July 2010 - Haskell-Cafe

analogous functions in Sound.File.Sndfile
by Maria Gabriela Valdes 07 Jul '10

07 Jul '10

Does anybody know what are the analogous functions and data types in the haskell library "Sound.File.Sndfile" of the libsndfile library in C ? data type : SNDFILE function: sf_open function : sf_read_short Thanks in advance, -- Maria Gabriela Valdes G. Linux Registered User #485743

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finding the right mathematical model
by Günther Schmidt 07 Jul '10

07 Jul '10

Hi list, the problem I have stems from the app I had developed. What my app does is to split the money a hospital receives for a case to the departments involved in a fair way. An additional requirement however was to allow the users of the app to re-map any revenue shares credited to certain departments to other departments. Such cases are sometimes due to politics within the hospital and also have more legitimate reasons, like saying the radiology should not receive shares for surgical procedures but those shares should be redirected to the "General surgery" department. The feature is already implemented, but I'm not pleased with it, especially since I did not develop a mathematical model for it. Details: It boils down to model mappings, or rather what sort of data structure would be suited for this kind of thing. Dept A is "mapped" to itself A -> A Dept B is mapped to Dept C B -> C Dept C is mapped to Dept C C -> C Dept D is mapped to Dept A D -> A It should not be possible to construct looping mappings, ie. 1. A -> B 2. B -> C 3. C -> A ...... What sort of model would be suitable to describe this, some sort of matrix? Günther

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Re: [Haskell-cafe] reading and playing music openAL
by Patai Gergely 07 Jul '10

07 Jul '10

Hello, > Hi ! We have a question about about openAL. We would like to know if anybody > knows how to read a WAV file by chunks of a determined size, and after doing > some processing with a specific chunk send that same chunk back to the sound > card so we can play the whole WAV continiously (just like a music player). The basic idea is that you have to define a source and repeatedly call queueBuffers on it. I wrote an OpenAL version of the Hemkay module player (example code without comments, with some horrible way to achieve double buffering can be found at [1]), which can be compiled if you have installed the hemkay-core package. Also, you might want to ask sound related questions on the Haskell art list [2]. Gergely [1] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27083 [2] http://lists.lurk.org/mailman/listinfo/haskell-art -- http://www.fastmail.fm - Does exactly what it says on the tin

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Debugging cause of indefinite thread blocking
by Ben Lever 07 Jul '10

07 Jul '10

Hi all, I am making use of the Data.Array.Repa module to achieve data-parallelism. On running my program I get the error: "thread blocked indefinitely on an MVar operation" Two questions: 1. What could be some of the potential causes for the above error when the only use of parallelism is Repa arrays? 2. What are the best strategies for debugging he cause(s) of such an error? Cheers, Ben. The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.

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Re: [Haskell-cafe] Criteria for determining if a recursive function can be implemented in constant memory
by Steffen Schuldenzucker 07 Jul '10

07 Jul '10

Hum. You are right and I'm probably asking the wrong questions. My original question was when it was possible to eliminate stack frames. For example, in the 'f' function from my first post, we know that none of the variables x and y will be needed after the recursive call to f, so we can just re-use the current stack frame for it, preventing f from using "additional" space for the recursion. However, stack frames are an implementation detail and I thought "constant-space" was an abstraction of my idea. Looks like it was not. On 07/06/2010 04:07 PM, Lennart Augustsson wrote: > Are you limiting your data structures to numbers? In that case, only > numbers of limited size, the answer is, of course, yes. You can > implement any such function in constant space and time. Just make a > lookup table. > > Sent from my iPad > > On Jul 6, 2010, at 6:37, Steffen Schuldenzucker > <sschuldenzucker(a)uni-bonn.de <mailto:sschuldenzucker@uni-bonn.de>> wrote: > >> >> Forwarding this message to the list. >> >> No, I didn't think about the size of integers. For now, let all >> numbers have some bounded size. >> >> -------- Original Message -------- >> Subject: Re: [Haskell-cafe] Criteria for determining if a recursive >> function can be implemented in constant memory >> Date: Tue, 6 Jul 2010 13:25:57 +1200 >> From: Richard O'Keefe <ok(a)cs.otago.ac.nz <mailto:ok@cs.otago.ac.nz>> >> To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de >> <mailto:sschuldenzucker@uni-bonn.de>> >> >> >> >> On Jul 6, 2010, at 12:23 AM, Steffen Schuldenzucker wrote: >> > Given the definition of a recursive function f in, say, haskell, >> > determine if f can be implemented in O(1) memory. >> >> How are you supposed to handle integer arithmetic? >> >> If you don't take the size of integers into account, >> then since a Turing machine can do any computation, >> it can run a Haskell interpreter, and since a Turing >> machine's tape can be modelled by a single integer >> (or more conveniently by two), any Haskell function >> can be implemented in O(1) Integers. >> >> If you do take the size of integers into account, >> then >> pow2 n = loop n 1 >> where loop 0 a = a >> loop (m+1) a = loop m (a+a) >> requires O(n) memory. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >> http://www.haskell.org/mailman/listinfo/haskell-cafe

