Re: Why no abstract non-tagged types?

[Shark8 <onewingedshark@gmail.com>]

On Thursday, February 20, 2014 8:04:08 PM UTC-7, Britt wrote:
> Recently I've been wishing I could declare some otherwise conventional enumeration types as "abstract" so they could only be used as a template for derived types.
> 
> Such abstract types couldn't be used directly for object declarations.  For example:
> 
>    type Valve_State_Base_Type is abstract (Unknown, Open, Closed);  -- in Ada 202X
> 
>    Valve_State : Valve_State_Base_Type;  -- illegal, type is abstract

You already have this ability in Ada... in fact you've had it since Ada 83:

Generic
Package Abstract_Enumeration is
  Type Enumeration is (Value_1, Value_2, Value_3, Value_4, Value_5);
End Abstract_Enumeration;

If you WITH Abstract_Enumeration you cannot directly use Abstract_Enumeration.Enumeration, you need to instantiate it first:

Package Enums is new Abstract_Enumeration;
Package Other_Enums is new Abstract_Enumeration;

Defines two instances of Enumeration, one fore Enums and another for Other_Enums, although they have the same names they are incompatible w/o type-casting.

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[More recent Ada-specs]
If you wanted something like subtypes, you could have a generic something like:

Generic
  with Package AE is new Abstract_Enumeration(<>);
  First : AE.Enumeration := AE.Enumeration'First;
  Last  : AE.Enumeration := AE.Enumeration'Last;
Package Enumeration_Subtype is
  Subtype Enumeration_Subtype is AE.Enumeration range First..Last; 
End Enumeration_Subtype;