[R] 2 setGeneric's, same name, different method signatures

[William Dunlap]

Earlier you wrote
> changing "me" to "b1" in A's bB(), allows A to use B's setGeneric()
and here mention "intra-class name clashes" (I'm not sure what you
mean by this).

In R, classes do not "own" (or contain) generic functions.  They can supply
methods for generic functions but a generic function is an object that
exists outside of any class.  C++ and Java do things differently.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Greg Minshall
> Sent: Friday, February 15, 2013 3:35 AM
> To: Martin Morgan
> Cc: r-help at r-project.org
> Subject: Re: [R] 2 setGeneric's, same name, different method signatures
> 
> Martin,
> 
> fantastic.  thank you *very* much!  that clears lots of things up for
> me.
> 
> (for the record: i think that setGeneric overwriting a previous is more
> surprising -- thus violating the principle of least surprise -- than one
> function overwriting a previous, in that we think of (or, one way to
> think of) OOP is that, after finagling to have no clashes on *class*
> names, we should not worry about other than intra-class name clashes.
> as i said, for the record.)
> 
> thanks again.
> 
> cheers, Greg
> ----
> Martin Morgan <mtmorgan at fhcrc.org> wrote:
> >
> > Hi Greg --
> >
> > this setGeneric is over-writing the first, as would
> >
> > f = function() "first"
> > f = function() "second"
> > f() # "second"
> >
> > If you'd like to dispatch on a single argument, then
> >
> > setGeneric("one", function(x, ...) standardGeneric("one"))
> > setMethod("one", "A", function(x, ...) "A-method")
> > setMetohd("one", "B", function(x, y, ...) "B-method")
> >
> > The '...' in the generic allow you to add arguments that are 'picked
> > off' by methods. The user could provide any value for y, not only an
> > object of class "B".
> >
> > If you'd like to dispatch sometimes on two arguments then
> >
> > setGeneric("two", function(x, y, ...) standardGeneric("two"))
> > setMethod("two", c("A", "ANY"), function(x, y, ...) "A,ANY-method")
> > setMethod("two", c("B", "B"), function(x, y, ...) "B,B-method")
> >
> > then two(new("A")), two(new("A"), new("A")) and two(new("A"),
> > new("B")) end up in A,ANY,two-method while two(new("B"), new("B"))
> > ends up in "B,B,two-method". Other combinations are errors. One might
> > instead not define A,ANY but instead
> >
> > setMethod("two", c("A", "missing"), function(x, y, ...) "A,missing-method")
> >
> > and then two(new("A"), new("A")) would be an error.
> >
> > Multiple dispatch is complicated, and perhaps best to avoid if possible.
> >
> > It's possible to write a generic and methods that dispatch on '...',
> > with the requirement that all classes are the same; this in the spirit
> > of comparing two B's and returning the smaller; see ?dotsMethods
> > though again this is not a trivial use case.
> >
> > Hope that helps enough.
> >
> > Martin
> 
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