- Haskell-Cafe - Haskell.org

Equivalence of two expressions
by Grigory Sarnitskiy 12 Jul '10

12 Jul '10

I'm not very familiar with algebra and I have a question. Imagine we have ring K. We also have two expressions formed by elements from K and binary operations (+) (*) from K. Can we decide weather these two expressions are equivalent? If there is such an algorithm, where can I find something in Haskell about it? If there is no such algorithm for a ring, maybe there is for a field?

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haskell on suse?
by gladstein＠gladstein.com 12 Jul '10

12 Jul '10

I've been spoiled by package managers that download and install everything for you, and I've forgotten how RPM works. In particular, I want to install Haskell on suse, and I read that "RPMs are available from openSUSE. " I followed the link, and found a bunch of folders of folders of RPM files. What exactly should I do to install Haskell? Sorry about the newbicity of this request, but I'm totally stumped. If suse isn't a well-supported platform, I'll switch. Thanks in advance.

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ANNOUCE: graphviz-2999.10.0.0
by Ivan Lazar Miljenovic 12 Jul '10

12 Jul '10

I'm pleased to announce a new version of my graphviz [1] library that provides bindings to the Graphviz [2] suite of tools for visualising graphs. [1]: http://hackage.haskell.org/package/graphviz [2]: http://graphviz.org/ Changes in this release: * I followed my own advice and put in bounds on the version of FGL it used... * Conversion of FGL-style graphs to `DotRepr` values is now achieved using the new `GraphvizParams` configuration type. This allows us to define a single parameter that stores all the conversion functions to pass around rather than having to pass around several functions. This also allows all the non-clustered and clustered functions to be collapsed together, whereas what used to be handled by the primed functions is now achieved by using the `setDirectedness` function. There are three default `GraphvizParams` available: - `defaultParams` provides some sensible defaults (no attributes or clustering). - `nonClusteredParams` is an alias of `defaultParams` where the clustering type is explicitly set to be `()` for cases where you don't want any clustering at all (whereas `defaultParams` allows you to set your own clustering functions). - `blankParams` sets all fields to be `undefined`; this is useful for situations where you have functions that will set some values for you and there is no sensible default you can use (mainly for the clustering function). * Expansion of the `DotRepr` class: - More common functions are now defined as methods (`getID`, etc.). - The ability to get more information about the structure of the `DotRepr` graph, as well as where all the `DotNode`s are, etc. - `graphNodes` now returns `DotNode`s defined only as part of `DotEdge`s, and will also merge duplicate `DotNode`s together. - `graphNodes` and `graphEdges` also return `GlobalAttributes` that apply to them. * The `Point` type now only has one constructor: `Point Double Double`. The `Int`-only constructor was present due to historical purposes and I thought that the `Pos` value for a `DotNode` would always be a pair of `Int`s; this turns out not to be the case (and accounted for some bugs in older versions of my SourceGraph tool; it's taken me this long to remember to fix this...). * `SortV` and `PrismOverlap` now only take `Word16` values rather than `Int`s, as they're not meant to allow negative values (the choice of using `Word16` rather than `Word` was arbitrary, and because it's unlikely those values will be large enough to require the larger values available in `Word`). * `NodeCluster` has been generalised to not have to take an `LNode` for the node type; the type alias `LNodeCluster` is available if you still want this. * Several documentation typos fixed, including one spotted by Kevin Quick. * The test-suite now allows running individual tests. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: SourceGraph-0.6.1.1 and Graphalyze-0.10.0.0
by Ivan Lazar Miljenovic 12 Jul '10

12 Jul '10

I'm pleased to announce updated versions of SourceGraph [1] (my graph-theoretic static analysis tool for Haskell) and Graphalyze [2] (a library for graph-theoretic analysis of relationships in discrete data). [1]: http://hackage.haskell.org/package/SourceGraph [2]: http://hackage.haskell.org/package/Graphalyze This version of SourceGraph now (should) only consider executables listed in .cabal files as being buildable, and both packages have been updated to use the latest versions of fgl (as well as following my own advice in putting an upper bound on the version of fgl being used...), graphviz and haskell-src-exts (SourceGraph only). Please note that SourceGraph-0.6.1.0 was a version I forgot to upload (and just realised this when I wondered why it wasn't appearing on Hackage...). The big change that should be apparent with this version of SourceGraph is that thanks to the changes I just made to the newly-uploaded fgl-5.4.2.3, multiple edges are finally being handled/visualised properly; for example see the nice thick lines in http://code.haskell.org/~ivanm/Sample_SourceGraph/SourceGraph/graphs/codeCo… (which represents among other things 32 recursive calls within getExp and 7 calls from getExp to maybeEnt). I forgot to put this in the TODOs, but here are my future plans for these packages. They're rather ambitious and I have no idea when I'll get around to them, but for all intents the only work I'm going to do on them until I do these splitting up goals is bug-fixes, etc. Graphalyze ---------- Graphalyze is going to fade away: I'm going to integrate its data -> graph conversion utilities into either my new graph library I'm going to work on during AusHack or into the successor for fgl Thomas Bereknyei and are working on (whatever we'll end up calling it), the Data.Graph.Analysis.Algorithms modules into graph-algorithms and fgl-algorithms packages and the reporting stuff into its own reporting library with different backends (pandoc, blaze, etc.). SourceGraph ----------- I want to split out and formalise the whole call-graph definitions and concepts into its own library and have sub-libraries for different languages to be able to have their own code -> call-graph conversions (so it can also be used for language-c, language-python, etc.). SourceGraph itself would then be more flexible in terms of what languages it can deal with (so it might even be possible to integrate C source files in a Haskell package into the call graph!). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Multidimensional Matrices in Haskell
by Mihai Maruseac 12 Jul '10

