Dynamic properties in Scala

Does Scala support somet开发者_开发知识库hing like dynamic properties? Example:

val dog = new Dynamic // Dynamic does not define 'name' nor 'speak'.
dog.name = "Rex" // New property.
dog.speak = { "woof" } // New method.

val cat = new Dynamic
cat.name = "Fluffy"
cat.speak = { "meow" }

val rock = new Dynamic
rock.name = "Topaz"
// rock doesn't speak.

def test(val animal: Any) = {
   animal.name + " is telling " + animal.speak()
}

test(dog) // "Rex is telling woof"
test(cat) // "Fluffy is telling meow"
test(rock) // "Topaz is telling null"

What is the closest thing from it we can get in Scala? If there's something like "addProperty" which allows using the added property like an ordinary field, it would be sufficient.

I'm not interested in structural type declarations ("type safe duck typing"). What I really need is to add new properties and methods at runtime, so that the object can be used by a method/code that expects the added elements to exist.

---

Scala 2.9 will have a specially handled Dynamic trait that may be what you are looking for.

This blog has a big about it: http://squirrelsewer.blogspot.com/2011/02/scalas-upcoming-dynamic-capabilities.html

I would guess that in the invokeDynamic method you will need to check for "name_=", "speak_=", "name" and "speak", and you could store values in a private map.

---

I can not think of a reason to really need to add/create methods/properties dynamically at run-time unless dynamic identifiers are also allowed -and/or- a magical binding to an external dynamic source (JRuby or JSON are two good examples).

Otherwise the example posted can be implemented entirely using the existing static typing in Scala via "anonymous" types and structural typing. Anyway, not saying that "dynamic" wouldn't be convenient (and as 0__ pointed out, is coming -- feel free to "go edge" ;-).

Consider:

val dog = new {
   val name = "Rex"
   def speak = { "woof" }
}

val cat = new {
   val name = "Fluffy"
   def speak = { "meow" }
}

// Rock not shown here -- because it doesn't speak it won't compile
// with the following unless it stubs in. In both cases it's an error:
// the issue is when/where the error occurs.

def test(animal: { val name: String; def speak: String }) = {
   animal.name + " is telling " + animal.speak
}

// However, we can take in the more general type { val name: String } and try to
// invoke the possibly non-existent property, albeit in a hackish sort of way.
// Unfortunately pattern matching does not work with structural types AFAIK :(

val rock = new {
   val name = "Topaz"
}

def test2(animal: { val name: String }) = {
   animal.name + " is telling " + (try {
       animal.asInstanceOf[{ def speak: String }).speak
   } catch { case _ => "{very silently}" })
}

test(dog)
test(cat)
// test(rock) -- no! will not compile (a good thing)
test2(dog)
test2(cat)
test2(rock)

However, this method can quickly get cumbersome (to "add" a new attribute one would need to create a new type and copy over the current data into it) and is partially exploiting the simplicity of the example code. That is, it's not practically possible to create true "open" objects this way; in the case for "open" data a Map of sorts is likely a better/feasible approach in the current Scala (2.8) implementation.

Happy coding.

---

First off, as @pst pointed out, your example can be entirely implemented using static typing, it doesn't require dynamic typing.

Secondly, if you want to program in a dynamically typed language, program in a dynamically typed language.

That being said, you can actually do something like that in Scala. Here is a simplistic example:

class Dict[V](args: (String, V)*) extends Dynamic {
  import scala.collection.mutable.Map

  private val backingStore = Map[String, V](args:_*)

  def typed[T] = throw new UnsupportedOperationException()

  def applyDynamic(name: String)(args: Any*) = {
    val k = if (name.endsWith("_=")) name.dropRight(2) else name
    if (name.endsWith("_=")) backingStore(k) = args.first.asInstanceOf[V]
    backingStore.get(k)
  }

  override def toString() = "Dict(" + backingStore.mkString(", ") + ")"
}

object Dict {
  def apply[V](args: (String, V)*) = new Dict(args:_*)
}

val t1 = Dict[Any]()
t1.bar_=("quux")

val t2 = new Dict("foo" -> "bar", "baz" -> "quux")
val t3 = Dict("foo" -> "bar", "baz" -> "quux")

t1.bar // => Some(quux)
t2.baz // => Some(quux)
t3.baz // => Some(quux)

As you can see, you were pretty close, actually. Your main mistake was that Dynamic is a trait, not a class, so you can't instantiate it, you have to mix it in. And you obviously have to actually define what you want it to do, i.e. implement typed and applyDynamic.

If you want your example to work, there are a couple of complications. In particular, you need something like a type-safe heterogenous map as a backing store. Also, there are some syntactic considerations. For example, foo.bar = baz is only translated into foo.bar_=(baz) if foo.bar_= exists, which it doesn't, because foo is a Dynamic object.