Nullable Reference Class fields in R

nullable.r

#' Create a nullable class field for a reference class.
#'
#' @param name name of class.
#' @examples \dontrun{
#' Foo = setRefClass('Foo', fields=c(a=nullable('list'), b='character'))
#' x = Foo$new(a=list(1,2,3), y="Non-NULL 'a'")
#' y = Foo$new(a=NULL, y="NULL 'a'")
#' }
nullable = function(name) {
  union_name = paste(name, 'NULL', sep='_')

  tryCatch({
    # use cached definition
    getClass(union_name)
  },
  error=function(e) {
    # register new definition
    setClassUnion(union_name , c(name, 'NULL'))
  })

  return(union_name)
}

condition = setRefClass('condition',
  fields=list(call=nullable('call'), traceback='list', message='character'),
  methods=list(
    initialize=function(call=NULL, traceback=list(), message='') {
      .self$call = call
      .self$traceback = traceback
      .self$message = message

      return(.self)
    }
  )
)