12 Jul '10

Hi, A friend of mine wanted to do some Cellular Automata experiments in Haskell and was asking me what packages/libraries are there for multidimensional matrices. I'm interested in both immutable and mutable ones but I don't want them to be trapped inside a monad of any kind. Any hints? -- MM

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EnumMap 0.0.2 and Containers-0.3
by Max Cantor 12 Jul '10

12 Jul '10

Is there a reason that EnumMap specifically requires containers < 0.3 or just a matter of not having been tested? It seems to compile just fine (but with one warning for a deprecated call). Max

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ANNOUNCE: UMM-0.3.0
by Uwe Hollerbach 12 Jul '10

12 Jul '10

Hi, all, I've just uploaded version 0.3.0 of umm, my small money-manager program, to hackage. This version does nicer plotting of data than before (depends on gnuplot). Have a look if you... errr... like money :-) regards, Uwe

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HaskellWiki spam
by Henk-Jan van Tuyl 12 Jul '10

12 Jul '10

L.S., There are hundreds of HaskellWiki users created, their names all start with "Buy" and their user pages contain spam. I suppose the antispam measures were reverted when a backup of the site was loaded. (B.T.W. the site still displays the old logo.) Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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hs-dotnet with Mono?
by Daniel Cook 12 Jul '10

12 Jul '10

Hi, Has anyone used hs-dotnet (http://haskell.forkio.com/dotnet/) with Mono on Linux? Cheers, - Dan

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Mapping a Texture in OpenGL?
by Hector Guilarte 12 Jul '10

12 Jul '10

Hello, I already send this to the HOpenGL list, but I'm kind of in a hurry with this and I'll probabbly get more responses from you... I've been looking for a way to map a texture into a Quad primitive with HOpenGL with no luck. I've already done this in C++ and had no problem. I did the tutorial in http://www.haskell.org/~pairwise/HOpenGL/HOpenGL.html, downloaded the sources, compiled them and it worked great (After changing a 2 lines to make it work with OpenGL's latest version), but when I change the code to use a .rgb file created by me with the program they provide from a .png file of 256x256 pixels I don't have the same luck... My OpenGL version is 2.4.0.1... I'm REALLY lost, clueless, so any kind of help would be greatly appreciated. I checked the textures in the examples of the red book, but they are "procedural" textures, and I need to map an image. One of the pictures I need to map is in http://dl.dropbox.com/u/1476919/Orange256.png Are six like that one and are for making a Rubik's Cube. After I finish it I'll upload it to hackage and let you all know. Thanks a lot, Héctor Guilarte

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Re: [Haskell-cafe] Re: lhs2TeX - lhs2TeX.fmt missing
by Tillmann Rendel 11 Jul '10

11 Jul '10

Hi Ivan, (why are you answering off-list?) Ivan Miljenovic wrote: > I was under the impression that with cweb, you can have one function > definition split into two, with another completely different block of > code in between them. I agree, that's something literate haskell can not do. (But it's different from what you wrote before). Tillmann

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Haskellers in Cleveland
by Jason Felice 11 Jul '10

11 Jul '10

I've just started a Cleveland Haskellers meetup, and for some reason I haven't thought to ask on this list yet. If there's anybody from the Greater Cleveland, OH area who is interested, please sign up! I hope to keep it small and informal. I've had good luck keeping it small so far. There's a meetup next Saturday, in fact. http://www.meetup.com/cle-haskell/ Thanks, -Jason

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new haskell project (Was Re: [Haskell-cafe] Subtype polymorphism in Haskell)
by Simon Courtenage 11 Jul '10

11 Jul '10

Hi, Just to add some details about the project I'm working on in case anyone is interested. The project is called Quanthas and is being hosted on sourceforge at http://sourceforge.net/projects/quanthas/. The aim of the project is to produce a Haskell implementation of Quantlib ( http://sourceforge.net/projects/quantlib/)- an open source library for quantitative finance written in C++. Haskell is starting to be used seriously in quantitative finance and risk modelling circles within the investment banking and finance community, so I thought there would be some value in producing such a version. If anyone is interested in helping out, we would be more than happy to hear from you, since the project has just started and there's a great deal to do (and learn!). Best regards Simon Courtenage On Tue, Jul 6, 2010 at 2:32 PM, Yitzchak Gale <gale(a)sefer.org> wrote: > Hi Simon, > > Did you intentionally not reply to the list? > > Simon Courtenage wrote: > > This is for a project to port an open-source C++ library to haskell. > > Great! We'd love to give you whatever support you need > for your efforts. > > > My initial plan is to more or less preserve the way the > > library works in the first draft of the port and see how > > far we can get like that > > That's fine, as long as you truly mean the way it works, > and not the way the code is structured. Haskell is a post-OO > language. Its abstractions are very different than class > structures in C++. There is no direct translation - any given > C++ class structure could correspond to many totally different > kinds of Haskell programs, depending on what the program > is trying to do. > > If you are trying to find a method to transliterate a strongly > OO-style C++ program more or less word for word into > Haskell in a way that the class structure of the C++ will > still be apparent in the result, you are likely in for a frustrating > experience. You will spend a lot more time than you expected, > and the results will be very unsatisfying. Many others have > ended up that way. > > On the other hand, if you are willing to be a little more flexible > in your thinking, you'll probably find the task much easier than > you thought, enjoy it, and reap many benefits from the process > that you never imagined. > > In any case, please keep us in the loop, we'd like to hear > how it's going. And, uh... would you be willing to share a > few more details about what it is that you're trying to port? ;) > > Thanks, > Yitz >

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building ghc-6.12.3 from source on Open Solaris
by Günther Schmidt 10 Jul '10

10 Jul '10

Hi, does anyone here have experience what it takes to build ghc-6.12.3 from source on Open Solaris, with binary ghc-6.12.1 installed? Günther

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ANN: HaRe-0.6, now on Hackage
by Chris BROWN 10 Jul '10

