# The usual Scheme way : build a function that takes commands as parameters (it's like message passing oriented programming)
queue <- function() {
    v <- list()
    f <- function(cmd, val=NULL) {
        if(cmd == "push") {
            v <<- c(v, val)
            invisible()
        } else if(cmd == "pop") {
            if(length(v) == 0) {
                stop("empty queue")
            } else {
                x <- v[[1]]
                v[[1]] <<- NULL
                x
            }
        } else if(cmd == "length") {
            length(v)
        } else if(cmd == "empty") {
            length(v) == 0
        } else {
            stop("unknown command")
        }
    }
    f
}

# Create two queues
a <- queue()
b <- queue()
a("push", 1)
a("push", 2)
b("push", 3)
a("push", 4)
b("push", 5)

a("pop")
# [1] 1
b("pop")
# [1] 3