Re: static modules and local vars

["62945" <62945 at bol dot com dot br>]

Per wrote:

> 62945 wrote:
>
> > I imagine this implies that closures coming from a
> > static module are safe as well
>
> Yes and no.  Closures and objects are conceptually
> equivalent.  (They're implemented similarly, and you can
> use one to simulate the other.)  If you create a closure
> in one thread, and pass that closure to another thread,
> you might get thread safety problems - though only
> if either thread modifies the captured variable (field)
> while sharing it.

If I understand the jvm model, only heap objects need to
be synchronized - method (stack) variables are private
to each thread. I think I get what you mean when you say
there might be problems in sharing a closure, but this
would apply to any heap object. If there is only one
(object) reference in _use_ at a given time no problem.
If you copy (pass) the reference be cautious.

So the scenario:

static module f:

(define (f x)
  (let ((c (abs x)))
    (lambda ()
      (if (positive? c) (set! c (- c 1)))
      (zero? c))))

non-static module a:

(define a (f 5))

(if (a) ... ) ; many times

non-static module b:

(define b (f 10))

(if (b) ... ) ; many times

should work properly since the counter "c" is local to
each thread (module "a" and "b" run concurrently) and I
see no sharing of values except if "(f ...)" returns the
same value/closure/object. I would like to think that
the returned lambda behaved as a value, different for
each "f" call.




 
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