fold.scala

import cats._
// import cats._
object f1 {
  trait Fold[I, O] {
    type M
    def tally(i: I): M
    def summarize(m: M): O
    def monoid: Monoid[M]
  }
  object Fold {
    def runList[I, O](fold: Fold[I, O], data: List[I]): O = {
      ???
    }
  }
}