I need help figuring out what is going on here.
g <- function(x, y=NULL, z=NULL) invisible(NULL)
h <- function(...) {
args <- as.list(match.call(g))[-1]
print(args)
## lapply(args, eval)
}
f <- function(...) {
h(...)
}
Function h() prints a named list of unevaluated arguments and then tries
to evaluate the arguments.
See what happens when f is called with z=3 and z=-3.
> f(1, z=3)
$x
[1] 1
$z
[1] 3
> f(1, z=-3)
$x
[1] 1
$z
..2
What is this ..2? Notice that h(1, z=-3) produces a different output:
> h(1, z=-3)
$x
[1] 1
$z
-3
The second question is easy. You have a function that creates a side-effect of printing from the inside of a function as the last called function. The print function returns its argument as a value so when the function exits the read-eval-print behavior of the console displays it again.
The first question delves into the inner workings a bit more. The items in the ellipsis get temporary names: ..1, ..2, ..3, etc. When using the minus sign in front of the 2 you are actually applying a unary operator to change its value. You get exactly the same result with log(2) as the second argument:
> f(1,log(2))
$x
[1] 1
$y
..2
I think the reason that the ..2 doesn't appear when the second argument to f is atomic is that a different evaluation process (or no evaluation) process is needed. The same behavior occurs with a character object.
> f(1,"a")
$x
[1] 1
$y
[1] "a"