Re: Ada202X : Adding functors

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

On Fri, 16 Nov 2012 07:20:58 -0500, Peter C. Chapin wrote:

> On 11/16/2012 03:59 AM, Dmitry A. Kazakov wrote:
> 
>>     type T is tagged ...
>>     procedure Operation (X : T);
>>
>> There is only one instance of Operation for T. This is easy when Operation
>> is not an object. Once you make it an object, there could possibly be more
>> than one instance of.
>>
>>     X : T'Class;
>> begin
>>     X.Operation;  -- Which one instance is called here?
> 
> Isn't this an issue even without first class functions? It sounds like 
> you are talking about dynamic dispatch... an OOP issue, not a functional 
> issue. I guess I'm still not following you.

It is about how many instances of primitive

   procedure (X : T)

are allowed to exist. It is different from dispatch which selects instances
of different functional types. E.g.

   procedure (X : T)
   procedure (X : S)

S /= T

>>> def filter(someList: List[Int], predicate: Int => Boolean) = ...
>>>
>>> This defines a function 'filter' that takes a list of integers and a
>>> function from Int to Boolean. The notation 'Int => Boolean' is a
>>> function type. And yes, you can write types like
>>>
>>> 	(Int => Boolean) => String
>>>
>>> for functions taking a (function from Int to Boolean) and returning
>>> String. It's no big deal.
>>
>> Can I inherit it?
> 
> Yes.
> 
> class MyClass extends (Int => Boolean) => String {
>    // ...
> }

And the semantics of? Considering this:

   type T is tagged ...
   procedure Operation (X : T);

   procedure Operation_1 (X : T) is new Operation (X : T) with ...;

would mean what?

>> Override it?
> 
> Not sure what you mean by this.

1. When you inherit from Operation you inherit higher-order functions
defined on Operation (not on T). May I override them?

2. Let I inherited from Operation and overrode Operation in T. What
happens?

3. The class of operations derived from:

   procedure (X : T)
   
is

   procedure (X : T)'Class

What is the semantics of? In which relation is it with

  procedure (X : T'Class)
  procedure (X : T'Class)'Class

>> Which are the classes of types
>>
>>     (Int => Boolean) => String
>>
>> operates on?
> 
> Scala treats function types as classes with a special 'apply' method. 

This is equivalent to Ada's access to subprogram. But I suggest that
"apply" cannot be overridden just like Ada's all cannot. Which flattens the
type structure. The recursion is killed because "apply" or "all" are not
really first-class.

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