I'd also like to know what you're going for. If there's a better tool for your situation, we'd be happy to let you know.
For example, if you want to check what's going on inside a function, examining the results of each statement, use debug and the commands listed under the help for ?browser.
my_fun <- function(x) {
y <- x + 2
z <- y^2
z
}
debug(my_fun)
my_fun(5)
# debugging in: my_fun(5)
# debug at #1: {
# y <- x + 2
# z <- y^2
# z
# }
Browse[2]> x
# [1] 5
Browse[2]> n
# debug at #2: y <- x + 2
Browse[2]> n
# debug at #3: z <- y^2
Browse[2]> y
# [1] 7
Browse[2]> n
# debug at #4: z
Browse[2]> z
# [1] 49
Browse[2]> Q
undebug(my_fun)