Re: %nil once again

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

Daniel Kraft <d@domob.eu> writes:

>>> scheme@(guile-user)> (null? %nil)
>>> #f
>>> scheme@(guile-user)> (equal? %nil (cdr (list 1)))
>>> #f
>>
>> I believe those work in the interpreter, and so are VM bugs.  Can you
>> check that with ,o interp #t ?
>
> The first one is indeed #t with the interpreter, the second one
> not. But unfortunatly I think that the elisp equivalent of
>
> (equal?/eqv?/eq? (cdr (list 1)) nil)
>
> (don't know which predicates take the place of eq?/eqv?/equal? in elisp yet)  should indeed yield true, as a perfectly valid way to check for (null? (cdr (list 1))), right?  So it seems that in this case even the Guile interpreter does not handle empty lists as it should for elisp -- 
> or don't we need to ensure that test is true?

Hm, interesting point.  Is it a problem in practice though?  If so,
what is the practical context?

i.e. is there a lot of existing code that uses (equal ... nil) to test
for the end of a list, rather than (null ...) ?

> So no need for ensuring myself that
> all '()'s get replaced by %nil's?

I would say not - because much of the existing Scheme/Elisp design is
based on _not_ having to translate data as it passes between
languages.

Or, to put it another way, if we think that we _do_ have to accept
translating data, I think we could remove lots of special Elisp
support from the libguile core.

Regards,
        Neil