Re: Items blocking release 1.6.1 (2002-04-21)

[Marius Vollmer <mvo@zagadka.ping.de>]

tomas@fabula.de writes:

> On Tue, Apr 23, 2002 at 08:16:20PM +0200, Marius Vollmer wrote:
> [...]
> > Yes.  The way the old 'bound?' was implemented was a bug.  The mistake
> > (my mistake) back then was to fix this bug in a sub-optimal way, by
> > just removing the functionality.  Now it is too late to change it
> > again; and changing it would be quite gratuitous, too.
> > 
> > Using #f as the default default value is a sensible thing, I'd say,
> > and should even be recommended.
> 
> As a provider of some functionality I'd sometimes like to be able
> to distinguish between `value was provided' and `value was not
> provided at all'. It'd be perfectly reasonable to agree on a
> value which means `not provided' (like Perl's undef or Pythons
> None): an user providing *such* a value hopefully knows what
> she's doing...

Yes.  You can do this easily when defining such a function.  I.e.

    (define not-provided (cons* 'not-provided))

    (define* (foo :optional (bar not-provided))
      (if (eq? not-provided bar)
          ...))

> ...but #f seems to be just wrong, since it's an often-used `logical'
> value.

To me, it seems just right as a default default value...

> Unspecified seems nice for something ``you don't specify'',
> doesn't it? (I know, you were against that on a previous posting).

You might use the 'unspecified value' (which is different from
SCM_UNDEFINED which was used previously) as the value of not-provided
above, but it seems to be too obscure, for my taste.  It doesn't feel
right to me to give #<unspecified> any specific meaning.

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