May 2010 - Beginners - Haskell.org

Why does this Ord-class instance crash?
by R J 22 May '10

22 May '10

I'm trying to declare Triangle as an instance of the Ord class without using "deriving (Ord)", so I can better understand the manual instantiation. As I understand it, all I need merely to define "<". Yet, when I do so using the code snippet below, this code hangs when I type "Scalene > Failure" in GHCi. Why is that, and what's the fix? Thanks. data Triangle = Failure | Equilateral | Isosceles | Scalene deriving (Eq, Show) instance Ord Triangle where Failure < Failure = False Failure < _ = True Equilateral < Failure = False Equilateral < Equilateral = False Equilateral < _ = True Isosceles < Scalene = True Isosceles < _ = False Scalene < _ = False _________________________________________________________________ The New Busy is not the old busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

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QuickCheck properties on monadic values
by Ashish Agarwal 21 May '10

21 May '10

I've been using the binary library, and would like to test whether my put and get functions are inverses. I thought of using QuickCheck but it is not clear how I can write the necessary property. I skimmed Claessen, Hughes, "Testing monadic code with QuickCheck", but their examples require a lot of extra work (e.g. their queue example requires an AST for a mini-queue language). Am I missing something obvious? Also, what's the difference between QuickCheck 1 and 2? I'm on gch 6.10 and doing `cabal install QuickCheck` gives me version 1.2.0.0, which lacks constructs described in the above paper. Thanks.

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Bit Manipulation
by Daniel Rozycki 21 May '10

21 May '10

Hi all, I am a newcomer to Haskell trying to manipulate data at the bit level. More specifically, I am trying to calculate a seven-bit value and a nine-bit value, concatenate the two values as a Word16, and put the result using putWord16. I know that this sounds weird (because it totally is), but it is required by the standard of a networking protocol that I am implementing. The byte seems to be the atomic unit in Haskell, but I need to go subatomic. Currently, I am storing the two values each as Word16s, bitshifting one of them left by nine (using shift in the Data.Bits module), and then adding the two. Is there a simpler and/or more elegant way of achieving this result? Thanks, Dan Rozycki

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class and instance question
by Walck, Scott 20 May '10

20 May '10

Hi folks, I'm trying to make doubles and triples act like vectors, as in (3,4) <+> (7,8) ==> (10,12) (3,2,1) <+> (9,8,7) ==> (12,10,8) 6 *> (1,2,3) ==> (6,12,18) I thought I should make a type class so that I could use <+> for both double addition and triple addition, and *> for both double and triple scalar multiplication. (Some of this functionality is provided by NumericPrelude, but I didn't need all of that, and I hoped this would be simple to write.) The code below gives the error NewVectorShort.hs:19:0: Type synonym `Vector2D' should have 1 argument, but has been given 0 In the instance declaration for `BasicVector Vector2D' Failed, modules loaded: none. I don't understand how what I'm trying to do is different from, say, the Monad instance for Maybe. (Maybe a) is a type, and (Vector2D a) is a type. Thanks, Scott {-# LANGUAGE TypeSynonymInstances #-} infixl 6 <+> infixl 6 <-> infixl 7 *> infixl 7 <* class BasicVector v where (<+>) :: v a -> v a -> v a (<->) :: v a -> v a -> v a (*>) :: Num a => a -> v a -> v a (<*) :: Num a => v a -> a -> v a v1 <-> v2 = v1 <+> fromInteger (-1) *> v2 v1 <* c = c *> v1 c *> v1 = v1 <* c type Vector2D a = (a,a) instance BasicVector Vector2D where (ax,ay) <+> (bx,by) = (ax+bx,ay+by) c *> (ax,ay) = (c*ax,c*ay)

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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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