Scala flatMap DSL syntax challenge

flatMapSyntax.scala

// The goal is to find "the best" syntax for line #21

trait M[A] {
  def map[B](f: A => B): M[B]
  def flatMap[B](f: A => M[B]): M[B]
}

class UseParams2[P1, P2](p1: M[P1], p2: M[P2]) {
  def toCall[R](f: Function2[P1, P2, R]): M[R] =
    for {
      v1 <- p1
      v2 <- p2
      r = f(v1, v2)
    } yield r
}

object UltimateSyntax {
  def entry(a: M[Int], b: M[String]): M[Boolean] = {
    // How can I make this look better?
    // Can I somehow write "logic(a, b)"?
    useParams(a, b).toCall(logic)
  }

  private def logic(a: Int, b: String): Boolean = b.toString.length < b.length
  private def useParams[P1, P2](p1: M[P1], p2: M[P2]) = new UseParams2[P1, P2](p1, p2)
}