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From: Natasha Kerensikova <lithiumcat@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Wed, 18 Aug 2010 12:02:48 +0000 (UTC)
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On 2010-08-18, Georg Bauhaus <rm.dash-bauhaus@futureapps.de> wrote:
>> 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.

Oh ok, thanks a lot for the explanation.

I thought discriminant really was like C's union, allocating enough room
for any possible sub-object to fit (and adding an overhead to explicit
which sub-object is currently used, to prevent C union's unfamous
undefined behaviour issues). Actually it's much more than a union.


Thanks a lot for your clarity,
Natacha