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From: mjl@cs.rit.edu
Newsgroups: comp.object
Subject: Re: "Ad Hoc Polymorphism" -- I'm Quoting Someone Else
Keywords: ad hoc polymorphism, reference to original use
Message-ID: <1514@cs.rit.edu>
Date: 9 Jan 90 14:20:28 GMT
References: <646@ajpo.sei.cmu.edu>
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Reply-To: mjl@prague.UUCP (Michael Lutz)
Organization: Rochester Institute of Technology, Rochester, NY
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In article <646@ajpo.sei.cmu.edu> eberard@ajpo.sei.cmu.edu (Edward Berard) writes:
>Jim Adcock has admonished me as follows:
>
>	"Can you please choose some other descriptor rather than "ad
>	hoc polymorphism" ???  This term is not descriptive, is being
>	applied to a technique that is certainly *not* "ad hoc," and
>	sounds like it is intended to be prejudicial terminology --
>	which I'm sure was not your intent!"
>
>If you recognize this type of polymorphism as having a different name,
>and can cite two, or more, earlier references, I will consider using
>a different name. (Heck! I'll even take _one_ earlier reference. ;-))

I doubt such a reference will be found, as the original use of the
phrase "ad-hoc polymorphism" is from Christopher Strachey ("Fundamental
concepts in programming languages." Lecture Notes for the International
School in Computer Programming, Copenhagen, Aug. 1967).  Note the date:
well before the first Smalltalk system, and coterminal with the
introduction of Simula.  I don't have the original handy, but I do have
a second hand source, the paper "On Understanding Types, Data
Abstraction, and Polymorphism", by Peter Wegner and Luca Cardelli, ACM
Computing Surveys, December 1985.  To quote from this paper (p. 475):

  Strachey distinguished, informally, between two major kinds of
  polymorphism.  *Parametric polymorphism* is obtained when a function
  works uniformly on a range of types; these types normally exhibit
  some common structure.  *Ad-hoc polymorphism* is obtained when a
  function works, or appears to work, on several different types (which
  may not exhibit a common structure) and may behave in unrelated ways
  for each type.

[If Strachey was not using ad-hoc polymorphism as a pejorative, he
certainly considered such polymorphism unseemly :-) mjl]

Wegner and Cardelli then define a new form of polymorphism, *inclusion
polymorphism*, to handle inheritance.  They group both parametric
and inclusion polymorphism under a superclass universal polymorphism,
because they both achieve a uniform type structure.  By way of
contrast, coercion and overloading are forms of non-uniform, ad-hoc
polymorphism. [A classic example is overloading "+" on strings to
mean catenation - mjl]

I strongly recommend this paper to those interested in a deeper
appreciation of the issues surrounding the notion of "type".

Mike Lutz
Rochester Institute of Technology
mjl@cs.rit.edu
rutgers!rochester!rit!mjl

Mike Lutz	Rochester Institute of Technology, Rochester NY
UUCP:		{rutgers,cornell}!rochester!rit!mjl
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