Re: Ada-Singleton-Why does it work like this?

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

On Thu, 26 Mar 2009 06:39:26 -0700 (PDT), Maciej Sobczak wrote:

> On 26 Mar, 10:28, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
> 
>> This highlights the point. "Singletonness" is not a property of the type
>> used to implement it.
> 
> Depends. The type involves not only the set of possible states, but
> also the set of possible operations. Is the creation as an operation
> part of this set?

No, creation is not an operation.

Operations are defined on values. A singleton is a variable that can take
different values changed by applying corresponding operations. The property
of being singleton is one of the variable (object).

> Can you defend the idea that creation does not belong to the type?

I think I can. Creation cannot be expressed in terms of values of the type.
As an example consider integer. There is no integer operation that creates
values. In fact, integer values exist outside the program, they always
existed before and will exist after any program being written or run.
Obviously + does not create a new 4 when 2 is added to 2. Similarly, there
is no integer subprogram that creates an integer object (like a variable).
That is a prerogative of the compiler which inserts some magic object
creation stuff as necessary. This thing is called "constructor." Functions
are not constructors, they return "existing" objects. Even newly introduced
Pickwickian functions in Ada 2005 do so. That's why return-statement was
invented.

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