[Haskell-cafe] OOP'er with (hopefully) trivial questions.....

[C.M.Brown]

> If however, you *really* want to keep your shapes as being seperate
> types, then you'll want to invoke the class system (note, not the same
> as OO classes).
>
> class Shape a where
>    area :: a -> Int
>
> newtype Circle = C Int
>
> instance Shape Circle where
>    area (C r) = pi * r^2
>
> newtype Rectangle = R Int Int
>
> instance Shape Rectangle where
>    area (R h w) = h * w
>
> newtype Square = Sq Int
>
> instance Shape Square where
>    area (Sq l) = l * l
>
> -- Now we can do something with our shapes
> doubleArea :: Shape a => a -> Int
> doubleArea s = (area s) * 2

Perhaps introduce an existensial quantification?

data Shape = forall a. Sh a => Shape a

class Sh a where
  area :: a -> Float

data Circle = Circle Float

instance Sh Circle
  area (Circle r) = pi*r*2

data Rect = Rect Float Float

instance Sh Rect
  area (Rect h w) = h * w

doubleArea :: Shape -> Float
doubleArea (Shape x) = (area x) * 2

I think this is more in the traditional OOP sense. But this way or Tom's:
one would have to convert functions like equality over Values of type
Shape into equality over different types (Circle and Rect). This can be
done using case analysis over the types with something like read.

Kind regards,
Chris.