Scala function that returns an anonymous object?

Question

I want to do some funky closure-like stuff. I want a method to return an anonymous object whose guts make reference to the parameters of the method. Here is the code that I wrote that illustrates my intent:

object SessionManagement {

  implicit class SessionManagementExtensions( val c : ChainBuilder ) {

    def set( dest: String ) = object {

      def from( src: String ) =
        c.exec( session => {
          val list = session( src ).as[Vector[String]]
          val i = if ( list.size == 0 ) -1 else Random.nextInt( list.size )
          val value = if ( i > 0 ) list(i) else "INVALID_" + dest
          session.set( dest, value )
        })

      def to[T]( v: Expression[T] ) =
        c.exec( session => session.set( dest, v ) )

    }

}

What I'm TRYING to do is have a call to "set" return an object that allows me to then chain together a call to ".to", like so:

.set( SOMETHING ).to( OTHER )

But I can't say

def foo = object { ... }

Is there a way in Scala to get what I am after? Do I have to define a class and instantiate it?

Answer 1

You can simply return a new anonymous object. Your syntax was almost right: just replace object by new:

def set( dest: String ) = new {
  def from( src: String ) =
    ...

  def to[T]( v: Expression[T] ) =
    ...
}

However, this will give a structural type at call site, which will have to use reflection to use the methods. To prevent this, define a trait with the API of your object:

trait SetResult {
  def from(src: String): ReturnTypeOfFrom
  def to[T](v: Expression[T]): ReturnTypeOfTo
}

def set( dest: String ): SetResult = new SetResult {
  def from( src: String ) =
    ...

  def to[T]( v: Expression[T] ) =
    ...
}

Note that I used ReturnTypeOfFrom and ReturnTypeOfSet because I've no idea what your methods return. Btw, this is a bad habit: public methods should always have an explicit result type.