[R-pkg-devel] Is a double dispatch possible with two S3 classes?

Hello,

I have stumbled upon an S3 class dispatch issue which is easily solved
using S4 classes, but I'd like to know if there is a way to solve it
without changing the type of class used.

The problem is as follows. There is an S3 class 'foo' which is defined
in a package over which I don't have control. Then there is an S3 class
'bar' derived from 'foo' in a package that can be modified.

Here is some code for 'foo':

as.foo <- function(x) {
   oldClass(x) <- "foo"
   x
}

print.foo <- function(x, ...) print(paste(x, "foo"))

"-.foo" <- function(e1, e2) "-.foo was called"


And here is some code for 'bar':

as.bar <- function(x) {
   oldClass(x) <- c("bar", "foo")
   x
}

print.bar <- function(x, ...) print(paste(x, "bar"))


Now the '-' operator must be defined in such a way that the behaviour is
different depending on the operand classes, and in particular
'bar'-'bar' needs to be different from 'bar'-'foo'. If I define the
following function:

"-.bar" <- function(e1, e2) "-.bar was called"

then I get the following results.

as.foo(1) - as.foo(2)     # uses '-.foo'
as.bar(1) - as.bar(2)     # uses '-.bar'
as.bar(1) - as.foo(2)     # uses '-.default' and issues
                           # Incompatible methods warning
as.foo(1) - as.bar(2)     # uses '-.default' and issues
                           # Incompatible methods warning

So that seems like a dead end.

If, overcoming an instinctive shudder of disgust, I redefine '-.foo' in
the "bar" package, I have checked that correctness is at the mercy of
the order of loading of the packages, even though the "bar" package
imports the "foo" package. So that doesn't seem to work either.

Before committing to making 'bar' an S4 class, does anyone know if there
is another option that would allow 'bar' to remain an S3 class?