Re: pi seems not to be a constant

On Tue, Oct 20, 2015 at 1:25 AM, Don Cohen
<[email protected]> wrote:
> Pierpaolo Bernardi writes:
>  > On Tue, Oct 20, 2015 at 12:28 AM, Don Cohen
>  > <[email protected]> wrote:
>  > > Pierpaolo Bernardi writes:
>  > >  > 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.
>  > >
>  > > I don't understand what you think will go wrong.
>  > > Please specify.
>  >
>  > Portable, standard, CL code expects the value of
>  > MOST-POSITIVE-LONG-FLOAT to be the most positive long float value.
>
> I already demonstrated that this expectation is violated in clisp
> as things are right now.
>
> I didn't actually propose to "fix" that.
> I'd rather "fix" the expectations of users/programmers who set
> long-float-digits.
>
>  > With your proposal, the example case I described above won't work.
>
> Your code above does not demonstrate this.  I was asking for such a
> demonstration.  How about a demonstration of some code that might
> be useful for something other than demonstrating non-compliance?

Here it is:

;; portable CL code library
(defun circle-area (radius)
  (* radius radius pi))

currently in Clisp I can use the above code in this way:

(setf (ext:long-float-digits) 1000)

;; print area with 1000 bits precision
(print (circle-area 42l0))

This will break under your proposal.  I think this is an important use
case and I will be unhappy if this stopped working.

If you don't agree that this is a useful feature of Clisp, then we
will have to agree to disagree.

Cheers
P.

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