[R] assigning from multiple return values

[Richard A. O'Keefe]

Gabor Grothendieck <ggrothendieck at myway.com> wrote:
	fn <- function(x,y) {
		assign(as.character(substitute(x)), "zz", sys.frame(-1))
		assign(as.character(substitute(y)), 3, sys.frame(-1))
	}
	fn(a,b)  # sets a to "zz" and b to 3
	
	"list2<-" <- function(x,y,value) {
		assign(as.character(substitute(y)), value[[2]], sys.frame(-1))
		value[[1]]
	}
	fn <- function()list("zz",3)
	a <- 1 # first arg must exist prior to invoking list2. Its value not important.
	list2(a,b) <- fn()
	
There is still another way, which doesn't use substitute, as.character, or
assign.  Instead of returning two results, accept an extra argument which
is a function that decides what to do with them.

    > f <- function(handler) { handler("zz", 3) }
    > f(function(x, y) { a <<- x; b <<- y })
    > a
    [1] "zz"
    > b
    [1] 3
    > g <- function() {
    +     u <- v <- NULL # make it clear these are local to g
    +     f(function(x,y) {u <<- x; v <<- y})
    +     list(u, v)  # see what we got
    + }
    > g()
    [[1]]
    [1] "zz"

    [[2]]
    [1] 3

Whenever I would have wanted to pass "a variable" to an R function,
this is what I do instead.  It works so well that I am still happily
ignorant of how to use substitute().  It's also much more compiler-friendly.