- Beginners - Haskell.org

Implementing a Local Propagation Network
by Patrick LeBoutillier 19 May '10

19 May '10

Hi all, After learning some Haskell recently, I decided to revisit a book about functional programming techniques for Perl: Higher Order Perl. I didn't fully understand the book at the time but now my Haskell experience has proved to be very insightful. Towards the end of the book the author implements a local propagation network. Here is the Perl source code: http://hop.perl.plover.com/Examples/Chap9/Local-Propagation/ The PDF of the specific chapter is here: http://hop.perl.plover.com/book/pdf/09DeclarativeProgramming.pdf I would like to experiment with something similar in Haskell, but the way this network is designed is all about state and references: - Wires have a values that can change over time; - Wires have references to nodes; - Nodes have references to wires; I'm a bit stuck as to how to approach the "object has a list references to other objects" situation from Haskell. I tried this: type Name = String data Node = Node Name [Wire] data Wire = Wire Name Node Double [Node] But that doesn't seem like it would work since when I change a Wire I must find all "copies" of it (in the Node objects) and update them also. Perhaps I should just refer to Wires/Nodes by name and use an association list to lookup them up, but that seems cumbersome. Anybody have any suggestions? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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WxHaskell problems was Re: [Haskell-beginners] How to model this in haskell, get rid of my OO thinking?
by James Fenn 19 May '10

19 May '10

For me on ubuntu karmic and haskell platform 2010.1, WxWidgets doesn't seem to be responding to keypresses. I have only tried it with WxAsteroids as I thought it would be useful for me to study. Does anyone else have the same problem? Cheers, James On 19 May 2010 09:02, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > On Tue, 18 May 2010 05:21:38 +0200, <haskell(a)lonely-star.org> wrote: > >> Hi, >> >> I learning haskell and I am trying to understand how model certain >> things in it. >> As I have been doing a lot of C++ programming so far. Let's imagine we >> want to write a game. In the game there are spaceships and rocks (image >> something like astroids :) ). Now both spaceships and rocks have a >> position and can move. Spaceships can shoot, while rocks can explode. > > There is a well documented implementation, called wxAsteroids; see > http://www.haskell.org/haskellwiki/WxAsteroids > > Regards, > Henk-Jan van Tuyl > > > -- > http://Van.Tuyl.eu/ > http://members.chello.nl/hjgtuyl/tourdemonad.html > -- > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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How to model this in haskell, get rid of my OO thinking?
by haskell＠lonely-star.org 19 May '10

19 May '10

Hi, I learning haskell and I am trying to understand how model certain things in it. As I have been doing a lot of C++ programming so far. Let's imagine we want to write a game. In the game there are spaceships and rocks (image something like astroids :) ). Now both spaceships and rocks have a position and can move. Spaceships can shoot, while rocks can explode. In C++, I would do (simplified): class ScreenObject { float x,y; void move(dx,dy){x+=dx;y+=dy;} }; class Spaceship : public ScreenObject { void shoot(){...} }; class Rocket : public ScreenObject { void explode(){...} }; But what would I do in haskell? Ok, I can define a typeclass "ScreenObjectType" that has a move function (taking an object, retuning an moved object). But I do not want to implement "move" for both Spaceship and Rocket. Can I somehow give a default implementation for move that works on any datatype having an "x" and "y" element? Or what would I do? Can I somehow define a "base datatype" holding a x and y member form which Spaceship and Rocket derive? I feel like I am thinking to much OOP here. But the point is, I guess, that I want to avoid code duplication! So I guess, it comes down to the questions: How would you model the scenario described above in haskell? Thanks! Nathan

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HOpenGL questions
by Nathan Huesken 19 May '10

