Re: Choice of OO primitives in Ada95

[Bernd Holzmueller <holzmuel@informatik.uni-stuttgart.de>]

Ray Toal wrote:
> 
> donh@syd.csa.com.au (Don Harrison) wrote:
> 
> >1) Why should you have to specify that a type is 'tagged'? Can't the compiler work
> >that out for itself? eg. by seeing whether the type is extended elsewhere. The
> >developer is forced to worry about what should be an implementation issue.
> 
> Requiring 'tagged' is a GOOD THING!  Inheritance weakens encapsulation.
> The default case (no tagged) is that you design a type, and you
> provide all and only those operations that work on the type and you
> don't make any details available to anyone else - in short you
> fully control the type.  Now if your intent is to ALLOW derivation
> then you must mark it tagged.  This alerts the reader that this
> type may be derived from.  Imagine a language in which you could
> inherit from any type you wanted to! :-)  Whether or not a type
> should be tagged is a DESIGN decision; I totally disagree that it
> should be an implementation decision.  By the way a compiler can not
> in general determine if the "type is extended elsewhere" since in
> Ada extensions can appear in other compilation units.

Actually, the compiler _can_ work out if a tag is necessary. This is the case only if 
a type is used polymorphically, i.e., class-wide in Ada 95 terminology (and has 
_nothing_ to do with type-extension). A different implementation model than is given 
in the LRM would create a tag only in these cases, which would allow 'normal' 
(monomorphic) uses of a tagged type be as efficient as the use of an untagged type. 
The consequence is that a special kind of "tagged types" is not necessary if 
efficiency is of concern. This is discussed in more detail in a paper at Ada-Europe 96.

Regarding the requirement to mark a type tagged to control type extension: I cannot 
see any advantage in restricting the flexibility (in the sense of potential reuse) 
of the language without gaining anything. Whether or not a programmer is going to 
use a type for type extension is not relevant for the author of the original type 
and should therefore not explicitly prohibitable by him. It could be of interest 
for a _user_ of the original type because dispatching can be a concern, but not 
necessarily because he may wish to use this type only monomorphically, and thus 
no dispatching will ever occur.

Bernd

-- 
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 Bernd Holzmueller,
 Institute of Computer Science, University of Stuttgart      
 email : holzmuel@informatik.uni-stuttgart.de
 http://www.informatik.uni-stuttgart.de/ifi/ps/bernd.html
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