as.missing

Paul Gilbert <pgilbert at bank-banque-canada.ca> writes:

Peter Dalgaard wrote:

Paul Gilbert <pgilbert at bank-banque-canada.ca> writes:

I.e., when x is missing in g, and g calls f(3,x), f will use its
default value for x.

Yes, that is the behaviour I am looking for. That is, f  should do
what it normal would do if it were called with x missing.

But if x has a default in g then that default should presumably be
used?

Yes.  The value of x in g would get passed to f, default or otherwise.
If that value is something that indicates x is missing, then it should
be treated as if it is missing in f. This means f should use its
default value, rather than throw an error saying x is missing.

And what if x is given a value in the evaluation frame of g
before it is used by f (which can happen, you know, even after the
evaluation of f has begun)? Now imagine a longer chain of calls.

I think what you're asking for is essentially dynamic scoping for
missing arguments: you'd have to backtrack along the call chain to
find the first instance where x is either given a value or has a
default. This sounds messy.

You understand this better than I do, but I don't think I am asking to
do this. Currently I think f  looks back too far and finds x is
missing and g does not have a default value for x, so it throws an
error.  Why can't f find its own default value for x?

Because it's being told to use the value of the argument instead. I
think. 

This stuff is treacherous. E.g. what would you expect from this?

g <- function(x) {f <- function(y) {x <<- 1; y} ; f(x)}
g()