[racket-dev] weak boxes in a script

David,

I believe you are seeing the difference between modules and the REPL.
At the REPL, each expression is compiled, run, and discarded.  This
yields the behavior you expect.  But a "#lang" form produces a module,
which is compiled and kept for the rest of the session.  Thus, even
after garbage collection, the compiled form of the first term still
exists:

(define b (make-weak-box 'not-used))

Thus there is a reference to the symbol not-used, and it is not
garbage collected.

Carl Eastlund

On Wed, Mar 16, 2011 at 5:09 PM, David Vanderson
<david.vanderson at gmail.com> wrote:
> I'm seeing a difference between when the value in a weak box is collected.
>  When I run the following interactively, the value is collected like I
> expect.  But it is not collected when running in a script, or in DrRacket.
>  This is v5.1 on Ubuntu x64, compiled from source.
>
> Can someone explain the difference?
>
> ~$ racket
> Welcome to Racket v5.1.
>> (define b (make-weak-box 'not-used))
>> (weak-box-value b)
> 'not-used
>> (collect-garbage)
>> (weak-box-value b)
> #f
>
>
> ~$ cat weak-box.rkt
> #lang racket
>
> (define b (make-weak-box 'not-used))
> (weak-box-value b)
> (collect-garbage)
> (weak-box-value b)
>
> ~$ racket weak-box.rkt
> 'not-used
> 'not-used
>
> Thanks,
> Dave