Re: Elimination of "use" clauses

[Richard D Riehle <laoXhai@ix.netcom.com>]

In article <dale-1807991510420001@192.168.0.2>,
	dale@cs.rmit.edu.au (Dale Stanbrough) wrote:

>Export directives -could- do it (and it is not a bad idea at all),
>but we shouldn't forget that Ada already has the ability to export
>-just- the operators we want, and it's called using a private type
>and only advertising the appropriate operations.

I fully agree that the private type is underused for this purpose.
However, you will need to declare the arithmetic and logical 
operators for a private type.  This is the ideal place for using
a nested Ops package to restrict the set of operators to exactly
those you want available in your design.

package P is
   type T is private;
   -- operations on T
   package Ops is
     function ">" (L, R : T) return Boolean;
     function "+" (L, R : T) return T;
   end Ops;
end P;

Now, use P.Ops makes sense and exports only the operators the 
designer feels are appropriate but still restricts them to an
Ops package.  Of course, if they were not in the nested Ops package,
one could still invoke the "use type" clause.  In the end, I think it
it will be a matter of preference rather than a matter of which is
better.

>However this has the disadvantage that you can't use the private
>type in all the places that you might want to use an integer type.

 That is  exactly the point.  We need to explicitly define the operators
 for the private type so it is not exactly like a free integer.

Richard Riehle
richard@adaworks.com
http://www.adaworks.com