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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: S-expression I/O in Ada
Newsgroups: comp.lang.ada
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Date: Sun, 22 Aug 2010 16:10:49 +0200
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Date: 2010-08-22T16:10:48+02:00
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On Sun, 22 Aug 2010 10:24:16 +0000 (UTC), Natasha Kerensikova wrote:

> On 2010-08-19, Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>> On Thu, 19 Aug 2010 10:16:00 +0000 (UTC), Natasha Kerensikova wrote:
>>> And if there is no other way, would it make sense to get rid of the
>>> discriminant with something like:
>>> 
>>> type S_Expression is tagged null record;
>>
>> The type tag in S_Expression'Class is a kind of discriminant. You replaced
>> one with another.
> 
> That's more or less the idea: it seems I can't escape using a tag, and I
> thought having only a tag might be simpler and cleaner than having both
> a tag and a real discriminant, hence my proposition.

You often add discriminants later. One possible way to implement a tree:

   type Abstract_Node is
      abstract new Ada.Finalization.Limited_Controlled null record;

   type Leaf is new Abstract_Node with ...;

   type Branch (Cardinality : Positive) is new Abstract_Node with record
       Children : Array_Of_Node_Ptr (1..Cardinality);
   end record;

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