[fluxus] OOP / polymorphism

[Dave Griffiths]

> Hi all,
>
> Inspired by: http://www.pawfal.org/index.php?page=SchemeThinking
>
> I made a dynamic method dispatcher, enabling polymorphism.
>
> See attachment, could be useful I hope.

Nice. I've been wondering about wrapping up fluxus calls into higher level
objects for teaching and livecoding - or whether it's a bad idea to put
another level in the way. I've been writing quite long fluxus scripts
lately, and I tend to do everything in an object orientated manner to make
it nice and encapulated, but do thing explicitly (objecttype-method object
arg1 arg2 ...)

There is an interesting way to do it using local state variables that I
remembered after writing that page that comes from the SICP book section
3:
http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-20.html#%_sec_3.1.1

Basically, your constructors can return a function with the object
embedded as local state:

---8<---

(define (make-account balance)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch m)
    (cond ((eq? m 'withdraw) withdraw)
          ((eq? m 'deposit) deposit)
          (else (error "Unknown request -- MAKE-ACCOUNT"
                       m))))
  dispatch)

Make-account can be used as follows:

(define acc (make-account 100))
((acc 'withdraw) 50)
50
((acc 'withdraw) 60)
"Insufficient funds"
((acc 'deposit) 40)
90
((acc 'withdraw) 60)
30

--->8---

cheers

dave