Re: Ada 'hello world' for Android; success!

[Stephen Leake <stephen_leake@stephe-leake.org>]

Georg Bauhaus <rm.dash-bauhaus@futureapps.de> writes:

>> And here is the Ada version (just the package body):
>>
>> package body Hello is
>>
>>     overriding procedure OnCreate
>>       (This               : access Typ;
>>        savedInstanceState : access Standard.Android.Os.Bundle.Typ'class)
>>     is
>>        type Super_Ref is access all Android.App.Activity.Typ;
>>     begin
>>        Android.App.Activity.Oncreate (Super_Ref (This), savedInstanceState);
>>
>>        Android.App.Activity.SetContentView (Android.App.Activity.Ref (This), 16#7f030000#);
>>     end OnCreate;
>>
>> end Hello;
>>
>> By defintion, every non-scalar type in Java _is_ a pointer; Bundle and
>> 'this' are pointers. The fact that the code doesn't say 'access' just
>> hides that fact. Java also hides the existence of 'this'.
>
> Java does not really hide the existence of "this", 

All I mean is it is not present in the parameter list; that's hidden.

> ..  Hence, with Android, I'd find this to be just as good:
>
> package body Hello is
>
>    overriding procedure OnCreate
>      (This               : in out Hello.Typ;
>       savedInstanceState : Android.Os.Bundle.Typ'Class)
>    is
>    begin
>       Android.App.Activity.OnCreate (
>            Android.App.Activity.Typ(This),
>            savedInstanceState);
>
>       Android.App.Activity.SetContentView (
>           Android.App.Activity.Typ(This),
>            16#7f030000#);
>    end OnCreate;
>
> end Hello;

Yes, that is one possible design.

But it certainly does not match the Android Java API.

And in my real Ada program at work, I started with that style, but ended
up needing to store pointers to objects, so lots of 'access' crept in.

> Why is Super_Ref called Super_Ref, BTW? Isn't this name another hint at
> writing Java in Ada?

Yes. In this case, because I'm explicitly following the Android example.
But in general, the 'super' keyword in Java is a nice feature. There has
been discussion of adding that feature to Ada; I forget what the problem
was that ended up killing it. 

>> http://www.stephe-leake.org/ada/access_vs_object.html).
>
> (When the problem domain is grammars, my goal of an application is to allow
> the user to build a structure of subprograms that represent a grammar,
> so the application can then call the subprogram matching the start symbol
> (or other symbols) to parse input strings.

I'd love to see your solution to this problem. It's easy to talk about
general principles and design goals; only complete actual solutions
flesh out all the problems and details.

>> In brief, _any_ framework that uses dispatching on class-wide
>> objects needs reference semantics. In Ada, that is indicated by the
>> explicit word 'access'.
>
> +1 to Randy's remarks; tagged types being by-reference types for
> a reason, in Ada, is the first thing I thought when I saw
> "access Typ['Class]".
>
> And 'Class makes dispatching explicit enough IMHO.

Then you really did not understand the point of my access_vs_object
example. I tried as hard as I could to eliminate 'access'; I succeeded,
but at too great a cost. 

Do you agree with my conclusion in that example? If not, why not?

-- 
-- Stephe