10 Jul '10

Dear Haskellers, As part of our project on Refactoring Functional Programs http://www.cs.kent.ac.uk/projects/refactor-fp/ we are pleased to announce the availability of HaRe 0.6 on Hackage. http://hackage.haskell.org/package/HaRe-0.6 Please see the README.txt for build/use instructions and known issues, and let us know about any problems, bugs, suggestions or additional platforms you can confirm as working. Happy Refactoring! The HaRe Team (Chris Brown, Huiqing Li, Simon Thompson) Background: Refactoring is the process of changing the structure of programs without changing their functionality, i.e., refactorings are meaning-preserving program transformations that implement design changes. For more details about refactoring, about our project and for background on HaRe, see our project pages. HaRe - the Haskell Refactorer: HaRe is our prototype tool supporting a collection of refactorings for Haskell 98 (see README.txt for known issues and limitations). It is implemented as a separate refactoring engine (on top of Programatica's Haskell frontend and Strafunski's generic traversal strategy library), with small scripting frontends that call this engine from either Vim or Emacs.

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Transformers versus monadLib versus...
by Gregory Crosswhite 10 Jul '10

10 Jul '10

Hey everyone, What is the current state regarding transformers versus monadLib versus mmtl versus ... etc.? Transformers seems to be the "blessed" replacement for mtl, so when is it worthwhile to use the other libraries instead? It hadn't even occurred to me to look closely at packages other than transformers for this purpose until I read a message earlier today from someone who said that he uses monadLib whenever he needs to use monad transformers, and it has now made me curious about them. Cheers, Greg

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Memoization in Haskell?
by Angel de Vicente 10 Jul '10

10 Jul '10

Hi, I'm going through the first chapters of the Real World Haskell book, so I'm still a complete newbie, but today I was hoping I could solve the following function in Haskell, for large numbers (n > 108) f(n) = max(n,f(n/2)+f(n/3)+f(n/4)) I've seen examples of memoization in Haskell to solve fibonacci numbers, which involved computing (lazily) all the fibonacci numbers up to the required n. But in this case, for a given n, we only need to compute very few intermediate results. How could one go about solving this efficiently with Haskell? Thanks in advance, Ángel de Vicente -- http://www.iac.es/galeria/angelv/ High Performance Computing Support PostDoc Instituto de Astrofísica de Canarias --------------------------------------------------------------------------------------------- ADVERTENCIA: Sobre la privacidad y cumplimiento de la Ley de Protección de Datos, acceda a http://www.iac.es/disclaimer.php WARNING: For more information on privacy and fulfilment of the Law concerning the Protection of Data, consult http://www.iac.es/disclaimer.php?lang=en

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Fix plugins package.
by Andy Stewart 10 Jul '10

10 Jul '10

Hi all, I want to use *plugins* package (http://hackage.haskell.org/package/plugins-1.4.1) Unfortunately, it looks broken. Anybody can fix it? Thanks, -- Andy

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bug in ghci ?
by Pasqualino "Titto" Assini 10 Jul '10

10 Jul '10

Hi, I just noticed that in ghci: data Test = Test String instance Show Test show $ Test "Hello" Will result in infinite recursion. Is this a known bug? Thanks, titto

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Transformers versus monadLib versus...
by Gregory Crosswhite 10 Jul '10

10 Jul '10

Hey everyone, What is the current state of opinion regarding transformers versus monadLib versus mmtl versus ... etc.? Transformers seems to be the "blessed" replacement for mtl, so when is it worthwhile to use the other libraries instead? (It hadn't even occurred to me to look closely at packages other than transformers for this purpose until I read a message earlier today from someone who said that he uses monadLib whenever he needs to use monad transformers, and it has now made me curious about them.) Cheers, Greg

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Code Example and Haskell Patterns
by Ali Razavi 09 Jul '10

09 Jul '10

Hi, Hackage is a sizable repository of Haskell code; makes me wonder if there is a way to use it more effectively for pedagogical purposes. For example, I really would like to study State monad, monad transformers, applicative, arrows etc. in action--i.e., in the context of a real application rather than small toy examples commonly found in tutorials. If there isn't such a source, can someone point me to an application whose implementation you would consider being a paragon for said concepts? Best, Ali

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FIX Protocol in Haskell (was Re: [Haskell-cafe] Talking to Java from Haskell?)
by Daniel Cook 09 Jul '10

09 Jul '10

> 1) How stable/defined is FIX? Not very. There are several protocol versions, various vendors have their own custom message types, there are service packs released fairly regularly updating the protocol for new message types, etc. > 2) How large of a subset of FIX do you use? I will be using a relatively small subset of the available FIX messages, and my subset will change a lot at the beginning but then stabilize. > work overall. If FIX has only one canonical implementation, changes > frequently, or you'll only use a small portion of it, then b) would > likely be simplest. FIX has one canonical open source implementation (actually 2, but they're the same program ported to C++ and Java). Another option is to use Haskell's FFI to wrap the C++ implementation. QuickFIX is designed exactly the way you *wouldn't* design it to make interop with Haskell easy. The API is based on callbacks to a class which one extends. The C++ uses lots of templates, and the .h files are full of macros. On the plus side, the program has a .idl, and is already SWIG-wrapped for C#, Python, and Ruby. Do you (or anyone) have experience wrapping highly stateful C++ programs with Haskell FFI? Are there examples around? (Xmonad comes to mind, but I don't know if X11's API is C or C++ ..) I think I'm going to have to recreate the various message types in Haskell's type system anyway. Given that, maybe I should simply use Haskell to implement a small, clean, extensible subset of this protocol (what I need). I can then test it heavily against the QuickFIX implementation. Opinions? -- Dan On Fri, Jul 9, 2010 at 2:31 PM, John Lato <jwlato(a)gmail.com> wrote: >> From: Daniel Cook <danielkcook(a)gmail.com> >> >> Hi, >> >> Someone has written a large Java library (QuickFIX/J) which speaks a >> gnarled, ugly protocol (FIX). There don't appear to be any FIX >> protocol libraries in Hackage. I need my Haskell program to talk to a >> 3rd-party system that only speaks FIX. >> >> Should I: >> >> a) Reimplement the protocol directly Haskell? (This appears to be non-trivial) >> >> b) Wrap the Java library with some code to use a lightweight message >> queue (zeromq) to send messages to my Haskell program? (This would >> require essentially re-implementing an abstracted subset of the the >> protocol into 0MQ messages) > > I think you need to answer these two questions to make a decision: > > 1) How stable/defined is FIX? > 2) How large of a subset of FIX do you use? > > If FIX is relatively stable or has multiple (non-C) implementations in > current use, and you'd be using a large-enough subset, then I expect > implementing a Haskell implementation would be the smallest amount of > work overall. If FIX has only one canonical implementation, changes > frequently, or you'll only use a small portion of it, then b) would > likely be simplest. > > Another option would be to expose hooks to your Haskell program > through the FFI and have it called by Java. > > John >

