Re: [SPARK] Code safety and information hiding

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

On Mon, 21 Aug 2006 19:31:10 GMT, Jeffrey R. Carter wrote:

> Maciej Sobczak wrote:
>> 
>> It's interesting to see this discussion, especially after the posts 
>> revealing that it's not really clear what "global" means.
> 
> We were discussing "true" global variables: variables in package 
> specifications.

Packages have scope. Is a variable in the specification of a nested package
"truly" global? Clearly, both "global" and "local" are relative to the
given scope. The third possible status is "orthogonal" (exists in a sibling
scope).

>> Let's say I have a program composed of some subroutines or packages. 
>> Each subroutine has some local variables and each package has some state 
>>  and together they all cooperate according to what the whole is supposed 
>> to do.
>> Then I decide to break the system into separate processes, and for the 
>> sake of discussion what was a subroutine (or a package) originally 
>> becomes a separate process, all communicating using some form of IPC and 
>> still cooperating to achieve the supposed goals.
> 
> Now each main subprogram that was a subprogram (Ada doesn't have 
> subroutines) in the original has its own local variables.

Which are global relatively to the process, and local relatively to the
distributed system as a whole.

> Each main subprogram that was a package with state now has its own local 
> variables that contain that state.
> 
>> The point is that while chopping the system into pieces, what was local 
>> (or state) variable originally became global in the resulting smaller 
>> program.

Which is true without any chopping. Descending a hierarchy of scopes makes
some things global without changing anything.

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