search for: _with_a

...default" and "missing without default". If an argument x is missing without a default, then missing(x) is true, if you pass x to another function, it will pass the value of the "missing argument". (which is different than simply being missing!) If an argument x is missing _with_a default, then missing(x) is still true, but if you pass x to another function, the default value is passed, not the missing argument symbol. If you add default arguments to your example, you'll see this effect: f1 <- function(x, y, z){ if(missing(x)) cat("f1: x is missing\n&qu...

There's a StackOverflow question https://stackoverflow.com/q/22024082/2554330 that references this text from ?missing: "Currently missing can only be used in the immediate body of the function that defines the argument, not in the body of a nested function or a local call. This may change in the future." Someone pointed out (in https://stackoverflow.com/a/58169498/2554330)