Re: %nil once again

[Andreas Rottmann]

Daniel Kraft <address@hidden> writes:

> Hi Neil,
>
> Neil Jerram wrote:
>> Daniel Kraft <address@hidden> writes:
>>> 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?)
>
Yes (although I'd say you need the unquote because you want to access
the variable, not the other way around </nitpick>).

>>
>> Do you mean why don't we just use the symbol nil?  If so, the answer
>> is because in Scheme, (cons 'a 'nil) should be (a . nil), not (a).
>
> No, I mean why '(1 2 3 . %nil) does yield (1 2 3 . %nil) while `(1 2 3
> . ,%nil) gives the expected (1 2 3).  But that does not matter much
> besides astonishing me, as this is only something related to the
> Scheme implementation of %nil, I guess.
>
The *symbol* '%nil has absolutely no special status in Scheme. It just
happens that Guile provides (in the default environment) a value bound
as %nil that has special properties.

Regards, Rotty
-- 
Andreas Rottmann -- <http://rotty.yi.org/>