Re: Type invariants and private extensions?

[Shark8 <onewingedshark@gmail.com>]

On Tuesday, September 12, 2017 at 2:09:08 PM UTC-6, Victor Porton wrote:
> Jeffrey R. Carter wrote:
> 
> > 
> > "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.

You certainly can:

    Type Type_Enumeration is ( TInteger, TReal, TBoolean, Nothing );
    
    Type Example( Item_Type : Type_Enumeration ) is tagged record
	case Item_Type is
	    when TInteger => I : Integer;
	    when TReal    => R : Float range Float'Range;
	    when TBoolean => B : Boolean;
	    when Nothing  => Null;
	end case;
    end record;
    
> Moreover, the object in consideration is a wrapper over a certain C API. It 
> surely cannot be described in variant record terms.

Sure it can; there's a reason that there's a separation between specification and body, part of which is so you can hide something like an interface to C and present something sensible to your program/clients.

> 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.

Variant records can have discriminants, too. (In fact, discriminants are half of how variant-records do their thing, the other half being the "internal case statement/construct".)

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

I fail to see how you couldn't achieve this by use of (a) variant records, and (b) constrained subtypes.

eg:
  SubType Integer_Example is Example( TInteger );
  Function X(Input : Integer) Return Integer_Example;