[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: ambiguous sign notation support?



> From: William D Clinger <will@xxxxxxxxxxx>>
>> Paul Schlie wrote:
>> Any possibility of considering a somewhat more numerically
>> consistent abstract notation for ambiguously signed values?
>> 
>> (i.e. ~nan.0 replaces the positively signed +nan.0 notation)
>> 
>> Thereby enabling the optionally supported designations:
>> 
>> ~inf.0                                             ~inf.0
>> -inf.0                  -0.0 0 +0.0                +inf.0
>>    <|------------------------|-----------------------|>
>>    <--------- -nan.0 -------> <-------- +nan.0 ------->
>>    <---------------------- ~nan.0 -------------------->
> 
> This is an interesting question.  The IEEE-754 standard
> does not mandate *any* means for determining the sign of
> a NaN, but recommends a CopySign function that could be
> used for that purpose.
>> ...
> SRFI 77 does not require all NaNs to print as +nan.0.
> In fact, I don't believe SRFI 77 forbids any of the
> NaNs you wrote above to print as ~nan.0.  In that
> sense, I believe SRFI 77 already allows the extension
> you desire.
> 
> The extension you desire should not be mandated,
> however, because the IEEE standards explicitly allow
> a very large variety of encodings for the NaN values
> they specify.
> 
> Will

I agree that they need not be mandated, however the designation of
+nan.0 as the default NaN notation would seem to imply a positive
numerical ambiguity; thereby suspect if R6RS formalized '~' as
being a formally accepted notation for numerical sign ambiguity,
then ~nan.0 could be formally specified as the notation for a
generally ambiguous value; from which other somewhat more precise
ambiguities may then be utilized as a compatible extension?

-paul-