19 May '10

Hi, I send this mail to the HOpenGL mailinglist. I did not get a reply. I hope it is OK to ask this question here. I am new to Haskell, but not new to OpenGL. I am reading the tutorials on the HOpenGL page and have a few questions. - First of all, all tutorials are using GLUT. Is that really the way to go with HOpenGL? They define (among other things) a onIdle and a onDraw function. That seems to me (at least for a game) not the way to go. onIdle updates the game logic, and onDraw draws the screen. As far as I can see, it does not make sense to update the game logic more than once between consecutive calls to onDraw. Same thing is true for class to onDraw between consecutive class to onIdle. Is this accounted for in the approach? - In addition there is the onInput function. The onInput, onIdle and onDraw functions communicate (in the tutorials) using variables declared with newIORef. I come from the OO language C++ and almost no experience with functional languages. But this puzzles me. Basically they define global variables for the exchange between the callback functions, correct?!? So, for every piece of information I want to pass between these functions, I have to declare a newIORef variable? In C/C++ I find GLUT not a very good approach and prefer plain OpenGL. Does it make sense to go with plain OpenGL in Haskell too? Are there any resources on how to do that? Thanks! Nathan

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Haskell described as a "rigid" language
by aditya siram 19 May '10

19 May '10

Haskell is considered by many as an inflexible language [1] . I describe a flexible language as one that supports any design you want (even a bad one) - if you can think it, you can code it and run it (bugs and all). I share this opinion about Haskell but pursue it because I feel that one day it will open up and let me think more about the problem and less about how to get GHC to approve it. So I guess the question to you practitioners is: Would you agree that it is a rigid language as describe in the link below, or is that just an illusion that goes away with experience? -deech [1] http://therighttool.hammerprinciple.com/statements/this-language-has-a-very…

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Are monads pure?
by Amy de Buitléir 18 May '10

18 May '10

When I first started learning Haskell, I was under the impression that monads provided a way to encapsulate some imperative code so it didn't "contaminate" your functional code. But I think that's a misconception that I got because the first monad I dealt with was IO. Lately I've been reading more about monads, and (aside from IO), they are purely functional, aren't they? If they have state, they deal with it in a purely functional way. Behind the scenes, they thread the state through your computations so that you don't you don't have to bother with it. Is that correct? TIA.

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Type ambiguity - Int vs. Integer
by Thomas 17 May '10

17 May '10

Hello! I was very surprised to see that Int and Integer seem to differ with respect to ambiguity. And I have no idea what is going on. This works as intended: data AW = AI Integer deriving (Show, Eq, Ord) class AWC a where toAW :: a -> AW fromAW :: AW -> a instance AWC Integer where toAW = AI fromAW (AI v) = v > toAW 5 5 However, the following gives an error: data AW = AI Int deriving (Show, Eq, Ord) class AWC a where toAW :: a -> AW fromAW :: AW -> a instance AWC Int where toAW = AI fromAW (AI v) = v > toAW 5 Ambiguous type variable 't' in the constraints: 'Num t' arising from the literal '5' 'AWC t' arising from the use of 'toAW' Ok, I can fix this easily with > toAW (5::Int) But I still don't understand what's going on? Why does one type work and the other does not? Is there a way around this without type annotations? Any hint would be greatly appreciated! Thanks in advance, Thomas

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diagrams installation trouble
by Scott N. Walck 17 May '10

17 May '10

Hi folks, I'm trying to install Brent Yorgey's diagrams library. I'm having trouble getting the Haskell bindings to the Cairo library installed correctly and/or visible. As you can see below, I have the Haskell Cairo stuff installed at /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1 I did this with something like $ sudo aptitude install libghc6-cairo-dev In other words, I used the ubuntu package system to install this. You can also see that ghc-pkg does not see this installation. I thought I might be able to fix the problem by registering these packages with 'ghc-pkg register', but I can't get that to work. Finally (and this is not shown below) I tried to compile gtk2hs from source. I seem to have had a successful './configure' step, but then I fail at the 'make' step, for a similar reason. The system can't find the cairo and similar files at /usr/lib/haskell-packages One more point about my system might be important. I installed GHC 6.10 using the ubuntu installer, then compiled 6.12 from source, then installed the most recent haskell-platform. I would greatly appreciate any help you all could provide. Thanks, Scott walck@walck-nsf:~/computer/haskell/Yorgey$ cd diagrams-new/ walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ runhaskell Setup.lhs configure Configuring diagrams-0.2.9... Setup.lhs: At least the following dependencies are missing: cairo >=0.9 && <0.11, colour >=2.2.1 && <2.4, containers ==0.2.* walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ runhaskell Setup.lhs configure --user Configuring diagrams-0.2.9... Setup.lhs: At least the following dependencies are missing: cairo >=0.9 && <0.11, containers ==0.2.* walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg list /usr/local/lib/ghc-6.12.1/package.conf.d: Cabal-1.8.0.2 GLUT-2.1.2.1 HTTP-4000.0.9 HUnit-1.2.2.1 OpenGL-2.2.3.0 QuickCheck-2.1.0.3 array-0.3.0.0 base-3.0.3.2 base-4.2.0.0 bin-package-db-0.0.0.0 bytestring-0.9.1.5 cgi-3001.1.7.2 containers-0.3.0.0 deepseq-1.1.0.0 directory-1.0.1.0 (dph-base-0.4.0) (dph-par-0.4.0) (dph-prim-interface-0.4.0) (dph-prim-par-0.4.0) (dph-prim-seq-0.4.0) (dph-seq-0.4.0) extensible-exceptions-0.1.1.1 ffi-1.0 fgl-5.4.2.2 filepath-1.1.0.3 (ghc-6.12.1) (ghc-binary-0.5.0.2) ghc-prim-0.2.0.0 haskell-platform-2010.1.0.0 haskell-src-1.0.1.3 haskell98-1.0.1.1 hpc-0.5.0.4 html-1.0.1.2 integer-gmp-0.2.0.0 mtl-1.1.0.2 network-2.2.1.7 old-locale-1.0.0.2 old-time-1.0.0.3 parallel-2.2.0.1 parsec-2.1.0.1 pretty-1.0.1.1 process-1.0.1.2 random-1.0.0.2 regex-base-0.93.1 regex-compat-0.92 regex-posix-0.94.1 rts-1.0 stm-2.1.1.2 syb-0.1.0.2 template-haskell-2.4.0.0 time-1.1.4 unix-2.4.0.0 utf8-string-0.3.4 xhtml-3000.2.0.1 zlib-0.5.2.0 /home/walck/.ghc/x86_64-linux-6.12.1/package.conf.d: GLFW-0.4.2 binary-0.5.0.2 colour-2.3.1 darcs-2.4.1 dataenc-0.13.0.2 dimensional-0.8 gnuplot-0.3.4 hashed-storage-0.4.11 haskeline-0.6.2.2 mmap-0.4.1 monoid-transformer-0.0.2 non-negative-0.0.5.1 numeric-prelude-0.1.3.4 numtype-1.0 storable-record-0.0.2.2 terminfo-0.3.1.2 transformers-0.2.1.0 utf8-string-0.3.6 utility-ht-0.0.5.1 walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ dpkg -L libghc6-cairo-dev /. /usr /usr/share /usr/share/doc /usr/share/doc/libghc6-cairo-dev /usr/share/doc/libghc6-cairo-dev/changelog.gz /usr/share/doc/libghc6-cairo-dev/copyright /usr/share/doc/libghc6-cairo-dev/changelog.Debian.gz /usr/lib /usr/lib/haskell-packages /usr/lib/haskell-packages/ghc6 /usr/lib/haskell-packages/ghc6/lib /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1 /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/libHScairo.a /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/HScairo.o /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Cairo.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Paths.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Patterns.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Text.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Transformations.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Fonts /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Fonts/FontOptions.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/Image.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PDF.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PNG.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PS.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/SVG.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/Surface.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Utilities.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Matrix.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Types.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo.hi walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ls -l /usr/lib/haskell-packages/ghc6/lib/ total 20 drwxr-xr-x 3 root root 4096 2010-05-04 08:25 cairo-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 16:48 curl-1.3.5 drwxr-xr-x 4 root root 4096 2010-05-04 08:25 glib-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 16:00 gtk-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 08:25 mtl-1.1.0.2 walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg register /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf Reading package info from "/usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf" ... done. cairo-0.10.1: missing id field cairo-0.10.1: dependency "base-4.1.0.0" doesn't exist (use --force to override) cairo-0.10.1: dependency "mtl-1.1.0.2" doesn't exist (use --force to override) cairo-0.10.1: dependency "glib-0.10.1" doesn't exist (use --force to override) cairo-0.10.1: dependency "bytestring-0.9.1.4" doesn't exist (use --force to override) walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg register /usr/lib/haskell-packages/ghc6/lib/glib-0.10.1/glib.package.conf Reading package info from "/usr/lib/haskell-packages/ghc6/lib/glib-0.10.1/glib.package.conf" ... done. glib-0.10.1: missing id field glib-0.10.1: dependency "base-4.1.0.0" doesn't exist (use --force to override) walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ -- Scott N. Walck Associate Professor of Physics Lebanon Valley College Annville, PA 17003 phone: 717-867-6153 fax: 717-867-6075 email: walck(a)lvc.edu

