'State' design pattern in R

dp_state.R

# General functions
atom <- function(obj) {
  atom <- new.env()
  atom$obj <- obj
  atom
}

deref <- function(atom) {
  atom$obj
}

swap <- function(atom, fn, ...) {
  atom$obj <- do.call(fn, c(list(atom$obj), list(...)))
}

multimethod <- function(dispatch_fn, ...) {
    
  args <- list(...)
  dispatch_values <- args[seq(1, length(args), 2)]
  functions <- args[seq(2, length(args), 2)]
  
  function(obj) { 
    functions[[which(dispatch_values == dispatch_fn(obj))]](obj)
  }
    
}

# Implementation of example
subscribe <- function(user) {
  if (deref(user)$user_state == "not_subscribed") {
    swap(user, purrr::list_modify, user_state = "subscribed")
  }
}

greet <- 
  multimethod(function(x) x$user_state, 
              "subscribed", function(x) paste0("Greetings, ", x$name, "!!!"), 
              "not_subscribed", function(x) paste0("Greetings, ", x$name, ".")) 
              
# Run the example
user <- atom(list(name = "John Doe", user_state = "not_subscribed"))

greet(deref(user)) 
subscribe(user)
greet(deref(user))

As discussed here:
https://www.mpj-analytics.co.uk/2019/09/18/design-patterns-r-state/

Inspired by:
http://mishadoff.com/blog/clojure-design-patterns/#episode-3-state