There are lot of subtle issues involved here. We should think through
carefuly exactly what semantics we want for missing value propagation
before making any changes. Making usage easy at top level is
genearlly a good thing, but for usage within functions eliminating
error messages by making more automated choices may not be a good
thing--it may mask real mistakes.
There are also issues with the internal implementation if missing
arguments we ned to think carefuly about before exposing them at teh R
level. The fact that internally there is a missing argument token
does not mean it is a good thing to expose that detail at the R level
(and it already is in call objects and creates some issues with
computing on the language.
Like I said, it's complicated, so let's not leap before we look
carefully.
Best,
luke
On Fri, 27 Oct 2006, Paul Gilbert wrote:
Peter Dalgaard wrote:
Paul Gilbert <[EMAIL PROTECTED]> writes:
Peter Dalgaard wrote:
Paul Gilbert <[EMAIL PROTECTED]> 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()
I'm confused. Neither f nor g have a default here, so I don't think
this is related to what I'm talking about. Currently, in your example, f
find x with a value of 1, and I am not suggesting changing that. I'm
only suggesting that if f finds x is missing, it should look at it's own
default argument.
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Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
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