Re: Redefinition of Equality (Long)

[Dave Gibson <dgibson@cis.ohio-state.edu>]

Robert A Duff wrote:
> 
> In article <52hgmoINN7tg@snoopy.cis.ohio-state.edu>,
> david scott gibson <dgibson@snoopy.cis.ohio-state.edu> wrote:
> 
> It seems highly undesirable for "=" to modify its parameters.  If you
> want to implement "=" in terms of some destructive operations, then you
> ought to make a copy of the parameters first.  Why is this a problem?

Why pay the price of making a copy of an arbitrarily large object when 
it is not necessary??  (Making a few copies of access values will do.)
"=" should, of course, leave the abstract value of its parameters 
unmodified.  It should be the responsibility of the person implementing 
Set "=" to ensure that.  However, I do not see any good reason 
to prevent the operation from manipulating the value passed in 
in order to compute the function result - especially if it avoids
unnecessary copying.  The way I'm writing code, the value passed in is
always an access value although this is not visible at the client
level. 

> I certainly agree that functions should allow 'in out' parameters, but
> not so you can write evil "=" operations.  But even if 'in out' were
> allowed, you would still have your problem, because for *tagged* types
> an overriding operation has to be subtype conformant with what it's
> overriding.

Are you saying that a hypothetical 'in out' mode "=" would only
overload 
and not override the automatically generated "=" function?  Or would it
not even be allowed to overload?  Either way, that would be a problem.

> (2) About half of the reviewers were rabidly anti-side-effect zealots.
> Despite the fact that Ada functions allow side effects on globals, and
> on stuff pointed to by parameters, this contingent believed that
> preventing *some* forms of side effects (namely, on parameters) is
> better than nothing.

Well, I'm almost in agreement with them.  I'd like to write functions 
which do not have any side-effects at the abstract level.  How this is
achieved concretely, however, should not be unduely hampered by the 
compiler.  

> I was very much in favor of removing this restricion.  And I've found it
> frustrating when writing Ada 95 code.  I happen to think that *some*
> kinds of side effects are OK, and I find it annoying that I have to use
> access parameters, and put "aliased" and "'Access" all over the place.

The use of access types didn't help me out in this situation because 
I want to be able to move around object values by exchanging the access 
values pointing to the objects.  I also found it very annoying and
distracting placing "aliased" and "'access" all over the place.  

Thanks for your insights.

Dave
--
dgibson@cis.ohio-state.edu