R: Avoid accidently overwriting variables

Question

Is there any way to define a variable in R in your namespace, such that it can't be overwritten (maybe ala a "Final" declaration)? Something like the following psuedocode:

> xvar <- 10
> xvar
[1] 10
xvar <- 6
> "Error, cannot overwrite this variable unless you remove its finality attribute"

Motivation: When running R scripts multiple times, it's sometimes too easy to inadvertently overwrite variables.

Answer 1

Check out ? lockBinding:

a <- 2
a
## [1] 2
lockBinding('a', .GlobalEnv)
a <- 3
## Error: cannot change value of locked binding for 'a'

And its complement, unlockBinding:

unlockBinding('a', .GlobalEnv)
a <- 3
a
## [1] 3