Guarding State

Part of the Object Oriented Programming section of Coddy's R journey. Lesson 20 of 57.

A setter is the one place where a value can change, so it is the place to enforce the rules. It checks first and calls stop() before anything is changed, and the object stays valid:

make_account <- function(balance = 0) {
  withdraw <- function(x) {
    if (x <= 0) stop("amount must be positive", call. = FALSE)
    if (x > balance) stop("insufficient funds", call. = FALSE)
    balance <<- balance - x
  }
  list(withdraw = withdraw, balance = function() balance)
}
acc <- make_account(50)
msg <- tryCatch(acc$withdraw(80), error = function(e) conditionMessage(e))
cat(msg, "/", acc$balance())

Output:

insufficient funds / 50

lockBinding("rate", e) makes one variable of an environment read-only. Assigning to it afterwards is an error, while the other variables stay writable:

loan <- new.env()
loan$rate <- 0.05
loan$owed <- 1000
lockBinding("rate", loan)
loan$owed <- 900
msg <- tryCatch({ loan$rate <- 0; "changed" }, error = function(e) conditionMessage(e))
cat(msg, "\n")
cat(loan$rate, loan$owed, "\n")

Output:

cannot change value of locked binding for 'rate' 
0.05 900

lockEnvironment(e) stops new variables from being added. With bindings = TRUE it also locks every existing one, which makes the whole object read-only:

cfg <- new.env()
cfg$theme <- "dark"
lockEnvironment(cfg, bindings = TRUE)
for (step in c("change", "add")) {
  msg <- tryCatch({
    if (step == "change") cfg$theme <- "light" else cfg$size <- 12
    "done"
  }, error = function(e) conditionMessage(e))
  cat(step, ":", msg, "\n")
}

Output:

change : cannot change value of locked binding for 'theme' 
add : cannot add bindings to a locked environment

local() runs code in a fresh environment and returns its last value. Helpers defined inside stay hidden, which is how a file can keep a function private even though source() puts everything else into the global environment:

circle_area <- local({
  square <- function(x) x * x
  function(r) pi * square(r)
})
print(round(circle_area(2), 2))
print(exists("square"))

Output:

[1] 12.57
[1] FALSE

Challenge

Easy

Complete the factory make_account(owner) in Account.R. The balance starts at 0 and a private history records every change as +50 or -20. The returned list has four functions:

deposit(x) stops with amount must be positive unless x is above 0;
withdraw(x) stops with amount must be positive or insufficient funds;
balance() returns the balance;
history() returns the history as a character vector.

Check before changing anything and use call. = FALSE. The supplied code reads the owner, then lines dep 50 or wd 20, prints ok or error: and the message for each, tries to change a copy of the history, and prints the balance and the history.

Your code goes in Account.R. main.R holds the supplied input/output code and cannot be edited.

Try it yourself

source("Account.R")

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
acc <- make_account(input[1])
for (line in input[-1]) {
  p <- strsplit(line, " ")[[1]]
  amount <- as.numeric(p[2])
  msg <- tryCatch({
    if (p[1] == "dep") acc$deposit(amount) else acc$withdraw(amount)
    "ok"
  }, error = function(e) paste("error:", conditionMessage(e)))
  cat(line, ": ", msg, "\n", sep = "")
}
h <- acc$history()
h[1] <- "tampered"
cat("balance: ", acc$balance(), "\n", sep = "")
cat("history: ", paste(acc$history(), collapse = ", "), "\n", sep = "")