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

[Lyndon Maydwell]

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