Miguel Mitrofanov
I've noticed it, but there are some problems with this representation, so I decided not to mention it. It's OK as far as we don't want functions working on two areas - I don't see, how we can implement, say, intersect :: Shape -> Shape -> Bool in this way. However, it's a useful pattern.
Yes, there are different trade offs, it depends what you want to do. The AlgDT makes intersect simple: intersect :: Shape -> Shape -> Bool intersect (Circle x) (Circle y) = ... intersect (Circle x) (Rectangle x y) = ... : As Derek hints at, this isn't so nice in C++ and friends, you probably will end up with // Apologies for any mistakes in the code, it's been a while. class Circle : public Shape { : bool intersect(s){ if(dynamic_cast<Circle>(s)){...} else if (dynamic_cast<Rectangle>(s)){...} : } } etc. In addition to being very verbose and tedious code to write, you will have no idea if you have managed to cover all cases. Your 'intersect' function is spread all over the place. -k -- If I haven't seen further, it is by standing in the footprints of giants