Re: Task interface and entries with aliased parameters

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

On Wed, 21 Oct 2015 12:45:58 -0700 (PDT), Hadrien Grasland wrote:

>> He possibly meant Java interfaces as opposed to existing Ada 95 abstract
>> types. Ada 95 abstract type did everything Java interface do and more.
>> 
>> Interface as a separate type is indeed worthless. Interface as properties
>> of a type, a package, a subroutine is more than useful. It is impossible to
>> program anything without the concept of the interface.
>> 
>> If Ada were a better language you would be able to strip the implementation
>> from any type:
>> 
>> Interface = Type - Implementation
> 
> I like the idea of interfaces as separate types because it solves all the
> problems of multiple inheritance,

There is no problem with MI, whatsoever. MI problems is an urban legend.

> and provides a very nice way to specify
> a shared interface to multiple objects and have the compiler enforce that
> shared interface as the objects are refactored, rewritten, etc.

Dangerous and useless. The [public] interface is everything declared
publicly. If something publicly declared does not belong to the interface,
the language is poorly designed and broken.

> Regarding the last point, would you mean having a type attribute like
> Type'Interface, that defines an interface composed of all of the type's
> primitive operations ?

There is no need in such attribute. The type already denotes its interface. 

What you need is a construct to inherit from a type dropping the
implementation or some parts of. Presently in Ada you always inherit both
the interface and the implementation. The later can be slightly altered by
overriding operations and/or extending the representation. This is a very
limited model unsuitable for scalar and by-value types.

> But if so, is it really reasonable to define an interface from an
> implementation?

You simply could not do that. Implementation is not interface, per
definition of. If a language does not support clear syntactic separation of
interface and implementation, it is a bad language. Ada does support it,
with some exceptions, though. So, you should always be able to see what is
interface and what is an implementation of.

> Shouldn't the interface be considered as a separate
> entity that is shared by multiple types?

For each interface there exists single type it belongs to. You cannot have
a type interface without the type. Ada's declaration of an interface is
just a type declaration.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de