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Date: Wed, 05 Sep 2007 23:38:34 +0200
From: Georg Bauhaus <bauhaus.rm.tsoh@maps.futureapps.de>
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Subject: Re: Self pointer in limited record
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Date: 2007-09-05T23:33:31+02:00
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Dmitry A. Kazakov wrote:

[snip all kinds of buzz words :-]

>>> The point was "to have one instance" and "to be a FSM" are unrelated.
>> There is a point of view from which the concept "to have one instance"
>> and "to be a FSM" are unrelated. When writing an Ada program, you may
>> have a point of view from which the concept "to have one instance" and
>> "to be a FSM" coincide on a suitable conceptual level. It is the level
>> on which an abstract state machine package is explained.
> 
> Can I take a protected object or a task to implement either?

You can take a package and implement either.


> (I have no respect to singletons. They might have some sacral meaning for
> some OO proponents, but that was always beyond my understanding.)

They can avoid surprises and nasty issues because they enforce body
visibility of data objects. Example:

<start of declarative part>

        package S is
                type T is limited private;
                procedure op(x: in out T);
        private
                type T is limited record
                        null;
                end record;
        end S;

        package body S is separate;

        -- ... a different programmer has a clever idea
	-- ... and adds another ad hoc procedure for
	-- ... debugging porposes

        procedure bar(x: S.T) is
        begin
                raise Program_Error;
        end bar;

        use S;

        item: T;
begin
        bar(item);
end;

This is of course an artificial example. However, it serves to
demonstrate a few effects. Once the object type T is visible
outside the package (being declared in the public part of S), objects
can be declared outside of S. Adding a subprogram like Bar will in a
sense break grouping (where to look for bar which takes a T).
If the author of S had intended an upper bound on instances of T,
say one instance, then enforcing this limit will require more work.
These issues go away with a simple abstract state machine style package.