[Zope-dev] RFC: Python/Zope Interfaces

[Ken Manheimer]

On Tue, 28 Nov 2000, Dieter Maurer wrote:

> Michel Pelletier writes:
>  > Dieter Maurer wrote:
>  > > Michel Pelletier writes:
>  > >  > Also, defining the interface seperately keep the two things apart,
>  > >  > impementation and interface, and doesn't allow you to sneak in a new
>  > >  > method unless you also sneak it into the interface, thus making a
>  > >  > stronger "contract" with the user.
>  > > I am a bit astonished by this statement:
>  > > ....
>  > ....
>  > I'm not sure if you are making these statement because you
>  > disagree with the current proposal or because you disagree with what I
>  > said above.
> I disagree with the "separating implementation and interface
> makes for a stronger contract".

The separation need not mean that the specification is hard to access from
the implementations, either for documentation or for runtime enforcement.  
However, it *does* run contrary to the literate programming notion of
weaving together the description of software with the code.  When you have
multiple implementations that satisfy an interface, having a copy of the
interface spec with each implementation can lead to the drift described
above.  If you *want* discrete interface definitions with integrity across
implementations, you have to avoid this multiple, potentially conflicting
copies of the specification.

The principle is that an interface should be a discrete entity in its own
right, not subject to dis-integration for the sake of different
implementations.  Tieing an interface specification to the implementations
can mean that separate *copies* of the specification drift.

I think you're asking for something internally contradictory, and not
seeing that - but, as i said in my first comment in this thread, i'm not
expert, and may be misinterpreting what you're asking.

> Separation by itself does not make the contract stronger.

If it preserves integrity, it does.

Ken
klm@digicool.com