Queue structure (as Linked List) in R using R5 classes

################# QUEUE IMPLEMENTATION USING R5 ###################################
nodebase <- setRefClass("NodeBase", fields = list(value="list"))
node <- setRefClass("Node", contains = 'NodeBase',fields = list(nextnode="NodeBase"))
linkedList <- setRefClass("LinkedList", fields = list(head="Node",tail="Node", count="numeric"))

createLinkedList <- function(){
  headNode <- new("Node",value=list())
  headNode$nextnode <- headNode

  linkedList <- new("LinkedList",head=headNode,tail=headNode,count=0)
  return(linkedList)
}

isEmpty <- function(linkedList){
  return(linkedList$count == 0)
}

append <- function(linkedList,item){

  # create the new node
  n <- new("Node",value=list(item),nextnode=linkedList$head)

  linkedList$tail$nextnode <- n
  linkedList$tail <- n

  linkedList$count <- linkedList$count+1

  invisible(linkedList)
}

removeFirstItem <- function(linkedList){
  if(isEmpty(linkedList))
    stop("Cannot remove elements from an empty list")
  val <- linkedList$head$nextnode$value[[1]]

  nextNextNode <- linkedList$head$nextnode$nextnode
  linkedList$head$nextnode <- nextNextNode

  linkedList$count <- linkedList$count-1
  if(linkedList$count == 0)
    linkedList$tail <- linkedList$head
  return(val)
}

toItemsList <- function(linkedList){
  l <- vector(mode="list",length=linkedList$count)
  curr <- linkedList$head$nextnode
  if(linkedList$count > 0){
    for(i in 1:linkedList$count){
      l[[i]] <- curr$value[[1]]
      curr <- curr$nextnode
    }
  }
  return(l)
}

# override length and print method
lengthMethod <- setMethod("length", signature(x="LinkedList"), function(x){x$count})
showmethod <- setMethod("show", signature(object="LinkedList"), function(object){print(paste("LinkedList with ", object$count, " elements"))})
###############################################################################

###### USAGE EXAMPLE ######

queue = createLinkedList()

append(queue,1)
append(queue,2)
append(queue,3)

toItemsList(queue)

print(queue)

removeFirstItem(queue)
removeFirstItem(queue)
removeFirstItem(queue)

isEmpty(queue)

length(queue)


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