Re: [Maxima-discuss] sometimes 1//2 =/= 1/2

[Barton W. <wi...@un...>]

I have tested changing (let ((1//2  ... to (let ((bfhalf ....  It fixes the trouble and eliminates the need for the macro half that is used in absarg.   I hope to commit this fix along with a few others in a few weeks.


________________________________
From: Raymond Toy <toy...@gm...>
Sent: Thursday, December 4, 2025 10:28 AM
To: max...@li... <max...@li...>
Subject: Re: [Maxima-discuss] sometimes 1//2 =/= 1/2

Caution: Non-NU Email


On 12/4/25 5:43 AM, Barton Willis via Maxima-discuss wrote:

OK, maybe I found the culprit:

(defun bfloat-erf (z)
  ;; Warning!  This has round-off problems when abs(z) is very small.
  (let ((1//2 ($bfloat '((rat simp) 1 2))))
  ;; The argument is real, the result is real too
    ($realpart
      (mul
        (simplify (list '(%signum) z))
        (sub 1
          (mul
            (div 1 (power ($bfloat '$%pi) 1//2))
            (bfloat-gamma-incomplete 1//2 ($bfloat (power z 2)))))))))



And similarly for complex-bfloat-erf .  Changing  1//2 to bfhalf in this code didn't result in any testsuite failures, but I have a few pending changes to the function  absarg.
Nice bit of detective work! Yeah, renaming 1//2 to something else should take care of the problem.

Also, loading gamma.lisp gives the warning: (SBCL)

 in: DEFUN GAMMA-INCOMPLETE
;     (COND ((COMPLEXP MAXIMA::X) #C(0.0d0 0.0d0)) ((REALP MAXIMA::X) 0.0d0))

I think there are lots of these.

The issue here is that the compiler doesn't know what X is, so the cond cases of complexp and realp covers the case of numbers, but there are lots of other types which aren't covered.

This can be fixed in two ways. Add (declare (number x)). Add a T clause to cond. This should probably signal an error.

Or maybe even replace the cond with etypecase?

&#8203;