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

-----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