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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Type invariants and private extensions?
Date: Tue, 12 Sep 2017 23:08:56 +0300
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Date: 2017-09-12T23:08:56+03:00
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Jeffrey R. Carter wrote:

> On 09/12/2017 08:54 PM, Victor Porton wrote:
>> 
>> A SPARQL query may return boolean, graph, bindings, etc. The return type
>> is determined by the text of the query.
>> 
>> I want to make derived types of a query which are validated that their
>> return is of a certain type (e.g. a boolean). This certainly useful in a
>> language like Ada intended for reliability and maximum runtime check
>> during debugging.
> 
> "I want to" is not the same as "there is no way to solve the problem in
> current Ada". Ada has a feature that provides exactly what you need. It's
> called a variant record.

It is a tagged type. AFAIK, a type cannot be both a variant record and 
tagged.

Moreover, the object in consideration is a wrapper over a certain C API. It 
surely cannot be described in variant record terms.

Moreover, bindings may contain more than one row of values which can be 
enumerated like an iterator. I am not sure if this can be expressed in terms 
of a variant record.

The proposed feature increases reliability of programming. It is undoubtful 
that adding more type invariants increases reliability.

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