Re: Ada Interfaces and the Liskov Substitution Principle

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

On Sun, 03 Jun 2007 15:04:00 -0700, Maciej Sobczak wrote:

> On 3 Cze, 09:09, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
> 
>>> Yes, replacement can change the tag. It's the tag of referred object
>>> that can change. An access variable can point to Circle and then to
>>> Triangle.
>>
>> No, this is illegal. At least in Ada,: access Circle and access Triangle
>> are different types. They cannot be assigned to each other.
> 
> I was talking about access Object'Class with Circle and Triangle being
> both derived from Object.

What is the difference between the types access T'Class and T'Class?

> We are going in circles. I say T'Class should be limited.
> Even if T is not.

If I'd say access T'Class should be limited even if T is not?
 
> You can theoretize as long as you want and you can say (even rightly)
> that replacement is doubly-dispatching, but there *is* a difference
> between O(n+m) and O(n*m) - and this difference is what makes
> assignment of T'Class simply unrealistic.

No, that makes it unrealistic to manage this complexity using crude
patterns instead of a consistent approach. The problem is here, the choice
is between language support and manual cascaded case-statements.
 
>> The issue of identity / references you are trying to bring in is
>> irrelevant. Multiple dispatch assignment is fully applicable to wheel and
>> car objects. When you "replace" wheels, you could perform different tasks
>> depending on the types of the old and new wheels .How these tasks can be
>> encapsulated into one replacement action?
> 
> They are not. That's the feature of replacement.
> 
> This, however, brings another point - if you want to execute different
> assignment tasks depending on both LHS and RHS *types*, then let's
> extend it a bit: I want to execute different tasks depending on
> current *values* of both LHS and RHS. After all, if assignment is
> supposed to be doubly-dispatching and if it's also supposed to be no
> different than hypothetical Assign(X, Y), then I should be able to do
> this, right?

Wrong. These are independent. Double dispatch controls the choice of the
specific body, it tells nothing about how this body has to be assembled.
There could be different ways of doing this, with advantages and
disadvantages. In some cases you might really want LHS being finalized
before the overridden part of the assignment, like when it holds an
expensive resource. Then you have an issue of enforcing finalization on
LHS.

My approach, not shared here, is that the programmer should have more
control over this, when he wants. The constructors, destructors,
assignments and aggregates are all polymorphic subroutines composed
differently from normal primitive and class-wide operations. In a better
language one should be able to describe all these decompositions uniformly,
with a higher-level mechanism of polymorphic subprograms.

Handling discriminants and types tags is a separate issue of constraint
description and propagation.

> Right?
> Unfortunately, this is completely broken in Ada.

It is not broken, it does not exist. Adjust is not an assignment, it is
hack to provide functionality without much rethinking.

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