How to create an S4 array which has slots containing arrays - r

Question

How do I create an S4 class which has arrays as slots? Below, I have an example class. I'd like to be able to construct in such a way that I get two "person" elements, each of which has appropriate array members.
The code below gives me the following error: "Error in validObject(.Object) :
invalid class “person” object: invalid object for slot "children" in class "person": got class "character", should be or extend class "array"
setClass("person", representation(name="character", age="numeric", children = "array"))
setMethod(
f = "[",
signature="person",
definition=function(x,i,j,...,drop=TRUE){
initialize(x, name=x#name[i], age = x#age[i], children = x#children[i])
}
)
setMethod(f = "length", signature = "person", definition = function(x){
length(x#name)
})
setMethod(f = "dim", signature = "person", definition = function(x){
length(x#name)
})
kids1 = as.array(c("Bob", "Joe", "Mary"))
person = new("person", name="John", age=40, children = kids1)
person#children[2]
kids2 = as.array(c("Steve", "Beth", "Kim"))
people = new("person", name=c("John", "Fred"), age=c(40, 20), children = as.array(c(kids1, kids2), dim = 2))
people[1]#age
people[2]#children[1]

Answer 1

You probably don't want #children to be an array. With a dim attribute of length 1, it's essentially the same as a vector, and you lose the ability to distinguish different people's children. Consider making this slot a list instead.
setClass("person",
representation(name="character", age="numeric", children = "list"))
person = new("person", name="John", age=40, children = list(kids1))
person#children
people = new("person", name=c("John", "Fred"), age=c(40, 20),
children = list(kids1, kids2))
people[1]

Answer 2

Add drop=FALSE to your subset of the children slot; this is a consequence of standard R rules for array subsetting
setMethod(
f = "[",
signature="person",
definition=function(x,i,j,...,drop=TRUE){
initialize(x, name=x#name[i], age = x#age[i],
children = x#children[i,drop=FALSE])
}
)
Also I'm not sure that your as.array() is doing what you think? The dim argument is being ignored.

Answer 3

Hong's answer works. I did need to add a list wrapper in the "[" subset function. Once that's done, everything works fine.
kids1 = as.array(c("Bob", "Joe"))
kids2 = as.array(c("Steve", "Beth", "Kim"))
setClass("person", representation(name="character", age="numeric", children = "list"))
setMethod(
f = "[",
signature="person",
definition=function(x,i,j,...,drop=TRUE){
initialize(x, name=x#name[i], age = x#age[i], children = list(x#children[i]))
}
)
people = new("person", name=c("John", "Fred"), age=c(40, 20), children = list(kids1, kids2))
people[1]#name
people[1]#age
people[1]#children
people[2]#name
people[2]#age
people[2]#children