Re: Extension of non-limited type needs limited component

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

"Jean-Pierre Rosen" <rosen@adalog.fr> writes:

> The benefit, and drawback, of tagged types is that they allow dynamic
> typing, therefore requiring (isn't it surprising) dynamic checking. More
> flexibility, less security. If you want full compile-time checking, don't
> use tagged types.

Well, it's not quite that bad.  Dmitry Kazakov posted the example of a
procedure with 2 controlling operands.  Yes, that requires a tag check.
Downward conversions require a tag check.  But normal dispatching on a
single controlling operand (which is by far the most common case)
requires no run-time check.  So, for most of your code, you can freely
use tagged and class-wide types, and *still* get full compile-time
checking.  That's the advantage of Ada (and C++ and Java) over Smalltalk
(and CLOS and ...).

In other words, "...don't use tagged types" is too strong.

> At least in Ada, you have a choice!

Yes.

- Bob