Re: Eiffel and Java + Ada dispatching

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <55p0i3$2u4@tjnews.is.s.u-tokyo.ac.jp> jezequel@piccolo.is.s.u-tokyo.ac.jp (Jean-Marc Jezequel) writes:

  > Just to change the subject, does anyone know why the designers of
  > Ada95 did not choose to include the true multiple-dispatch a la
  > CLOS (or point to a reference on this)?  At leat the syntax have
  > been a problem; whereas it is in Java/C++/Eiffel.

    It was felt to be too much work for too little gain.  With the
current rules a programmer can provide a multiply dispatching routine,
and it is not that uncommon.  But there is a little extra work for the
implementor of the operation, but it is linear.  (In other words if
you have a routine:

     function Foo(X: Foob; Y: Foob'CLASS) return Boolean;

    That underneath calls:

     function Foobar(X: Foob'CLASS; Y: Foob; Additional: Parameters)
        return Boolean;

   Then simulating 'true' multiple dispatch may require overriding Foo
and Foobar for a new descendent of Foob, not just redefinition of Foo.
But the amount of work to be done, assuming you know what you are
doing is the same.

   For example, I created a heterogenous list class, and thought that
it would be nice to have the operation:

   function "&"(X,Y: Object'Class) return List;

   where X and Y can be of any type descended from Object.  The body
is REAL easy:

   function "&"(X,Y: Object'Class) return List is
   begin return Append(New_List(X),Y); end "&";

   Append and New_List are functions that are dispatching and may
require overriding, but as you can see, the "&" above dispatches on
both parameters, but doesn't require writing N squared operations for
N new subclasses.  In fact, it will usually be the case that no new
operations need to be writen or overridden when a new object type is
added to the list.  In more complex cases, the programmer may have
some work to do to figure out how to keep the implementation burden of
adding a new class under control, but in general you simply can't
manage the N-squared case and don't want to.

   (This is not to say that there aren't cases where you want to
implement boundary conditions with special values. For example a list
type that can contain other lists, but that I have never seen any case
where you want the general non-linear complexity.)

--

					Robert I. Eachus

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