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cabal and ghc disagree on the name of the module Main
by robin morisset 06 Jul '10

06 Jul '10

Hello, I have the following directories set : Project/ Project/Language Project/Language/Copilot/ in Project there is the copilot.cabal file. in Project/Language/Copilot there are a bunch of .hs files, including Main.hs All the modules in these files are named Language.Copilot.NameOfTheFile for cabal to work. My issue happened when I chose to add a Main.hs, containing Language.Copilot.Main and to try to compile it with ghc --make. I got the error message "output was redirected with -o, but no output will be generated because there is no Main module." So I replaced Language.Copilot.Main by Main. ghc was happy, but cabal sent me the following error message : "File name does not match module name: Saw: `Main' Expected: `Language.Copilot.Main'" So I finally kept the Language.Copilot.Main module name, and tried to use the -main-is ghc option. The result was an unreadable linker error message : "/usr/lib/ghc-6.12.1/libHSrtsmain.a(Main.o): In function `main': (.text+0x10): undefined reference to `ZCMain_main_closure' /usr/lib/ghc-6.12.1/libHSrtsmain.a(Main.o): In function `main': (.text+0x18): undefined reference to `__stginit_ZCMain' collect2: ld returned 1 exit status" Does anyone know what this last message mean, and how to make both cabal and ghc happy with my module name ? Thank you in advance, Robin

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Canonical Graphviz code
by Ivan Miljenovic 06 Jul '10

06 Jul '10

Graphviz (http://graphviz.org/) has the option to convert provided Dot code for visualising a graph into a canonical form. For example, take the sample Dot code: ---- | digraph foo { | a [color=blue]; | subgraph cluster_bar { | a [label="hi!"] | b [color=blue, label="bye!"]; | b -> c; | } | a -> b; | a -> c; | c -> d; | d [color=red] `---- Calling "dot -Tcanon" on this produces: ,---- | digraph foo { | node [label="\N"]; | subgraph cluster_bar { | a [label="hi!", color=blue]; | b [label="bye!", color=blue]; | b -> c; | a -> b; | a -> c; | } | d [color=red]; | c -> d; | } `---- Note in particular the following: * The addition of the top-level "node [label="\N"]" attribute; this happens automatically even if all nodes have their own custom label. * The two `a' nodes are merged into the inner-most cluster/subgraph that they are defined in (since they represent the same node). * The attributes for `b' are re-ordered. * The "a -> b" and "a -> c" edges are shifted into the cluster, since all of the related nodes are in that cluster (note that the `c' node is implicitly defined there since an edge involving it is defined there). * The "c -> d" edge is shifted to being _after_ the definition of the `d' node (in fact, in each grouping all nodes are defined before edges). I've recently thought up a way that I can duplicate this functionality (in terms of what it does, not necessarily the actual output) in my graphviz library (http://hackage.haskell.org/package/graphviz) however I'm not sure how closely to follow what Graphviz does. What would the community prefer out of the following (including a mixture, such as options 2 and 3): 1) Match Graphviz's output as much as possible (note that the ordering of the attributes won't happen this way, since I'll be sorting them alphabetically rather than working out Graphviz's arcane rules). 1a) As with 1), but don't worry about defining extra attributes such as "node [label="\N"]". 2) Explicitly define nodes that are only mentioned in edges (e.g. actually define a `c' node, even if it doesn't have any extra attributes). Note that this already happens if such an edge is specified inside a different cluster than the already known node is in; in that case then the stand-alone node is defined in the cluster its edge is defined in, and the actual edge is moved into the outer context that combines the two clusters. 2a) As with 2), but define all edges in the overall graph rather than grouping them internally inside clusters, etc. 3) Group common attributes together as much as possible (e.g. inside the bar cluster, define another subgroup which has a top-level attribute "node [color=blue]" and define the `a' and `b' nodes in there). Not sure how well this will work with edges though, and is probably not possible in conjunction with option 2a). This is a bit tricky and subtle: if `a' and `b' are already in such a subgraph, but an edge specifying one of them is defined _before_ the subgraph is defined, then that node will have an explicit color attribute defined of "" (i.e. empty string), though I would classify this as a bug. 4) As an alternate to option 3), remove all explicit top-level node and edge attributes and move them all to the affected node and edge definitions themselves; this may require option 2). I'm also willing to hear and consider other possible variants; however, I'm only going to code (at most) one. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: ghc-gc-tune: a tool for analyzing the impact of GC flags
by Don Stewart 06 Jul '10

