Lambda the Ultimate

The present code implements a classless, delegation-based OO system, similar to those of Self or Javascript. This is a full-fledged OO system with encapsulation, object identity, inheritance and polymorphism. It is also a purely functional system: there is not a single assignment or other mutation in the code below.
http://pobox.com/~oleg/ftp/Scheme/index.html#pure-oo

The code archive
http://pobox.com/~oleg/ftp/Computation/Subtyping/Shapes/
contains a third directory, Shapes, with three source code files. The files illustrate various ways of handling a collection of polymorphic values. The collection is populated by Rectangles and Ellipses. They are instances of concrete classes implementing a common abstract base class Shape. A Shape is an existential type that knows how to draw, move and resize itself. A file Shapes-oop.cc gives the conventional, OOP-like implementation, with virtual functions and such. A file Shapes-no-oop.cc is another implementation, also in C++. The latter follows BRules, has no assignments or virtual functions. Any particular Shape value is created by a Shape constructor and is not altered after that. Shapes-no-oop.cc achieves polymorphic programming with the full separation of interface and implementation: If an implementation of a concrete Shape is changed, the code that constructs and uses Shapes does not even have to be recompiled! The file defines two concrete instances of the Shape: a Square and a Rectangle. The absence of mutations and virtual functions guarantees that any post-condition of a Square or a Rectangle constructor implies the post-condition of a Shape. Both shapes can be passed to a function set_dim(const Shape& shape, const float width, const float height); Depending on the new dimensions, a square can become a rectangle or a rectangle square. You can compile Shapes-no-oop.cc and run it to see that fact for yourself.
It is instructive to compare Shapes-no-oop.cc with Shapes-h.hs, which implements the same problem in a purely functional, strongly-typed language Haskell. All three code files in the Shapes directory solve the same problem the same way. Two C++ code files -- Shapes-oop.cc and Shapes-no-oop.cc -- look rather different. On the other hand, the purely functional Shapes-no-oop.cc and the Haskell code Shapes-h.hs are uncanny similar -- in some places, frighteningly similar.

• Why not this simpler Shapes-no-oop2.cc instead of your Shapes-no-oop.cc ?

• In my examples, I tried to get as strong a typing as possible. In your example that would be having Square::offset_by return a Square instead of returning Shape.

  This kind of problem you don't have with classical OO since after using square.offset_by(...), you still know square is Square. Whereas with functional-OO, you loose this information. This is bad because you usually have special functions/methods for Square that you don't have for other classes.

The problem that do exist is when you want to write a polymorphic function that works on any Shape. C++ function templates solve this problem (not very easy to write though).