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

[Raymond T. <toy...@gm...>]

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|?

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