R devel - Sep 2009 - Finding the environment of a promise

Hi all,

Is it possible to determine the environment in which a promise will be
evaluated?  e.g.

f <- function(code) { force(code) }
f({
  a <- 1
  b <- 2
})

Is there any way to tell from within f that a and b will be created in
the global environment?

Thanks,

Hadley

-- 
http://had.co.nz/

If I understand your question, you can get the environment with sys.frame:

f <- function(code){
    print(sys.frame())
    force(code)
}

f({
 a <- 1
 b <- 2
})

On Wed, Sep 9, 2009 at 10:30 AM, Hadley Wickham <hadley@rice.edu> wrote:
> Hi all,
>
> Is it possible to determine the environment in which a promise will be
> evaluated?  e.g.
>
> f <- function(code) { force(code) }
> f({
>  a <- 1
>  b <- 2
> })
>
> Is there any way to tell from within f that a and b will be created in
> the global environment?
>
> Thanks,
>
> Hadley
>
> --
> http://had.co.nz/
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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On 9/9/2009 9:30 AM, Hadley Wickham wrote:
> Hi all,
> 
> Is it possible to determine the environment in which a promise will be
> evaluated?  e.g.
> 
> f <- function(code) { force(code) }
> f({
>   a <- 1
>   b <- 2
> })
> 
> Is there any way to tell from within f that a and b will be created in
> the global environment?
I don't think so in R code, but C code to do it would be possible.  It 
needs to be in C code to avoid forcing the promise.

I think we'd be reluctant to make an R function available to do this, 
because it requires non-standard evaluation, and it's not something a 
normal function should care about.  (The promise belongs to the caller, 
not to the function:  why would the function need to play with it?  It 
should be happy with the value, or maybe a text representation of the 
promise, for labelling purposes.)

Duncan Murdoch