Currently, if one wants to test if an argument to an outer function is missing from within an inner function, this works:> g5 <- function(a) {+ inner <- function(a) { + if (missing(a)) + "outer arg is missing" + else + "found outer arg!" + } + inner(a) + }> g5(3)[1] "found outer arg!"> g5()[1] "outer arg is missing" While if inner is defined as function() (without arguments) one gets an error: 'missing' can only be used for arguments. However, ?missing contains a note that this behavior is subject to change. I'm particularly interested in whether the code shown above will continue to work. While it does what I want in this case, the behavior seems a bit surprising since textually the call to inner does provide an argument. So it seems possible that might change. Can anyone provide more insight into how things may change? Thanks.