Re: Ada202X : Adding functors

[Adam Beneschan <adam@irvine.com>]

On Monday, November 12, 2012 2:09:15 PM UTC-8, Martin wrote:
> I'm increasingly using functors in my C++ life and they are very cool. I'd like to be able to use them in my Ada life too but the language doesn't support it - or can someone tell me I'm wrong?
> 
> I think it could be done by 'simply' allowing something like this:
> 
> package P is 
>    type T is tagged ...;
> 
>    function Create return T;
>    procedure "()" (This : T; I : Integer);  -- The functor call
>    function "()" (This : T) return Integer; -- Alternative functor that's a function
> 
> private
> 
> ...
> 
> end P;
> 
> with P;
> procedure Demo is
>    My_T : P.T := P.Create
> begin
>    My_T (42);  -- call to procedure version
>    if My_T () > 100 then ... end if;  -- call to function version
> end Demo;
> 
> 
> 
> I guess one of the trickier bits of adding this would be the possible need to step away from the current Ada convention of not requiring empty parenthesis...but new language revisions always change the 'current' good style...
> 
> Any thoughts?

package P is 
   type T is tagged ...; 
   function Create return T; 
   procedure Call (This : T; I : Integer);  -- The functor call 
   function Call (This : T) return Integer; -- Alternative functor that's   
                                            -- function
private 
... 
end P; 

with P; 
procedure Demo is 
   My_T : P.T := P.Create ;
begin 
   My_T.Call (42);  -- call to procedure version 
   if My_T.Call > 100 then ... end if;  -- call to function version 
end Demo; 


This already works in Ada, and doesn't require any major language changes that would give the ARG headaches about empty parentheses, possible syntax conflicts with user-defined references and indexing, the interpretation of constructs involving function calls that return P.T followed by parentheses, etc., etc., etc.  I'll concede that doing it this way may be considered less "cool" than being able to avoid typing that extra identifier ("Call").  Being able to say "This is a variable, but OH LOOK I can use it like a function" certainly does seem cool, in a way.  I don't see any other advantage, though.

                         -- Adam