Re: Simple example on interfaces

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

On 2021-01-26 10:37, J-P. Rosen wrote:
> Le 25/01/2021 à 23:06, Dmitry A. Kazakov a écrit :
>> Ada interface is a type that has interface and no implementation. [It 
>> is a silly idea inherited from Java.]
>>
> To make it look a little less silly, think of it as a promise: a type 
> that implements an interface promises to provide a certain number of 
> operations.

I agree. I meant that Ada 95 had that already:

    type Interface is abstract tagged null record;

There was no need to introduce it as a separate concept. I think the 
real reason was laziness. Vendors did not want to implement full 
multiple inheritance. Adding a simple constraint on the base types 
looked bad and also breached privacy:

    type Is_It_Interface is abstract tagged private;
private
    type Is_It_Interface is abstract tagged null record;

> Then you can define algorithms that work on any type that fulfills the 
> promises.
> 
> To me, the big benefit of interfaces is that it is NOT inheritance; you 
> say that your type provides some operations, without needing to classify 
> it with an is-a relationship.

But you do. When you say that T provides F that in other words means T 
*is-a* member of a class that provides F. Interface is merely a 
formalization of that.

> (I can hear screamings of pure-OO people who will not agree with me ;-) )

OO muddied a lot of water. To me things are quite pragmatic. How do I 
spell in the language the fact that Long_Integer is an integer? If 
Integer is an integer and Long_Integer is an integer can I write a 
program that works on integers? Can it be the *same* program for each 
instance of? Simple, natural questions.

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