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From: johnson@uiucdcsp.cs.uiuc.edu
Newsgroups: comp.lang.c++
Subject: Re: oo definition request
Message-ID: <77300011@uiucdcsp>
Date: 24 May 88 16:47:00 GMT
References: <4800021@uiucdcsm>
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Nf-ID: #R:uiucdcsm:4800021:uiucdcsp:77300011:000:1188
Nf-From: uiucdcsp.cs.uiuc.edu!johnson    May 24 11:47:00 1988


A polymorphic procedure is one that takes arguments of many different
types.  There are several different kinds of polymorphism.  ML features
"parametric" polymorphism. (I hope I get all these names right.)  C++
features "inclusive" polymorphism, where an instance of a subclass can
be treated as if it were in a superclass.  A type system for Smalltalk
would have to treat "ad-hoc" polymorphism, which is where unrelated
objects can all implement the same operation, but in entirely different
ways.  The question, "Which is more important in an object-oriented
programming language, inheritance or message-passing?" is the same as
saying "Which kind of polymorphism is most important, inclusive or ad-hoc"?

Languages like Trellis/Owl show that ad-hoc polymorphism can be
translated into inclusive polymorphism with multiple inheritance.
If a procedure needs to be able to operate on objects in class A or
class B then you can just define a common superclass to describe
their commonality.  On the other hand, parametric polymorphism is
still needed even if the other two are allowed.

For further information, see the paper by Cardelli and Wegner in
Computing Surveys a few years ago.