interface Money {
  __type: 'money';
  amount: number;
  unit: string;
}

interface Liter {
  __type: 'liter';
  amount: number;
  unit: string;
}

function moneyOrLiter(param: Money | Liter) {
  if (param.__type === 'money') {
    param;
  } else {
    param;
  }
}
// (parameter) param: Money
// (parameter) param: Liter