Rationale for Ada 2012

John Barnes
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4.4 Discriminants

There is one minor change in this area which was mentioned in the Introduction.
In Ada 2005, a discriminant can only have a default if it is not tagged. But in Ada 2012, a default is also permitted in the case of a limited tagged type.
Ada typically uses defaults as a convenience so that in many cases standard information can be omitted. Thus it is convenient that the procedure New_Line has a default of 1 since it would be boring to have to write New_Line(1);all the time.
In the case of discriminants however, a default as in
type Polynomial(N: Index := 0) is
      A: Integer_Vector(0 .. N);
   end record;
also indicates that the type is mutable. This means that the value of the discriminant of an object of the type can be changed by a whole record assignment. However, tagged types in Ada 2005 never have defaults because we do not want tagged types to be mutable. On the other hand if a tagged type is limited then it is immutable anyway. And so it was concluded that there is no harm in permitting a limited tagged type to have a default discriminant.
This may seem rather academic but the problem arose in designing containers for queues. It was felt desirable that the protected type Queue should have a discriminant giving its ceiling priority and that this should have a default for convenience. As illustrated in the Introduction this resulted in a structure as follows
   with package Queue_Interfaces is new ...
   Default_Ceiling: Any_Priority := Priority'Last;
package AC.Unbounded_Synchronized_Queues is
   protected type Queue(Ceiling: Any_Priority := Default_Ceiling)
          with Priority => Ceiling
       is new Queue_Interfaces.Queue with ...
Now the problem is that a protected type such as Queue which is derived from an interface is considered to be tagged because interfaces are tagged. On the other hand a protected type is always limited and its discriminant provides a convenient way of providing the ceiling priority. So there was a genuine need for a change to the rule.
Note incidentally that the default is itself provided with the default value of Priority'Last since it is a generic parameter with its own default.

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