Field constraints

[trickytank]

I'd like to find a way to constrain fields, that is, passing tests in a similar way to the Moose Object-oriented system in Perl. This is a suggested enhancement or extension in another package to R6.

What I want to achieve can be done using active fields, though the coding is kind of convoluted and the field value isn't shown in the R6.print method.
An example of this is:

library(R6)
library(checkmate)

CONSTRAINT <- R6Class(
  "CONSTRAINT",
  public = list(

  ),
  private = list(
    x_ = 0
  ),
  active = list(
    x = function(value) {
      if (missing(value)) {
        return(private$x_)
      } else {
        check <- checkmate::checkNumber(value)
        if(is.character(check)) {
          stop("Value attempting to be assigned to x is not valid: ", check)
        }
        value
      }
    }
  )
)

A <- CONSTRAINT$new()
A$x <- 5 # Successful
A$x <- 1L # Successful
A$x <- pi # Successful
A$x <- c(3,5) # Throws an error, as required. 
A$x <- "SD" # Throws an error, as required. 

What I would be hoping for is something like this:

CONSTRAINT_IDEAL <- R6Class(
  "CONSTRAINT",
  public = list(
    x = 0
  ),
  constraints = list(
    x = checkmate::checkNumber # this can be any function that returns TRUE when the field is within the constraints.
  )
)

My main problem is I don't know if this is a possible extension of public/private fields or whether this would have to stick to active bindings (with a rewrite of the R6.print method to show the values of these constrained fields).

I'm willing to have an attempt at developing this myself, though I need to understand much better how R6 works under the hood.