Re: [Haskell-beginners] How to model this in haskell, get rid of my OO thinking?

Hey, Yes, that makes sense. Still, how would the "World" object look? How do I do a list of abstract base classes in haskell? On Tue, 18 May 2010 22:08:31 +0800 Lyndon Maydwell wrote:

You would likely not use a void function and instead map over the objects in the world:

updateWorld :: World -> World updateWorld = fmap update

This way your functions can remain pure.

On Tue, May 18, 2010 at 9:28 PM, wrote:

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Hi,

Thanks, I think I am starting to get it :). But let me extend my example. Assume, (in C++) ScreenObject also has a abstract function "update" implemented in SpaceShip and Rocket and doing something completly different for both of them. Now there are a lot of ScreenObjects in the "world" which have to be updated in every frame. This is done by having a list of pointers to ScreenObjects (objects) and a updateWorld function which looks like this (simplified):

void updateWorld() {    for(o in objects)        o->update(); }

How would you model this in haskell? Thanks! Nathan

On Tue, 18 May 2010 11:33:08 +0200 edgar klerks wrote:

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He Nathan,

I would create an data type (SpaceObject), which holds the Position and an object. Then I create a typeclass for SpaceObject, which has the function move in it. Functions which all objects have are moved in the typeclass:

type Position = (Double, Double)

data SpaceObject a = SO a Position

data RocketObject = RocketObject {                 stuff :: String         }

data SpaceShipObject = SpaceShipObject {                 bla :: Int         }

type Rocket = SpaceObject RocketObject type SpaceShip = SpaceObject SpaceShipObject

class ScreenObject a where         move :: a -> Position -> a

instance ScreenObject (SpaceObject obj) where         move (SO obj (x,y) ) (dx, dy)  = SO obj (x + dx, y + dy) ~

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On Tue, May 18, 2010 at 5:21 AM, wrote:

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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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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