Love the index-passing approach! Combinator ideas?

Hey! Saw src/index.ts. Passing pos is definitely the way to go for perf. startsWith(str, pos) is a gem.

Are you planning to add map or sequence? I use a similar pattern in my stream utils and avoiding string slicing is huge for throughput.

Maybe something like:

export const map = <T, U>(parser: Parser<T>, fn: (t: T) => U): Parser<U> => (input, pos) => {
  const result = parser(input, pos);
  if (!result.ok) return result;
  return ok([fn(result.value[0]), result.value[1]]);
};