On 7/6/2017 11:41 AM, Marko Rauhamaa wrote:
Chris Angelico <ros...@gmail.com>:
The formal definition is that objects have identities, and that
assignment (including function parameters and return values) gives you
a reference to the same object.
My example didn't contain a single assignment, but a variation of your
statement would make a good part in a definition of identity.
"A person just walked into the revolving door and came back out
again." "Is it the same person?" "I don't know. What's the definition
of identity?"
Of course it's the same person. You don't need to identify that person
by a social security number in order to say "the SAME PERSON came back
out". You identify him/her by... identity.
Here's how identity is dealt with in First-Order Logic:
<URL: https://en.wikipedia.org/wiki/First-order_logic#Semantics>
In other words, identity is mapped to the "sameness" in a domain of
discourse.
In Second-Order Logic, you can define identity directly:
∀x ∀y x = y ↔ ∀P (P(x) ↔ P(y))
Programming languages are different beasts, of course, but "objects" and
"identity" are such important foundational topics that you'd expect a
bit more than hand-waving when defining the data model.
As a good example of the style I'm looking for, take a look at:
<URL: https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html>
Marko
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Terry Jan Reedy
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