Scoping.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.
> 
> #  Copyright (C) 1997-8 The R Core Team
> 
> 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+ 	     cat(amount,"withdrawn.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)
30 withdrawn.  Your balance is 70 


> ross$balance()
Your balance is 70 


> robert$balance()
Your balance is 200 


> ross$deposit(50)
50 deposited. Your balance is 120 


> ross$balance()
Your balance is 120 


> try(ross$withdraw(500)) # no way..
Error in ross$withdraw(500) : You don't have that much money!