Question

Is it possible to map the key value pairs of a Map to a Scala constructor with named parameters?

That is, given

class Person(val firstname: String, val lastname: String) {
    ...
}

… how can I create an instance of Person using a map like

val args = Map("firstname" -> "John", "lastname" -> "Doe", "ignored" -> "value")

What I am trying to achieve in the end is a nice way of mapping Node4J Node objects to Scala value objects.

Answer 1

The key insight here is that the constructor arguments names are available, as they are the names of the fields created by the constructor. So provided that the constructor does nothing with its arguments but assign them to fields, then we can ignore it and work with the fields directly.

We can use:

def setFields[A](o : A, values: Map[String, Any]): A = {
  for ((name, value) <- values) setField(o, name, value)
  o
}

def setField(o: Any, fieldName: String, fieldValue: Any) {
  // TODO - look up the class hierarchy for superclass fields
  o.getClass.getDeclaredFields.find( _.getName == fieldName) match {
    case Some(field) => {
      field.setAccessible(true)
      field.set(o, fieldValue)
    }
    case None =>
      throw new IllegalArgumentException("No field named " + fieldName)
  }

Which we can call on a blank person:

test("test setFields") {
  val p = setFields(new Person(null, null, -1), Map("firstname" -> "Duncan", "lastname" -> "McGregor", "age" -> 44))
  p.firstname should be ("Duncan")
  p.lastname should be ("McGregor")
  p.age should be (44)
}

Of course we can do better with a little pimping:

implicit def any2WithFields[A](o: A) = new AnyRef {
  def withFields(values: Map[String, Any]): A = setFields(o, values)
  def withFields(values: Pair[String, Any]*): A = withFields(Map(values :_*))
}

so that you can call:

new Person(null, null, -1).withFields("firstname" -> "Duncan", "lastname" -> "McGregor", "age" -> 44)

---

If having to call the constructor is annoying, Objenesis lets you ignore the lack of a no-arg constructor:

val objensis = new ObjenesisStd 

def create[A](implicit m: scala.reflect.Manifest[A]): A = 
  objensis.newInstance(m.erasure).asInstanceOf[A]

Now we can combine the two to write

create[Person].withFields("firstname" -> "Duncan", "lastname" -> "McGregor", "age" -> 44)