call return from child frame

Question

How can I return a value in a function, through another function, see example here :

first_try <- function() eval(return(1),parent.frame())
second_try <- function() source(textConnection("return(2)"),parent.frame())

fun1 <- function(x){
  first_try()
  second_try()
  3
}

fun1()
# [1] 3

fun1 should stop at first_try and return 1, and if second_try had worked it would have returned 2.

Is such a thing possible ?

Answer 1

rlang::return_from() provides this functionality :

return_a <- function() rlang::return_from(parent.frame(),"a")

fun <- function(){
  return_a()
  "b"
}
fun()
#> [1] "a"

^{Created on 2020-01-03 by the reprex package (v0.3.0)}

We can also fix my first try by quoting return(1) and use rlang::eval_bare()rather than base::eval()

From the doc :

eval_bare() is a lower-level version of function base::eval(). Technically, it is a simple wrapper around the C function Rf_eval(). You generally don't need to use eval_bare() instead of eval(). Its main advantage is that it handles stack-sensitive (calls such as return(), on.exit() or parent.frame()) more consistently when you pass an enviroment of a frame on the call stack.

first_try <- function() rlang::eval_bare(quote(return(1)),parent.frame())
fun1 <- function(x){
  first_try()
  second_try()
  3
}

fun1()
#> [1] 1

^{Created on 2020-01-03 by the reprex package (v0.3.0)}