###-*- R -*-
					# Here is a little example which shows a fundamental difference between
					# R and S.  It is a little example from Abelson and Sussman which models
					# the way in which bank accounts work.	It shows how R functions can
					# encapsulate state information.
##
					# When invoked, "open.account" defines and returns three functions
					# in a list.  Because the variable "total" exists in the environment
					# where these functions are defined they have access to its value.
					# This is even true when "open.account" has returned.  The only way
					# to access the value of "total" is through the accessor functions
					# withdraw, deposit and balance.  Separate accounts maintain their
					# own balances.
##
					# This is a very nifty way of creating "closures" and a little thought
					# will show you that there are many ways of using this in statistics.

open.account <- function(total) {

    list(
	 deposit = function(amount) {
	     if(amount <= 0)
		 stop("Deposits must be positive!\n")
	     total < <- total + amount
	     cat(amount,"deposited. Your balance is", total, "\n\n")
	 },
	 withdraw = function(amount) {
	     if(amount > total)
		 stop("You don't have that much money!\n")
	     total < <- total - amount
	     cat(amount,"withdrawn.  Your balance is", total, "\n\n")
	 },
	 balance = function() {
	     cat("Your balance is", total, "\n\n")
	 }
	 )
}

ross <- open.account(100)
robert <- open.account(200)

ross$withdraw(30)
ross$balance()
robert$balance()

ross$deposit(50)
ross$balance()
ross$withdraw(500)