06 Jul '10

Inspired by a comment by Simon Marlow on Stack Overflow, about the time and space tradeoffs we make with garbage collection, particularly with a generational GCs, I wrote a small program, ghc-gc-tune, to traverse the garbage collector variable space, to see the relationship between settings and program performance. Given a program, it will show you an (optionally interactive) graph of how -A and -H flags to the garbage collector affect performance. http://donsbot.wordpress.com/2010/07/05/ghc-gc-tune-tuning-haskell-gc-setti… Feedback and patches welcome! -- Don

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CGI differences FastCGI
by Jonas Fager 06 Jul '10

06 Jul '10

Hi. This is probably "I must be blind department" I have a very basic strip down echo cgi that runs with apache2. When I try to modfi it to be a FastCGI instead and run it with spawn-fcgi with nginx It don't behave as expected. I suppose that the line that don't get any data is mn <- getInputFPS "file" when running under FastGCI. So is the code wrong? or have I missed something with the setup? I can run the example of fastcgi provided in http://mult.ifario.us/p/wiring-haskell-into-a-fastcgi-web-server with no problem. Description: Ubuntu 10.04 LTS 2.6.32-22-generic-pae #36-Ubuntu SMP The Glorious Glasgow Haskell Compilation System, version 6.12.1 +++++ import Control.Concurrent import Network.CGI -- import Network.FastCGI import Text.Html import qualified Data.ByteString.Lazy as BS import qualified Data.ByteString.Lazy.Char8 as BS8 import Control.Monad (liftM) import Data.Maybe (fromJust) import Char import Data.List import System.Time import qualified Data.ByteString.Char8 as C import Text.JSON.Generic import Text.JSON insertthis fb = do let f = BS8.unpack fb return (f) fileForm = form ! [method "post", enctype "multipart/form-data"] << [textfield "file", submit "" "Upload"] saveFile cont = do r<-liftIO $ insertthis cont return $ paragraph << ("Inserted " ++ (show r)) page t b = header << thetitle << t +++ body << b test :: CGI CGIResult test = do setHeader "Content-Type" "text/html; charset=utf-8" mn <- getInputFPS "file" h <- maybe (return fileForm) saveFile mn output $ renderHtml $ page "Upload" h main = runCGI $ handleErrors test --main= runFastCGI $ handleErrors test +++++

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Subtype polymorphism in Haskell
by Simon Courtenage 06 Jul '10

06 Jul '10

Hi, I am porting a C++ program to Haskell. My current task is to take a class hierarchy and produce something equivalent in Haskell, but I don't seem to be able to get a grip on how type classes and instances contribute to the solution. Can anyone enlighten me? Roughly, the class hierarchy in C++ is of the form class A { public: virtual int do_x(int,int) = 0; }; class B { public: int do_x(int x,int y) { ...} }; class C { public: int do_x(int x,int y) { ...} }; Any help would be greatly appreciated. Thanks Simon courtenage(a)gmail.com

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