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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Type invariants and private extensions?
Date: Thu, 14 Sep 2017 16:56:29 +0300
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Date: 2017-09-14T16:56:29+03:00
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Shark8 wrote:

> On Tuesday, September 12, 2017 at 4:59:17 PM UTC-6, Victor Porton wrote:
>> 
>> Thanks.
>> 
>> But this seems not to solve my problem:
>> 
>> The base type for Example cannot be defined as a discriminated type for
>> certain Type_Enumeration, because it is possible that when creating the
>> object it may be yet unknown what the value of the discriminant should
>> have (my main problem is to invent somethings if this value is known at
>> object creation, but we must support the unknown case too). AFAIK, it is
>> not possible to change the discriminant later.
> 
> What are you doing that you need to create an object w/o knowing the
> types? I've used the variant-record approach in several interpreters to
> excellent effect; I've also used tagged-type hierarchies, too.

Consider an interactive user interface.

The user enters the query command.

After this I create the object (at this point its "kind" is not yet known). 
After this I parse the query with the object. At this point I can determine 
the kind of the object (which depends on the query text), but at this point 
the object already exists, so that I cannot assign the discriminant.

-- 
Victor Porton - http://portonvictor.org