Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!samsung!cs.utexas.edu!rice!uw-beaver!uw-june!ewan
From: ewan@cs.washington.edu (Ewan Tempero)
Newsgroups: comp.object
Subject: Re: Understanding the Object-Oriented Life-Cycle
Message-ID: <9828@june.cs.washington.edu>
Date: 16 Nov 89 20:37:09 GMT
References: <5026@internal.Apple.COM> <315@shrike.AUSTIN.LOCKHEED.COM>
Reply-To: ewan@june.cs.washington.edu.cs.washington.edu (Ewan Tempero)
Organization: University of Washington, Computer Science, Seattle
Lines: 23

In article <315@shrike.AUSTIN.LOCKHEED.COM> aihaug@AUSTIN.LOCKHEED.COM (Daniel A Haug) writes:
>By far (IMHO), the largest advantage to this is in code extensibility,
>or reusability.  It is so incredibly useful to define an algorithm
>that doesn't care about the types on which it operates.  For example,
>I need only implement a priority queue ADT once.  The algorithm is the
>same, but the types can be whatever.  As result, I only need to write
>one QUEUE and DEQUE procedure that don't depend on any typing.  Then,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>every object that is passed into the queue would need to support 
>some sort of comparison method that QUEUE would call to determine
>proper placement.

But QUEUE and DEQUE aren't operations on _elements in the priority
queue_! They are operations on objects of type "priority queue". So
there is nothing "typeless" about them at all, you have just been
using the wrong type. Many people seem to have this misconception
which I think may be due to Pascal including the type of the
element as part of the type of the array. There is a (OO) programming
language called Emerald  whose type system will allow you to do
what you require. See "Distribution and Abstract Types in Emerald"
by Black, Hutchinson, Jul, Levy, and Carter. Trans. Software 
Engineering, Dec 1986.
--ewan