IsTraversableLike

A trait which can be used to avoid code duplication when defining extension methods that should be applicable both to existing Scala collections (i.e., types extending GenTraversableLike) as well as other (potentially user-defined) types that could be converted to a Scala collection type. This trait makes it possible to treat Scala collections and types that can be implicitly converted to a collection type uniformly. For example, one can provide extension methods that work both on collection types and on Strings (Strings do not extend GenTraversableLike, but can be converted to GenTraversableLike)

IsTraversable provides two members:

1. type member A, which represents the element type of the target GenTraversableLike[A, Repr]
2. value member conversion, which provides a way to convert between the type we wish to add extension methods to, Repr, and GenTraversableLike[A, Repr].

Usage

One must provide IsTraversableLike as an implicit parameter type of an implicit conversion. Its usage is shown below. Our objective in the following example is to provide a generic extension method mapReduce to any type that extends or can be converted to GenTraversableLike. In our example, this includes String.

  import scala.collection.GenTraversableLike
  import scala.collection.generic.IsTraversableLike

  class ExtensionMethods[A, Repr](coll: GenTraversableLike[A, Repr]) {
    def mapReduce[B](mapper: A => B)(reducer: (B, B) => B): B = {
      val iter = coll.toIterator
      var res = mapper(iter.next())
      while (iter.hasNext)
        res = reducer(res, mapper(iter.next()))
      res
    }
  }

  implicit def withExtensions[Repr](coll: Repr)(implicit traversable: IsTraversableLike[Repr]) =
    new ExtensionMethods(traversable.conversion(coll))

// See it in action!
List(1, 2, 3).mapReduce(_ * 2)(_ + _) // res0: Int = 12
"Yeah, well, you know, that's just, like, your opinion, man.".mapReduce(x => 1)(_ + _) // res1: Int = 59

Here, we begin by creating a class ExtensionMethods which contains our mapReduce extension method. Note that ExtensionMethods takes a constructor argument coll of type GenTraversableLike[A, Repr], where A represents the element type and Repr represents (typically) the collection type. The implementation of mapReduce itself is straightforward.

The interesting bit is the implicit conversion withExtensions, which returns an instance of ExtensionMethods. This implicit conversion can only be applied if there is an implicit value traversable of type IsTraversableLike[Repr] in scope. Since IsTraversableLike provides value member conversion, which gives us a way to convert between whatever type we wish to add an extension method to (in this case, Repr) and GenTraversableLike[A, Repr], we can now convert coll from type Repr to GenTraversableLike[A, Repr]. This allows us to create an instance of the ExtensionMethods class, which we pass our new GenTraversableLike[A, Repr] to.

When the mapReduce method is called on some type of which it is not a member, implicit search is triggered. Because implicit conversion withExtensions is generic, it will be applied as long as an implicit value of type IsTraversableLike[Repr] can be found. Given that IsTraversableLike contains implicit members that return values of type IsTraversableLike, this requirement is typically satisfied, and the chain of interactions described in the previous paragraph is set into action. (See the IsTraversableLike companion object, which contains a precise specification of the available implicits.)

Note: Currently, it's not possible to combine the implicit conversion and the class with the extension methods into an implicit class due to limitations of type inference.

Implementing IsTraversableLike for New Types

One must simply provide an implicit value of type IsTraversableLike specific to the new type, or an implicit conversion which returns an instance of IsTraversableLike specific to the new type.

Below is an example of an implementation of the IsTraversableLike trait where the Repr type is String.

implicit val stringRepr: IsTraversableLike[String] { type A = Char } =
  new IsTraversableLike[String] {
    type A = Char
    val conversion = implicitly[String => GenTraversableLike[Char, String]]
  }