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Sorry to continue on this subject, but...

This page is part of the web mail archives of SRFI 73 from before July 7th, 2015. The new archives for SRFI 73 contain all messages, not just those from before July 7th, 2015.

1. I'm glad to see that -0 will be removed.  In the current reference
implementation, (+ 1 -1) returns 0, as does (+ -1 1), and it's very hard
for me to see why 0 is privileged over -0 in this case.

2. As you can probably guess from the fuss created by -0, exact
infinities sure as hell are not reciprocals of 0.  Infinities can be
created by calculating a limit of lim x->(+)0 (1/x), but the existence
of a limit is required.  This is because 0 _really is_ -0, and
consequently, 1/0 _really is_ 1/-0, that is, undefined.

So you might have two things to do, which are relatively orthogonal:

(a) define an exact NaN value, which is the result of division by zero,
and provide rules for handling NaN wrt basic arithmetic and boolean
operations (things like (> 0 NaN) are especially problematic);

(b) define exact (negative and positive) infinity, which are greater in
magnitude than any finitary exact number, but cannot be manipulated much
in any way; for example, (+ 1 inf) cannot evaluate to inf, because this
will break the numeric system.  These infinities cannot be obtained as a
result from any calculation, but one can use them as literals.

I think the latter might have some value, for example as initialisation
values for algorithms searching for a maximum value in a list etc.


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