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

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

On 12/4/25 8:51 AM, Barton Willis wrote:

> 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.

Hmm. float.lisp defines |bfhalf| spec var. And it's value is updated 
whenever fpprec1 is called. Maybe you don't need that at all.

And we should probably rename |bfhalf| to |*bfhalf*| along with the 
other spec vars updated in fpprec1. And rename |1//2| to |*1/2*|. Or 
make |1//2| a |defconstant|. But sbcl is really picky about what can be 
a |defconstant|.

>
>
> ------------------------------------------------------------------------
> *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;
&#8203;