How do you explicitly specify a parameterized type for an existential type in Scala?

Question

In Programming in Scala, the following example is given to show how to reference a Java class with wildcards. The method javaSet2ScalaSet takes a type T. Typically, you can always explicitly supply a parameterized type. But in this case where (new Wild).contents is existential, a normal type parameter is not acceptable.

The compiler is able to perform some magic and infer an appropriate parameter when javaSet2ScalaSet is called without a type parameter. Viewing what the compiler supplies with scalac –Xprint:typer shows that it assigns a value of ?0 for T. This can’t be provided manually.

Is this a special case that only works for an inferred type, or is there some way to explicitly supply the type?

  // This is a Java class with wildcards
  public class Wild {
    Collection<?> contents() {
      Collection<String> stuff = new Vector<String>();
      stuff.add("a");
      stuff.add("b");
      stuff.add("see");
      return stuff;
    }
  }

  import scala.collection.mutable.Set
  import java.util.Collection

  abstract class SetAndType {
    type Elem
    val set: Set[Elem]
  }

  def javaSet2ScalaSet[T](jset: Collection[T]): SetAndType = {
    val sset = Set.empty[T]  // now T can be named!

    val iter = jset.iterator
    while (iter.hasNext)
      sset += iter.next()

    return new SetAndType {
      type Elem = T
      val set = sset
    }
  }

val setAndType = javaSet2ScalaSet((new Wild).contents)

You can write javaSet2ScalaSet[T forSome {type T}] (which is the same as javaSet2ScalaSet[_]). Is that what you mean? — Alexey Romanov
You can't write javaSet2ScalaSet[T forSome {type T}]. The compiler complains with unbound wildcard type. Same with javaSet2ScalaSet[_]). — ToddLC

Daniel C. Sobral Daniel C. Sobral · Accepted Answer · 2011-08-04T18:46:47

It's just a syntactic limitation, not an inherent one. Here's how you can declare it:

def javaSet2ScalaSet[C <: Collection[T] forSome { type T }](jset: C): SetAndType = {
  val sset = Set.empty[T forSome { type T }]

  val iter = jset.iterator
  while (iter.hasNext)
    sset += iter.next()

  return new SetAndType {
    type Elem = T forSome { type T }
    val set = sset
  }
}

How do you explicitly specify a parameterized type for an existential type in Scala?

2 Answers