[R] eval and evironments: call local function in a global function

[Renaud Gaujoux]

Thanks Duncan, I agree that touching the environments is risky and not 
robust. I rather go for another solution.
But the function solution still require to pass an extra object to the 
user's function. I'd like the user to simply be able to call function 
setVar as if it was a standard R function (visible from any of his -- 
possibly nested -- functions), but this function would act on a variable 
local to the main call, that I can setup on runtime. This variable 
should be protected from direct access as much as possible (my idea with 
the local layer was something to implement kind of a private variable). 
Maybe it's just impossible ?
-- Sorry to insist :) --

Renaud

Duncan Murdoch wrote:
> On 8/20/2009 4:27 AM, Renaud Gaujoux wrote:
>> Hi,
>>
>> in my project I want the user to be able to write hook functions that 
>> are in turn called in my main code. I'd like the user's hooks to be 
>> able to call some function that set a variable outside their running 
>> environment.
> The trick is that this variable is not global, but defined
>> on runtime before calling the hooks, and I don't want to leave any 
>> trace (i.e. global variables) after the main code has finished.
>
> The best way to do this is to pass the function (setVar below) as an 
> argument to the user's function.  If it's the user's function, you 
> have no business messing with it by changing its environment.  How do 
> you know the user didn't spend hours working out some crazy scheme of 
> creating a nested function with a carefully crafted environment, and 
> evaluation of his function depends on you leaving it alone?
>
>
>>
>> I thought that the following would work but it doesn't. I guess I got 
>> too messy with environment and enclosures:
>>
>> # global function defined by the user
>> fun.global <- function(){
>>     message('fun.global')
>>     setVar(5) #
>> }
>
> Pass setVar as an arg:
>
> fun.global <- function(setVar) {
>     message('fun.global')
>     setVar(5)
> }
>
>>
>>
>> # my main code
>> main <- function(){
>>     message('main')
>>  
>>     # define a function to set some local variable
>>     setVar <- local({
>>     l.var <- 0
>>     function(value){
>>         message('setVar')
>>        l.var <<- value
>>     }
>>     })
>>     .workspace <- environment(setVar)
>>     environment(setVar) <- new.env()
>>  
>>     eval(fun.global(), enclos=environment(setVar))
>>     print(get('l.var', envir=.workspace, inherits=FALSE))
>> }
>
> I wouldn't bother with the extra layer of local(), just put l.var in 
> main's evalution frame.  (Since you're the one writing setVar, you can 
> avoid any name clashes.)  That is:
>
> main <- function() {
>     message('main')
>     l.var <- 0
>     setVar <- function(value) {
>     message('setVar')
>     l.var <<- value
>     }
>     fun.global(setVar)
>     print(l.var)
> }   
>
>
> Duncan Murdoch
>
>>
>> main()
>>
>> I get the following output:
>>  > main
>>  > fun.global
>>  > Error in fun.global() : could not find function "setVar"
>>
>> There is definitely a problem of lookup somewhere. I first tried 
>> without eval, as I thought that function setVar would then be defined 
>> in a parent environment of the call to fun.global, but it does not 
>> work either.
>> Can anybody tell me what's the problem and how I should do my stuff?
>>
>> Thanks,
>> Renaud
>>
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>