Re: pi seems not to be a constant

On Mon, Oct 19, 2015 at 8:46 PM, Don Cohen
<[email protected]> wrote:
>
>  > Yes, now it is a sound proposal. But, if implemented in plain Clisp it
>  > would make variable precision floats a PITA to use, in addition to
>
> I don't see how it makes variable precision any worse than before.

A typical use case for adjustable precision floats is the following:

(setf (ext:long-float-digits) some-appropriate-number-of-bits)
(large-body-of-standard-portable-cl-code-using-long-floats)

If long-float-digits is changed and the "constants" are not adjusted
then STANDARD PORTABLE CL code will break.

>  > invalidating all previously written Clisp programs using variable
>  > precision floats.
>
> Actually, only those using variable precision float CONSTANTS,

All of them will be broken until someone audits them and proves that
they are not.

>  > One solution could be for the ansi mode to transform all these
>  > variables in constant variables, and remove the EXT:LONG-FLOAT-DIGITS
>  > function from the image.
>
> I don't see that removing EXT:LONG-FLOAT-DIGITS is really necessary.
> It's an extension, and if you don't use it then everything seems to
> conform.

And if you use it then everything breaks.  So removing it will avoid troubles.

>  > Still, I don't get the need to do this work, supposing someone is
>  > willing to do it (It seems I haven't got all the emails in this thread
>  > and I may have missed some clarifying explanation of this point)
>
> I don't claim it's necessary, only an improvement.  A small one, but
> also, I think, requiring a small amount of work.

Maybe a simpler solution could be to replace CL:CONSTANTP with
something similar to

(setf (symbol-function 'cl::system-constantp) (symbol-function 'cl:constantp)

(defun cl:constantp (x)
  (case s
     ((CL:PI CL:MOST-POSITIVE-LONG-FLOAT ...) t)
     (otherwise (cl::system-constantp x))))

when compliance wrt this issue is desired.

Would this break something?

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