I'm curious, why do you think this is inapplicable to Arc? In a prototype-based class system, you could use these hidden classes to model namespaces for a purely functional space, and then function dispatch would be really fast.

Further, it opens up interesting possibilities for modules.

It's possible though that you could implement arc hash tables using js objects, since they, too, are just glorified hash tables in a different way.

Basically calls on the arc side are compiled thusly:

  (fn (bar)
    (foo bar))
   =>
  (lambda (bar)
    (ar-funcall1 __foo bar)) ; the 1 here means there is one argument

  (let ((callsite1 (ar-funcaller1)))
    (lambda (bar)
      (callsite1 __foo bar)))

  ; scheme!!
  (define (ar-funcaller1)
    (lambda (f arg1)
      ;dispatch based on f, argument types, etc.
      ...))

By using a callsite lambda object, we can keep track of previous multimethod etc. lookups.

And then we can make the compiler emit code especially for (foo 'bar) forms:

  (fn (foo) foo!bar)
    =>
  (let ((callsite1 (ar-quotecaller1 'bar)))
    (lambda (foo)
      (callsite1 foo)))

  (define (always-false . rest) #f)
  (define (ar-quotecaller1 quoted)
    ; use only a single variable to prevent
    ; race conditions in multi-threaded code
    ; (might not always work if using true
    ; threads instead of green ones though)
    (let ((checker-and-dispatch (cons always-false #f)))
      (lambda (ob)
        ; copy to our own local variable to
        ; prevent race conditions (assuming
        ; the read is atomic anyway)
        (let* ((my-checker-and-dispatch checker-and-dispatch)
               (checker (car my-checker-and-dispatch)))
          (if (not (checker ob))
              (begin
                (set! my-checker-and-dispatch
                      (patch-quotecaller1 ob quoted))
                (set! checker-and-dispatch my-checker-and-dispatch)))
          ((cdr my-checker-and-dispatch) ob quoted)))))
  (define (patch-quotecaller1 ob quoted)
    (cond
      ((table-with-hidden-class? ob) ; implemented however you will
        (let* ((hidden-class (hidden-class-get ob))
               (hidden-class-index (hidden-class-index-lookup hidden-class quoted)))
          (cons
            ; checker
            (lambda (ob _)
              (and (table-with-hidden-class? ob)
                   (eq? hidden-class (hidden-class-get ob))))
            ; dispatcher
            (lambda (ob _)
              (hidden-class-ref ob hidden-class-index)))))
      (... ;other conditions: functions, methods, other data types...
        )))