JavaScript const in R

One thing I like about JavaScript is the const declaration method, which allows you to declare a variable one time, and that variable can’t be reassigned after that. I.e, this piece of code will throw an error:

node -e "const x = 12; x  = 14"

## [eval]:1
## const x = 12; x  = 14
##                  ^
## 
## TypeError: Assignment to constant variable.
##     at [eval]:1:18
##     at Script.runInThisContext (vm.js:124:20)
##     at Object.runInThisContext (vm.js:314:38)
##     at Object.<anonymous> ([eval]-wrapper:9:26)
##     at Module._compile (internal/modules/cjs/loader.js:805:30)
##     at evalScript (internal/process/execution.js:60:25)
##     at internal/main/eval_string.js:16:1

The cool thing about this is that you can’t override the variable by mistake: once it’s set, it’s set. On the other hand, R allows you to override almost any variable (well, except some reserved variables).

I asked Twitter if there was any implementation of that concept in R. The use case, for example, would arise when you have a value that takes some time to compute. If I do my computation, I can accidentally override it later on. Event more if you’re using notebook, where you create symbols and values all along your document.

a <- some_very_complex_computation()
# [...] Going on the weekend
a <- "Hello there!"

Here, I have no way to prevent myself from erasing the value in a. Of course, there are always rigor, explicit variable name, and not-assigning-things-without-thinking but you know how it is in the real world, and there is no Cmd + Z there.

Romain pointed out that ?lockBinding existed, and that it was what I was looking for. And that does.

Here’s how it works: it takes a character string referring to a symbol, and an environment, and prevents from assigning any new value to this symbol in the given environment.

x <- 12
lockBinding("x", .GlobalEnv)
x <- 13

## Error in eval(expr, envir, enclos): cannot change value of locked binding for 'x'

And here’s a small wrapper to do that:

lock <- function(x){
  lockBinding(
    deparse(
      substitute(x)), 
    env = parent.frame()
  )
}

plop <- 12
lock(plop)
plop <- 13

## Error in eval(expr, envir, enclos): cannot change value of locked binding for 'plop'

pouet <- function(){
  plop <- 14
  print(plop)
  lock(plop)
  plop <- 13
}
pouet()

## [1] 14

## Error in pouet(): cannot change value of locked binding for 'plop'

So there I could do

a <- some_very_complex_computation()
lock(a)
# [...] Going on the weekend
a <- "Hello there!"

And there, I have prevented myself from erasing my a variable. Of course, it’s not the same as JavaScript const, as there is always a way to unlock the symbol.

x <- 12

## Error in eval(expr, envir, enclos): cannot change value of locked binding for 'x'

lock(x)
x <- 13

## Error in eval(expr, envir, enclos): cannot change value of locked binding for 'x'

unlockBinding("x", .GlobalEnv)
x <- 13
x

## [1] 13

But I think it’s a rather elegant solution for preventing yourself from unwanted variable overwriting.

See also:

• Make a Constant in R with Active Binding

Some answers to the Twitter thread also suggested using R6… but that will be for another post :)