Re: Ada Interfaces and the Liskov Substitution Principle

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

On 27 Maj, 12:04, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> To ban something in the language you have to translate the ban into
> language terms. That could be either by syntax or by semantic rules.
> Because syntactically banned assignments are indistinguishable from legal
> ones, the only mechanics you could use for this is matching types. The
> types in question are from the same hierarchy (covariant). This lets mixing
> such types. So it *must* be contravariant to be banned = the operation
> (":=") will not be inherited in the argument of interest upon inheritance.

OK, now I see your point. I have nothing against following this path -
assignment statement *is* special anyway. What could go wrong if we
say that assignment is not inherited?

> And I don't see why it is a mess. What is wrong in assignment of
> UTF8_String to Wide_String, both from String'Class?

Probably design issue. Character encoding can/should be a strategy
inside a String, you don't need to subclass the whole thing just for
it. There are many aspects on which you could parameterize String and
with some of them being orthogonal you could quickly end up with
milion leaf classes.

> The very definition of a class says that types there share class-wide and
> primitive operations. If that is not the case: ":=" is not shared, then
> either it is not a class (= don't derive! if need not) or there is no ":="
> (= make it limited! if unsure). Looks much like a design problem.

Yes. That's why I say that I have never seen any reasonable hierarchy
that supports assignments.

> There is nothing special in
> assignments.

Yes, there is - they are generated. This is deceptive, because it
makes one think that it should all work by magic.
Another reason why they are special is that they are intuitively
associated with some particular effects and these effects cannot be
reasonably provided in an automated way. That's why I think that
assignments of class-wide types should be forbidden.

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