variant1.re

let firstBracketPair = inputChars => {
  let rec iter = (currentState, chars, currentIndex) =>
    switch (currentState, chars) {
    | (`FoundBracket(startIndex), [')', ..._]) =>
      Some((startIndex, currentIndex))
    | (_, ['(', ...tail]) =>
      iter(`FoundBracket(currentIndex), tail, currentIndex + 1)
    | (_, [_, ...tail]) =>
      iter(currentState, tail, currentIndex + 1)
    | (_, []) =>
      None
    };
  inputChars(`NoBrackets, inputChars, 0);
};