2017-01-27 0:15 GMT+01:00 Martin McClure <martin@hand2mouse.com>:
On 01/26/2017 12:47 PM, Nicolas Cellier wrote:
2017-01-26 21:22 GMT+01:00 Martin McClure <martin@hand2mouse.com <mailto:martin@hand2mouse.com>>:
[...]
The current implementation of #sign is
self > 0 ifTrue: [^ 1]. (self < 0 or: [((self at: 1) bitShift: -31) = 1]) ifTrue: [^ -1]. ^ 0
I'd propose factoring this into two simpler methods:
sign self > 0 ifTrue: [^ 1]. self < 0 ifTrue: [^ -1]. ^ 0
maybe self isNan ifTrue [^-1 raisedTo: self signBit], or the standard tells it should be 0 too?
This addition makes sense to me.
The standards help a *little* and suggest on alternative.
ANSI Smalltalk acts like NaNs don't exist (basically, it lets implementers do what they want for those values).
ANSI Smalltalk relies on ISO/IEC 10967 of 1994 for numerics (even when it *cannot* rely on 10967, it does, sigh). That spec only defines a "sign" operation for floats with a definite numeric value, and says that operations are *permitted* to accept infinities and NaNs, but does not require this, nor say what the answer should be.
The 2007 update of 10967 is somewhat more helpful. It replaces the "sign" operation with one called "signum" which returns 1, -1, or NaN. It returns 1 for positive zero and positive infinity, and -1 for negative zero and negative infinity. If given a qNaN, it returns a qNaN. If given a signaling NaN, it returns a qNaN but notifies the application of an "invalid" situation.
If we extend this to Smalltalk, I *think* this would have us answer qNaN for qNaN, and for a sNaN signal a resumable exception (a Notification, perhaps?) that if resumed would answer qNaN. This also seems to make some sense -- NaNs are supposed to propagate, and asking the just plain "sign" (as opposed to signBit or isSignMinus) of a NaN is more or less nonsense. But if you prefer (-1 raisedTo: self signBit) I won't complain.
I'm OK with answering NaN, it was just a question. An incomplete specification is an open door to useless differences
That means that we'd have to refactor most (all?) senders of sign...
Certainly should audit senders of sign, but I'd expect that most senders don't really care about what the sign of -0.0 or NaNs are. The answer would remain the same for all other receivers.
Regards,
-Martin
-0.0 sign is answering -1 at least since 1998, so we might have get used to it ;) I've done the job on Squeak trunk, the answer is we need to revise some senders....