On 17 Dec 2007, at 10:46, Nicholls, Mark wrote:
I can obviously at a later date add a new class Triangle, and not have to touch any of the above code….
Yes, and you can indeed do a similar thing in Haskell. The natural thing to do here would be to define a type Shape... data Shape = Circle Int | Rectangle Int Int | Square Int area :: Shape -> Int -- Note, this is an interesting type if you want the area of circles area (Circle r) = pi * r^2 area (Rectangle h w) = h * w area (Square l) = area (Rectangle l l) 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 Hope that helps Bob