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Other transactional arrays?
by Emil Melnicov 09 Jul '10

09 Jul '10

Currently, Haskell have transactional arrays in "Control.Concurrent.STM.TArray" implemented as "Array i (TVar e)" which is array of transactional variables. But what if I need to place an array into TVar itself? This is something like "TVar (IOArray i e)", but with ability to read/write array elements inside an STM transaction, and then commit changes with "atomically". I've tried to make up this arrays in the following way: > {-# LANGUAGE MagicHash #-} > {-# LANGUAGE UnboxedTuples #-} > {-# OPTIONS_GHC -fno-cse #-} > module Data.Vector.STM > ( TVector > , newTVector > , newTVectorIO > , readTVector > , readTVectorIO > , writeTVector > ) where > import GHC.Conc > import GHC.Prim > import GHC.Types (Int(..), IO(..)) > data MutableArray a = MutableArray (MutableArray# RealWorld a) > data TVector a = TVector (TVar# RealWorld (MutableArray a)) > stm = STM > {-# NOINLINE stm #-} > newTVector :: Int -> a -> STM (TVector a) > newTVector (I# i#) x = stm $ \s1# -> > case newArray# i# x s1# of { (# s2#, a# #) -> > case newTVar# (MutableArray a#) s2# of { (# s3#, t# #) -> > (# s3#, (TVector t#) #) }} > newTVectorIO :: Int -> a -> IO (TVector a) > newTVectorIO (I# i#) x = IO $ \s1# -> > case newArray# i# x s1# of { (# s2#, a# #) -> > case newTVar# (MutableArray a#) s2# of { (# s3#, t# #) -> > (# s3#, (TVector t#) #) }} > readTVector :: TVector a -> Int -> STM a > readTVector (TVector t#) (I# i#) = stm $ \s1# -> > case readTVar# t# s1# of { (# s2#, (MutableArray a#) #) -> > case readArray# a# i# s2# of { (# s3#, a #) -> > (# s3#, a #) }} > readTVectorIO :: TVector a -> Int -> IO a > readTVectorIO (TVector t#) (I# i#) = IO $ \s1# -> > case readTVarIO# t# s1# of { (# s2#, (MutableArray a#) #) -> > case readArray# a# i# s2# of { (# s3#, a #) -> > (# s3#, a #) }} > writeTVector :: TVector a -> Int -> a -> STM () > writeTVector (TVector t#) (I# i#) x = stm $ \s1# -> > case readTVar# t# s1# of { (# s2#, (MutableArray a#) #) -> > case writeArray# a# i# x s2# of { s3# -> > case writeTVar# t# (MutableArray a#) s3# of { s4# -> > (# s4#, () #) }}} It seems like it works, but I'm in doubt about it's correctness. Unfortunately, I don't know much about STM mechanics, so I'm asking Cafe users (you :-) for help.

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Re: [Haskell-cafe] Talking to Java from Haskell?
by John Lato 09 Jul '10

09 Jul '10

> From: Daniel Cook <danielkcook(a)gmail.com> > > Hi, > > Someone has written a large Java library (QuickFIX/J) which speaks a > gnarled, ugly protocol (FIX). There don't appear to be any FIX > protocol libraries in Hackage. I need my Haskell program to talk to a > 3rd-party system that only speaks FIX. > > Should I: > > a) Reimplement the protocol directly Haskell? (This appears to be non-trivial) > > b) Wrap the Java library with some code to use a lightweight message > queue (zeromq) to send messages to my Haskell program? (This would > require essentially re-implementing an abstracted subset of the the > protocol into 0MQ messages) I think you need to answer these two questions to make a decision: 1) How stable/defined is FIX? 2) How large of a subset of FIX do you use? If FIX is relatively stable or has multiple (non-C) implementations in current use, and you'd be using a large-enough subset, then I expect implementing a Haskell implementation would be the smallest amount of work overall. If FIX has only one canonical implementation, changes frequently, or you'll only use a small portion of it, then b) would likely be simplest. Another option would be to expose hooks to your Haskell program through the FFI and have it called by Java. John

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Something that kind of resembles OO
by Michael Mossey 09 Jul '10

09 Jul '10

I notice in algebraic data defined with named fields, you can use the same name inside different constructors and then apply it to any data of that type. data Vehicle = Car { ident :: String, wheel :: Circle } | Plane { ident :: String, stick :: Line } f :: [Vehicle] -> [String] f = map ident This is a little like fields in a base class. I also see that a wrong use of accessor functions will compile but give a runtime error: test = wheel (Plane "foo" (Line 1)) Will give a runtime error.

