Handling of missing arguments in R

args.R

# How the "missingness" status of arguments can be passed down
# the call stack. (Thanks Martin Morgan!)

bar <- function(q,...) {
  cat(q)
  foo(...)
}

foo <- function(a=123) {
  if (missing(a))
    "I'm missing something"
  else
    "a ok"
}

bar(q=123)
123[1] "I'm missing something"
bar(q=123, a=456)
123[1] "a ok"