Re: [Chicken-users] DBI

[Graham Fawcett]

On Thu, Feb 28, 2008 at 2:40 AM, felix winkelmann <address@hidden> wrote:
> On Wed, Feb 27, 2008 at 10:31 PM, Peter Bex <address@hidden> wrote:
>  > On Wed, Feb 27, 2008 at 04:23:37PM -0500, Graham Fawcett wrote:
>  >  > >  Can you even check for void?  Afaik there's no VOID? procedure.
>  >  >
>  >  > You can; just compare with another (void) value:
>  >  >
>  >  > (define void? (cute eq? (void) <>))
>  >
>  >  That sounds rather brittle.  Afaik "void" is defined as "no value".
>  >
>  >  One possible and plausible implementation of VOID is this:
>  >
>  >  (define (void) (values))
>  >
>  >  (eq? (values) (values)) is #t, but I'm not sure how safe it is to
>  >  depend on that.  Especially since VOID is some kind of "undefined"
>  >  value, nobody says this representation can't change.  I don't think
>  >  you should depend on this implementation detail.
>  >
>
>  Absolutely. Tha values definition of void above is btw not equivalent and
>  would break in many places.

I stand corrected. :-)

It would be a smart idea to change the implementation, then, so that
the unspecified value could not be tested with (eq?). That would
prevent it from being 'misused'.

There does seem to be a good case for an immediate value that *can* be
tested this way, though. John et. al. wouldn't have used (void) in
eggs if there weren't. Record instances aren't really a great answer
(though I suggested them myself) since different records of the same
type will fail an identity test.

Graham