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From: bertrand@eiffel.UUCP (Bertrand Meyer)
Newsgroups: comp.software-eng,comp.lang.eiffel
Subject: Re: OOP and software reuse
Message-ID: <365@eiffel.UUCP>
Date: 12 Jul 90 19:49:33 GMT
References: <39400113@m.cs.uiuc.edu> <112789@linus.mitre.org> <558@argus.mrcu>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 41

From <558@argus.mrcu> by  paj@mrcu (Paul Johnson):
> 
> Meyer writes of extracting and brushing up
> classes after an application is finished.

	What I wrote was, I hope, somewhat more nuance'.

	The observation to which Mr. Johnson refers is descriptive rather
than prescriptive: many good reusable components
have evolved from more specialized ones through a process of
generalization.

	If you can write components that are reusable right
from the start, then great. But an ``existence proof'' is required:
you cannot be sure that a component is reusable until you have shown it to
be at least usable. If the component's initial avatar was as an element
of a specific but reasonably successful system, it has passed that first
test - certainly not a sufficient condition, but a necessary one.

	The comment also has a more practical side. To develop good
reusable components, it is certainly better to have as
your official charter the development of reusable software components.
Most people, however, work in a context where
managerial concerns are more short-term; the decision makers want to see
working programs more than they want to see high-quality reusable
components. This is what (in published discussions of these topics)
I have called the traditional ``project culture'', as opposed
to the ``component culture'' fostered by object-oriented techniques.
Since most people work in an environment where the project culture is the
dominant one, the more modest approach to reusability - building up
reusable components from specific ones, as opposed to more
ambitious efforts having reusability as their avowed goal -
may be the only practical one. I have seen it work quite well.

	I agree with Mr. Johnson that its success requires the designer
to have a mental model of future component usage which goes beyond the
original application. The only way to validate this model is to apply
the components successfully to two or more widely different applications.
-- 
-- Bertrand Meyer
bertrand@eiffel.com