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does () match with a??
by Hector Guilarte 09 Jul '10

09 Jul '10

Hey everyone, I'm making a Game Monad for an assignment (yes, homework) . Here's a little explanation of what I need to do (I can't use anything from Control.Monad.*, I need to do everything myself): I need to define my newtype Game and make it's Monad instance I need to make the function : runGame :: Game a -- A particular game -> Int -- Initial amount of lives -> Maybe (a , Int ) -- Result and remaining lives. I need to make the instance for this class: class Monad m = > GameMonad m where extraLife :: m () getLives :: m Int checkPoint :: m a -> m a die :: m a now what I've done (I was inspired by the State Monad) I defined my Game type as follows: > newtype Game r = Game { execGame :: Int -> Maybe (r,Int) } My Monad instance like this: > instance Monad Game where > return a = Game $ \r -> Just (a,r) > m >>= k = Game $ \r -> let x = execGame m r > in case x of > Just (a, r') -> execGame (k a) r' > Nothing -> Nothing and my GameMonad instance: > instance GameMonad Game where > extraLife = Game $ \l -> Just ((),l+1) > getLives = Game $ \l -> Just (l,l) > die = do > n <- getLives > Game $ \_ -> Just ((),n-1) -- Here's the problem so, what's bothering me? Look at the type signature of die in the GameMonad class, it's supposed to return something of type (m a), but I don't know what to return in that case, and whatever I try to return it doesn't work, because when I try to compile it says that it couldn't match expected type 'a' against infered type 'whatever' ('whatever' being anything, from (), to string, or a number). shouldn't 'a' match with anything I put there? Note that I can't change the signatures because they were giving to me that way and I already checked with teacher if they were right. (I fixed it returning Maybe (Maybe a, Int) instead, but I can't change the signature of the function runGame) Thanks you, Hector Guilarte

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Re: [Haskell-cafe] Multidimensional Matrices in Haskell
by John Lato 09 Jul '10

09 Jul '10

Hello, There are a lot of options. The "array" package, which is included with GHC, provides both mutable and immutable arrays of arbitrary dimensions. A quick scan of hackage shows ArrayRef, ix-shapable, and judy as alternatives, among others. Immutable arrays can be pure, but all mutable array interfaces will require a monad of some type. I usually think ST-based mutability is easiest to work with. John > From: Mihai Maruseac <mihai.maruseac(a)gmail.com> > > Hi, > > A friend of mine wanted to do some Cellular Automata experiments in > Haskell and was asking me what packages/libraries are there for > multidimensional matrices. I'm interested in both immutable and > mutable ones but I don't want them to be trapped inside a monad of any > kind. > > Any hints? > > -- > MM >

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use of modules to save typing
by Michael Mossey 09 Jul '10

09 Jul '10

I'm fairly beginnerish to Haskell, and come from OO. I have a complaint about Haskell, but I think I found a good solution. Any suggestions welcome. I have RSI and like to minimize typing. The use of classes as name spaces helps to do that. Also I can use some Emacs abbreviation magic easily with OO and not so easily with Haskell. I'll explain in a second. In Haskell, when defining data for complex programs I like to use named fields to allow for changing data definitions without having to change all code. But named fields are top-level functions (I think). They must be chosen not to clash. My habit has been to prefix them with the name of the constructor. So in a program for playing back musical documents that needs to track some state, we have: data PlayState = PlayState { playState_cursor :: Int , playState_verts :: [Loc] , playState_len :: Int , playState_doc :: MusDoc } Note all these "playState_" prefixes. Lots of typing, which is not good. In OO, you could type x.cursor() In Haskell you have to type playState_cursor x which also, I feel, is harder to read. Now suppose I want to use PlayState with a State monad. -- Increment the cursor. incrCursor :: State PlayState () incrCursor = cur <- gets playState_cursor len <- gets playState_len let newCur = min (cur+1) (len-1) p <- get put $ p {playState_cursor = newCur} Okay, I'm sorry, that is just a lot of typing for what it is doing. Not good for people with RSI, and not all that readable. I could define a function to make modifying the state a little easier. playState_update_cursor :: Int -> PlayState -> PlayState playState_update_cursor i p = p {playState_cur=i} Then incrCursor would look like: incrCursor :: State PlayState () incrCursor = cur <- gets playState_cursor len <- gets playState_len let newCur = min (cur+1) (len-1) modify (playState_update_cursor newCur) Notice how often the characters "playState_" get typed. This would be a great situation for Emacs abbreviations. When you define an abbreviation in Emacs, such as defining "xps" to expand to "PlayState", emacs will watch for the characters xps. It will then replace "xps" with "PlayState" when you type a non-alphanumeric character following "xps". So if I type "xps." the moment I hit "." it changes to "PlayState." But I would have a hard time using this feature with "playState_" because it is always followed by an alphanumeric character. So my idea, now, is to put the definition of PlayState in its own module and import it qualified as PlayState. ---------------- module PlayState -------------- data PlayState = PlayState { cursor :: Int , verts :: [Loc] , len :: [Int] , doc :: MusDoc } update_cursor i p = p {cursor = i} ----------------------------------------------- I got rid of the "playState_" prefixes because I am not worried about using generic field names like "doc". They won't clash if I always import this qualified. And that reduces the necessary typing in the definition. Now my monad looks like testMonad = do cursor <- gets PlayState.cursor len <- gets PlayState.len let newCur = min (cur+1) (len-1) modify $ PlayState.update_cursor newCur Now I can define an abbreviation for PlayState. This is a big help. Also, I find this more readable. To me PlayState.cursor is more readable than playState_cursor For one thing, syntax highlighting helps in the former case. For another, the latter case requires that you recognize a naming convention, but the former case says clearly: "cursor is within the namespace PlayState, so this combination must be describing a cursor related to PlayState."

