R specify function environment

Question

I have a question about function environments in the R language. I know that everytime a function is called in R, a new environment E is created in which the function body is executed. The parent link of E points to the environment in which the function was created.

My question: Is it possible to specify the environment E somehow, i.e., can one provide a certain environment in which function execution should happen?

Answer 1

A function has an environment that can be changed from outside the function, but not inside the function itself. The environment is a property of the function and can be retrieved/set with environment(). A function has at most one environment, but you can make copies of that function with different environments.

Let's set up some environments with values for x.

x <- 0
a <- new.env(); a$x <- 5
b <- new.env(); b$x <- 10

and a function foo that uses x from the environment

foo <- function(a) {
    a + x
}
foo(1)
# [1] 1

Now we can write a helper function that we can use to call a function with any environment.

with_env <- function(f, e=parent.frame()) {
    stopifnot(is.function(f))
    environment(f) <- e
    f
}

This actually returns a new function with a different environment assigned (or it uses the calling environment if unspecified) and we can call that function by just passing parameters. Observe

with_env(foo, a)(1)
# [1] 6
with_env(foo, b)(1)
# [1] 11
foo(1)
# [1] 1