Re: pi seems not to be a constant

Pierpaolo Bernardi writes:
 > On Sun, Oct 18, 2015 at 8:02 PM, Don Cohen
 > <[email protected]> wrote:
 > > Pierpaolo Bernardi writes:
 > 
 > >  > At the cost of every computation involving pi and long floats
 > >  > returning wrong answers!
 > >
 > > Why do you think they're wrong?
 > 
 > They would be wrong if PI had not the required long float precision.

Wrong in what sense?
I think that "wrong" means different from intended.

 (setf (EXT:LONG-FLOAT-DIGITS) 128)
 (setf s2 (sqrt 2l0))
 (setf (EXT:LONG-FLOAT-DIGITS) 64)
 (* 2 s2)
Is this now wrong because s2 was computed with higher precision?
If this is what I intended then I think it's right.


 > PI not being a constant is a much more benign non conformity. Surely
 > you can work around it?

I think you could use ext:pi for the cases where you want it to
change.
In fact, it occurs to me that it would make more sense to use
(pi &optional precision) to compute pi (values could still be cached),
and allow special binding of *ext:long-float-digits* to control
default precision. 

BTW, it now occurs to be that things like MOST-POSITIVE-LONG-FLOAT
have to be reinterpreted compared to other lisp implementations:
(setf (EXT:LONG-FLOAT-DIGITS) 128) 
(setf mp128 MOST-POSITIVE-LONG-FLOAT)
(setf (EXT:LONG-FLOAT-DIGITS) 64)
(> mp128 MOST-POSITIVE-LONG-FLOAT)
=>
T 

It would make sense to me to treat these other "constants" in the same
way I now propose to treat pi.

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