Re: Software Reuse -- do we really know what it is ? (long)

[jbn@glacier.STANFORD.EDU (John B. Nagle)]

     The trouble with this idea is that we have no good way to express
algorithms "abstractly".  Much effort was put into attempting to do so 
in the late 1970s, when it looked as if program verification was going to
work.  We know now that algebraic specifications (of the Parnas/SRI type)
are only marginally shorter than the programs they specify, and much
less readable.  Mechanical verification that programs match formal
specifications turned out not to be particularly useful for this reason.
(It is, however, quite possible; a few working systems have been 
constructed, including one by myself and several others described in
ACM POPL 83).

     We will have an acceptable notation for algorithms when each algorithm
in Knuth's "Art of Computer Programming" is available in machineable form
and can be used without manual modification for most applications for which
the algorithm is applicable.  As an exercise for the reader, try writing
a few of Knuth's algorithms as Ada generics and make them available to 
others, and find out if they can use them without modifying the source
text of the generics.

     In practice, there now is a modest industry in reusable software
components; see the ads in any issue of Computer Language.  Worth noting
is that most of these components are in C.  

					John Nagle