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From: Natasha Kerensikova <lithiumcat@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Thu, 19 Aug 2010 10:16:00 +0000 (UTC)
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Date: 2010-08-19T10:16:00+00:00
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On 2010-08-19, Natasha Kerensikova <lithiumcat@gmail.com> wrote:
> On 2010-08-18, Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> wrote:
>> Yes, ideally we want a list of S_Expression. However, the language
>> doesn't allow us to create that directly. I can't instantiate Vectors
>> with S_Expression before I declare S_Expression, and I can't declare
>> S_Expression until I've instantiated Vectors for lists.
>
> OK, I understand the catch-22 problem here. I guess it will be the same
> with linked lists instead of vectors.
>
> Is there any other (perhaps more difficult for a beginner) way to
> address that issue?

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;

package Lists is new Ada.Containers.Indefinite_Doubly_Linked_List
                     (Element_Type => S_Expression'Class);

type Atom_Node is new S_Expression with record
   Value : Atom;
end record;

type List_Node is new S_Expression with record
   List : Lists.Vector;
end record;


There definitions would of course be private, though I guess
S_Expression, Atom_Node, List_Node and their lineage would be public.
That would allow subprogram to explicit whether they work on/return
atoms, lists, or any kind of S_Expression.

Does it make sense? Is it better or worse or just different? Would
conversion from *_Node to S_Expression be to difficult or too heavy to
make this realistic?


Thanks for your help,
Natacha