How to force dispatch to an internal generic in R?

Question

I have a class 'myClass' in R that is essentially a list. It has an assignment operator which is going to do some things and then should assign the value using the regular list assignment operator

`$<-.myClass`<-function(x,i,value){
   # do some pre-processing  stuff

   # make the assignment using the default list assignment
   x[[i]]<-value
   x
 }

But I can't actually use x[[i]]<-value as it will dispatch to the already existing [[<-.myClass method.

In similar S3 dispatching cases, I've been able use UseMethod or specifically call [[<-.list, or [[<-.default but those don't seem to exist because $<- and [[<- are primitive generics, right? And I'm sure I'll be sent to a special R hell if I try to call .Primitive("$<-"). What is the correct way to dispatch the assignment to the default assignment method?

Have you tried using `NextMethod()`? That would be the obvious place to start. — hadley, Dec 17 '13 at 13:58
Except that $ does non-standard evaluation, so this is going to be hard — hadley, Dec 17 '13 at 15:56
I have tried NextMethod, but maybe I wasn't calling it correctly? Current solution is to remove the class from x, call [[x]] — skyebend, Dec 17 '13 at 16:05

score 2 · Answer 1 · answered Dec 17 '13 at 16:07

2

It doesn't look like there is a particularly elegant way to do this. The data.frame method for $<- looks like this:

`$<-.data.frame` <- function (x, name, value) {
  cl <- oldClass(x)
  class(x) <- NULL
  x[[name]] <- value
  class(x) <- cl
  x
}

(with error checking code omitted)

This should only create one copy of x, because class<- modifies in place, and so does the default method for [[<-.

answered Dec 17 '13 at 16:07

hadley

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> myFrame myFrame tracemem(myFrame) [1] "<0xb3889e8>" myFrame[['a']] 0xaf37df0]: tracemem[0xaf37df0 -> 0xaf38110]: [[ 0xaf38490]: [[ – skyebend Dec 17 '13 at 16:11
@skyebend, When you set the `class` to `NULL`, `x` will remain a NAMED list, and `[[ – mnel Dec 17 '13 at 22:28

How to force dispatch to an internal generic in R?

1 Answers1