Re: The "()" operator revisited.

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Warren W. Gay VE3WWG" <warren@ve3wwg.tk> writes:

> Robert A Duff wrote:
> > "Warren W. Gay VE3WWG" <warren@ve3wwg.tk> writes:
> >>Was operator assignment ever considered for Ada? If it was, and shot
> >>down, what were the primary reasons against it?
> > See AARM-7.6(17.a).
> > - Bob
> 
> I looked at this, but this doesn't really address the question.
> This info talks about controlled types, adjust etc. and what
> it should or shouldn't do, implementation details etc.
> 
> Perhaps I need to rephrase the question: If C++ proponents
> thought it necessary (or useful) to include an assignment
> operator, why was it that the Ada language (Ada83) was
> developed without one? Can anyone answer this or point back
> to early Ada discussions about this?

Sorry, I didn't realize you were talking about Ada 83.  Why can't the
user redefine ":=" in Ada 83?  I don't know, but I guess the designers
didn't like users redefining stuff like that.  Same reason the user
can't redefine "in" or aggregate notation or literal notation or array
indexing notation, I guess.  It seems inconsistent with the fact that
you *can* redefine "=" and "+" and so forth.

In Ada 9X, we decided the user *should* be able to redefine the behavior
of ":=".  AARM-7.6(17.a) explains why we didn't do that in the "obvious"
way -- some sort of syntax like:

    procedure ":="(...);

which would correspond directly to the way C++ does it.
It has to do with mutable record types (defaulted discrims).
That's unfortunate; if you want to redefine ":=" for a non-mutable
type, it's pretty annoying to be told you can't do that because
mutable types exist in the language.

The C++ way has some advantages over the Ada way (where the user's
control is split between Finalize and Adjust) because it's sometimes
convenient to get your hands on both sides of the assignment at the same
time.

- Bob