The three-dot question

Hi: If you want testFun to know about b, then you would have to do
b<-list(...)$b inside
TestFun itself.  But the dot dot dot argument is not really for that
purpose. 

The use of dotdotdot is for the case where a function INSIDE testFun
has a formal argument named say b. Then you can pass the ... at the
top level and the function inside will receive the ... but
automatically slurp the b out of the dot dot dot and know about it.
Below is an example of the use I'm talking about. It was created by a
mentoR when I was trying to understand the dotdotdot concept. 
hope it helps you. 

#======================================================================

Note that f does not directly know about x and g does.  That is g
knows about x even though f does
not know and g got it from f.

g <- function(x, ...) { cat("g: exists('x') = ", exists("x"), "\n");
list(...) }
f <- function(...) { cat("f: exists('x') = ", exists("x"), "\n");
g(...) }
f(x = 3, y = 4)
f: exists('x') =  FALSE
g: exists('x') =  TRUE
$y




On Mon, Jan 14, 2013 at 5:21 AM, Feng Li <m at feng.li> wrote:
        Dear all,
        
        Why does not the three-dot accept arguments from the parent
        environment?
        I am just confused with this error, can someone give me a
        hint?
        

        > rm(list=ls())
        > testFun <- function(a, ...)

        +   {
        +     if(a){
        +         print(a)
        +       }else
        +       {
        +         print(b)
        +       }
        +   }

        >
        > myTask <- function(a)

        +   {
        +     b <- 3
        +     testFun(a, b = b)
        +   }

        > myTask(FALSE)

        Error in print(b) : object 'b' not found
        
        
        Thanks in advance!
        
        Feng
        
        --
        Feng Li
        Department of Statistics
        Stockholm University
        SE-106 91 Stockholm, Sweden
        http://feng.li/