Re: Operation can be dispatching in only one type

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 22 Nov 2009 16:47:21 -0500, Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> P.S. ":=" for initialization of limited objects might look misleading (and
>> does look misleading to me), but that would be another discussion on Ada
>> design, in which nobody would agree with me. So I prefer not to go into it.
> 
> Well, I would agree.
> 
> Ada uses the term "assignment" to refer to both "initial assignment /
> initialization" and "assignment_statement / overwriting".
> I'd prefer to use different symbols for the two.
> We're not going to change Ada in that regard, for compatibility reasons,
> but I'm thinking in my hobby language design to use the term "assignment"
> for the initial one, and "reassignment" for the subsequent overwriting
> one, and use different symbols for the two.
> 
> So, for a limited type, "assignment" is legal, "reassignment" is not.
> 
> What do you think?

I would prefer conventional:

allocation
construction (initialization)
assignment
destruction (finalization)
deallocation

Semantically a limited object is never assigned. Its state comes into
existence per construction and disappears per destruction.

The word assignment for most people is associated with state change,
assuming that there was some state before. So

   X : T := F (Y);

looks equivalent to

   X : T;
begin
   X := F (Y);

But they are not. I would prefer proper constructors, e.g.

   X : T (Y); -- Y is a constraint of T, parameter of the constructor

I don't like functions returning limited objects.

But there also are two other forms of standard assignments. Depending on
whether the left part is available:

   procedure ":=" (Left : in out T; Right : T);

and

   function ":=" (Right : T) return T;

Ada uses the second form, but obviously there are important cases where the
first form is preferable (and conversely).

And further, there are three variants per each concerning dispatch:

   procedure ":=" (Left : in out T; Right : T);  -- Full MD
   procedure ":=" (Left : in out T; Right : T'Class);  -- Target-controlled
   procedure ":=" (Left : in out T'Class; Right : T);  -- Source-controlled

It would be difficult to sort this out! (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de