R help - Jul 2005 - Problems with eval() in connection with match.call()

Dear all, I have a problem when passing parms from one function to another when
the argument list is just '...'. Consider this example:
 
foo<-function(){
 xx <- 111222
 bar(x=xx)
}
bar <- function(...){
  cl <- match.call(expand.dots=TRUE)
  print(cl)
  x <- eval(cl$x)
  print(x)
}
foo()
> bar(x = xx)
> Error in eval(expr, envir, enclos) : Object "xx" not found
My expectation was, that xx would be evaluated to 111222 in foo before being
passed on to bar, but obviously it is not so. Should I do something explicitely
in foo() to 'evaluate' xx or need I do something special in bar()??
 
Thanks in advance, S??ren

On 7/4/05, S??ren H??jsgaard <Soren.Hojsgaard at agrsci.dk>
wrote:
> Dear all, I have a problem when passing parms from one function to another
when the argument list is just '...'. Consider this example:
> 
> foo<-function(){
>  xx <- 111222
>  bar(x=xx)
> }
> bar <- function(...){
>   cl <- match.call(expand.dots=TRUE)
>   print(cl)
>   x <- eval(cl$x)
>   print(x)
> }
> foo()
> 
> > bar(x = xx)
> > Error in eval(expr, envir, enclos) : Object "xx" not found
> 
> My expectation was, that xx would be evaluated to 111222 in foo before
being passed on to bar, but obviously it is not so.
Welcome to the world of lazy evaluation.  Arguments are evaluated only
when needed (i.e. when first referenced) not at the time of building
the function call.   That's why the default value of an argument can
depend on previously calculated results within the function.
> Should I do something explicitely in foo() to 'evaluate' xx or need
I do something special in bar()??
Change the eval(cl$x) to eval(cl$x, parent.frame())
> bar
function(...){
    cl <- match.call(expand.dots=TRUE)
    print(cl)
    x <- eval(cl$x, parent.frame())
    print(x)
}
> foo()
bar(x = xx)
[1] 111222
> Thanks in advance, S??ren
> 
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S??ren H??jsgaard wrote:
> Dear all, I have a problem when passing parms from one function to another
when the argument list is just '...'. Consider this example:
>  
> foo<-function(){
>  xx <- 111222
>  bar(x=xx)
> }
> bar <- function(...){
>   cl <- match.call(expand.dots=TRUE)
>   print(cl)
>   x <- eval(cl$x)
>   print(x)
> }
> foo()
> 
> 
>>bar(x = xx)
>>Error in eval(expr, envir, enclos) : Object "xx" not found
> 
> 
> My expectation was, that xx would be evaluated to 111222 in foo before
being passed on to bar, but obviously it is not so. Should I do something
explicitely in foo() to 'evaluate' xx or need I do something special in
bar()??
Hi S??ren.

You need to use eval.parent(cl$x) (which is the same as eval(cl$x, 
envir=parent.frame())) to get what you want.  You want the evaluation to 
happen in the caller's frame of reference, not in bar's frame.

Generally when you eval() an expression, evaluation takes place in the 
current frame:  the function's local frame when it's in a function, the 
global frame when you do it at a command line.  You can use the envir= 
argument to change where it takes place.

Things look different when you have named parameters, e.g.

bar <- function(x) {
print(x)
}

because in this case, the argument gets turned into a "promise" at the
time of the call, and a promise knows its own evaluation frame.  There's 
nothing really special about named parameters though, e.g.

 > bar <- function(...) {
+  cl <- list(...)
+  print(cl$x)
+ }
 > foo()
[1] 111222


By using match.call, you ignore the environment, and only see the 
expression.

I imagine you could put together an example where eval.parent() gets the 
evaluation wrong, e.g. because you want to evaluate in the parent's 
parent.  I was a little surprised that it didn't happen here:

 > foo
function(){
  xx <- 111222
  bar2(x=xx)
}
 > bar2
function(...) { bar(...) }
 > bar
function(...){
   cl <- match.call(expand.dots=TRUE)
   print(cl)
   x <- eval.parent(cl$x)
   print(x)
}
 > foo()
bar(x = ..1)
[1] 111222



I guess match.call() works hard to be helpful.

Duncan Murdoch