Re: Real OO

[thomas@class.com.au (Thomas Beale)]

In article <Dov7D0.AtM@assip.csasyd.oz>, donh@syd.csa.com.au (Don Harrison) writes:
>Norman H. Cohen wrote:
>
>:When Don Harrison asked how to achieve selective export in Ada, I
>:proposed a technique, but I never asked why one would want selective
>:export.
>:
>:Selective export does indeed help document the modular structure of a
>:program, and make the program easier for a reader to understand, but it
>:seems to me that the documentation is in the wrong place.  In general,
>:the writer of a program component should be as unaware as possible of the
>:context in which that component will be used, especially if loose
>:coupling and reusability are valued.
>

This area may seem initially to be one where Eiffel takes the wrong
tack. But consider two things:

a) we shouldn't be obsessed with single-class encapsulation at the
   expense of allowing coarse-grained objects consisting of
   collaborating fine-grained instances. Such coarse-grained entities
   represent closely co-operating and "co-designed" abstractions; for
   example most design patterns and small software components would
   fall into this category. I would suggest that the really important
   boundary of re-use is not around the single class, but around small
   groups of classes designed to realise a coarser abstraction.

b) the Eiffel export mechanism can be used to specify an abstract 
   parent class e.g. 

         feature {LIST}

   This effectively says that any class conforming to the design
   intention of LIST may have access to the following features. Thus
   selective export does not restrict the client to be a single
   effective class, but to classes conforming to a design idea.

- Thomas Beale

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