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

[nospam@thanks.com.au (Don Harrison)]

(Sorry for not responding to you earlier. I wasn't ignoring you - just 
short on time.)

W. Wesley Groleau wrote:

:> I disagree, the short form is not code, but documentation. As such, 
:> they can't be regarded as the same thing.
:
:In Ada, a package spec is code AND documentation.  It is, in a way, 
:a contract with all clients of the package, and a precondition for
:the implementation.  

Agree. The use of "precondition" is a bit loose, however..

:However, in Eiffel, since the short form comes
:FROM the implementation, "documentation" is a good word. 

Some of us in this thread have associated Ada package specs with Eiffel short
forms. This is not strictly true. More accurate may be:

  - The human readable view of package specs is analogous to the short 
    form (hence my use of the term documentation), and

  - The software client view of package specs is analogous to the interface
    embedded in the class text, loosely called the class implementation.
    (This embedded interface is what is extracted to become the short form.)

Why bother making this distintion? Because the ability of software clients 
to use a class doesn't depend on whether a short form has been generated
but on the existence of the embedded interface. These two aspects of clientship 
which are combined in Ada are separated in Eiffel. 

Further, considering that an Eiffel class designer establishes the client 
aspects of a class first (conceiving the (embedded) interface), and then 
proceeds to implement it (probably first in stubbed form), we see that rather 
than the interface trailing the implementation, it precedes it, as in Ada.

Consequently, I disagree with John Volan's observation..

:.. Eiffel treats the notion of
:an "interface-only view of a class which describes its contract" as
:something of an afterthought.


Due to the fact that the short form is always generated from the class text,
there is a slight advantage in this approach:

  - The system is always executable, even if only with stubbed bodies. 
    Yes, I know you can generate such stubs with Ada tools, but..

  - Humans can generate more meaningful stubs, returning sensible default 
    values - zero rather than Integer'First, for example.


:In Ada, since the specification conceptually precedes and constrains the
:implementation, the word we've chosen--specification--is more 
:precise than the broader "documentation."

Strictly speaking, you could say Eiffel short forms *document* the 
specification - they are not the specification itself. The specification
is embedded in the class text and that's what is used by the compiler.

This is why I think it's important for an Eiffel environment to be able to 
freeze that embedded specification. As any Ada programmer knows, this is 
crucial in a team environment.


Don.                     (Reverse to reply)
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Don Harrison             au.com.csa.syd@donh