On Tue, 06 Jan 2009 01:24:58 +0000, Mark Wooding wrote: > Steven D'Aprano <ste...@remove.this.cybersource.com.au> wrote: > >> I've already mentioned NaNs. Sentinel values also sometimes need to >> compare not equal with themselves. Forcing them to compare equal will >> cause breakage. > > There's a conflict between such domain-specific considerations (NaNs, > strange sentinels, SAGE's equations), and relatively natural assumptions > about an == operator, such as it being an equivalence relation.
Such assumptions only hold under particular domains though. You can't assume equality is an equivalence relation once you start thinking about arbitrary domains. > I don't know how to resolve this conflict without introducing a new > function which is (or at least strongly encourages developers to arrange > for it to be) an equivalence relation. But there cannot be any such function which is a domain-independent equivalence relation, not if we're talking about arbitrarily wacky domains. Even something as straight-forward as "is" can't be an equivalence relation under a domain where identity isn't well-defined. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
