How to create a custom R 'property' (e.g. dim())

Question

So I basically want to create a 'property' in R which I haven't really found documentation or other SO Q/A explaining how this works.

So an example of what I'm looking for to make a function property with the behavior found in some base R functions (eg dim(), names(), attr(), etc...), where you can do property(variable) = value and then have property(variable) == value be true.

I was hoping that this would work:

x = 1
property = function(x) attr(x, 'property')
`property<-` = function(x, value) attr(x, 'property') <<- value
property(x) = 2
property(x)

But it didn't and in hindsight I have no idea how I would do this since x is copied by value.

Answer 1

You may use `attr<-` in your function.

x <- 1
property <- function(x) attr(x, 'property')
`property<-` <- function(x, value) `attr<-`(x, 'property', value) 
property(x) <- 2
property(x)
# [1] 2

Or, using your code, you need to do return(x) after assigning something to it.

`property<-` <- function(x, value) {attr(x, 'property') <- value;x}
property(v) <- 2
property(v)
# [1] 2