monadic.scala

case class Person(name: String, age: Int)
case class Couple(p1: String, p2: String)

def sameAgeCouples(
  people: Dataset[Person], 
  couples: Dataset[Couple]
): Dataset[Couple] =
  run {
    for {
      p1 <- liftQuery(people)
      p2 <- liftQuery(people)
      c <-  liftQuery(couples) 
         if (c.p1 == p1.name && 
             c.p2 == p2.name && 
             p1.age == p2.age)
    } yield {
      c
    }
  }