Re: Making 'eq' == 'eql' in bignum branch

[Paul Eggert <eggert@cs.ucla.edu>]

Stefan Monnier wrote:

> Same here, except rather than "I'm fine with" I'd say that I'm in favor
> of it (for the same reason that I made `apply` work when called with
> a single argument).

I looked into implementing something along those lines and ran into a problem: 
the C standard does not require support for floating-point infinity, and a few 
mildly-plausible Emacs targets lack it. The most significant is IBM z/OS when 
compiled in default mode. There is also the Unisys Clearpath (formerly 
Burroughs) and Dorado (formerly Univac) lines. These environments are still 
supported, and help explain why the C standard does not require support for a 
working INFINITY macro in <math.h>, and why Common Lisp does not require 
infinity constants.

That being said, I now see that the Emacs source code has been using <math.h>'s 
INFINITY macro since Emacs 25.1. Although this was just an oversight (and I just 
now fixed it on master), nobody has complained.

With that in mind we could simply be upfront about it and explicitly require 
IEEE floating point, excluding old mainframe environments and some embedded 
platforms. Then we could change 'min' and 'max' to return infinity, and we can 
also simplify the C code a bit by assuming IEEE_FLOATING_POINT. On the other 
hand perhaps it's better to leave sleeping dogs lie, and not jump completely 
into the IEEE bed, and wait for the next C standard to revamp the floating point 
API again (which it looks like it will) before worrying about this.

As for a named constant, I now see a problem with it: it wouldn't work in 
expressions like (let ((gamow '(1 2 3 infinity))) ...). So perhaps we should 
just ask people to remember to use 1e+INF instead, as it doesn't have this 
syntactic glitch.