setValidity and validObject

Part of the Object Oriented Programming section of Coddy's R journey. Lesson 30 of 57.

setValidity("Class", function(object) ...) adds the validity rule after the class is declared, which keeps a long setClass() call short. Like setGeneric(), it returns a value that is printed at the top level, so wrap it in invisible():

setClass("Account", slots = c(owner = "character", balance = "numeric"))
invisible(setValidity("Account", function(object) {
  if (object@balance < 0) return("balance must not be negative")
  TRUE
}))
msg <- tryCatch(new("Account", owner = "Ada", balance = -1), error = function(e) conditionMessage(e))
cat(msg)

Output:

invalid class “Account” object: balance must not be negative

Changes should go through methods that check the result. A withdraw() method changes the slot, calls validObject(), and returns the object only when it is still valid:

setClass("Account", slots = c(balance = "numeric"))
invisible(setValidity("Account", function(object) {
  if (object@balance < 0) "balance must not be negative" else TRUE
}))
invisible(setGeneric("withdraw", function(acc, amount) standardGeneric("withdraw")))
setMethod("withdraw", "Account", function(acc, amount) {
  acc@balance <- acc@balance - amount
  validObject(acc)
  acc
})
a <- new("Account", balance = 50)
a <- withdraw(a, 20)
msg <- tryCatch({ a <- withdraw(a, 100); "done" }, error = function(e) "refused")
cat(msg, a@balance)

Output:

refused 30

An accessor pair can be made of generics: balance(x) reads and a replacement generic "balance<-" writes. Its method checks the object, so balance(a) <- -5 is refused like any other bad change:

setClass("Account", slots = c(balance = "numeric"),
  validity = function(object) if (object@balance < 0) "balance must not be negative" else TRUE)
invisible(setGeneric("balance", function(x) standardGeneric("balance")))
invisible(setGeneric("balance<-", function(x, value) standardGeneric("balance<-")))
setMethod("balance", "Account", function(x) x@balance)
setMethod("balance<-", "Account", function(x, value) {
  x@balance <- value
  validObject(x)
  x
})
a <- new("Account", balance = 10)
balance(a) <- 25
msg <- tryCatch({ balance(a) <- -5; "changed" }, error = function(e) "refused")
cat(balance(a), msg)

Output:

25 refused

validObject(x, test = TRUE) does not stop. It returns TRUE for a valid object, or the problem text, which is handy for a report:

setClass("Box", slots = c(w = "numeric"),
  validity = function(object) if (object@w <= 0) "w must be positive" else TRUE)
b <- new("Box", w = 2)
print(validObject(b, test = TRUE))
b@w <- 0
print(validObject(b, test = TRUE))

Output:

[1] TRUE
[1] "w must be positive"

Challenge

Easy

The class Account in Account.R has the slots owner and balance. Complete the file:

add a rule with setValidity() (wrapped in invisible()) that returns owner must not be empty or balance must not be negative;
create the generics deposit(acc, amount) and withdraw(acc, amount) and their Account methods: deposit stops with amount must be positive (use call. = FALSE) for amounts of 0 or less; both change the balance, call validObject() and return the account;
add a show() method that prints ada: 50 and a new line.

The supplied code reads the owner, then lines dep 50 or wd 20, and prints the account after each change or refused: and the problem.

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 <- tryCatch(new("Account", owner = input[1], balance = 0),
                error = function(e) { cat("error: ", sub(".*object: ", "", conditionMessage(e)), "\n", sep = ""); NULL })
if (!is.null(acc)) {
  for (line in input[-1]) {
    p <- strsplit(line, " ")[[1]]
    amount <- as.numeric(p[2])
    acc <- tryCatch({
      changed <- if (p[1] == "dep") deposit(acc, amount) else withdraw(acc, amount)
      print(changed)
      changed
    }, error = function(e) {
      cat("refused: ", sub(".*object: ", "", conditionMessage(e)), "\n", sep = "")
      acc
    })
  }
}