in repositories or any indication about their real-world performance, so I made some criterion benchmarks. You can find them here: https://github.com/nh2/psqueue-benchmarks Graphs: http://htmlpreview.github.com/?https://raw.github.com/nh2/psqueue-benchmark… Benchmark results ----------------- * PSQueue throws a stack space overflow if you try to put in 100000 Ints * PSQueue suffers from some significant worst case in terms of queue creation, sometimes creating a queue from random numbers just takes 5 times longer (1st graph). This only happens sometimes (despite Criterion) * queuelike creation is instant - it seems to work around my benchmark somehow * converting a queuelike queue to a list surprisingly takes 10 times longer than with the other packages * in terms of average performance, the three are quite close to each other (apart from the point above). queuelike seems fastest, fingertree-psqueue is second and PSQueue slowest, with a difference of +30% to the next one My questions to the maintainers ------------------------------- @Scott: Do you have an idea why PSQueue stack overflows? @Louis: Why did you switch from queuelike to pqueue? Could you put the code up somewhere manageable (repo)? Is it possible to make pqueue a full pSqueue implementation? Niklas

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Resource Limits for Haskell
by Edward Z. Yang 04 Apr '13

04 Apr '13

Hey folks, Have you ever wanted to implement this function in Haskell? -- | Forks a thread, but kills it if it has more than 'limit' -- bytes resident on the heap. forkIOWithSpaceLimit :: IO () -> {- limit -} Int -> IO ThreadId Well, now you can! I have a proposal and set of patches here: http://hackage.haskell.org/trac/ghc/wiki/Commentary/ResourceLimits http://hackage.haskell.org/trac/ghc/ticket/7763 There is a lot of subtlety in this space, largely derived from the complexity of interpreting GHC's current profiling information. Your questions, comments and suggestions are greatly appreciated! Cheers, Edward

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install cuda
by Peter Caspers 03 Apr '13

03 Apr '13

Hi, I am trying to install the cuda package on a Windows 7 enviroment. However I run into an error and can not figure out, what it is. Can someone help ? Thanks a lot Peter C:\Users\Peter\_dev\haskell>cabal install cuda Resolving dependencies... [1 of 1] Compiling Main ( C:\Users\Peter\AppData\Local\Temp\cuda-0.5.0.1-11856\cuda-0.5.0.1\Setup.hs, C:\Users\Peter\AppData\Local\Temp\cuda-0.5.0.1 -11856\cuda-0.5.0.1\dist\setup\Main.o ) Linking C:\Users\Peter\AppData\Local\Temp\cuda-0.5.0.1-11856\cuda-0.5.0.1\dist\setup\setup.exe ... Configuring cuda-0.5.0.1... cygwin warning: MS-DOS style path detected: C:\Users\Peter\AppData\Roaming\cabal/share/config.site Preferred POSIX equivalent is: /cygdrive/c/Users/Peter/AppData/Roaming/cabal/share/config.site CYGWIN environment variable option "nodosfilewarning" turns off this warning. Consult the user's guide for more details about POSIX paths: http://cygwin.com/cygwin-ug-net/using.html#using-pathnames checking for gcc... C:\Program Files (x86)\Haskell Platform\2012.4.0.0\mingw\bin\gcc.exe checking whether the C compiler works... no configure: error: in `/cygdrive/c/Users/Peter/AppData/Local/Temp/cuda-0.5.0.1-11856/cuda-0.5.0.1': configure: error: C compiler cannot create executables See `config.log' for more details cabal: Error: some packages failed to install: cuda-0.5.0.1 failed during the configure step. The exception was: ExitFailure 77

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Optimizing Fold Expressions
by J. J. W. 02 Apr '13

02 Apr '13

Dear all, I was wondering whether it was possible to write fold expressions more elegantly. Suppose I have the following datastructure: data Expr = Add Expr Expr | Sub Expr Expr | Mul Expr Expr | Eq Expr Expr | B Bool | I Int deriving Show type ExprAlgebra r = (r -> r -> r, -- Add r -> r -> r, -- Sub r -> r -> r, -- Mul r -> r -> r, -- Eq Bool -> r, -- Bool Int -> r -- Int ) foldAlgebra :: ExprAlgebra r -> Expr -> r foldAlgebra alg@(a, b, c ,d, e, f) (Add x y) = a (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Sub x y) = b (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Mul x y) = c (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (Eq x y) = d (foldAlgebra alg x) (foldAlgebra alg y) foldAlgebra alg@(a, b, c ,d, e, f) (B b') = e b' foldAlgebra alg@(a, b, c ,d, e, f) (I i) = f i If I am correct, this works, however if we for example would like to replace all Int's by booleans (note: this is to illustrate my problem): replaceIntByBool = foldAlgebra (Add, Sub, Mul, Eq, B, \x -> if x == 0 then B False else B True) As you can see, a lot of "useless" identity code. Can I somehow optimize this? Can someone give me some pointers how I can write this more clearly (or with less code?) So I constantly don't have to write Add, Sub, Mul, for those things that I just want an "identity function"? Thanks in advance! Jun Jie

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