[Rd] as.missing
Peter Dalgaard
p.dalgaard at biostat.ku.dk
Fri Oct 27 17:40:37 CEST 2006
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()
--
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