Trait scala.collection.generic.IsIterable

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 Iterable) 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 Iterable, but can be converted to Iterable)

IsIterable provides three members:

1. type member A, which represents the element type of the target Iterable[A]
2. type member C, which represents the type returned by transformation operations that preserve the collection’s elements type
3. method apply, which provides a way to convert between the type we wish to add extension methods to, Repr, and IterableOps[A, Iterable, C].

Usage

One must provide IsIterable 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 Iterable. In our example, this includes String.

  import scala.collection.{Iterable, IterableOps}
  import scala.collection.generic.IsIterable

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

  implicit def withExtensions[Repr](coll: Repr)(implicit it: IsIterable[Repr]): ExtensionMethods[Repr, it.type] =
    new ExtensionMethods(coll, it)

// 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 Repr, where Repr represents (typically) the collection type, and an argument it of a subtype of IsIterable[Repr]. The body of the method starts by converting the coll argument to an IterableOps in order to call the iterator method on it. The remaining of the implementation is straightforward.

The withExtensions implicit conversion makes the mapReduce operation available on any type Repr for which it exists an implicit IsIterable[Repr] instance. Note how we keep track of the precise type of the implicit it argument by using the it.type singleton type, rather than the wider IsIterable[Repr] type. We do that so that the information carried by the type members A and C of the it argument is not lost.

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 IsIterable[Repr] can be found. Given that the IsIterable companion object contains implicit members that return values of type IsIterable, this requirement is typically satisfied, and the chain of interactions described in the previous paragraph is set into action. (See the IsIterable 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 IsIterable for New Types

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

Below is an example of an implementation of the IsIterable trait where the Repr type is Range.

implicit val rangeRepr: IsIterable[Range] { type A = Int; type C = IndexedSeq[Int] } =
  new IsIterable[Range] {
    type A = Int
    type C = IndexedSeq[Int]
    def apply(coll: Range): IterableOps[Int, IndexedSeq, IndexedSeq[Int]] = coll
  }

(Note that in practice the IsIterable[Range] instance is already provided by the standard library, and it is defined as an IsSeq[Range] instance)