[R] best practice(s) for retrieving a local variable from a closure

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Benjamin Tyner
> Sent: Friday, April 08, 2011 6:48 PM
> To: r-help at r-project.org
> Subject: [R] best practice(s) for retrieving a local variable 
> from a closure
> 
> Greetings,
> 
> Say I have defined
> 
>   mp <- function(a) function(x) x^a
> 
>   f2 <- mp(2)
> 
> and I would like to retrieve the "a" which is local to f2. 
> Two options 
> come to mind;
> 
>   get("a", envir=environment(f2))
>   eval(substitute(a), environment(f2))
> 
> I'm curious if one of these is preferred over the other in terms of 
> efficiency, robustness, aesthetics, etc. Or perhaps someone can share 
> another way to skin this cat, besides wrappers around eval() like
> 
>   evalq(a, environment(f2))
>   with(environment(f2), a)

You can also use a list-like notation with environments:
  > environment(f2)$a
  [1] 2
  > environment(f2)[["a"]]
  [1] 2
  > environment(f2)$a <- 3:1
  > f2(2)
  [1] 8 4 2

The above feels a bit like snooping where I wasn't invited.
You could do something like
  mq <- function(a) {
     force(a)
     list(getA = function()a,
          setA = function(newA) a <<- newA,
          fun = function(x)x^a
     )
  }
to make it clear that you expect people to look at or change
fun's 'a'.
  > f3 <- mq(3)
  > f3$fun(5)
  [1] 125
  > f3$getA()
  [1] 3
  > f3$setA(4:1)
  > f3$fun(3)
  [1] 81 27  9  3

I don't code like this much so haven't developed a sense of
aesthetics about these variants.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> 
> Also, I'd like to confirm that I have used the correct terminology in 
> the subject line, that "a" is a local variable with respect 
> to f2, which 
> is a closure?
> 
> Thanks,
> Ben
>