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Colin Fay
Colin Fay

Posted on • Originally published at colinfay.me on

JavaScript cont in R

#r #rstats

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
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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!"
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Here, I have no way to prevent myself from erasing the value in a. Of course, there are always rigour, 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'
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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'
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So there I could do

a <- some_very_complex_computation() lock(a) # [...] Going on the weekend a <- "Hello there!"
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And there, I have prevented myself from erasing my a variable. Ofcourse, it’s not the same as JavaScript const, as there is always away 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
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But I think it’s a rather elegant solution for preventing yourself fromunwanted variable overwriting.

See also:

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

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