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

[Duncan Murdoch]

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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