Bank ledger in Mojo

An account struct that guards its own balance and keeps a running history.

struct Account(Movable):
    var owner: String
    var cents: Int
    var history: List[String]

    def __init__(out self, var owner: String, opening: Int):
        self.owner = owner^
        self.cents = opening
        self.history = List[String]()

    def deposit(mut self, amount: Int) raises:
        if amount <= 0:
            raise Error("deposit must be positive")
        self.cents += amount
        self.history.append("deposit " + String(amount))

    def withdraw(mut self, amount: Int) raises:
        if amount > self.cents:
            raise Error("insufficient funds")
        self.cents -= amount
        self.history.append("withdraw " + String(amount))

    def balance(self) -> String:
        return String(self.cents // 100) + "." + String(self.cents % 100)


def attempt(mut account: Account, action: String, amount: Int):
    try:
        if action == "deposit":
            account.deposit(amount)
        else:
            account.withdraw(amount)
        print("ok      ", action, amount)
    except e:
        print("rejected", action, amount, "--", String(e))


def main():
    var account = Account("Ada", 5000)
    attempt(account, "deposit", 1250)
    attempt(account, "withdraw", 800)
    attempt(account, "deposit", -40)
    attempt(account, "withdraw", 99999)
    attempt(account, "deposit", 375)

    print("")
    print("ledger for", account.owner)
    for entry in account.history:
        print("  ", entry)
    print("closing balance:", account.balance())

How it works

1. Account owns its owner name and a List[String] of entries.
2. deposit and withdraw are marked raises and reject bad amounts with Error.
3. attempt wraps each call in try, so a rejection prints instead of stopping the run.
4. The closing report replays the history and formats the balance in dollars.

Keywords and builtins used here

• Error
• Int
• List
• Movable
• String
• attempt
• balance
• def
• deposit
• else
• except
• for
• if
• main
• print
• raise
• return
• self
• struct
• try
• var
• withdraw

The run, in numbers

Lines

50

Characters to type

1256

Tokens

351

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 281 seconds.

Type this snippet

Step 1 of 2 in Encore, step 21 of 22 in Structs & traits.

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