Classes and objects

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Lecture 8 - Page 5 : 11

Functional Programming in Scheme
Object-oriented programming in Scheme

Classes and objects

Due to (1) the first class status of functions, and due to (2) the use of static binding of free names, it is possible to interpret a closure as an object

With this interpretation, it is possible to regard certain function definitions as classes

The send method which is used in the Point class.

The function apply calls a function on a list of parameters. This should be seen in contrast to a normal call, in which the individual parameters are passed.

(define (point x y)
  (letrec ((getx    (lambda () x))
           (gety    (lambda () y))
           (add     (lambda (p) 
                      (point 
                       (+ x (send 'getx p))
                       (+ y (send 'gety p)))))
           (type-of (lambda () 'point))
          )
    (lambda (message)
      (cond ((eq? message 'getx) getx)
            ((eq? message 'gety) gety)
            ((eq? message 'add)  add)
            ((eq? message 'type-of) type-of)
            (else (error "Message not understood"))))))

The definition of a 'class Point ' with methods getx , gety , add , and type-of . On this page we have also defined the syntactical convenience function send that sends a message to an object. In MzScheme, be sure that you define send before Point (such that send in the add method refers to our send , and not an already existing and unrelated definition of the name send ).

All necessary stuff to play with Point.

Points and Rectangle