How to fix an exponential slowness due to backtracking? about fastparse

[winitzki]

I tried memoization like this:

    val cache_minus = mutable.Map[Int, P[Int]]()
    val cache_plus  = mutable.Map[Int, P[Int]]()

    def x_minus[$](implicit p: P[$]): P[Int] = cache_minus.getOrElseUpdate(p.index, P(x_times ~ "-" ~ expr).map { case (x, y) => x - y })
    def x_plus[$](implicit p: P[$]): P[Int]  = cache_plus.getOrElseUpdate(p.index, P(x_times ~ ("+" ~ expr).rep).map { case (i, is) => i + is.sum })
   // Other grammar rules remain unchanged.

This fails whenever the input contains parentheses:

parse("123*(1+1)", program(_))
// 
java.lang.ClassCastException: class scala.runtime.BoxedUnit cannot be cast to class java.lang.Integer (scala.runtime.BoxedUnit is in unnamed module of loader 'app'; java.lang.Integer is in module java.base of loader 'bootstrap')

	at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:99)

But I was unable to find the code that does this unboxing to Int. Probably, this code is generated by a macro. In any case, I don't know how to fix this problem.