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Re: [Haskell-cafe] Merge hsql and HDBC -- there can only be one!
by Leonel Fonseca 09 Jul '10

09 Jul '10

On the subject of bridging relational persistence and fucntional programming I wish to communicate on what I'm working on (Tránsito Abierto Library) (or Open Transit): 1) Whole objective: To disminish to gap between functional and relational paradigms. 2) Specific objetive: Illustrate and explore the gap. 3) Narrow focus: I'm using Takusen and Oracle and Haskell (á la GHC). And Template Haskell (TH). 4) Hypothesis: There'll be always a gap because you're using distinct nature beasts, but you a bit more of convinience would yield great improvements. 4.1) Another thought on this is: A mapping always needs to be specified between data shapes (types/representations). 5) Findings/Achievements(?): 5.1) Generation of Haskell types and iterator code given the shape of relational table. This is not very useful itself. Because of (4.1), using SQL is an effective way to: (1) convey such mapping and (2) specify queries that involve more than one table. 5.2) Given a sql query text, use the facilities on Oracle database to describe the query. With such description, use Template Haskell to generate Haskell types and iterator code. Let us call Geco to this generated code that is built by means of an AST. 5.3) As 5.2 (spliced Geco) but saving the Geco AST. What for? It turns out that later, during program execution you can regenerate a fresh AST code and compare it to Geco AST. If there's a difference it means your database environment is changed and it is likely that your code is no longer safe. The idea here, your program reacts to the enviroment detecting if its unsafe to run. 5.4) Takusen covers many database datatypes but not all. 5.5) I was about to automate pl/sql bindings using a inspired Takusen wrapper found on the enumerator tests directory . It follows scheme 5.2. But it is uncomplete. 5.6) Takusen is limited to iterators up to 8 fields. By using TH, easily you can have any number of columns in your query. 5.7) I wrote a serialiser library on the DB side and a deserialiser library on Haskell side. The idea it's to push *any* pl/sql result onto a string and recover it on the Haskell side. Needs schemes 5.2 and 5.5 to work. Status: Not working. 6) I believe intelligent schema generation is possible following the scheme above. What you would get is a whole module that would enforce constraints (cardinality, restrictions, etc). 7) I haven't try the way Haskell -> DB. Neither the storing of functional values as it was done before in Persistent Haskell and others. Cheers, -- Leonel Fonseca.

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Re: Haskell-Cafe Digest, Vol 83, Issue 25
by Jeff Rubard 08 Jul '10

08 Jul '10

WHUSSUP?

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What is Haskell unsuitable for?
by aditya siram 08 Jul '10

08 Jul '10

Hi all, Haskell is a great language and in a lot of ways it still hasn't found a niche, but that's part of what is great about it. But I wanted to ask people are more experienced with Haskell - what kinds of problems is it unsuited for? Have you ever regretted using it for something? Meaning if you could write the program over you would do it in another language. thanks ... -deech

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C9 video on monads and coordinate systems
by Greg Meredith 08 Jul '10

08 Jul '10

Dear Haskellians, You may be interested in this video i did with Brian Beckman<http://channel9.msdn.com/shows/Going+Deep/E2E-Whiteboard-Jam-Session-with-B…>on monads, location and coordinate systems. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] Merge hsql and HDBC -- there can only be one!
by Oscar Finnsson 08 Jul '10

08 Jul '10

I'd love to see this too. I've only used HDBC and I think it works well. A common problem in the Haskell-community is that some produce low-level bindings/libs and some produce high-level (extremely type safe) bindings/libs but there's seldom good support for switching between or combining the two approaches. My experience is that when it comes to real world scenarios involving databases/xml/json/&c you are seldom able to figure out *all* types compile time and are thus forces to use the low-level lib for everything even though you got the types (e.g. of a table/view/function/...) at compile time for 90% of your code. That's a shame and something that I wish a merging between the packages would be able to solve. In other words: a good package should support more than one way to interface with the db and the different ways should be possible to seamlessly combine. My wish-list: * low level capabilities a la HDBC * high level capabilities (matching result sets to data types, db functions <-> haskell type classes or similar) * sql 2003/2008-, pl/sql-quasi quoter * code generation given a db schema. * schema generation from code (i.e. the opposite from the above). * fixture generation (Haskell -> db, db -> Haskell) * syb-support * BSD3 or DWTFYWT * github :) I'm prepared to spend a couple of hours a week helping out and I got some experience developing database layers that's used in telecom, gaming and logistics. -- Oscar P.S. I'm not saying that there should be only one; competition is always good. It would however be lovely if the community made an effort to (a) produce an package that can cover mosts needs that (b) most developers/package-maintainers used that (c) other developers can build upon with new features.

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Re: [Haskell-cafe] use of modules to save typing
by John Lato 08 Jul '10

08 Jul '10

> From: Michael Mossey <mpm(a)alumni.caltech.edu> > > Now my monad looks like > > testMonad = do > cursor <- gets PlayState.cursor > len <- gets PlayState.len > let newCur = min (cur+1) (len-1) > modify $ PlayState.update_cursor newCur > > Now I can define an abbreviation for PlayState. This is a big help. > Also, I find this more readable. To me > > PlayState.cursor > > is more readable than > playState_cursor > > For one thing, syntax highlighting helps in the former case. For > another, the latter case requires that you recognize a naming > convention, but the former case says clearly: "cursor is within the > namespace PlayState, so this combination must be describing a cursor > related to PlayState." I do this as well, although I would probably import like this: import qualified PlayState as PS which then allows for "PS.cursor" John

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Suggestions for an MSc Project?
by John Smith 08 Jul '10

08 Jul '10

My MSc requires a project dissertation, which is expected to take about 800 hours. I would like to work on something which is of use to the Haskell community. Any suggestions? My interests are in higher-level application programming, rather than low-level libraries or compiler hacking. (This is also the focus of my course.) I don't know whether Haskell is suitable for this (the only significant Haskell programs seem to be its own tool-set), but I had fun learning Haskell and I'd be interested in using it for my project if relevant. The details for the project's requirements are here http://www.ljmu.ac.uk/cmp/onlinemsc/students/97939.htm

