[R] Environment question

[Duncan Murdoch]

On 23/10/2015 11:08 AM, ALBERTO VIEIRA FERREIRA MONTEIRO wrote:
> From the code below:
> 
> y <- 1
> 
> f1 <- function() { 
>   cat("y =", y, "\n") 
> }
> 
> f2 <- function() { 
>   y <- 2
>   f1()
> }
> 
> f3 <- function() {
>   y <- 3
>   f <- f1
>   f()
> }
> 
> f4 <- function() {
>   y <- 4
>   f <- function() { cat("y =", y, "\n") }
>   f()
> }
> 
> f1()
> f2()
> f3()
> f4()
> 
> Clearly, f1(), f2() and f4() will display "y = 1", "y = 1" and "y = 4",
> but, not as much clearly but predictably, f3() also displays "y = 1".
> 
> Is there any way to rewrite the code of f3 in such a way that it
> displays "y = 3"?

After f <- f1, you can say

environment(f) <- environment()

That says that f should resolve non-local symbols by looking in the
environment that is current in that line, i.e. the local evaluation
frame of this invocation of f3.

One warning:  if in the real use case, the user is supplying f1, then
the user might be giving you something where the environment really
matters, and this substitution would cause the function to return
nonsense.  Or if the user's f1 makes use of other global variables that
happen to have the same name as other locals in f3, nonsense again.

Duncan Murdoch

> 
> An obvious but cumbersome way would be something like:
> 
> f3 <- function() {
>   y <- 3
>   # write the code of f1 to a temporary file
>   dump("f1", "temp.R")
>   # read the code of f1
>   str <- readLines("temp.R")
>   # replace the code that defines function f1 for a code that defines function f
>   str <- gsub("f1 <-", "f <-", str)
>   # write the new code to the temporary file
>   writeLines(str, "temp.R")
>   # read the source but use local to get things from f3's environment
>   # (with the default local = FALSE, "y" would get the value from globalenv())
>   source("temp.R", local = TRUE)
>   # ...?
>   f()
>   # PROFIT!
> }
>   
> Is there a more elegant way to do this?
> 
> Alberto Monteiro
> 
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