Re: An observation of Ada (may offend)

[stt@spock.camb.inmet.com (Tucker Taft)]

Peter Hermann (ucaa2385@alpha1.csv.ica.uni-stuttgart.de) wrote:

[Various proposals relating to limiting the use of child units.]

: I would not recommend that whole direction because
: one of the strengths of the child feature is exactly the fact,
: that the parent need not be touched further on.
: For Ada200X, the abs private part may be more convenient (if any)
: but, imho, this overburdening of the language is superfluous.

Here, here.

Child units are a namespace structuring mechanism.  A fundamental 
characteristic of this structuring mechanism is that the spec of the 
parent does not depend on the children.  Furthermore, if a child unit 
is not mentioned in some "with" clause, it is as though it doesn't 
exist at all, and will not be included in the program.  Hence, adding 
an "unwanted" child has no effect on any program that does not 
explicitly mention that child in some with clause.

Adding a child is simply extending the interface of the abstraction
represented by the tree of library units.  Without child units,
this is done by entering a new version of a package into a library
after adding more subprograms to the package.  With child units, you 
can accomplish the same thing by entering the new child into 
the library.  In both cases, if you want to control access
or update to the library, then you need to use the O/S or the Ada compiler
to do it.  

Access control is not a language issue, it is a tool or O/S issue.

Note that using child units is actually *safer* than the "old" approach
where one simply edited the package that represented the entire abstraction,
since as mentioned above, the only clients that can possibly be affected
are those that explicitly add a "with" clause for the new child.
With the "old" approach where someone goes in and adds or changes stuff
in the original package, it is much less obvious which clients might
be affected.

So a pragma that lists the names of child units is the moral equivalent
to a comment at the top of a package that lists the names of all of 
the subprograms in the package.  It proves nothing, and simply creates 
busy work during maintenance, since it must be updated every time
you want to add a new child (and don't tell me somehow that your tools can
prevent me from entering a new version for the parent package, but
they can't prevent me from adding a new child).  Some kinds of redundancy
are useful, but these kinds of redundant lists are a waste of energy.

If you want a list of the children, get a list from the program library
manager (a.k.a "list directory" on GNAT).  Similarly, if you want
a list of the subprograms in a package, use "grep."  If you want to
lock out unauthorized additions or changes to *your* program library, 
use "chmod" (or whatever).

This whole thing reminds me of attempts to add silly amendments
to the US constitution ;-).

: --
: Peter Hermann  Tel:+49-711-685-3611 Fax:3758 ph@csv.ica.uni-stuttgart.de
: Pfaffenwaldring 27, 70569 Stuttgart Uni Computeranwendungen
: Team Ada: "C'mon people let the world begin" (Paul McCartney)

-Tucker Taft   stt@inmet.com
Intermetrics, Inc.