Re: [Kawa-commonlisp-dev] 'NIL != '()

[Jamison Hope]

On Aug 7, 2012, at 5:10 PM, Charles Turner wrote:

> The following equality does not currently hold in Kawa CL
>
> (eq 'nil '()) ;=> nil
>
> but it should.
>
> I'm not very confident, but I think this is because nil is looked up
> in the EmptyNamespace (or current lisp package in my repository),
> whereas () is processed by LispReader#makeNil, which returns
> LList.Empty. Of course, the mapping in EmptyNamespace is nil =>
> LList.Empty, which is confusing me, I guess you'd need a RefExp to get
> the LList.Empty.
>
> The initialisation in CommonLisp:
>
>  public static final LList FALSE = LList.Empty;
>  public static final Expression nilExpr = new QuoteExp(FALSE);
>
> seems to point to the problem, obviously they were designed to be
> different objects, which seems wrong to me, but this is code Per
> wrote, so I'm not confident I understand what's happening here.
>
> Any suggestions on how to fix this?

Those are separate objects because one is a value (one you are likely
to encounter from user code) and the other is an Expression which is
internal to the workings of the compiler. In particular, nilExpr is
not 'nil:

$ kawa --lisp
#|kawa:1|# 'nil
nil
#|kawa:2|# (static-field "gnu.commonlisp.lang.CommonLisp" "nilExpr")
(Quote
  ())

The inequality between 'nil and '() is a completely separate issue.
Note:

#|kawa:1|# ()
()
#|kawa:2|# '()
()
#|kawa:3|# nil
()
#|kawa:4|# 'nil
nil
#|kawa:5|# (eval 'nil)
()

'nil (unlike any other quoted symbol) should be self-evaluating but it  
isn't.

Other CL implementations probably get away with this by just using the  
symbol
itself as the canonical representation of NIL; Kawa, however, chooses  
to use
LList.Empty so that Scheme lists and Lisp lists are interchangeable.

--
Jamison Hope
The PTR Group
www.theptrgroup.com