On 8/14/06, Smylers wrote:
David Green writes:
Thanks for that. In summary, if I've understood you correctly, it's that:
=:= two aliases to the same actual variable
=== one variable contains a copy of the other's actual contents
eqv both contain contents which represent the same thing but may have
come from different sources
And that being true at one level implies being true for the above
levels. Yes?
Right. (Where the ordering for "above" means =:= implies ===, and ===
implies eqv, but not in the other direction, of course.)
> ===
Example: Suppose I have some employee objects, and I employ two John
Smiths. They have the same name, work in the same department, and by
stunning coincidence everything my class knows about them just
happens to be the same.
Except that they wouldn't. Because each one would have a separate
payroll number, or some artificial thing invented just for the sake
of being different. So this example doesn't sound plausible to me.
Well, I didn't say it was a *good* payroll system! OK, it's a silly
contrived example; maybe my objects should have represented the
reticulated flanges I have in stock, since one piece of inventory is
the same as any other. Except I wouldn't really create an object for
each one, I'd just have a single group-object that contained a
$num_available counter....
(Anyone cleverer than I should feel free to jump in with a better example.)
So I now understand what this operator does. But I'm still
struggling to fathom where I would ever have a use for it.
Maybe you wouldn't -- eqv is what most of us will use most of the
time, I expect (being the ordinary everyday parallel to == and eq).
But since it is possible to use variables both by value and by
reference, there still needs to be a way to work with and compare
both of them when you want to do fancy advanced stuff.
-David
