Re: Ada Interfaces and the Liskov Substitution Principle

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 1 Cze, 09:29, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> You are trying to invent some alternative meaning for class-wide.

Not really.
There is a difference between this:

X : T'Class := Y;

and this:

X := Y;  -- with X, Y : T'Class;

The difference is coming from the fact that T'Class gets bound at
initialization only and cannot change its binding (tag for you) later
on (X cannot change the actual type).
In other words, X can become equivalent to Y by initialization, but
not by assignment. The latter is already too late - you might be
assigning Triangles to Circles.
This is what makes T'Class different from T.

Of course, you can say that the meaning of assignment and equivalence
are both user-defined and can go together, but this way of thinking
usually subverts the regular meaning of :=. Not the Ada way.

> > You don't need to assign it. It is enough if you can replace it.
>
> Care to explain difference?

You can replace a Circle with a Triangle but you better not try to
assign them.

Note that you normally replace a Circle with a Triangle by
*assignments* of references of the appropriate interface type. ;-)
This indirection solves the problem, because references (pointers)
themselves are values.

No, hierarchies and values don't go together. I might lack the
formalism to prove it, but my observations are confirming: I've never
seen it in the wild.

--
Maciej Sobczak
http://www.msobczak.com/