Add methods to an R6 subclass

Question

I started tinkering with R6 for a project at work and I can't understand the following behavior.

Let's say I define a superclass Person and a subclass PersonWithAge :

Person <- R6Class("Person",
                  public = list(
                    name = NA,
                    hair = NA,
                    initialize = function(name, hair) {
                      if (!missing(name)) self$name <- name
                      if (!missing(hair)) self$hair <- hair
                      self$greet()
                    },
                    set_hair = function(val) {
                      self$hair <- val
                    },
                    greet = function() {
                      cat(paste0("Hello, my name is ", self$name, ".\n"))
                    }
                  )
)
PersonWithAge <- R6Class("PersonWithAge",
                       inherit = Person,
                       public = list(
                         age = NA))

If I try to add a new method to the subclass PersonWithAge, I get the following error :

> PersonWithAge$set("public", "set_age", function(age) self$age <<- age)
Error in self[[group]][[name]] <- value : 
  invalid type/length (closure/0) in vector allocation

Now if I define a new subclass with a dummy method, I can add new methods to the subclass without any problems :

PersonWithHeight <- R6Class("PersonWithHeight",
                         inherit = Person,
                         public = list(
                           height = NA,
                           foo = function() print(1)
                           ))
PersonWithHeight$set("public", "set_height", function(height) self$height <<- height)
> caitlin <- PersonWithHeight$new("Caitlin", "auburn")
Hello, my name is Caitlin.
> caitlin$set_height(165)
> caitlin
<PersonWithHeight>
  Public:
    foo: function
    greet: function
    hair: auburn
    height: 165
    initialize: function
    name: Caitlin
    set_hair: function
    set_height: function

I tried changing the lockparameter in the R6Class class definition but to no avail. Session info is :

> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=French_France.1252  LC_CTYPE=French_France.1252    LC_MONETARY=French_France.1252 LC_NUMERIC=C                  
[5] LC_TIME=French_France.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] R6_2.0.1

loaded via a namespace (and not attached):
[1] tools_3.1.1

I also get the same behavior on another machine with this session info :

> sessionInfo()
R version 3.1.2 (2014-10-31)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C         LC_TIME=C            LC_COLLATE=C         LC_MONETARY=C        LC_MESSAGES=C       
 [7] LC_PAPER=C           LC_NAME=C            LC_ADDRESS=C         LC_TELEPHONE=C       LC_MEASUREMENT=C     LC_IDENTIFICATION=C 

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] R6_2.0.1

loaded via a namespace (and not attached):
[1] tools_3.1.2

My question is then the following :

1. Am I missing something or is it normal and expected behaviour ? In that case, why so ?
2. As long as I'm here : from what I understood, there is no real implementation of virtual classes and abstract methods in R6 ?

EDIT : All right, after looking at the source code of the package, I realised that, in this line :

self[[group]][[name]] <- value

group is one of public_methods, private_methods, public_fields, private_fields. So I guess that when a class is created without any public methods, adding a new public method to the class fails because the group does not actually exist.

Answer 1

I managed to get around the problem by pulling the source code from Winston Chang's repo and modifying the get_functions and get_nonfunctions functions in utils.R so that they return an empty list instead of NULL when no functions (resp. no non-functions) are found.