How to model this in haskell, get rid of my OO thinking?

haskell＠lonely-star.org

17 May 2010 17 May '10

11:21 p.m.

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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Isaac Dupree

17 May 17 May

11:31 p.m.

New subject: How to model this in haskell, get rid of my OO thinking?

On 05/17/10 23:21, haskell@lonely-star.org wrote:

...

So I guess, it comes down to the questions: How would you model the scenario described above in haskell?

how about, data Position = Position { positionX :: Float, positionY :: Float } move :: Position -> (Float, Float) -> Position data Spaceship = Spaceship Position ...other data members... data Rocket = Rocket Position ...other data members... (or perhaps not separate data-types but rather data SpaceThing = Spaceship { spacePosition :: Position, ...other data members...} | Rocket { spacePosition :: Position, ...other data members...} ... -- this latter has advantages and disadvantages regarding code reuse and code reliability. Experiment! -Isaac

edgar klerks

18 May 18 May

5:33 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

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, 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@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Flatliner ICT Service, Email: Edgar.klerks@gmail.com, Tel: +31727851429 Fax: +31848363080 Skype: edgar.klerks Website: flatlinerict.nl Adres: Koelmalaan 258, 1813JD, Alkmaar Nederland

haskell＠lonely-star.org

9:28 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

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:

...

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, 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@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Lyndon Maydwell

10:08 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

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:

...

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:

...
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, 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@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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

Nathan Huesken

10:29 a.m.

New subject: 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:

...
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:

...
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, 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@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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

Lyndon Maydwell

10:45 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

World could be anything really, maybe a list of objects, or something much more involved. From my use of fmap I've implied that it is an instance of the Functor typeclass, but that was only a suggestion. I'm not sure what you're after with respect the the abstract base classes, but there's this diagram floating around of the haskell98 typeclasses: [ http://www.uni-bonn.de/~manfear/haskell-classhierarchy.php ]. Maybe this will help? I usually use the ":info" command on a class in ghci to find out some more about it, but I still have trouble finding a good class to use for any particular problem. Hope this was helpful. On Tue, May 18, 2010 at 10:29 PM, Nathan Huesken wrote:

...

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:

...
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:

...
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, 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@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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

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

Stephen Tetley

11:11 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

On 18 May 2010 15:29, Nathan Huesken wrote: [SNIP]

...

Yes, that makes sense. Still, how would the "World" object look?

Hi Nathan I don't know how appropriate it would be for a game - but Malcolm Wallace and colleagues presented a scene graph in this paper: Huge Data but Small Programs: Visualization Design via Multiple Embedded DSLs http://eprints.whiterose.ac.uk/5000/1/padl09.pdf ... see 5.2 Rendering and Interaction It might make a concrete starting point. There is a different scene graph on Hackage, but it has a somewhat more complicated implementation. Best wishes Stephen

Nathan Huesken

7:08 p.m.

New subject: How to model this in haskell, get rid of my OO thinking?

Hey, I think I am getting grasp of it. Thanks for all the help!

Henk-Jan van Tuyl

19 May 19 May

4:02 a.m.

New subject: How to model this in haskell, get rid of my OO thinking?

On Tue, 18 May 2010 05:21:38 +0200, 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 --

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edgar klerks
haskell＠lonely-star.org
Henk-Jan van Tuyl
Isaac Dupree
Lyndon Maydwell
Nathan Huesken
Stephen Tetley