// @flow
export opaque type AccountBalance = number;
export opaque type AccountNumber: number = number;
export opaque type PaymentAmount: number = number;
// export opaque type AccountNumber = number;
// export opaque type PaymentAmount = number;

type Account = {
  accountNumber: AccountNumber,
  balance: AccountBalance,
};

function getAccount(accountNumber: AccountNumber): Account {
  return { balance: 100, accountNumber };
}

export function validateAccountNumber(input: number): AccountNumber {
  if (Math.floor(input) !== input) {
    throw new Error('Invalid account number, must be an integer!');
  }
  return input;
}

export function validatePaymentAmount(input: number): PaymentAmount {
  if (typeof input !== 'number' || isNaN(input)) {
    throw new Error('PaymentAmount must be a valid number');
  }
  if (input <= 0) {
    throw new Error("Can't pay zero or less!");
  }
  return input;
}

export function spend(accountNumber: AccountNumber, amount: PaymentAmount) {
  const account = getAccount(accountNumber);
  account.balance -= amount;
}