scope - R scoping: disallow global variables in function

Question

Is there any way to throw a warning (and fail..) if a global variable is used within a R function? I think that is much saver and prevents unintended behaviours...e.g.

sUm <- 10
sum <- function(x,y){
sum = x+y
return(sUm)
}

due to the "typo" in return the function will always return 10. Instead of returning the value of sUm it should fail.

Answer 1

My other answer is more about what approach you can take inside your function. Now I'll provide some insight on what to do once your function is defined.

To ensure that your function is not using global variables when it shouldn't be, use the codetools package.

library(codetools)

sUm <- 10
f <- function(x, y) {
    sum = x + y
    return(sUm)
}

checkUsage(f)

This will print the message:

<anonymous> local variable ‘sum’ assigned but may not be used (:1)

To see if any global variables were used in your function, you can compare the output of the findGlobals() function with the variables in the global environment.

> findGlobals(f)
[1] "{"  "+"  "="  "return"  "sUm"

> intersect(findGlobals(f), ls(envir=.GlobalEnv))
[1] "sUm"

That tells you that the global variable sUm was used inside f() when it probably shouldn't have been.