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From: johnson@p.cs.uiuc.edu
Newsgroups: comp.lang.smalltalk
Subject: Re: Polymorphism
Message-ID: <80500053@p.cs.uiuc.edu>
Date: 22 Apr 89 03:14:00 GMT
References: <5957@pdn.paradyne.com>
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Nf-ID: #R:pdn.paradyne.com:5957:p.cs.uiuc.edu:80500053:000:1951
Nf-From: p.cs.uiuc.edu!johnson    Apr 21 22:14:00 1989


Alan, I like what you say, but I don't like your definition of polymorphism.
"Full polymorphism means that any component or aspect of a program, program 
component or programming language must behave as a value."
Surely you don't mean that polymorphic languages require that syntax and
types are values, but they are both aspects of a program.  What about the
run-time stack?  Name bindings and scopes are not first class objects in
any common language.  There are times when it would handy if they were.

A polymorphic language is one in which procedures can be polymorphic, i.e.
can take arguments of many different types.  Since the type of a procedure
depends on the types of its arguments, this means that the procedure has
many types.  "Polymorphic" means "many shapes", which I guess is as close
to "many types" as Greek is going to get.  Object-oriented languages get
their polymorphism from the late-binding of message sending.  A procedure
only cares about the messages that its arguments understand, not which class
they are in.

What you are calling polymorphism seems to be trying to make everything
a procedure call.  Smalltalk implements if statements and array accesses
with procedure calls, and the user can redefine them or can define a new
class with the same operations (this is actually not true, since the
compiler cheats on ifTrue: and ifFalse:).  Polymorphism combined with
making everything a procedure call makes it easy to change a system
without breaking it.

Polymorphism does not assume that all values have a common set of abilities,
just all the values that the procedure will get as arguments.  Thus, a
summation routine can work with any collection of numbers, but won't work
with a collection of windows.  It is still polymorphic.

There are a number of ways to implement polymorphism, and several
varieties of polymorphism.  Object-oriented programming provides one
of the most useful and versatile.

Ralph Johnson