add max depth?

[timelyportfolio]

It would be very helpful to have a function that provides the max depth of a list. Perhaps, this is related to #236 .

[hadley]

Which I presume would look something like this:

depth <- function(x) {
  if (!is.list(x)) {
    1
  } else {
    max(map_int(x, depth)) + 1
  }
}