Re: Type invariants and private extensions?

[Victor Porton <porton@narod.ru>]

Victor Porton wrote:

> 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.

On the other hand, when it is done programmatically without a user 
intervention, the kind of object is usually known at the point of object 
creation. In this case I could use the discriminant approach. But this 
breaks the above case with a query entered interactively by a user.

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