RE: Q on NaN

[Drew Adams]

Still following up on my own post:

  (condition-case nil (setq foo (/ 0.0 0.0)) (arith-error nil))
  In older versions of Emacs (at least prior to April 2005 CVS),
  this would evaluate to nil. Now, it evaluates to -0.0NaN.
  I can modify the code like so:
  (and (condition-case nil (setq foo (/ 0.0 0.0)) (arith-error nil))
       (bar foo)) ; foo must be a number, not a NaN
  What function do I use for bar?
  `numberp' doesn't work, since (numberp -0.0NaN) is non-nil.

 To make the point simpler:
 (numberp (/0.0 0.0)) returns t. That seems like a bug to me.

If this is not considered a bug, and `numberp' should return non-nil for
NaN, as it currently does, then what are all the possible NaN values to
test?

The Elisp manual mentions that the read syntax for (/ 0.0 0.0) is 0.0e+NaN.
On my system it is in fact -0.0e+NaN. Are there additional NaN values, or
would this be a sufficient test for NaN-ness, to replace `numberp':

(let ((foo (/0.0 0.0)))
  (and (not (equal -0.0e+NaN foo)) (not (equal 0.0e+NaN foo))))

(Note that (= -0.0e+NaN foo) returns nil, while (equal -0.0e+NaN foo)
returns `t'.)