Bulat, a) building (i) a list of data is fundamentally different from building (ii) a list of anticipated results of each datum. I would be surprised to hear that this counts as a valid technique. BTW, you can do the *same* in a lazy OO language. (No lazy OO language at hand -- well lazyness can be simulated.) Anyway, even if people end up coding as you propose, it won't work in general. Think of mutation methods that change the state but preserve the type. Then your list will still be heterogonous. NO? b) You wrote:
this state is just don't need to appear in interface definition :) circle x y r
You are not talking about state but constructor arguments. Please have a look here: http://www.angelfire.com/tx4/cus/shapes/cpp.html Circle Interface (Circle.h) class Circle: public Shape { public: Circle(int newx, int newy, int newradius); int getRadius(); void setRadius(int newradius); void draw(); private: int radius; }; [See getRadius and setRadius) In OO, mutable state tends to leak to the interface, perhaps not as public fields, perhaps not even as public proxy properties (but in the shapes example, it *does*), but then still as sublcass-specific interface member. Anyway, I am happy to add your and Lennart's proposals as appendices to the OOHaskell saga. And finally, Bulat, I agree with your point that
original exercise was about OO way to solve some problem. i want to say that in Haskell it's better in most cases to use another, functional way
Thanks, Ralf (returning to his VB problem)
-----Original Message----- From: Bulat Ziganshin [mailto:bulatz@HotPOP.com] Sent: Thursday, June 23, 2005 4:55 AM To: Ralf Lammel Cc: Andrew Ward; Pal-Kristian Engstad; haskell-cafe@haskell.org Subject: Re[4]: [Haskell] Dynamic binding
Hello Ralf,
Thursday, June 23, 2005, 11:36:20 AM, you wrote:
just create list of draw functions itself:
[drawCircle (10,10) 5, drawSquare (20,20) 10]
RL> No! the exercise is about lists of shapes RL> not lists of results of drawing shapes. RL> This is clearly a major difference.
in cases where you need to call only one function on created objects, you can just insert in list calls to this functions (not their results! i suppose that drawXXX functions has "... -> IO ()" type)
in cases where you need to call several functions for this object, you can insert in list tuple or structure for each object, as i do in next example. original exercise was about OO way to solve some problem. i want to say that in Haskell it's better in most cases to use another, functional way
RL> Bulat wrote:
for more complex tasks - declare interface as a structure:
data ShapeInterface = Shape { draw :: IO (), moveTo :: Point -> IO (), calcArea :: Float }
RL> No! You miss the point that the different shapes RL> differ regarding state types. RL> You don't have a chance when you use one datatype.
this state is just don't need to appear in interface definition :)
see for example:
data ShapeInterface = Shape { draw :: IO (), calcArea :: Float }
circle x y r = Shape { draw = drawCircle x y r, calcArea = pi*r*r }
square x y size = Shape { draw = drawSquare x y size, calcArea = size*szie }
figures = [circle 1 2 3, square 4 5 6, circle 7 8 9]
if you need to maintain mutable state, this is also not a problem:
data ShapeInterface = Shape { draw :: IO (), moveTo :: (Int,Int) -> IO (), calcArea :: Float } circle x y r = do center <- ref (x,y) return Shape { draw = val center >>= drawCircle r , moveTo = (center=:) , calcArea = pi*r*r } main = do figures <- sequence [circle 1 2 3, square 4 5 6, circle 7 8 9] mapM_ draw figures mapM_ (moveTo (0,0)) figures mapM_ draw figures
ref=newIORef val=readIORef (=:)=writeIORef
RL> haskell-cafe?
as you wish :)
-- Best regards, Bulat mailto:bulatz@HotPOP.com