Re: Ada Interfaces and the Liskov Substitution Principle

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

On 26 Maj, 09:48, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> > I'm still for banning it.
>
> For this you have to make assignment contravariant (non-primitive
> operation) in one of its arguments. That would be a total mess, because it
> would require overloading assignment for each derived type.

I don't get it. I said I would ban it. There is no "for this". :-)

> For the same reason all signatures with a class-wide parameter are bad,
> because they lead to ambiguities in trivial cases:
>
> type S is new T with ...;
> X, Y : S;
>
> X := Y;  -- S'Class x S'Class vs T'Class x T'Class?
>
> You will need some sort of dominance rules to resolve that.

Why? Both X and Y are of the same type above and it is not a class
wide type in the sense that no dispatching is needed here.
In other words, the above assignment can be bound statically.

> P.S. Probably you have in mind a "stratified" assignment which statically
> checks that LHS and RHS are of the same type.

Not necessarily. I would allow different types if the
assignment_statement is overloaded for them for those who like to
introduce sort of implicit conversions.
What I worry about is the situation where *both* dynamic types are not
known statically. This is a mess.

> -----------
> *  Ada's assignment is doubly dispatching. The dispatching table is a
> square, the diagonal of consists of thunks:
>
>    Finalize (LHS);
>    bit-copy (LHS, RHS);
>    Adjust (LHS);
>
> Non-diagonal elements are:
>
>    raise Constraint_Error;

Which is a run-time thingy. It's good if it's there, but I want
stricter guarantees. You can get them by just banning
assignment_statement between class-wide types.

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