idea for safe repetition on Parser0

We don't allow p.rep on a p: Parser0[_] because if you parse nothing and that is valid, at parse time you would loop infinitely not making progress.

Another idea could be:

def safeRep: Parser0[(NonEmptyList[A], Boolean)] =
  (Parser.index, this, Parser.index)
    .tupled
    .flatMap {
      case (s, a, e) =>
         if (s == e) {
           // No progress, stop
           Parser.pure((NonEmptyList(a, Nil), false))
         }
         else safeRep.?.map {
           case None => (NonEmptyList(a, Nil), true)
           case Some((tail, r)) => (a :: tail, r)
         }
     }

the boolean tells if the last item made progress or not.

Start by reviewing Parser0 and the existing p.rep behavior described in the issue. Examine how Parser.index, tupled, flatMap, Parser.pure, and safeRep.?. would compose, then determine whether the proposed safe repetition semantics prevent non-progress loops while preserving the boolean result. Done means the design is agreed and the safe repetition behavior is implemented and verified.