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scalacollectionstraversable

TraversableOnce MonadOps in Scala collection library

I'm looking at Scala 2.12 Collection Library sources and noticed the following thing:

trait TraversableOnce[+A] extends Any with GenTraversableOnce[A] {

  //methods

  implicit class MonadOps[+A](trav: TraversableOnce[A]) {
    def map[B](f: A => B): TraversableOnce[B] = trav.toIterator map f
    def flatMap[B](f: A => GenTraversableOnce[B]): TraversableOnce[B] = trav.toIterator flatMap f
    def withFilter(p: A => Boolean) = trav.toIterator filter p
    def filter(p: A => Boolean): TraversableOnce[A] = withFilter(p)
  }
}

What is the reason the Monad Ops are put into implicit class? The Traversable subtrait of TraversableOnce has this all monadic methods defined not as imlpicit class (through TraversableLike though).

Solution

  • Iterator, as something we only expect to traverse once, is an example of something that extends TraversableOnce without extending Traversable. The methods on MonadOps will be used there.

    Note MonadOps is in the companion object, not the trait.

    Also note TraversableOnce has been deprecated in 2.13.0 in favour of IterableOnce.