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Re: Adding new entry to list
by Johnny Morrice 08 Jul '10

08 Jul '10

Hi there Jack, I have modified your program. However, where you were attempting to use direct recursion to compute totalSales, I have used functions from the prelude. I have also provided a recursive definition totalSalesRec. Here is a lesson on recursion from the Haskell wikibook http://en.wikibooks.org/wiki/Haskell/Recursion Also, what you have is an associative array which pairs (Artist, Title) with Sold. A linked list [(Artist, Title, Sold)] is an inappropriate data type for this purpose. Consider using Map from Data.Map in the form Map (Artist, Title) Sold. I have included comments which I hope you will find useful in understanding the code. Also note my use of comments like: -- | This is a function that does blah These are annotations for the Haskell documention generator, Haddock http://www.haskell.org/haddock/ What I'd do to run haddock on this file (I use linux and I have saved as artist.hs) is mkdir doc && haddock artist.hs -o doc -h Now you can look at documentation for your program in a web browser while fickering with it in GHCi. Be aware that my copy pasting this file onto here may well have mangled its syntax! Have fun, Johnny module Main (main ,testDatabase ,totalSales ,totalSalesRec ,printNames ,mainLoop) where -- | Title of a record type Title = String -- | An artist, who makes records type Artist = String -- | Number of copies of a record sold type Sold = Int type Sales = [(Title, Artist, Sold)] -- | A small sales database testDatabase :: Sales testDatabase = [("Kids", "MGMT", 3), ("This Charming Man", "The Smiths", 5), ("Gimme Shelter", "The Rolling Stones", 7)] -- Notice there are no explicit function arguments here. Look up 'partial application'. -- | Total sales of all records in the database. totalSales :: Sales -> Sold totalSales = -- The sum of all the sales sum . map sale where sale (_,_,s) = s -- | Recursive version of totalSales totalSalesRec :: Sales -> Sold totalSalesRec ((_,_,s):ss) = s + totalSalesRec ss totalSalesRec _ = 0 -- | Print out the contents of a sales database printNames :: Sales -> IO () printNames testDatabase = mapM_ print testDatabase -- Main application loop mainLoop :: Sales -> IO () mainLoop testDatabase = do putStrLn "1 - Show all tracks in database" putStrLn "2 - Show the total sales" putStrLn "3 - Add a new entry" putStrLn "4 - Exit" putStrLn "" putStrLn "Please select an option:" input <- getLine case read input of 1 -> do banner "Show All Tracks" printNames testDatabase putStrLn "" mainLoop testDatabase 2 -> do banner "Total Sales" print $ totalSales testDatabase putStrLn "" mainLoop testDatabase 3 -> do banner "New entry" putStrLn "Enter artist name" a <- getLine putStrLn "" putStrLn "Enter title" t <- getLine putStrLn "" putStrLn "Enter number of sales" -- Look up functors to understand fmap s <- fmap read getLine putStrLn "" -- 'cons' the new element to the start of the linked list testDatabase -- Look up data constructors. mainLoop $ (a,t,s) : testDatabase -- No 'do' is required for only one statement. Look up monads and how 'do' is syntactic sugar. 4 -> return () where -- Print banner banner s = do putStrLn "------------------" putStrLn s putStrLn "------------------" -- | Run main :: IO () main = mainLoop testDatabase

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Build problems on Hackage server
by Emil Axelsson 08 Jul '10

08 Jul '10

Hello Last week I uploaded new versions of feldspar-language and feldspar-compiler. Both packages build just fine on our local machines with GHC 6.10 and 6.12. But Hackage reports the following build failure: > cabal: dependencies conflict: ghc-6.12.2 requires array ==0.3.0.1 however > array-0.3.0.1 was excluded because ghc-6.12.2 requires array ==0.3.0.0 Any idea of what might be causing this? Thanks! / Emil

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checking types with type families
by Evan Laforge 08 Jul '10

08 Jul '10

I have a parameterized data type: > data Val result = VNum Double | VThunk (SomeMonad result) > type Environ result = Map Symbol (Val result) I have a class to make it easier to typecheck Vals: > class Typecheck a where > from_val :: Val result -> Maybe a > > instance Typecheck Double where > from_val (VNum d) = Just d > from_val _ = Nothing Now I can write > lookup_environ :: (Typecheck a) => Symbol -> Environ result -> Maybe a Now of course there's a question of how to write Typecheck for VThunk. I would like to be able to call 'lookup_environ' expecting a 'SomeMonad result' and get Nothing or Just depending on if it's present. > instance Typecheck (SomeMonad result) where > from_val (VThunk f) = Just f But I need something to say that the 'result' from the instance head is the same as the 'result' from the class declaration, because otherwise I get Couldn't match expected type `result' against inferred type `result1' So maybe a type family? > class Typecheck ... > type ResultOf a :: * > from_val :: Val (ResultOf a) -> Maybe a > > instance Typecheck (SomeMonad result) where > type ResultOf (SomeMonad result) = result > ... Now the question is, what's the ResultOf a Double? Well, since the output doesn't depend on 'result' at all, I would like to be able to write 'type ResultOf Double = anything', and then I can look up a VNum in an 'Environ anything'. Unfortunately, that type declaration isn't legal because it wants 'anything' to be bound somewhere. I tried explicitly introducing 'anything' with a forall, but no dice. So there are a number of possible solutions to this problem. One is, is there another way to get from the expression language type VNum, VThunk, etc. to a haskell type that doesn't use typeclasses in this way? Or, is there a way to assert in the instance of 'Typecheck (SomeMonad result)' that its 'result' is equal to the 'result' in the class declaration without using type families in this way? Or, is there a way to use type families in the way I'm using them to leave 'result' polymorphic in the cases where it can be left free in the result (e.g. VNum 42 :: Val anything)? I know lots of people have used haskell to write little interpreters, so maybe someone has solved this problem before. Basically, I have a Val that is "semi-polymorphic", but even if I go the totally monomorphic / dynamic type route by making a Result type with all the different types, I still have trouble converting from haskell types and back when I need to write the "Result -> result" function. I suppose I could play the same Typecheck game there, but I like the idea of an "Environ X" that doesn't admit the wrong kind of VThunk. It seems like I should be able to express that in the type system and be safer as well as save some packing and unpacking.