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foldl by foldr
by Matt Andrew 16 May '10

16 May '10

Hi all, I am working my way through 'Real World Haskell' as my first introduction to Haskell and am thoroughly enjoying both it and Haskell. I have, however, hit something of a brick wall in figuring out one of the examples used in chapter 4. The example code is flagged as 'non-trivial' to understand, but I am trying to understand each thing presented reasonably well before I move on. I was hoping someone could shed some light on the issue for me; I have done a fair amount of searching and have not found an answer, so apologies if this has come up before and I have not found it. The offending code is: myFoldl :: (a -> b -> a) -> a -> [b] -> a myFoldl f zero xs = foldr step id xs zero where step x g a = g (f a x) The thing I am having trouble understanding is what the 'id' function is doing in a function that expects 3 arguments and is given 4 (foldr). Here is the breakdown of where I get stuck: My understanding of what id does as a function is that it returns the identity of the object it is passed. Playing with ghci gives me the type signature (a -> a) and as expected id returns the argument it is given unaltered. Now I believe Haskell does not allow the passing of more arguments to a function than it expects. Therefore I am driven to the conclusion that Haskell is treating the call to 'foldr' above as either: foldr step (id xs) zero (i.e. the second argument is the thunk (id xs)) or: foldr (step id) xs zero (i.e. given that 'step' is defined above as taking 3 arguments, whereas the type signature for 'foldr' expects a function taking 2 arguments it is accepting (step id) as a partially applied function taking two arguments) The problem is that having simulated the above scope in ghci for an identical call to 'foldr,' explicitly parenthesising the call to 'foldr' in either of the above ways throws an error. In addition to this either way of understanding the function doesn't make much sense to me: the first seems entirely redundant, the second would result in 'id' being called by 'step' in a position where it has no arguments and thus would surely result in an error. I'm obviously missing an important part of the puzzle and would appreciate any help in understanding this code. Appreciate you taking the time to read this, Matt Andrew

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Help with slow algorithm
by Diego Echeverri 16 May '10

16 May '10

Hi guys. I've been playing with https://www.spoj.pl/ to get better with Haskell. I'm trying to do: https://www.spoj.pl/problems/PALIN/ Basically given a number k, you have to find a number bigger than k that is palindrome. The thing is that k can be 1000000 digits long. I have learnt a bunch of things doing this exercise but I'm still getting time-limit exception. Here's the code: http://gist.github.com/401880 I believe that the bottleneck is in the addOne function (Specially in the case where you have a bunch of nines). I have rewritten it to use STArray but isn't fast enough. doing: addOne $ B.replicate 500000 'a' takes too much time! Can anybody point me out of better (faster) ways to improve the code? I don't need my code fixed, just suggestions and clues about things to try. Thanks! Diego Echeverri

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