NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/object/4994

Internet Message Format | 1993-01-22 | 3.2 KB

From: [e-mail] (Chris Dollin)
Date: Fri, 22 Jan 1993 09:59:35 GMT
Subject: Re: Re: FAQ  Part 1 (of 2) [ polymorphism or latent typing? ]
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Organization: Hewlett-Packard Laboratories, Bristol, UK.
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In-Reply-To: Bob Hathaway's message of Fri, 22 Jan 1993 00:44:20 GMT
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In article ... Bob Hathaway <[e-mail]> writes:

   Or even better, try Eiffel [Meyer 88:p224] ""Polymorphism" means the ability
   to take several forms.  In object-oriented programming, this refers to the
   ability of an entity to refer at run-time to instances of various classes.
   In a typed environment such as Eiffel, this is constrained by inheritance."

and later:

   I think you're the one inflicting confusing terminology.  Everyone in recent
   history seems to define polymorphism as a property of objects (Stroustrup,
   Meyer, Booch, ...)

Your quote from Meyer (above) shows clearly that, for Meyer, polymorphism is a
property of *entities*, not *objects*. Meyer describes [p73, OOSC] an
``entity'' as the object-oriented equivalent of a variable. In Eiffel, entities
of a class hold references to objects.

Thus, in at least this one case, polymorphism is a property of *entities*, not
*objects*.

   Lets look at the real world.  There is some class of objects, B, that can
   have many forms, lets assume as given the case that they don't change once
   a particular form is taken.  Now lets define some kinds of forms that a B
   can take, and call these classes D1, D2, ...  Now take the case of where any
   of the D's can occur.  In the sciences, B is refered to as "polymorphic" or
   as a "polymorphic object".  I can declare a B object that can stand for any
   of the D forms.  Since B can take on many differing forms, it is defined as
   "polymorphic".  The property that B can assume different forms (and not the
   ability to become other forms, as in metamorphosis) is the canonical use of
   the term polymorphism.  So sorry, it doesn't look as though structural and
   behavioral replacement (or a becomes facility) is the standard usage of the
   term polymorphism.

Not in the natural sciences, perhaps. Different fields of inquiry are
characterised by different linguistic perversions, sorry, developments.
How else could it be? There are different clusters of people doing the
work, with their own linguistic history, and the subject matter itself
suggests its own metaphors.

It might be ``standard'' to use polymorphism this way outside computing,
but it's ``standard'' to use ``bit'' to mean a little piece of something,
``core'' to mean the centre of something (usually -- in my experience -- an
apple), ``pipe'' to mean a physical tube through which (generalised) fluids
flow or something to smoke tobacco in, and ``function'' to mean a bit of a do.

--

Regards,    | "You're better off  not dreaming of  the things to come;
Kers.       | Dreams  are always ending  far too soon." - Caravan.