Re: Type invariants and private extensions?

[Victor Porton <porton@narod.ru>]

Jere wrote:

> On Tuesday, September 12, 2017 at 6:59:17 PM UTC-4, Victor Porton wrote:
>> Shark8 wrote:
>> 
>> > 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.
>> 
>> 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.
>> 
> 
> I may be misunderstanding what you are looking for, but you can
> make discriminant types unconstrained to let you choose the type
> at run time:
> 
> Type Type_Enumeration is ( TInteger, TReal, TBoolean, Nothing );
>    
>     Type Example( Item_Type : Type_Enumeration := Nothing) 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;
> 
> Then all you have to do is declare an object with no
> specified discriminant:
> 
>  Some_Object : Example;  --notice no discriminant
> 
> In Ada, when you do this, you are able to assign new versions
> of the type (with different discriminants:
> 
> Some_Integer : Example := (Item_Type => TInteger, I => 100);
> Some_Float   : Example := (Item_Type => TFloat,   F => 20.2);
> 
> Some_Object := Some_Integer;
> Some_Object := Some_Float;
> 
> That's just a play example, you can handle building them however
> you want at run time.

It does not suffice for me, because at the point of actual object creation, 
the discriminant value may be yet unknown. Sometimes, I need first create 
the object and LATER (when the object was already created and I did some 
operations with it) assign it the type.

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