(no subject)

[Dirk Herrmann]

On Thu, 22 Feb 2001, thi wrote:

>    From: Dirk Herrmann <address@hidden>
>    Date: Thu, 22 Feb 2001 04:23:54 +0100 (MET)
> 
>    (define foo (cons 'a 'b))
>    (define bar (cons 'c foo))
>    (define baz (cons 'd foo))
>    (eq? (cdr bar) (cdr baz))
>      ==> #t
>    [but reading back does not preserve structure]
> 
> fyi, below is a semi-recent usenet post that shows one solution.

Actually, no.  It shows a solution for the problem that we have already
solved, namely how to store/restore a _single_ structure.  However, since
we know how to correctly restore a single struture, a workaround for the
moment can be to put all objects that are to be binary-written into an
array, and then write this array.  It may be that this is the only
solution for the problem that makes sense - look at the following example:

  (define foo (cons 'a 'b))
  (binary-write foo <some-port>)
  (set-car! foo 'c)
  (binary-write foo <some-port>)

The cell address of foo did not change, but the content did.  Thus, to
treat such situations right, we may not keep a hashtable with 'objects
already stored' between different calls to binary-write, because the
objects that were already stored may have been modified in between.

As a conclusion, the solution that is currently implemented is fine.
These issues should, however, be documented.

Best regards,
Dirk Herrmann