Trouble with OCaml nested if

With my original code I kept getting Error: Parse error: [expr level ;] expected after "in" (in [expr]) on the line let numDigits = numDigits - 1 in

Original:

let rec rev_int num =
  if num / 10 == 0 then
     num
  else
    let temp = num mod 10 in

    let numDigits = String.length(string_of_int num) - 1 in

    if num < 0 then
      let numDigits = numDigits - 1 in
    else
      let numDigits = numDigits + 0 in

    let num = (num - temp) / 10 in
    temp * int_of_float(10.0 ** float_of_int numDigits) + rev_int num

With variations of:

if num < 0 then
   let numDigits = numDigits - 1 in;
else
   let numDigits = numDigits + 0 in;

---

if num < 0 then
   let numDigits = numDigits - 1 in
else begin
   let numDigits = numDigits + 0 in end

I revised the code and now it works, but I was wondering if there was a way to do it with nested if and less redundancy.

Revised:

let rec rev_int num =
  if num / 10 == 0 then
    num
  else
    let temp = num mod 10 in

    let numDigits = String.length(string_of_int num) - 1 in

    if num < 0 then
      let numDigits = numDigits - 1 in
      let num = (num - temp) / 10 in
      temp * int_of_float(10.0 ** float_of_int numDigits) + rev_int num
    else
      let numDigits = numDigits + 0 in
      let num = (num - temp) / 10 in
      temp * int_of_float(10.0 ** float_of_int numDigits) + rev_int num