Re: [Scheme-reports] EQV? on numbers should be based on operational equivalence

[John Cowan <cowan@x>]

Mark H Weaver scripsit:

> Are you sure that all NaNs are operationally equivalent?  IEEE 754
> defines two different kinds of NaN: quiet NaNs and signalling NaNs.

Yes.  However, in practice no operation returns a signaling NaN unless
at least one argument is a signaling NaN, and even then it may trap
rather than return one.  I also found out that different chips disagree
on the meaning of the quiet/signaling bit: on most procesors 1 means
quiet, but on the PA-RISC and MIPS chips it means signaling, since IEEE 754:1985
was silent on the point.

> Keep in mind that any operation involving NaNs that the standard leaves
> unspecified is a place where a conforming implementation might
> distinguish between NaNs with different bit patterns.

In principle.

> > R6RS mandates that, but R7RS currently leaves it unspecified.
> 
> How would you suggest implementing memoization in a portable R7RS
> program?

Actually, both draft 6 and the current draft say (in section 6.2.4)
that 0.0 and -0.0 are distinct to EQV? if they are distinct at all.
I didn't remember that until today.

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