Course of Raku / Addendum 🆕 / Types and text machines / Objects and classes / Exercises / A bank account

Solution: A bank account

Here is a possible solution to the task.

Code

class Account {
    has $!balance = 0;

    method deposit($amount) {
        $!balance += $amount;
    }

    method withdraw($amount) {
        if $amount > $!balance {
            say 'declined: insufficient funds';
        }
        else {
            $!balance -= $amount;
        }
    }

    method balance { $!balance }
}

my $account = Account.new;
$account.deposit(100);
$account.withdraw(30);
$account.withdraw(200);

say $account.balance;

🦋 You can find the source code in the file bank-account.raku.

Output

declined: insufficient funds
70

Comments

1. has $!balance = 0 declares the attribute private — with the ! twigil it has no generated accessor at all, so the only way to change the balance is through the methods the class provides. A method balance gives the outside world a way to read it, but no way to set it.

2. This is deliberate. Had we written has $.balance, the . twigil would add a public accessor — read-only by default, but one careless is rw on it would expose direct assignment like $account.balance = 1_000_000, letting anyone move money without going through deposit or withdraw. Keeping the attribute private rules that out by construction: the balance can only ever change by the account’s own rules, such as the insufficient-funds check in withdraw.

3. The second withdrawal asks for more than the balance, so it is declined and the balance stays at 70.

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