() and T.

[Richard M. Stallman <RMS at MIT-AI>]

I believe that () should be distinuished from NIL because
it is good if every data type is either all true or all false.
I don't like having one symbol be false and others true.

Another good result from distinguishing between () and NIL is
that the empty list can be LISTP.

For these reasons, I think that the Lisp machine should convert
to option 2 for NIL.

The situation for T is different.  Neither of those advantages
has a parallel for the case of T and #T.  It really doesn't matter
what non-() object is returned by predicates that want only to return
non-falsity, so the symbol T is as good as any.  There is no reason
to have #T as distinct from T.  However, option 3 is not really ugly.
Since one non-() value is as good as another, there is no great need
to require what value the implementation must use.  I prefer option 1,
but I think option 3 is nearly as good.

Meanwhile, let's have the predicates SYMBOLP, NUMBERP, STRINGP and CONSP
return their arguments, to indicate truth.  This makes possible the
construction
  (RANDOM-FUNCTION (OR (SYMBOLP expression) default))
where default might eval to a default symbol or might be a call to ERROR.
To do this now, we must write
  (RANDOM-FUNCTION (LET ((TEM expression))
		     (IF (SYMBOLP TEM) TEM
		       default)))
LISTP should probably return its argument when it is a non-() list.
(LISTP ()) should return some non-() list, also.
ATOM should return its argument if that is not ().
(ATOM ()) should return T.  Then ATOM's value is always an atom.

The general principle is: if a predicate FOO-P is true if given
falsehood as an argument, FOO-P should always return an object
of which FOO-P is true.
If, on the other hand, FOO-P is false when given falsehood as an
argument, then FOO-P should always return its argument to
indicate truth.

These two principles can be applied whether or not () and NIL
are the same.  If applied, they minimize the issue about T and #T.