bank.kt

/* 1) Domain entities: Bank and Account */
typealias Amount = BigDecimal
​
data class Bank(val accounts: MutableMap<String, Account> = HashMap())
​
data class Account(val id: String, val name: String) {
    var balance: Amount by TxDelegate(initialValue = Amount.ZERO) { 
        // triggers rollback on validation failure
        it >= Amount.ZERO 
    }
}
​

/* 2) Commands: CreateAccount, Deposit, Withdrawal, Transfer */
data class CreateAccount(val id: String, val name: String) : BankCommand {
    override fun applyTo(bank: Bank) {
        bank.accounts[id] = Account(id, name)
    }
} 
  
data class Deposit(override val accountId: 
                   String, val amount: Amount) : AccountCommand {
    override fun applyTo(account: Account) {
        account.balance += amount
    }
}

data class Withdrawal(override val accountId: 
                      String,val amount: Amount) : AccountCommand {
    override fun applyTo(account: Account) {
        account.balance -= amount
    }
}

data class Transfer(val fromAccountId: String, 
                    val toAccountId: String, 
                    val amount: Amount) : BankCommand {
    override fun applyTo(bank: Bank) {
        Deposit(toAccountId, amount).applyTo(bank)
        Withdrawal(fromAccountId, amount).applyTo(bank)
    }
}