Re: Subprogram Renaming

[bobduff@world.std.com (Robert A Duff)]

In article <316AEA8D.7600@csehp3.mdc.com>,
James A. Squire <m193884@CSEHP3.MDC.COM> wrote:
>Still (!) Nobody has answered the $64,000.00 question:  WHY IS THIS SUCH
>A GOOD THING?

It's not SUCH A GOOD THING IN ALL CAPS.  It's just a minor short-hand.
No big deal.  You seem to be expecting some AMAZING NEW CAPABILITY --
but all you get is a shorthand notation.

>...  In other words, why did they waste their time adding this
>ability to rename a subprogram body.  Why should I do:
>
>package q is
>  procedure j;
>end q;
>
>package body q is
>  procedure k;
>  procedure j renames k;
>end q;
>
>when I can do:
>
>package q is
>  procedure k;
>end q;
>
>package body q is
>  procedure k is ...
>end q;

Well, what if K were in another package, and you want the implementation
of J to just call K?  In Ada 83, you have to write "procedure j(...) is
begin k(...); end j;", whereas in Ada 95 you can write "procedure j(
renames K;".  Nobody is saying this is some amazing wonderfulness.  It's
just a shorthand notation that seems nice in some situations.

>Now that I KNOW I can do in Ada83.  Since the parameter spec has to
>match exactly, I see no point to doing a rename in the body and hiding
>it from the user.  All you are changing is the name it goes by.  What's
>the point?

The point is layering one abstraction on top of another, where *some*
subprograms don't actually do anything, except pass the buck to another
lower-level subprogram.  Usually the *other* subprogram is in another
package -- unlike your example, which renames a subprogram from the
*same* package, which, as you noted, is rather silly.

>> As Robert Dewar said, this has nothing to do with syntax rules -- the
>> syntax for renamings is the same.  The difference is where they're
>> allowed, and what they mean.  To find the rules, you have to look at the
>> text under subprogram renamings (as opposed to just looking at the BNF
>> syntax rules).
>
>I read those - 8.5.4 and 6.3.1.  What does "subtype of the profile"
>mean?

I guess you're referring to 6.3.1(17).  It just mean the subtypes of the
parameters of whatever subprogram you're talking about.  If you want to
use renaming-as-body, you have to have matching parameter subtypes.
This rule is stricter than the "normal" (Ada 83) renaming rules for
renaming-as-declaration.

- Bob

P.S. I don't blame you for being confused.  These rules are written in a
rather language-lawyerly fashion.  :-(