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author | Heather Miller <heather.miller@epfl.ch> | 2013-01-09 22:08:56 +0100 |
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committer | Heather Miller <heather.miller@epfl.ch> | 2013-01-09 22:08:56 +0100 |
commit | 79a722feb99d6aeccb9ffcb691601c3b907b9d93 (patch) | |
tree | 4b0d0d773791d2db24cd523c02646ff88c823783 /src/library | |
parent | f219ade08ed8174c7bded654e8070f4b61843513 (diff) | |
download | scala-79a722feb99d6aeccb9ffcb691601c3b907b9d93.tar.gz scala-79a722feb99d6aeccb9ffcb691601c3b907b9d93.tar.bz2 scala-79a722feb99d6aeccb9ffcb691601c3b907b9d93.zip |
SI-6946, SI-6924 Greatly improves IsTraversableLike docs
Diffstat (limited to 'src/library')
-rw-r--r-- | src/library/scala/collection/generic/IsTraversableLike.scala | 101 |
1 files changed, 86 insertions, 15 deletions
diff --git a/src/library/scala/collection/generic/IsTraversableLike.scala b/src/library/scala/collection/generic/IsTraversableLike.scala index b45279229b..c70772d8f9 100644 --- a/src/library/scala/collection/generic/IsTraversableLike.scala +++ b/src/library/scala/collection/generic/IsTraversableLike.scala @@ -9,26 +9,97 @@ package scala.collection package generic -/** Type class witnessing that a collection representation type `Repr` has - * elements of type `A` and has a conversion to `GenTraversableLike[A, Repr]`. +/** 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 `String`s (`String`s + * do not extend `GenTraversableLike`, but can be converted to `GenTraversableLike`) * - * This type enables simple enrichment of `GenTraversable`s with extension - * methods which can make full use of the mechanics of the Scala collections - * framework in their implementation. + * `IsTraversable` provides two members: + * + * 1. type member `A`, which represents the element type of the target `GenTraversableLike[A, Repr]` + * 1. 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`. * - * Example usage, * {{{ - * class FilterMapImpl[A, Repr](val r: GenTraversableLike[A, Repr]) { - * final def filterMap[B, That](f: A => Option[B])(implicit cbf: CanBuildFrom[Repr, B, That]): That = - * r.flatMap(f(_).toSeq) + * 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 filterMap[Repr, A](r: Repr)(implicit fr: IsTraversableOnce[Repr]): FilterMapImpl[fr.A,Repr] = - * new FilterMapImpl(fr.conversion(r)) * - * val l = List(1, 2, 3, 4, 5) - * List(1, 2, 3, 4, 5) filterMap (i => if(i % 2 == 0) Some(i) else None) - * // == List(2, 4) - * }}} + * 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]] + * } + *}}} * * @author Miles Sabin * @author J. Suereth |