return_from
Jump to or from a frame
Description
While base::return() can only return from the current local
frame, these two functions will return from any frame on the
current evaluation stack, between the global and the currently
active context. They provide a way of performing arbitrary
non-local jumps out of the function currently under evaluation.
Usage
return_from(frame, value = NULL) return_to(frame, value = NULL)
Arguments
frame |
An environment, a frame object, or any object with an
|
value |
The return value. |
Details
return_from() will jump out of frame. return_to() is a bit
trickier. It will jump out of the frame located just before frame
in the evaluation stack, so that control flow ends up in frame,
at the location where the previous frame was called from.
These functions should only be used rarely. These sort of non-local gotos can be hard to reason about in casual code, though they can sometimes be useful. Also, consider to use the condition system to perform non-local jumps.
Life cycle
The support for frame object is soft-deprecated. Please pass
simple environments to return_from() and return_to().
These functions are in the questioning lifecycle because we are considering simpler alternatives.
Examples
# Passing fn() evaluation frame to g():
fn <- function() {
val <- g(current_env())
cat("g returned:", val, "\n")
"normal return"
}
g <- function(env) h(env)
# Here we return from fn() with a new return value:
h <- function(env) return_from(env, "early return")
fn()
# Here we return to fn(). The call stack unwinds until the last frame
# called by fn(), which is g() in that case.
h <- function(env) return_to(env, "early return")
fn()