R: test what objects a function takes from the enclosing environment [duplicate]

Question

When defining an R function, I sometimes miss that it relies on objects from the enclosing environment. Something like:

a <- 1
fn <- function(x) x + a

If this happens unintentionally, it can lead to problems which are difficult to debug.

Is there a simple way to test whether fn uses objects from the enclosing environment?

Something like:

test(fn=fn, args=list(x=1))
## --> uses 'a' from enclosing environment

Answer 1

One possibility is to use the findGlobals function from the codetools package which is designed to:

Finds global functions and variables used by a closure

This works in your example:

#install.packages('codetools')
codetools::findGlobals(fn)
[1] "+" "a"

If we define a inside the function, it goes away:

fn <- function(x) {
    a = 1
    x + a
}

codetools::findGlobals(fn)
[1] "{" "+" "="

But I haven't used it in anything more complex, so I can't say how accurate it will be with a more complex function. The docs come with the following caveat:

The result is an approximation. R semantics only allow variables that might be local to be identified (and event that assumes no use of assign and rm).