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

[Leonardo Silvestri]

On 02/16/2017 02:34 PM, Duncan Murdoch wrote:
> On 16/02/2017 11:52 AM, Leonardo Silvestri wrote:
>> 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?
>
> I don't think so, though you could conceivably check whether things will
> work when bar is loaded, and fail with a message saying "bar must be
> loaded first!" or whatever.
>
> Duncan Murdoch
>

Thanks for the answer. Yes, that would be a possibility (using 'foo' in 
'Depends' rather than 'Imports' does guarantee the loading order). One 
downside though is that two packages can't play at that game 
simultaneously...

Leo