[R] Is there a way to figure out what nonlocal variables and functions have been used in a function?

[blue sky]

x=10
f=function() {print('in f')}
g=function() {f(); print(x)}
g()

Suppose I have the above code, I want to know what functions and
variables have been used in the function g (in this case, f and x). Is
there a function to do so?