Feature request: pass `name` to property setter

[teunbrand]

I've found it useful to declare properties outside a class definition and recycle the property definition for various classes or for similar properties within one class.
The issue I'm running in to is that property setters don't have access to the name of the property that is being set.
In the example below, I cannot recycle the colour property for fill, because the setter function hardcodes the name.

library(S7)

property_colour <- new_property(
  class = class_character,
  setter = function(self, value) {
   # Must I hardcode the property name?
    prop(self, "colour") <- as.character(value)
    self
  }
)

rectangle <- new_class(
  "rectangle",
  properties = list(
    colour = property_colour, 
    fill = property_colour
  )
)

x <- rectangle("red", "blue")
#> Error: <rectangle> object properties are invalid:
#> - @fill must be <character>, not <NULL>

^{Created on 2025-07-23 with reprex v2.1.1}

My current solution to this problem is to make a property factory like so:

library(S7)

property_colour <- function(name) {
  force(name)
  new_property(
    class = class_character,
    setter = function(self, value) {
      prop(self, name) <- as.character(value)
      self
    }
  )
}

rectangle <- new_class(
  "rectangle",
  properties = list(
    colour = property_colour("colour"), 
    fill = property_colour("fill")
  )
)

x <- rectangle("red", "blue")

^{Created on 2025-07-23 with reprex v2.1.1}

But this solution is a bit unwieldy because I still have to declare the "colour" and "fill" names twice: once in the list names and once as argument to property_colour(). If we could somehow automatically access the property name that is being set from the context of the setter function, this would be a much smoother experience.

[wjschne]

Not sure if this helps, but my workaround has been to create a list of reusable properties and then use rlang::inject. For example,

library(S7)

color_prop <- new_property(class = class_character)

reusable_properties <- list(
  colour = color_prop,
  fill = color_prop,
  not_relevant_property = new_property(class = class_double)
)

rectangle <- new_class(
  "rectangle", 
  properties = rlang::inject(
    list(!!!reusable_properties[c("colour", "fill")], 
         rectangle_specific_property = class_double)
    )
  )
x <- rectangle("red", "blue", 6)
x
#> <rectangle>
#>  @ colour                     : chr "red"
#>  @ fill                       : chr "blue"
#>  @ rectangle_specific_property: num 6
x@colour <- "green"
x
#> <rectangle>
#>  @ colour                     : chr "green"
#>  @ fill                       : chr "blue"
#>  @ rectangle_specific_property: num 6

^{Created on 2025-07-23 with reprex v2.1.1}

[teunbrand]

If you use rlang::list2() instead you can skip the inject() step. But that still won't work out with custom setters that need the name of the property that they're accessing.

[hadley]

I needed this recently too and I agree it would be useful.

[t-kalinowski]

I also wanted this recently.

It's far from elegant, but it is possible to resolve the name using the .setting_prop attr.

https://github.com/t-kalinowski/quickr/blob/da6712746fe5e58f7c501ace1145f1b9556cdcb0/R/classes.R#L9

[teunbrand]

Thanks @t-kalinowski, so I could in principle make the example work like so:

library(S7)

property_colour <- new_property(
  class = class_character,
  setter = function(self, value) {
    name <- as.character(attr(self, ".setting_prop")[[1]])
    prop(self, name) <- as.character(value)
    self
  }
)

rectangle <- new_class(
  "rectangle",
  properties = list(
    colour = property_colour, 
    fill = property_colour
  )
)

(x <- rectangle("red", "blue"))
#> <rectangle>
#>  @ colour: chr "red"
#>  @ fill  : chr "blue"

^{Created on 2025-07-25 with reprex v2.1.1}

Which is workable, but relies on internal structures that may change.

[t-kalinowski]

Small note: the [[1]] above will resolve the wrong name if called from a setter calling another setter. You'll want the last element in the pairlist. Use tail(, 1) or something like last <- \(x) x[[length(x)]]