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..