G'day all.
Thursday, June 23, 2005, 5:38:03 AM, you wrote:
> To handle the problem of drawing all shapes, in c++, I would have a list
> of shape pointers:
> struct shape{ virtual void draw(...);};
> struct circle : public shape {...};
> struct square : public shape {...};
> std::list<shape *> shapes;
> for(std::list<shape *>::iterator it = shapes.begin();it !=
> shapes.end();++it)
> { (*it)->>draw(...); }
> This general pattern of dynamic binding I use over and over again. Could
> you give me some example code of this type of thing handled in Haskell's
> way? Assuming that the number of classes deriving from shape might get
> quite large.
class Drawable s where
draw :: s -> IO ()
data Circle = Circle Point Radius
instance Drawable Circle where
draw (Circle centre radius) = ...
If you only need interface inheritance, this should do. If you also need
implementation inheritance, then you can model it with an upcast method:
data Shape = Shape Stuff
class Shape s where
toShape :: s -> Shape
draw :: s -> IO ()
instance Shape Shape where
toShape s = s
draw s = ...
data Circle = Circle Shape Point Radius
instance Shape Circle where
toShape (Circle s _ _) = s
draw (Circle _ centre radius) = ...
In your original example, draw() wasn't abstract virtual, so I assume
there's a reasonable default draw() method for your "shape" class. If
there isn't, then it's probably better in Haskell to split the typeclass:
class Shape s where
toShape :: s -> Shape
class (Shape s) => DrawableShape s where
draw :: s -> IO ()
And only define DrawableShape on types where "draw" makes sense.
Cheers,
Andrew Bromage
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