Practical uses of a Dynamic type in Scala

Question

Besides integration with dynamic languages on the JVM, what are the other powerful uses of a Dynamic type in a statically typed language like Scala?

Answer 1

I guess a dynamic type could be used to implement several of the features found in JRuby, Groovy or other dynamic JVM languages, like dynamic metaprogramming and method_missing.

For example creating a dynamic query similar to Active Record in Rails, where a method name with parameters is translated to an SQL query in the background. This is using the method_missing functionality in Ruby. Something like this (in theory - have not tried to implement anything like this):

class Person(val id: Int) extends Dynamic {
  def _select_(name: String) = {
    val sql = "select " + name + " from Person where id = " id;
    // run sql and return result
  }

  def _invoke_(name: String)(args: Any*) = {
    val Pattern = "(findBy[a-zA-Z])".r
    val sql = name match {
      case Pattern(col) => "select * from Person where " + col + "='" args(0) + "'"
      case ...
    }
    // run sql and return result
  }
}

Allowing usage like this, where you can call methods 'name' and 'findByName' without having them explicitly defined in the Person class:

val person = new Person(1)

// select name from Person where id = 1
val name = person.name

// select * from Person where name = 'Bob'
val person2 = person.findByName("Bob")

If dynamic metaprogramming was to be added, the Dynamic type would be needed to allow invoking methods that have been added during runtime..

Answer 2

Odersky says the primary motivation was integration with dynamic languages: http://groups.google.com/group/scala-language/msg/884e7f9a5351c549

[edit] Martin further confirms this here

Answer 3

You might also use it for syntactic sugar on maps:

class DynamicMap[K, V] extends Dynamic {
  val self = scala.collection.mutable.Map[K, V]()
  def _select_(key: String) = self.apply(key)
  def _invoke_(key: String)(value: Any*) = 
    if (value.nonEmpty) self.update(key, value(0).asInstanceOf[V])
    else throw new IllegalArgumentException
}

val map = new DynamicMap[String, String]()

map.foo("bar")  // adds key "foo" with value "bar"    
map.foo         // returns "bar"

To be honest this only saves you a couple of keystrokes from:

val map = new Map[String, String]()
map("foo") = "bar"
map("foo")