Re: IEEE Inf and NaN support for numbers.c

[Marius Vollmer <mvo@zagadka.ping.de>]

Marius Vollmer <mvo@zagadka.ping.de> writes:

> Ok.  I think we should follow PLT Scheme here.  I will apply your
> patch and make the necessary changes.

Done.  Please play with it and tell me whether you like the changes I
made.

	numbers.c (mem2ureal): Recognize "inf.0" and "nan.xxx".
	(scm_even_p, scm_odd_p): Treat infinity as even and odd.
	(iflo2str): Don't output a '+' for negative numbers or for Inf and
	NaN.  They will provide their own sign.
	(scm_divide): Only allow divides by inexact zeros.  Dividing by
	exact zeros still signals an errors.

Here is the NEWS entry:

    ** There is support for Infinity and NaNs.

    Following PLT Scheme, Guile can now work with infinite numbers and
    'not-a-numbers'.

    There is new syntax for numbers: "+inf.0" (infinity), "-inf.0"
    (negative infinity), "+nan.0" (not-a-number), and "-nan.0" (same as
    "+nan.0").  These numbers are inexact and have no exact counterpart.

    Dividing by an inexact zero returns +inf.0 or -inf.0, depending on the
    sign of the dividend.  The infinities are integers, and they answer #t
    for both 'even?' and 'odd?'. The +nan.0 value is not an integer and is
    not '=' to itself, but '+nan.0' is 'eqv?' to itself.

    For example

        (/ 1 0.0)
        => +inf.0

        (/ 0 0.0)
        => +nan.0

        (/ 0)
        ERROR: Numerical overflow

PLT Scheme does also distinguish plus and minus zero:

    (= 0.0 -0.0)
    => #t

but

    (eqv? 0.0 -0.0)
    => #f

I think we should do this as well.  John, how would one test for the
sign of zero?

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