R help - Apr 2011 - 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)

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

On Fri, Apr 8, 2011 at 9:48 PM, Benjamin Tyner <btyner at gmail.com>
wrote:
> 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)
>
> 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?
>
Solutions involving eval tend to be less preferred.  "a" is termed a
free variable of f2.

-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com

> -----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
>