type monthlyInterestRate = | MonthlyInterestRate(float);
type monthlyPaymentsRemaining = | MonthlyPaymentsRemaining(int);
type amountBorrowed = | AmountBorrowed(float);

type mortgageCalculator = {
  monthlyInterestRate: float,
  monthlyPaymentsRemaining: int,
  amountBorrowed: float
};

let calcWithRecord = (mc) => {
  (mc.monthlyInterestRate *. mc.amountBorrowed) /. (1.0 -. (1.0 +. mc.monthlyInterestRate) ** (-1.0 *. float_of_int(mc.monthlyPaymentsRemaining)))
};

let calcWithUnion = (mir, mpr, ab) => {
  let local_mir = switch mir {
    | MonthlyInterestRate(mir) => mir
  };
  let local_mpr = switch mpr {
    | MonthlyPaymentsRemaining(nmp) => nmp
  };
  let local_ab = switch ab {
    | AmountBorrowed(ab) => ab
  };
  (local_mir *. local_ab) /. (1.0 -. (1.0 +. local_mir) ** (-1.0 *. float_of_int(local_mpr)))
};

let mc = {
  monthlyInterestRate: 0.003,
  monthlyPaymentsRemaining: 12 * 30,
  amountBorrowed: 200000.0
};

Js.log(calcWithRecord(mc));
Js.log(calcWithUnion(MonthlyInterestRate(0.003), MonthlyPaymentsRemaining(12 * 30), AmountBorrowed(200000.0)));