Path: utzoo!attcan!uunet!wuarchive!mailrus!jarvis.csri.toronto.edu!qucis!dalamb
From: dalamb@qucis.queensu.CA (David Lamb)
Newsgroups: comp.sw.components
Subject: Re: What should a component library loo
Message-ID: <423@qusuntrc.queensu.CA>
Date: 23 Nov 89 15:25:59 GMT
References: <130200020@p.cs.uiuc.edu> <7096@hubcap.clemson.edu>
Reply-To: dalamb@qucis.queensu.CA (David Lamb)
Organization: Queen's University, Kingston Ontario
Lines: 17

From johnson@p.cs.uiuc.edu:
> In particular, he said "It seems to me that separation of functional
> specification from implementation is at least a Good Thing, [...]
> This is a very common claim.  I disagree with it.  This is only important
> in a library of data structures, 

Separation of functional specification from implementation is the most
widely-perceived example of separation of properties guaranteed not to change
from properties allowed to change.  The typical examples are all from data
structures, but the principle is more general.  The separation gives the
implementor and maintainer free to change things without having to rewrite
lots of the rest of the system.  Parnas has examples of device drivers,
for example, where you need to write most of the software for a system before
the engineers have pinned down the last detail of the hardware interface
to the radar.  The "abstract interface" for the radar device includes
functional specifications, but also any other assumptions the device driver
needs to make about its clients and vice-versa.