Re: Controlled types as interfaces

["Randy Brukardt" <randy@rrsoftware.com>]

"Brad Moore" <brad.moore@shaw.ca> wrote in message 
news:1O1Jv.11735$8G3.5145@fx12.iad...
> On 14-08-20 06:50 AM, Victor Porton wrote:
...
> The incompatibilities are described in the discussion section of the AI, 
> but these had to do with  problems of changing Root_Stream_Type into an 
> interface.
>
> However, I believe you are correct that if we modified Root_Stream_Type to 
> remain an abstract type that inherits from a Root_Stream_Interface, then 
> these incompatibilities would not exist.

No, sorry, they're the same. The problem is that Ada doesn't allow hidden 
inheritance (in any way) of an interface.

If Root_Stream_Type is an interface, or inherits from an interface, then it 
cannot be used in the full type definition of a private type. That's the 
backwards incompatibility.

> Backward incompatibilities are to be avoided if at all possible, so I
> think if we were to go forward with this AI, your suggestion to make 
> Root_Stream_Type inherit from a Root_Stream_Interface, might be the way to 
> go.

We've already discussed that as-nausem. My personal opinion (in the case of 
Root_Stream_Type) is that there isn't any good reason to use this in a 
hidden way, so we could just make it an interface and swallow the 
incompatibility. (When I suggested that at a meeting, though, it got no 
traction.)

But that's not true for Controlled. The vast majority of the types I define 
are tagged private where the full type is derived from Controlled (or 
Limited_Controlled).

The most recent versions of AI12-0023-1 were trying to find ways to lift the 
ban against hidden interfaces for a restricted set of interfaces. It's the 
only way to do this without incompatibilities for existing uses. My 
recollection is that it wasn't going well (but we haven't worked on that 
since the Stockholm meeting more than a year ago).

                                            Randy.