R Reference Class multiple inheritance: how to cal

I have a reference class Child which inherits from parents SuperA and SuperB. When in the initialize method of Child, I would like to invoke the initialize methods of both SuperA and SuperB in turn.

Thus, for instance, I have:

SuperA <- setRefClass("SuperA",
    fields = list(a = "ANY"),
    methods = list(
        initialize = function(a) {
            print(a)
            initFields(a = a)
        }
    )
)

SuperB <- setRefClass("SuperB",
    fields = list(b = "ANY"),
    methods = list(
        initialize = function(b) {
            print(b)
            initFields(b = b)
        }
    )
)

Child <- setRefClass("Child",
    contains = c("SuperA", "SuperB"),
    methods = list(
        initialize = function(a, b) {
            # attempt to invoke parent constructors one by one:
            SuperA$callSuper(a)
            SuperB$callSuper(b)
        }
    )
)

Child(1, 2)   # want values 1 and 2 to be printed during construction of superclasses

However all I get is:

Error in print(a) : argument "a" is missing, with no default

So does anyone have any ideas on how to invoke a method belonging to a particular parent?

I'm not a heavy reference class user, but in S4 (on which reference classes are based) the best approach is often to avoid defining initialize methods (which have a complicated contract, both 'initialize' and 'copy' of named and unnamed arguments), to treat the constructor returned by setClass() as for internal use only, and to provide a user-facing constructor. Thus

.SuperA <- setRefClass("SuperA", fields = list(a = "ANY"))

.SuperB <- setRefClass("SuperB", fields = list(b = "ANY"))

.Child <- setRefClass("Child", contains = c("SuperA", "SuperB"))

Child <- function(a, b)
    ## any coercion, then...
    .Child(a=a, b=b)

The result is

> Child(a=1, b=2)
Reference class object of class "Child"
Field "b":
[1] 2
Field "a":
[1] 1

From ?ReferenceClasses, the method $export(Class) coerces the object to an instance of 'Class', so

.SuperA <- setRefClass("SuperA", fields = list(a = "ANY"),
    methods=list(foo=function() .self$a))

.SuperB <- setRefClass("SuperB", fields = list(b = "ANY"),
    methods=list(foo=function() .self$b))

.Child <- setRefClass("Child", contains = c("SuperA", "SuperB"),
    methods=list(foo=function() {
        c(export("SuperA")$foo(), export("SuperB")$foo())
    }))

Child <- function(a, b)
    .Child(a=a, b=b)

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