S4 class: update slot in another method without returning the object (R)

Question

For an R package I am working on (which creates an S4 class), I would like to delay loading some data until the user actually asks for it (because it may or not be required and loading it takes a little while). This would require me to set a slot's value within its getter (aka accessor) method if it has not previously been loaded. But I cannot get the new value to "stick". Here is an MRE:

setClass("A", representation(value = "numeric"))

setGeneric("value<-", function(x, value) standardGeneric("value<-"))
setGeneric("value", function(x) standardGeneric("value"))

setMethod("value<-", signature("A", "numeric"),
          function(x, value) 
          {
            x@value = value
            x
          })    

setMethod("value", signature(x = "A"),
          function(x) 
          {
            if(!length(x@value))
              value(x) <- 20
            x@value
          })

This produces the following result:

> a <- new("A")
> value(a)
[1] 20
> a
An object of class "A"
Slot "value":
numeric(0)

So the value() function returns the desired new value (20), but this value is not actually updated in the object. Doing x@value <- value instead of value(x) <- 20 in the getter did not succeed either.

It seems the problem is that I am not returning the updated object in my getter (the way my setter does), but I have something else to return in my getter (the value).

What is the Right Way To Do This™?

Thank you!

EDIT:

After futher study of S4 pass-by-value semantics, I have come to the conclusion that this is Simply Not Possible™. If a slot is updated, the object must be returned, and you can't return something else instead. Can someone confirm my conclusion is correct?

Look at the documentation for setRefClass or the R6 package. S4 does not have traditional OOP mutability. — Alexis
Thanks @Alexis I have to use S4 for my current project, but those docs brought to mind a possible work around (see my answer below). — Eric

Eric Eric · Accepted Answer · 2018-05-23T19:49:41

The comment by @Alexis mentioned R6 classes. While I have been asked to S4 classes for my current project (targeting BioConductor), the following sentence from the R6 ReferenceClasses docs caught my eye: 'Reference classes are implemented as S4 classes with a data part of type "environment".'

So if I really want mutable elements while using S4 classes, I could emulate R6 as follows:

setClass("A", representation(inMutable = "ANY", 
                             .env = "environment"))

A = function(inMutable = NULL, mutable = NULL) {
  x <- new("A", 
           inMutable = inMutable,
           .env = new.env())
  [email protected]$mutable  = mutable
  x@inMutable <- inMutable
  x
}

setGeneric("inMutable<-", function(x, value) standardGeneric("inMutable<-"))
setGeneric("inMutable", function(x) standardGeneric("inMutable"))
setGeneric("mutable<-", function(x, value) standardGeneric("mutable<-"))
setGeneric("mutable", function(x) standardGeneric("mutable"))

#setter
setMethod("inMutable<-", signature("A", "numeric"),
          function(x, value) 
          {
            x@inMutable <- value
            x
          })    

#getter
setMethod("inMutable", signature("A"),
          function(x) 
          {
            if (!length(x@inMutable))
              message("Hmmm, you haven't initialized a value for 'inMutable'.",
                      " I'm afraid I can't do anything about that now.")
            x@inMutable
          })    

#setter
setMethod("mutable<-", signature("A", "numeric"),
          function(x, value) 
          {
            [email protected]$mutable <- value
            x
          })    

#getter (mutable!)
setMethod("mutable", signature("A"),
          function(x) 
          {
            if (!length([email protected]$mutable)) {
              message("Ah. You haven't initialized a value for 'mutable'. ",
                      "Lucky for you I can initialize it for you now.")
              [email protected]$mutable = 12
            }
            [email protected]$mutable
          })

Then I can do this:

> a <- A()
> mutable(a)
Ah. You haven't initialized a value for 'mutable'. 
Lucky for you I can initialize it for you now.
[1] 12
> mutable(a)
[1] 12
>
> inMutable(a)
Hmmm, you haven't initialized a value for 'inMutable'. 
I'm afraid I can't do anything about that now.
NULL
> inMutable(a) <- 18
> inMutable(a)
[1] 18
>

Obviously, reference classes and R6 provide a much richer and more robust solution but in a pinch this seems like a viable S4 option. Of course, I haven't thoroughly tested this in the wild to see where it might break down.

S4 class: update slot in another method without returning the object (R)

1 Answers