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

[Per Bothner]

On 08/07/2012 03:15 PM, Jamison Hope wrote:

> > 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.

Correct.

> In particular, nilExpr is not 'nil:

But it should be.

> 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)
> ()

These should all be the same: CommonLisp.FALSE.

Whether this value *prints* as () or nil (or NIL)
might depend on printer settings, but that is a separate issue.

> 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.

One could argue it should be the otherwise; I'm not sure what is best.

However, as long as things are the way the are, the reader does need to
special case nil. Basically nil is first resolved to COMMON-LISP:NIL.
But then "at the last minute" we replace the result by CommonLisp.FALSE.

Something like, using your repository code:

      Object s = LispPackage.currentPackage.get().intern(symname).get(0);
      if (s == CommonLisp.NIL_AS_SYMBOL)
        s = CommonLisp.FALSE:
      return s;

where CommonLisp.NIL_AS_SYMBOL is CommonLispPackage.getSymbol("NIL").

For things like property-list access on the symbol 'nil (which I'm
not sure if is allowed) use CommonLisp.NIL_AS_SYMBOL.

For example (symbol-plist sym) needs to do something like:

(define (symbol-plist symbol)
 (gnu.mapping.PropertyLocation:getPropertyList
    (if (eq symbol CommonLisp:FALSE) CommonLisp:NIL_AS_SYMBOL symbol)))

The helper method gnu.commonlisp.lang.Symbol.getSymbol does something 
similar.
Perhaps we need this method:

  public static Symbol asSymbol(Object sym)
  {
    return sym == CommonLisp.FALSE ? CommonLisp.NIL_AS_SYMBOL : 
(Symbol) sym;
  }

--
        --Per Bothner
address@hidden   http://per.bothner.com/