Re: %nil once again

[Neil Jerram]

Daniel Kraft <address@hidden> 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