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Date: Wed, 18 Aug 2010 13:22:01 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
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Date: 2010-08-18T13:22:01+02:00
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On 18.08.10 12:49, Natasha Kerensikova wrote:

>>     type S_Expression (Is_Atom : Boolean) is new Root with record
>>        case Is_Atom is
>>        when False =>
>>           List : Lists.Vector;
>>        when True =>
>>           Value : Atom;
>>        end case;
>>     end record;
> 
> I don't understand why you need Indefinite_Vectors here. They have been
> presented to me as heterogeneous vector containers, but here Lists would
> be homogeneous, containing only S_Expression items.
> I'm not sure about the need of Root either, but my guess is that it's to
> provide a common root to all objects contained in the
> Indefinite_Vectors.
> 
> But what makes S_Expression an indefinite type?

The type S_Expression is not constrained: Is_Atom has no fixed value.
It is an unknown discriminant.  (LRM 3.3 says---in order to give a
reason I guess---"An indefinite subtype does not by itself provide
enough information to create an object".)
An S_Expression object that has Is_Atom = True can have a layout
different from that of an object with Is_Atom = False, and has
different components.  The objects are heterogeneous in this sense.

(Imagine creating two subtypes:
  subtype SA is S_Expression (Is_Atom => True);
  subtype SL is S_Expression (Is_Atom => False);
Then,
  A : SA;
  L : SL;
will be objects that might or might not take up the same amount
of storage, but more importantly, A has a Value component, L
does not, it has a List component, perhaps unlike C unions, cast.)

When instantiating a container generic without Indefinite_ in its name,
a definite subtype is needed.


Georg