Limiting variable scope

Question

I'm trying to write a function, which limits the scope of R variables. For example,

source("LimitScope.R")
y = 0
f = function(){
   #Raises an error as y is a global variable
   x = y
}

I thought of testing the variable environment, but wasn't really sure of how to do this.

The why

I teach R to undergrads. In their first couple of practicals a few of them always forget about variable scope, so their submitted functions don't work. For example, I always get something like:

n = 10
f = function(x){
  #Raises an error
  #as I just source f and test it for a few test cases.
  return(x*n)
}

I was after a quick function that would 'turn off' scope. As you can imagine it doesn't have to be particularly robust, as it would just be offered for the few practicals.

Answer 1

I'm not sure that you want to do this in general, but the local() function should help, as should the codetools library.

In your example, try

f = local( function() { ... }, baseenv())

It does not do exactly what you want, but it should get you closer.

Answer 2

You can force a variable to be the local version with this function:

get_local <- function(variable)
{
  get(variable, envir = parent.frame(), inherits = FALSE)  
}

Compare these cases

y <- 0    
f <- function()
{
  x <- y
}    
print(f())        # 0

y <- 0    
f <- function()
{
  y <- get_local("y")
  x <- y
}    
print(f())        # Error: 'y' not found

Depending on what you are doing, you may also want to check to see if y was an argument to f, using formalArgs or formals.

g  <- function(x, y = TRUE, z = c("foo", "bar"), ...) 0

formalArgs(g)
# [1] "x"   "y"   "z"   "..."

formals(g)
#$x
#
#
#$y
#[1] TRUE
#
#$z
#c("foo", "bar")
#
#$...

EDIT: The more general point of 'how to turn off lexical scoping without changing the contents of functions' is harder to solve. I'm fairly certain that the scoping rules are pretty ingrained into R. An alternative might be to use S-Plus, since it has different scoping rules.