Signatures

type Account {
    owner: String
    balance: Decimal
}

fn withdraw(account: Account, amount: Decimal) -> Account
    require amount > 0
    require amount <= account.balance
    ensure result.balance == account.balance - amount
{
    account with { balance: account.balance - amount }
}

fn main() {
    const before = Account(owner: "Asha", balance: 500)
    const after = withdraw(before, 120)
    print(after.balance)
}