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

[Benjamin Tyner]

Thanks Bill and Gabor!

William Dunlap wrote:
>> -----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
>>
>>