Pass arguments to callr::r() background process and access them within?

Question

R code can be run in a background process like so

callr::r(function(){ 2 * 2 })
# [1] 4

When I attempt this with args I can't figure out how to access them. I tried a few obvious things:

callr::r(function(){ 2 * 2 }, args = list(x=3))

callr::r(function(){ 2 * x }, args = list(x=3))

callr::r(function(){ 2 * args$x }, args = list(x=3))

callr::r(function(){ 
  args <- commandArgs()
  2 * args$x 
  }, 
  args = list(x=3))

# Error: callr subprocess failed: unused argument (x = base::quote(3))
# Type .Last.error.trace to see where the error occurred

I also tried to debug using browser() but in this case it didn't work in the usual way.

Question

How can arguments be passed to a background process invoked with callr::r() and those arguments be accessed within the background process?

Answer 1

You have to move the argument inside both the list of args and inside function(), like so:

callr::r(function(x){ 2 * x }, args = list(x = 3))
# [1] 6

Or like this:

x <- 3
callr::r(function(x){ 2 * x }, args = list(x))
# [1] 6

Same idea for multiple arguments:

x <- 3
y <- 4
z <- 5

callr::r(function(x, y, z) { 2 * x * y * z }, args = list(x, y, z))
# [1] 120

Source: here