Re: U : Unbounded_String := "bla bla bla"; (was: Is the Writing...)

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Tue,  7 Oct  3 03:57:32 +0400, "Alexandre E. Kopilovitch"
<aek@vib.usr.pu.ru> wrote:

>Generalizing the Strings/Unbounded_Strings issue, I would propose a new notion
>of "enveloped" private type. That is, a private type Y may be declared as an
>envelope (new keyword) of some base type X:
>
>  type Y is private envelope of X;
>
[...]

Looks similar to my proposal for defining subtypes.

Why to call "envelope" something which is a subtype? (:-))

It should be something like:

type Y is private subtype X;

Note that for the sake of genericity one need more than two
conversions. There should be four:

X_From_Y, X_To_Y - this pair is used when an Y gets substituted for X,
i.e. when Y *inherits* a subprogram from X. One of these conversions
might be absent. Then Y becomes an in-subtype or an out-subtype of X:

type Y is private in subtype X;
   -- Only in-subroutines are inherited. So only X_From_Y has to be
   -- defined

Y_From_X, Y_To_X - this pair is used when X gets substituted for Y,
i.e. when Y exports something to X. This is a way to create
supertypes.

type Y is private in out supertype X:

If all four conversions are present both types become fully
interchangeable, which is actually required in case String vs.
Unbounded_String.

So a definition of Unbounded_String should be:

   type Unbounded_String is
      private in out subtype String, in out supertype String;

   procedure Append (Source : in out Unbounded_String; 
      New_Item : in Unbounded_String);
      -- No need to define a variant with New_Item of String,
      -- because it will be automatically exported to String.

private
   type Unbounded_String is
      new Ada.Finalization.Controlled with record ...

Note also that there is no need to require X_From_Y (X_To_Y (X))=X. It
is an implementation detail. The standard subtype does not insure
that. It raises Constraint_Error when a conversion is not possible.
For example, Append exported to String:

X : String (80);

Append (X, "something"); -- Constraint_Error

Here X was converted to Unbounded_String, then Append was called, then
it failed to convert it back to String.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de