R's UseMethod() does not dispatch on changed class() (PR#167)

[Bill Venables]

Blundering late into this discussion I'm going to take a chance
here and venture an opinion on what does happen in S3 UseMethod,
regardless of what interpretations are made of A.6 in the white
book.  This is by the "suck it and see" method, the only one I
trust.  (The chance I take is in supposing this to be useful...)

1. At the time of the UseMethod call the principal argument is
   examined, and evaluated if necessary, and the class it has at
   that stage is the one used for method dispatch.

2. All non-missing arguments are passed on to the method with the
   values they were supplied with at the time of the original
   call, not the changed values they may have at the time of
   dispatch, (including the principal argument).

3. Missing arguments that have been given values at the time of
   method dispatch, as well as local variables, retain their
   current values when control passes to the method.

4. Supplying a second argument to UseMethod() allows some other
   argument than the first to act as the principal argument.

5. Default values for arguments in the method supercede defaults
   supplied in the generic.

The method directly occupies the frame of the generic, but
arguments actually supplied to the generic re-assume their
original supplied values (even though changes to the principal
argument prior to method dispatch may determine mathod dispatch.)

So, for example, if the principal argument is not missing, has no
class at the call and is given class "xyz" by the generic, the
"xyz" method is dispatched, but the object corresponding to the
principal argument inside the method is the original classless
object.  (I would have thought that this was the major reason to
have a function like data.class in the first place.  What other
serious use does it have?)  To verify *from within the method*
that the principal argument had the class implied by the method,
you have to inspect the .Class guaranteed variable, not the
current class of the principal argument itself, which may be
wildly at variance.

"Missing" arguments on the original call remain missing unless
the generic has given them values prior to method dispatch, in
which case they behave like local variables and retain their
local supplied values.  If defaults are needed, the most recent
one is used.

Now, how does R differ?

Bill Venables.
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