AW: [plt-scheme] Changing field values in objects

On Fri, 2 Feb 2007, Robert Matovinovic wrote:

> The cookbook unfortunately doesn't help with that.

Hi Robert,


Are you sure?  I'm pretty sure I had something about this... checking... 
ah, ok, I see.


There are some example of methods that indirectly allow people to mutate 
objects.  For example, the example where people can meet other people:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
       (define person%
         (class object%
           (init-field name)
           (define friends '())
           (super-new)

           (define/public (add-friend other)
             (set! friends (cons other friends)))

           (define/public (meet-all)
             (for-each
               (lambda (f) (printf "hi ~a~%" (get-field name f)))
               friends))))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


I didn't understand fields as well back when I wrote the guide.  But 
currently, in my own programming, I try to avoid fields if I can because 
they're equivalent to "public" attributes in other programming languages. 
I like encapsulation.  *grin*


So in a hypothetical revision of the guide, I'm planning not to use fields 
at all, so that the example above will look like:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
       (define person%
         (class object%
           (init name)

           (define -name name)
           (define friends '())

           (define/public (get-name) -name)

           (define/public (add-friend other)
             (set! friends (cons other friends)))

           (define/public (meet-all)
             (for-each
               (lambda (f) (printf "hi ~a~%" (send f get-name)))
               friends)))

           (super-new))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

which matches much more closely with how classes in Java are done: private 
attributes, public accessors.  Plus this example shows a case where one 
object is politely sending one message to another object, rather than 
impolitely digging into concrete implementation details of that object.


I just have to motivate myself to get this hypothetical revision all 
done...  Stupid procrastination... *grin*


Best of wishes!