Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ncar!tank!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!wsmith
From: wsmith@m.cs.uiuc.edu
Newsgroups: comp.lang.c++
Subject: Reusable design vs. reusable code
Message-ID: <4800046@m.cs.uiuc.edu>
Date: 8 Dec 88 18:27:00 GMT
Lines: 51
Nf-ID: #N:m.cs.uiuc.edu:4800046:000:2534
Nf-From: m.cs.uiuc.edu!wsmith    Dec  8 12:27:00 1988


I would like to contrast two desirable properties of a language: the property
that software in the language is reusable, with the property that the 
software is easily rewriteable so that designs are reusable.

I believe both are desirable, but may conflict with each other.

To me, reusable software means that implementations may be used for many 
different purposes.  It may be difficult to make the first implementation
or the language may have a steep learning curve (as I understand Smalltalk
does to an extent) but once it is done, you have an library that may save
effort in the future.

Another desirable property to me is that the software is rewritable.  For this
to be true, the effort reused is in the design phase.  A design may be 
implemented in several different ways and each of the effort to make
new implementations of the design would be painless in a language with this 
property.  If a algorithm is easy to implement, reimplement, or modify in 
a language, there are several benefits to be gained:  

	A library of designs may be created that may then be adapted to 
		different uses, which leads to the second point:

	I think it is easier to reuse designs than implementations.  If I have 
		a simple schema (design) for symbol tables and I need to use 
		that design in a slightly different manner for a prototype, 
		it could be much easier to reimplement the modified design 
		than to try to fit an existing C++ or Smalltalk implementation 
		into my new framework.

	Prototyping will be much easier.  The work of making a prototype is to
		convert a design into a cheap and dirty implementation.  By
		making programs easily implementable, that initial expense is
		minimized.  Later, the same design may be implemented in a
		more sophisticated manner as profiling and performance monitors
		indicate that improvements are necessary.

	At the point where implementation becomes easy enough, a higher level
		implementation language could be used to guide an automatic
		generator of lower level code.

I believe it will be difficult to develop a language where both reusable
designs and reusable code are possible.    The problem is capturing all 
possible modification to a design and mapping them onto parameterizations of
a library.   At some point, a modified design will force a reimplementation
of the library.  Unless the reimplementation is made easy, the design won't
be very reusable.   The effort needed to make code reusable inhibits
this goal.

Bill Smith
wsmith@cs.uiuc.edu
uiucdcs!wsmith