On Friday, March 6, 2015 at 10:13:55 PM UTC+5:30, Steven D'Aprano wrote: > Rustom Mody wrote: > > > On Friday, March 6, 2015 at 3:57:12 AM UTC+5:30, rand...@fastmail.us > > wrote: > >> It's been brought up on Stack Overflow that the "in" operator (on > >> tuples, and by my testing on dict and list, as well as dict lookup) uses > >> object identity as a shortcut, and returns true immediately if the > >> object being tested *is* an element of the container. However, the > >> contains operation does not specify whether object identity or equality > >> is to be used. In effect, the built-in container types use a hybrid > >> test: "a is b or a == b". > >> > >> My question is, is this a *correct* implementation of the operator, or > >> are objects "supposed to" use a basis of equality for these tests? > > > > nan is an illegal or bogus value. > > NANs *represent* bogus values, they aren't bogus themselves. > > > > As usual legalizing the illegal is always fraught with increasing > > conundrums. > > > > The most (to me) classic instance of this is denotational semantics. > > In DS one tries to give semantics to programs by mapping programs to > > math-functions across some domains > > > > However some programs crash. What should be the semantics of such a > > program. We say its a partial function – undefined at the crash-points. > > But partial functions are not nearly as tractable (to mathematicians!) as > > total functions. > > So we invent a bogus value ⊥ (called bottom) and totalize all functions > > by mapping to this. > > > > Very nice… > > > > So nice in fact that we wish to add ⊥ to our programming language > > > > And now all hell breaks loose because the question x == ⊥ is the halting > > problem. > > Oh nonsense. x == ⊥ is easily performed with the equivalent of: > > type(x) == BottomType and bit_representation(x) == bit_representation(⊥)
You dont grok your theory of computation very well do you? def foo(x): return x + x def bar(x): return x + x def baz(x): return 2*x One can imagine an implementation where id(foo) == id(bar) [I am assuming that id is a good enough approx to bit_representation] Can you imagine an implementation where id(bar) == id(baz) ? If you can I will just start increasing the gap between bar and baz and in that game we will be re-executing the details of the halting problem (and more generally the Rice theorem) Simpler for you to revise your Theory of computation I think <wink> -- https://mail.python.org/mailman/listinfo/python-list
