On the mechanics of function evaluation and argument matching

def f(x,y,z): x

f(2)

On Wed, Jul 17, 2013 at 10:20 AM, Ben Bolker <bbolker at gmail.com> wrote:

Brian Rowe <rowe <at> muxspace.com> writes:

Thanks for the lead. Given the example in ?missing though,
wouldn't it be safer to explicitly define a
default value of NULL:

myplot <- function(x, y=NULL) {
 if(is.null(y)) {
   y <- x
   x <- 1:length(y)
 }
 plot(x, y)
}

[snip]

In my opinion the missing() functionality can indeed be
fragile (for example, I don't know how I can manipulate an
existing call to make an argument be 'missing' when it was
previously 'non-empty')

Like so:

thecall <- quote(x[i,j])
thecall[[3]] <- quote(expr=)
thecall

x[, j]

and using an explicit NULL is often
a good idea.  This makes the documentation a tiny bit less
wieldy if you have lots of parameters ...

I could certainly imagine a variant of R in which missing and NULL are
unified, and NULL is the default for any binding that exists but was not
given a value. I would probably prefer it on the grounds of being smaller
and more consistent.  (At the C implementation level, R_MissingArg and
R_NilValue are just two parallel uses of the null object pattern with
different behavior, which is a bit silly)

But one advantage the missing value behavior can have is that it "fails
early", i.e. it generates an error closer to where a function wants to use
a value it was not provided, rather than "failing late," where a NULL
propagates though your data and you have to do more debugging work to find
out where it came from. This kind of fragility can be a good thing as it's
easier to debug problems that happen closer to the point of failure.

For instance,

myplot <- function(y, x=1:length(y)) plot(x,y)
myplot()

Error in plot(x, y) (from #1) :
 error in evaluating the argument 'x' in selecting a method for function
'plot': Error in length(y) (from #1) : 'y' is missing

I didn't think about what myplot should do with no arguments. As it turns
out it is an error, as R refuses to pass a missing value along to length()
or plot(), which is reasonable.

Compare with a default-NULL version.

myplot <- function(y=NULL, x=1:length(y)) plot(x,y)
myplot()

Instead of failing early and generating a stack trace pointing you at the
problem, myplot() now generates a graph with points at (0,0) and (1,1) --
most surprising! This is because R happily forwards NULL to length() and
plot() where it refused to earlier. In more complicated code nulls can pass
along several layers before causing problems, making those problems more of
a headache to debug.

Peter

	[[alternative HTML version deleted]]