Passing a Scala function to a Java 8 method

Question

The following Scala code works and can be passed to a Java method expecting a function. Is there a cleaner way to do this? Here's my first pass:

val plusOne = new java.util.function.Function[Int,Int] {
  override def apply(t:Int):Int = t + 1

  override def andThen[V](after:function.Function[_ >: Int, _ <: V]):
    function.Function[Int, V] = ???

  override def compose[V](before:function.Function[_ >: V, _ <: Int]):
    function.Function[V, Int] = ???
}

Here's my second pass - it uses a generic wrapper for the Java-8 Function interface to make the Scala syntax simpler:

// Note: Function1 is Scala's functional interface,
// Function (without the 1) is Java 8's.
case class JavaFunction1[U,V](f:Function1[U,V]) extends Function[U,V] {
  override def apply(t: U): V = f(t)
  override def compose[T](before:Function[_ >: T, _ <: U]):
    Function[T, V] = ???
  override def andThen[W](after:Function[_ >: V, _ <: W]):
    Function[U, W] = ???
}

val plusOne = JavaFunction1((x:Int) => x + 1)
val minusOne = JavaFunction1((x:Int) => x - 1)

Can we do better?

As a follow-up, is there any chance that Scala will someday use the invoke-dynamic op-code as Java 8 does for its first-class function application? Will that make everything magically work, or will there still need to be a syntactic conversion?

Answer 1

You can make the conversion implicit:

implicit def toJavaFunction[U, V](f:Function1[U,V]): Function[U, V] = new Function[U, V] {
  override def apply(t: U): V = f(t)

  override def compose[T](before:Function[_ >: T, _ <: U]):
    Function[T, V] = toJavaFunction(f.compose(x => before.apply(x)))

  override def andThen[W](after:Function[_ >: V, _ <: W]):
    Function[U, W] = toJavaFunction(f.andThen(x => after.apply(x)))
}

implicit def fromJavaFunction[U, V](f:Function[U,V]): Function1[U, V] = f.apply

You shouldn't actually need to override compose and andThen, but maybe Scala compiler doesn't know about Java 8 default interface methods yet. (EDIT: it should work in 2.10.3.)

Also, you should be able to assign Scala lambdas (i.e. x => ...) to Function and any other SAM types in Scala 2.11, and Java 8 lambdas to Function1. (EDIT: this actually got added in Scala 2.12, not 2.11.)