On Dec 8, 2:51 pm, Robert Kern <[EMAIL PROTECTED]> wrote: > Rhamphoryncus wrote: > > "We've always done it that way" is NOT a use case! Certainly, it's a > > factor, but it seems quite weak compared to the sort use case. > > I didn't say it was. I was explaining that sorting was probably *not* a use > case > for the boolean predicates at the time of writing of the standard. In fact, it > suggests implementing a Compare() function that returns "greater than", "less > than", "equal" or "unordered" in addition to the boolean predicates. That > Python > eventually chose to use a generic boolean predicate as the basis of its > sorting > routine many years after the IEEE-754 standard is another matter entirely.
I interpret that to mean IEEE 754's semantics are for different circumstances and are inapplicable to Python. > In any case, the standard itself is quite short, and does not spend much time > justifying itself in any detail. A pity, as it is often invoked to explain language design. > > I suppose what I'm hoping for is an small example program (one or a > > few functions) that needs the "always false" behaviour of NaN. > > Steven D'Aprano gave one earlier in the thread. I see examples of behaviour, but no use cases. > Additionally, (x!=x) is a simple > test for NaNs if an IsNaN(x) function is not available. That's a trick to work around the lack of IsNaN(x). Again, not a use case. > Really, though, the > result falls out from the way that IEEE-754 constructed the logic of the > system. It is not defined that (NaN==NaN) should return False, per se. Rather, > all of the boolean predicates are defined in terms of that Compare(x,y) > function. If that function returns "unordered", then (x==y) is False. It > doesn't > matter if one or both are NaNs; in either case, the result is "unordered". And if I arbitrarily dictate that NaN is a single value which is orderable, sorting just above -Infinity, then all the behaviour makes a lot more sense AND I fix sort. So you see the predicament I'm in. On the one hand we have a problem and an obvious solution. On the other hand we've got historical behaviour which everybody insists *must* remain, reasons unknown. It reeks of the Parable of the Monkeys. I think I should head over to one of the math groups and see if they can find a reason for it. -- http://mail.python.org/mailman/listinfo/python-list
