Add an example of object oriented program

[jpellegrini]

Hi @egallesio !
One more example. This one has a mock ansi-color? procedure, but it would work better after PR #679 :)

This is the usual "geometric shapes" example, but one can actually draw the shapes on a terminal-based system! With colors!

• This is an example of medium difficulty. It illustrates several object orientation concepts as they are implemented in STklos.
• The geometric shapes are not pretty. It's an example of a object oriented program, and the focus is not on doing ASCII prettyfication of the figures.
• I have added some exercises at the end of the file. :)
• Not the best possible OO design, but I was focusing more on trying to exemplify several different CLOS concepts. :)

Concepts illustrated:

1. Class definition, #:accessor, #:init-keyword, #:init-form
2. Class inheritance
3. Generic methods, including the use of next-method inside initialize
4. Using only the class of an object to get different behavior (the initializers for <triangle> and <parallelogram> are mostly the same, but the objects will be drawn differently because they are instances of different classes)
5. display-object and object-equal?
6. One case in which syntax-rules would get much more complex than low-level macros
7. Specific STklos procedures: 1+, 1-, key-get

[jpellegrini]

• Not the best possible

In particular, I don't like the #:points list-of-points idea... But anyway, it's already there.

[jpellegrini]

To see how it works, do this:

(load "ascii-draw.stk")

(define v (make <canvas> #:rows 20 #:columns 40))
(define p (make <point>  #:x 10 #:y 25))
(define i (make <circle> #:center p #:radius 8 #:color 'red))

(define q (make <point>  #:x 5  #:y 8))
(define r (make <point>  #:x 10  #:y 7))
(define s (make <point>  #:x 7  #:y 4))
(define a (make <parallelogram> #:points (list q r s) #:color 'blue))

(draw v i)
(display v)
(draw v a)
(display v)

(reset! v)
(display v)

(define g (make <polygon> #:points (list q s r p) #:color 'blue))
(draw v g)
(display v)

(set! (canvas-background v) white)
(display v)