Inheritance and callSuper

Part of the Object Oriented Programming section of Coddy's R journey. Lesson 40 of 57.

contains = "Account" makes a Reference Class inherit every field and method of Account. The subclass adds its own fields and methods:

Account <- suppressMessages(setRefClass("Account",
  fields = list(owner = "character", balance = "numeric"),
  methods = list(deposit = function(x) balance <<- balance + x)
))
Savings <- suppressMessages(setRefClass("Savings", contains = "Account",
  fields = list(rate = "numeric"),
  methods = list(add_interest = function() deposit(balance * rate))
))
s <- Savings$new(owner = "Ada", balance = 100, rate = 0.05)
s$deposit(100)
s$add_interest()
cat(s$balance, is(s, "Account"))

Output:

210 TRUE

A subclass overrides a method by defining one with the same name. Inside it, callSuper(...) calls the parent's version, so the child only adds what is new:

Account <- suppressMessages(setRefClass("Account",
  fields = list(balance = "numeric"),
  methods = list(withdraw = function(x) {
    if (x > balance) stop("insufficient funds", call. = FALSE)
    balance <<- balance - x
  })
))
Savings <- suppressMessages(setRefClass("Savings", contains = "Account",
  methods = list(withdraw = function(x) {
    if (x > 500) stop("limit is 500", call. = FALSE)
    callSuper(x)
  })
))
s <- Savings$new(balance = 300)
s$withdraw(200)
for (x in c(600, 400)) cat(tryCatch({ s$withdraw(x); "ok" }, error = function(e) conditionMessage(e)), "\n")
cat(s$balance, "\n")

Output:

limit is 500 
insufficient funds 
100

initialize is inherited and overridden like any method. A child's initialize calls callSuper(...) to let the parent set its fields, then sets its own defaults:

Account <- suppressMessages(setRefClass("Account",
  fields = list(owner = "character", balance = "numeric"),
  methods = list(initialize = function(...) {
    initFields(...)
    if (length(balance) == 0) balance <<- 0
    invisible(.self)
  })
))
Savings <- suppressMessages(setRefClass("Savings", contains = "Account",
  fields = list(rate = "numeric"),
  methods = list(initialize = function(...) {
    callSuper(...)
    if (length(rate) == 0) rate <<- 0.01
    invisible(.self)
  })
))
s <- Savings$new(owner = "Bo")
cat(s$owner, s$balance, s$rate)

Output:

Bo 0 0.01

Objects of different subclasses answer the same method call in their own way, so a loop over a mixed list needs no checks of the class:

Account <- suppressMessages(setRefClass("Account",
  fields = list(balance = "numeric"),
  methods = list(month_end = function() balance <<- balance - 2)
))
Savings <- suppressMessages(setRefClass("Savings", contains = "Account",
  methods = list(month_end = function() balance <<- balance * 1.01)
))
bank <- list(Account$new(balance = 100), Savings$new(balance = 100))
for (a in bank) a$month_end()
cat(vapply(bank, function(a) a$balance, numeric(1)))

Output:

98 101

Challenge

Easy

Accounts and savings accounts are Reference Classes in two files. Complete the methods:

Account in Account.R (fields owner, balance): initialize sets the balance to 0 when it is not given; deposit(x); withdraw(x) stops with insufficient funds; month_end() takes a fee of 1; describe() returns ada: 99.
Savings in Savings.R (contains Account, field rate): withdraw(x) stops with limit is 500 above 500 and otherwise calls callSuper(x); month_end() adds interest, balance * rate, instead of the fee; describe() adds (savings) with callSuper().

Use call. = FALSE. Both definitions are wrapped in suppressMessages(). The supplied code reads savings ada 0.1 or account bo, then dep 100, wd 30 or month, and prints describe() or refused: and the message after each.

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

Try it yourself

source("Account.R")
source("Savings.R")

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
p <- strsplit(input[1], " ")[[1]]
acc <- if (p[1] == "savings") Savings$new(owner = p[2], rate = as.numeric(p[3])) else Account$new(owner = p[2])
for (line in input[-1]) {
  q <- strsplit(line, " ")[[1]]
  msg <- tryCatch({
    if (q[1] == "dep") acc$deposit(as.numeric(q[2]))
    if (q[1] == "wd") acc$withdraw(as.numeric(q[2]))
    if (q[1] == "month") acc$month_end()
    acc$describe()
  }, error = function(e) paste("refused:", conditionMessage(e)))
  cat(line, ": ", msg, "\n", sep = "")
}