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

[Nathan Huesken]

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 <maydwell at gmail.com> 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,  <haskell at lonely-star.org> wrote:
> > 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 <edgar.klerks at gmail.com> wrote:
> >
> >> 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)
> >> ~
> >>
> >> ~
> >>
> >> ~
> >>
> >> ~
> >>
> >> On Tue, May 18, 2010 at 5:21 AM, <haskell at 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. 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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> >> > Beginners at haskell.org
> >> > http://www.haskell.org/mailman/listinfo/beginners
> >> >
> >>
> >>
> >>
> >
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>