recursive default argument reference

-----Original Message-----
From: r-devel-bounces@stat.math.ethz.ch 
[mailto:r-devel-bounces@stat.math.ethz.ch] On Behalf Of Gabor 
Grothendieck
Sent: Tuesday, November 09, 2004 5:53 PM
To: r-devel@stat.math.ethz.ch
Subject: Re: [Rd] recursive default argument reference

Vadim Ogranovich <vograno <at> evafunds.com> writes:

: 
: Hi,
: 
: It seems that a formal function argument can not default to 
an "outer"
: variable of the same name:
: 
: > x <- "foo"
: > ff <- function(x=x) x
: > ff()
: Error in ff() : recursive default argument reference
: >
: 
: Is this intentional? Why?
: 

Because then one arg could not safely refer to another. Here 
x has y as its default value:

   x <- 10
   y <- 7
   f <- function(x = y, y = 3) print(x)
   f() # prints 3

Note that you could do this:

   x <- "foo"
   ff <- function(x.=x) x.
   ff() # returns "foo"
   ff(x=44) # returns 44
   ff(55) # returns 55

For a real example look at merge.data.frame.