length of `...`

On 03/05/2018 11:01 AM, William Dunlap via R-devel wrote:

In R-3.5.0 you can use ...length():

    > f <- function(..., n) ...length()
    > f(stop("one"), stop("two"), stop("three"), n=7)

    [1] 3

Prior to that substitute() is the way to go

    > g <- function(..., n) length(substitute(...()))
    > g(stop("one"), stop("two"), stop("three"), n=7)

    [1] 3

R-3.5.0 also has the ...elt(n) function, which returns
the evaluated n'th entry in ... , without evaluating the
other ... entries.

    > fn <- function(..., n) ...elt(n)
    > fn(stop("one"), 3*5, stop("three"), n=2)

    [1] 15

Prior to 3.5.0, eval the appropriate component of the output
of substitute() in the appropriate environment:

    > gn <- function(..., n) {

    +   nthExpr <- substitute(...())[[n]]
    +   eval(nthExpr, envir=parent.frame())
    + }

    > gn(stop("one"), environment(), stop("two"), n=2)

    <environment: R_GlobalEnv>

Bill, the last of these doesn't quite work, because ... can be passed
down through a string of callers.  You don't necessarily want to
evaluate it in the parent.frame().  For example:

x <- "global"
f <- function(...) {
    x <- "f"
    g(...)
}
g <- function(...) {
    firstExpr <- substitute(...())[[1]]
    c(list(...)[[1]], eval(firstExpr, envir = parent.frame()))
}

Calling g(x) correctly prints "global" twice, but calling f(x)
incorrectly prints

[1] "global" "f"

You can get the first element of ... without evaluating the rest using
..1, but I don't know a way to do this for general n in pre-3.5.0 base R.