%nil once again

[Daniel Kraft <d@domob.eu>]

Hi,

I think I got the test-suite as well as a basic macro implementation 
(the compiler framework is really cool, that was fairly easy to do); 
recursive macros do not yet work, but otherwise it looks fine.

However, I want to tackle quasi-quotes (besides others) now; and in 
Elisp %nil is not only #f of Scheme but also the end-of-list marker (I 
guess that's why simply using Scheme's #f for %nil does not work).

I did some experiments, and it seems that Scheme respects it partially:

scheme@(guile-user)> `(1 2 3 . ,%nil)
(1 2 3)

(is %nil in Scheme a variable because it needs the unquote?)

However:

scheme@(guile-user)> (null? %nil)
#f
scheme@(guile-user)> (equal? %nil (cdr (list 1)))
#f

It would be cool to teach Guile somehow to treat %nil as the "standard" 
end-of-list value, such that both of these queries would optimally 
return true.  At least, is there some way to construct lists terminated 
by %nil using something like the list primitive?  Other things needed 
would be for instance terminating rest-arguments by %nil rather than '() 
and the like.

So:  How is this handled by the interpreter?  Is there maybe some 
runtime-option to make Guile use %nil as end-of-list for lists 
constructed?  Or could we introduce some means to do so?

If that's not a good idea because of performance or other 
considerations, I guess I'll have to implement some conversion routine 
and use that?  This on the other hand will probably hit Elisp's performance.

Any suggestions and ideas welcome!  Maybe I just fail to see something...

Thanks,
Daniel

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