Re: Eiffel and Java

["Norman H. Cohen" <ncohen@watson.ibm.com>]

Joachim Durchholz wrote:

> Though I'd expect that similar problems exist for dispatching as well as
> for overloading. In both cases, it is necessary to decide which of several
> similar functions is the right one, overloading at compile time,
> dispatching (dynamic binding) at run-time.

But this choice is straightforward in the case of dispatching:  You
dispatch to the version for the type indicated by the tags of the
controlling parameters (or the controlling result).  If there is more
than one controlling parameter and the tags do not match, it is a
run-time error, raising an exception.

The only potential bit of ambiguity is in deciding whether a "tag
indeterminate" call--a call on a function that has a controlling result
and no controlling parameters except perhaps other tag indeterminate
calls--is dispatching.  The rule is that a tag-indeterminate call is not
a dispatching call unless it has to be.  A tag-indeterminate call has to
be a dispatching call if it is a parameter in some surrounding call that
has to be dispatching (e.g. because the surrounding call has a classwide
controlling parameter).  Here is an example:

   package P is
      type T is tagged ...;
      type CWP is access all T'Class;
      procedure S (X1, X2: in T);
      function F1 (X: T) return T;
      function F2 (X: Integer) return T;
   end P;

   A : CWP;
   B : T;

(A call on F2 is always tag-indeterminate.  A call on F1 is
tag-indeterminate if and only if its parameter is tag indeterminate, as
in F1(F2(0)) or in the outermost call of F1(F1(F2(0))).)

The call 
   S( F1(F2(0)), F2(1) );
is not dispatching.  Each call on F2, and the call on F1, return a value
of type T.  The call on S has two parameters of type T, causing the
version of S declared in P to be invoked.  The call
   S( F1(F2(0)), A.all);
IS dispatching, because the second parameter to S, A.all, is classwide. 
That means that the call on F1 must dispatch to the version of F1 for
the type indicated by the tag value A'Tag.  That, in turn, means that
the call on F2 must be dispatch to the version of F2 for that type.  The
rules in the RM are hard to understand, but the bottom line is that
dispatching occurs only in calls where there is data whose tag is not
determinable at compile time (looking at a single statement in
isolation).
 
> > Well, let's just say that it was appropriate that you posted your
> > message on October 31. ;-)
> 
> Is there anything special about Oct 31? It is called "Reformation Day"
> here in Germany (has to do with Luther and Protestants), but I'm not sure
> you mean that or something else...

You had asked whether you were "seeing ghosts".  October 31 is Halloween
(a variation on "All Hollows' Eve", i.e., the evening before All Saints
Day).  In the United States (I had thought this was a European tradition
too, and I am surprised to learn otherwise) it  has become associated
with ghosts, goblins, witches, and so forth (as well as with
institutionalized extortion by children who threaten to vandalize the
property of those who do not offer candy in return for protection).
 
-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen