Re: Using the GNU GMP Library for Bignums in Emacs

[Achim Gratz]

Ken Raeburn writes:
> If we’re going to treat NaNs as having distinguishable bit patterns so
> we can tell whether two are the same or not, should we also have
> printable/readable forms that distinguish them? If not, how do we
> create and use distinguishable NaNs?

That would expose the NaN payload, using the non-specified bits of the
mantissa of an NaN to transport (sideband) information.  You will
probably find that no one really expects these bits to be used…

> For that matter, are we guaranteed that operations like (/ 0.0 0.0)
> will always generate NaNs with the same bit pattern?

On the same platform? Likely.  On different platforms, surely not.

> I would’ve guessed that it might be preferable to go the other
> direction, and use one canonical NaN value in Lisp, which would thus
> always be eq/eql to all other NaN expressions.

Ignoring the payload bits would probably be OK, but that still gives you
at least two different NaN (positive and negative) to deal with.


Regards,
Achim.
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