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From: dgil@pa.reuter.COM (Dave Gillett)
Newsgroups: comp.lang.modula2,comp.edu
Subject: Re: Implementing Abstract Lists
Keywords: modula
Message-ID: <290@saxony.pa.reuter.COM>
Date: 1 Aug 90 18:18:04 GMT
Organization: Reuter:file Inc  (A Reuter Company) Palo Alto, CA
Lines: 27

Have you read _Abstract Data Types in Modula-2_, by R. Harrison of the 
Department of Electronics and Computer Science, University of Southampton, UK?
  
I picked up this book because it discusses implementation of list, queue, 
graph, and various sorts of tree structures in Modula-2.  I'd been hoping, 
particularly with the title's reference to abstract types, to find a general 
"list" implementation that might apply to lists of any sort, and so on.
  
So I was severely disappointed to discover that the "List" module presented 
does not in fact deal with lists in the abstract, but with lists of things of 
type "ItemType" which is imported from some other module.  So in a typical 
real-world program, if I have three types of things that I want lists of, I 
need three (almost but not quite identical) copies of the List module as 
presented.
  
This is absolutely not acceptible, and I fear that it may illustrate the 
difference between academic theory and engineering practice.
  
I've pretty much resolved that my own "List" module is going to need, as a 
parameter, the offset from the start of a record (of whatever type are in this 
list) to a field which contains the link field(s) and any other information 
that the List module needs.  (One of the side effects is that I can have a 
single set of records participate in multiple lists, in different orders.)
  
So I'm looking for a simple, and preferably portable, way to (a) calculate, 
and (b) use, the offset.  Pointer arithmetic is possible, but I'm developing 
on the PC so that approach may not be simple or portable.  Any suggestions?