Objects which contain objects (Spritely Goblins)

4.4 Objects which contain objects

Objects can also contain and define other object references, including in their outer constructor procedure.

Here is the definition of a “counting greeter” we call ^cgreeter:

(define (^cgreeter _bcom our-name)
  (define times-called
    (spawn ^mcell 0))
  (methods
   ((get-times-called)
    ($ times-called 'get))
   ((greet your-name)
    ;; increase the number of times called
    ($ times-called 'set
       (+ 1 ($ times-called 'get)))
    (format #f "[~a] Hello ~a, my name is ~a!"
            ($ times-called 'get)
            your-name our-name))))

As we can see near the top, times-called is instantiated as an ^cell like the one we defined earlier. The current value of this cell is returned by get-times-called and is updated every time the greet method is called:

goblins[1]> (define julius
              (spawn ^cgreeter "Julius"))
goblins[1]> ($ julius 'get-times-called)
; => 0
goblins[1]> ($ julius 'greet "Gaius")
; => "[1] Hello Gaius, my name is Julius!"
goblins[1]> ($ julius 'greet "Brutus")
; => "[2] Hello Brutus, my name is Julius!"
goblins[1]> ($ julius 'get-times-called)
; => 2