Re: truth of %nil

[Neil Jerram <neil@ossau.uklinux.net>]

Mark H Weaver <mhw@netris.org> writes:

> Certainly writing (a b c . #nil) as (a b c) would be most natural and
> convenient, and maybe it's the best compromise, but I'm not entirely
> sure it's safe.
>
> What if we have an association list mapping symbols to booleans that
> came from elisp?  Such a alist might look something like
> ((a . #t) (b . #nil)), and can reasonably be assumed to be written
> and then read back in, but doing so would then result in
> ((a . #t) (b . ())), magically changing the false to a true.

Hmmm...  From the elisp point of view it's still false, of course.  From
the scheme point of view your point stands.

> This also violates the idea the CARs and CDRs should be treated the
> same way.

Also a good point.

> I'm tempted to suggest that "write" should write (a . #nil) as
> "(a . #nil)", and "display" should write it as "(a)".

For now I'm happy with any reasonable position (such as this), because I
don't think we've got any data to help decide between the options.
Hopefully it won't be too long before we have some real non-trival
Guile/Scheme/Elisp interactions.

      Neil