Re: Abusing tagged types

[Jean-Pierre Rosen <rosen@adalog.fr>]

Maciej Sobczak a �crit :
> I'm asking about the design principles - the *purpose* of tagged type
> is to achieve polymorphism with dispatching calls. Is it OK to use
> tagged type without this motivation?
One of the nice things in Ada is the ability to make a difference 
between a method and a subprogram (and yes, I'm afraid about the new 
generation of programmers who know only classes as unit of 
modularization, and call every subprogram a method).

IMHO, the term method should be reserved to operations that have a 
general semantics, but whose implementation (the way of doing the thing 
- the /method/ for doing the thing) depends on the type to which it 
applies. Methods make sense only in the presence of dynamic dispatching, 
i.e. the ability to tell an object: "do this your own way".

O.M notation stresses this by making clearer that a method is applied to 
a given object. Although subprograms may have a parameter of a 
non-tagged type which is in some sense distinguished, they are not 
methods by my definition, and I would not favour using that notation (to 
be honnest, I've never been a great fan of the O.M notation in any case 
- it is misleading if you have more than one controlling operand, which 
is a great superiority of Ada over other OO languages).

-- 
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            J-P. Rosen (rosen@adalog.fr)
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