How to use a closure to make an assignment active binding in R6

Question

The active binding tmp_priv_2 does not work as expected:

active_creator <- function(clmn) {   
  function(value) {
    if (missing(value)) {
      private[[paste0(".", clmn)]]
    } else {
      if (!is.na(private[[paste0(".", clmn)]])) 
        warning(paste0("Overriding existing tmp_replace: ", 
                       private[[paste0(".", clmn)]]))
      private[[paste0(".", clmn)]] <- value                
    }
  }
}

tmpClass <-
  R6Class("tmpClass",
          private = list(.tmp_priv = NA_character_),
          active = list(
            tmp_priv = function(value) {         
              clmn <- "tmp_priv"
              if (missing(value)) {
                private[[paste0(".", clmn)]]
              } else {
                if (!is.na(private[[paste0(".", clmn)]])) 
                  warning(paste0("Overriding existing tmp_replace: ", 
                                 private[[paste0(".", clmn)]]))
                private[[paste0(".", clmn)]] <- value
              }       
            },
            tmp_priv_2 = active_creator("tmp_priv"))
  )

TC <- tmpClass$new()

The following results are expected:

> TC$tmp_priv
[1] NA
> TC$tmp_priv <- "asdf"
> TC$tmp_priv <- "zxcv"
Warning message:
In (function (value)  : Overriding existing tmp_replace: asdf

But the following returns an error instead of behaving like the tmp_priv binding:

> TC$tmp_priv_2 <- "asdf"
Error in (function (value)  : object 'clmn' not found

Yet:

> as.list(environment(tmpClass$active$tmp_priv_2))
$clmn
[1] "tmp_priv"

The idea of using one function to define all the active bindings is related to: Function for standard active binding in R6

EDIT (2025-07-30): After receiving the two very helpful answers, comments, and some debugging with Claude AI, I found this old github issue which discusses in great detail closures and R6 active bindings. In short, it is exactly as user2554330 describes.

Answer 1

clmn exists in the function's environment, but it's not available when R6 calls the function. That said, the solution is the same as the linked question:

library(R6)

active_creator <- function(clmn) {
  eval(substitute(function(value) {
    if (missing(value)) {
      private[[paste0(".", clmn)]]
    } else {
      if (!is.na(private[[paste0(".", clmn)]])) {
        warning(paste0("Overriding existing tmp_replace: ", 
                       private[[paste0(".", clmn)]]))
      }
      private[[paste0(".", clmn)]] <- value
    }
  }))
}

tmpClass <- R6Class("tmpClass",
                    private = list(.tmp_priv = NA_character_),
                    active = list(
                      tmp_priv = function(value) {
                        clmn <- "tmp_priv"
                        if (missing(value)) {
                          private[[paste0(".", clmn)]]
                        } else {
                          if (!is.na(private[[paste0(".", clmn)]])) 
                            warning(paste0("Overriding existing tmp_replace: ", 
                                           private[[paste0(".", clmn)]]))
                          private[[paste0(".", clmn)]] <- value
                        }       
                      },
                      tmp_priv_2 = active_creator("tmp_priv")
                    )
)

TC <- tmpClass$new()

TC$tmp_priv
#> [1] NA
TC$tmp_priv <- "asdf"
TC$tmp_priv <- "zxcv"
#> Warning in (function (value) : Overriding existing tmp_replace: asdf

TC$tmp_priv_2 <- "qwerty"  
#> Warning in (function (value) : Overriding existing tmp_replace: zxcv

TC <- tmpClass$new()

TC$tmp_priv_2
#> [1] NA
TC$tmp_priv_2 <- "azerty"
TC$tmp_priv_2 <- "qwertz"
#> Warning in (function (value) : Overriding existing tmp_replace: azerty

TC$tmp_priv <- "zxcv"
#> Warning in (function (value) : Overriding existing tmp_replace: qwertz

^{Created on 2025-07-30 with reprex v2.1.1}