Re: Records that could be arrays

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

On 26 Feb 2006 18:26:43 -0800, Steve Whalen wrote:

> I don't think I'd want Ada to have a facility to do array and record
> overlays beyond what exists (in the example I gave), because to me
> arrays and records are NOT synonymous. I can't think of a single "real
> world" example where the use of the data and the nature of the data in
> the application didn't clearly suggest that either it was fundamentally
> an array or a record.

Consider a linear algebra library. Aren't matrices arrays? Good. Aren't
sparse matrices arrays? Hmm.

Interface /= implementation. I was talking about array interface. The
implementation of the interface can be any.

> I guess that part of the reason I don't want this feature is because I
> agree with with Jean-Pierre Rosen's statement that arrays are really
> for iterative structures, so I don't see much need for this since most
> of the various types of examples given in this thread are clearly (to
> me) more safely and properly defined as records.

Defined /= declared

> I don't know that I'd want to burden the compiler with trying to ensure
> that an array and record had bit equal representation clauses either
> given or implied before allowing such an overlay.  If equal internal
> representation is NOT the case, I'd hate to think of the inefficiency
> of a program using both "array style" AND "record style" access code
> with the compiler having to do a lot of "conversion" of representation
> in order to support the hypothesized "overlay" facility.

There is no any burden for the compiler, because it is up to the programmer
to implement the interface. There is a definite burden for the language
designers to unify the type system and language syntax.

> Ada compilers have enough to keep straight already. Or at least I don't
> see enough utility in such a feature to justify the added complexity.
> Of course I may be misunderstanding what array/record overlay facility
> you have in mind.

What I mean is:

   type Sparse_Matrix is private array (...) of ...;
private
   type Sparse_Matrix is tagged record ...;
   -- Here the compiler will politely ask me to provide an implementation
   -- of access to element primitive operations of Sparse_Matrix.

or

   type Joystick is private record
      X : Integer;
      Y : Integer
   end record;
private
   task type Joystick is ...;
   -- Here I will be required to provide access to components methods,
   -- which I will implement via rendezvous! (:-))

Ada should finally get ADTs right!

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