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[C Binding] Turning the mutable to immutable?
by Yves Parès 08 Jul '10

08 Jul '10

Hello, I don't know if some of you are familiar with the SFML library (stands for Simple and Fast Multimedia Library) --> http://sfml-dev.org As SDL, SFML is a 2D graphics library, but conversely to SDL it provides a hardware-accelerated drawing, through OpenGL. Well, I'm currently writing its Haskell binding, and I'm stuck with design issues. What I'm heading to is a full IO binding, and that's what I'd like to avoid. I particularly like the way HGL works, with the Draw monad which (IMO) enables powerful composability of the drawings. But HGL is simpler in the way that, for instance, it doesn't deal with images/sprites. So that's why I would like to pick up advice concerning the design of such a binding. First, this is how SFML (in C/C++) works: The main types are RenderWindow, Drawable, Image, Sprite, Shape, Font and Text. An Image basically represents an array of pixels. It is just a resource, and is not directly drawable. An image is loaded from a file, and provides methods for accessing/altering its pixels. Its main purpose is to be bound as an OpenGL texture. A RenderWindow is the window on which the Drawables are... well... drawn. A Sprite, now, is a Drawable. One Sprite is linked to one Image. A Sprite NEVER alters its Image, it just provides positionning/rotation/resizing/colorizing methods that will be translated, when drawing on the RenderWindow, to glTranslate/glRotate/glScale/glColor, etc. So we can see a Sprite as an instance of an Image, since if a same Image is to be drawn 3 times on the same frame, we are advised to create three Sprites of it, which will differ by their position. A Font is loaded from a file, it is a simple resource, just like Image. Text and Shape are Drawables. So, now, questions: 1) For those who know HGL, can the monad Draw principle be adapted to my case? Or am I heading for disaster? 2) How would I handle images? SFML API enables the user to alter the pixels of the image, and I obviously don't wanna copy the entire Image each time a pixel is changed. 3) Is there another library on hackage that handles images in a functional way? (I mean not *all in IO*)

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monadic design patterns for the web -- a 2 day, hands-on workshop
by Greg Meredith 08 Jul '10

08 Jul '10

Dear Haskellians, Biosimilarity and Stellar Scala Consulting will be running a 2 day workshop in Seattle in September on monadic design patterns for the web. You can get the details here <http://monadic.eventbrite.com/>. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Adding new entry to list
by Mrwibbly 07 Jul '10

07 Jul '10

Hi all I'm having a problem trying to add a new record to a list. I need to be able to add a new record to the database. My code is below, I also need to work out the total sales for everything in the Sales list. As you can see, I've tried to do it but I can't seem to work it out. type Title = String type Artist = String type Sold = Int --type Record = (Title, (Artist, Sold)) type Sales = [(Title, Artist, Sold)] testDatabase :: Sales testDatabase = [("Kids", "MGMT", 3), ("This Charming Man", "The Smiths", 5), ("Gimme Shelter", "The Rolling Stones", 7)] totalSales :: Sales -> Title -> Artist -> Sold totalSales [] title artist = 0 --totalSales ((t,a,n):testDatabase) title artist = title artist --addSale :: Sales -> Sales --addSale (Title, Artist, sold) = (title, artist, sold + 1) --newRecord :: String -> String -> Sales -> Sales --newRecord testDatabase title artist = (title, artist) ++ testDatabase --newRecordAction :: Sales -> IO() --newRecordAction testDatabase = do --putStrLn "Enter a new title: " --title <- getLine --putStrLn "Enter an artist: " --artist <- getLine --return $ newRecord title artist testDatabase getTitle :: Sales -> Title getTitle (title,_) = title printNames :: Sales -> IO() printNames testDatabase = mapM_ print testDatabase mainLoop :: Sales -> IO() mainLoop testDatabase = do putStrLn "1 - Show all tracks in database" putStrLn "2 - Exit" putStrLn "" putStrLn "Please select an option:" input <- getLine case read input of 1 -> do putStrLn "------------------" putStrLn "Show All Tracks" putStrLn "------------------" printNames testDatabase putStrLn "" mainLoop testDatabase 2 -> do return () main :: IO() main = mainLoop testDatabase Thank you, Jack -- View this message in context: http://old.nabble.com/Adding-new-entry-to-list-tp29099032p29099032.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Re: [Haskell] analogous functions in Sound.File.Sndfile
by John Lato 07 Jul '10

07 Jul '10

> From: stefan kersten <sk(a)k-hornz.de> > > following up on haskell-cafe ... > > > i just noticed though that the lazy versions don't work correctly because they > don't take closing the file handle into account properly. i think they should be > removed from the API, for incremental processing an iteratee interface would > probably be more appropriate. Incidentally I've been meaning to write an iteratee interface to hsndfile for some time. It would be an excellent fit, and it should be quite simple to implement. This would be a good small project for someone interested in audio and iteratees. John

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reading and playing music openAL
by Maria Gabriela Valdes 07 Jul '10

07 Jul '10

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). Thanks in advance, -- Maria Gabriela Valdes G. Linux Registered User #485743

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Re: [Haskell-cafe] finding the right mathematical model
by John Lato 07 Jul '10

07 Jul '10

If you ignore the identity mappings (which all technically create trivial loops), these mappings would form a directed acyclic graph (DAG). I would look at some of the graph libraries, e.g. fgl, to see if they have anything appropriate. John > From: G?nther Schmidt <gue.schmidt(a)web.de> > 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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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 07 Jul '10

07 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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