Re: Interface/Implementation (was Re: Design by Contract)

["W. Wesley Groleau x4923" <wwgrol@pseserv3.fw.hac.com>]

Patrick Doyle wrote:
> Bertrand Meyer  <Bertrand.Meyer@eiffel.com> wrote:
> >The goals are clear:
> 
>   I agree with most of these goals, but I have an issue with one
> of them:

I'm still looking at them (actually, waiting for enough free time)
but I also disagree for similar reasons with this one:

> >        G5. Minimize the programmer's work. .....
>              ..... Duplication is almost always
> >            bad, especially with respect to ease of maintenance,
> >            one of the central goals of software engineering.
> >            It also hampers reuse, reliability (when you
> >            duplicate code, you duplicate its bugs, and
> >            the more software you write the more likely
> >            it is that it will include bugs), and ease
> >            of change.

>  1. Too much typing
>  4. Too hard to change

This is what a C bigot would say.  An Ada bigot (indeed, everyone
on the Ada side of the average) would say, "Absolutely not an issue"
(1) & (4) are almost the same, but discouraging change (4) in a SPEC 
(interface) is a Good Thing.

>  2. Duplicate code duplicates bugs
>  3. More code means more bugs

Both of these are incorrect in this case.  A bug that is repeated
is still only one bug  IF  it is repeated in the parts of the 
spec/body that are required to conform by the language.

I could add another argument that others have already offered:

5. A corollary of Murphy's law is that duplicate information
   will inevitably become non-duplicate.

This argument is applicable in C, where header files can easily
not match the implementation, yet still compile.  It does not
apply to Ada specs and bodies, since duplication that is required
is compiler-enforced to be a perfect match.  You can't even 
deviate as much as

  spec:

      Proc ( X : Pkg.X_Type );

  body:

      package P renames Pkg;

      Proc ( X : P.X_Type ) is ...

And duplication that is not required is not allowed (such as
redeclared variables).
-- 
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    Wes Groleau, Hughes Defense Communications, Fort Wayne, IN USA
Senior Software Engineer - AFATDS                  Tool-smith Wanna-be
                    wwgrol AT pseserv3.fw.hac.com

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