在调用环境中评估 function

Question

I have the function below.

test_fun <- function() {
  a <- 1
  b <- 2
}

Is there a way I run this function and a and b will be assigned in the parent environment (in this case globalenv)? I don't want to modify the function (no assign with envir or <<- ), but call it in a way that what I want will be achieved.

Answer 1

A better pattern to use, from the point of encapsulation, might be to have your custom function return a 2D vector containing the a and b values:

test_fun <- function() {
    a <- 1
    b <- 2
    return(c(a, b))
}

result <- test_fun()
a <- result[1]
b <- result[2]

The preferred method for passing information/work done in a function is via the return value, rather than trying to manage the issue of different scopes.

Answer 2

Normally R functions return their outputs. Functions such as test_fun are discouraged but if you want to do it anyways use trace as shown below. This will cause the code given in the exit argument to run at function exit. No packages are used.

trace("test_fun", exit = quote(list2env(mget(ls()), globalenv())), print = FALSE)
test_fun()
a;b
## [1] 1
## [1] 2

Alternatives

Some alternatives for the exit= argument are the following.

(a) below is similar to above except that rather than leaving the objects loose in the global environment it first creates an environment env in the global environment and puts them there.

In (b) below, the objects are copied to environment(test_fun) which is the environment in which test_fun is defined . (If test_fun is defined in the global environment then it gives the same result as the code at the top of the answer.)

In (c) below, the parent frame of test_fun is the environment of the caller of test_fun and can vary from one call to another if called from different places. (If called from the global environment then it gives the same result as the code at the top of the answer.)

It would also be possible to add the current frame within test_fun to the search path using attach but there are a number of gotchas associated with that so it is not shown.

# (a) copy objects to environment env located in global environment
assign("env", new.env(), globalenv())
trace("test_fun", exit = quote(list2env(mget(ls()), env)), print = FALSE)

# (b) copy objects to environment in which test_fun is defined
trace("test_fun", 
  exit = quote(list2env(mget(ls()), environment(test_fun)), print = FALSE)

# (c) copy object to parent.frame of test_fun; 5 needed as trace adds layers
trace("test_fun", exit = quote(list2env(mget(ls()), parent.frame(5))),
  print = FALSE)