R does not report error when an argument of a function is not provided but used for subsetting a vector

Question

Why there is no error reported when b is not supplied but required inside the function? Thanks!

f2 <- function(a,b) {a[b]}; f2(a=rep(1, 2))

I understand that there is no error in this function:

f <- function(x) {
  10
}
f(stop("This is an error!"))

due to lazy evaluation But this

f <- function(x) {
  force(x)
  10
}
f(stop("This is an error!"))

or this

f <- function(x) {
  x
  10
}
f(stop("This is an error!"))

will produce an error. Because in both cases x is used within the function. Both the above two examples are from http://adv-r.had.co.nz/Functions.html. Since b is also used within f2, should it be necessary to add force inside f2? Thanks!

Answer 1

x[b] returns (a duplicate of) x if b is missing. From the R source:

static SEXP VectorSubset(SEXP x, SEXP s, SEXP call)
{
    R_xlen_t stretch = 1;
    SEXP indx, result, attrib, nattrib;

    if (s == R_MissingArg) return duplicate(x);

https://github.com/wch/r-source/blob/ec2e89f38a208ab02449b706e13f278409eff16c/src/main/subset.c#L169

From the documentation, in which 'empty' means 'missing', not NULL:

An empty index selects all values: this is most often used to replace all the entries but keep the attributes.