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.

--
XQuery Blueberry DOM                            John Cowan
Entity parser dot-com                           cowan@ccil.org
    Abstract schemata                           http://www.ccil.org/~cowan
    XPointer errata
Infoset Unicode BOM                                 --Richard Tobin

_______________________________________________
Scheme-reports mailing list
Scheme-reports@scheme-reports.org
http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports