scala macros: Add function to class

问题

I'm new to scala macros and I'm using scala 2.10.0-RC3.

I want to write a macro that adds a function to a class. Usage example:

trait MyTrait {
  def addF = macro { /*add "def f = 3" to class*/ }
}

class MyClass extends MyTrait {
  addF //Adds the "def f" to MyClass
}

object Main {
  val t = new MyClass
  assert(t.f==3)
}

I need this in the following scenario. My first try didn't use macros but didn't work, because I can't inherit the same trait twice.

trait AddF[T] {
  def f(t: T) { /* ...do sthg ... */ }
}

class MyClass extends AddF[Int] with AddF[String]

With the macro solution I could write

class MyClass extends MyTrait {
  addF[Int]()
  addF[String]()
}

Is there a way to do this with scala macros? Or is there another way to achieve this?

回答1:

It is currently impossible to add, modify or remove definitions visible outside the macro. I.e. you can create a class or a method local to the expansion (e.g. emit a ClassDef tree as a part of the result returns by your macro), but there's no facility to affect the outside world.

However we plan to experiment with this functionality as roughly sketched in http://scalamacros.org/future.html. Also there's already a solid prototype that can generate new top-level classes. Contact me for details if you would like to take a look.

回答2:

In case I'm not completely confused, simple overloading should provide the desired behavior? For instance, this would work:

trait MyTrait {
  def f(i: Int)
  def f(i: String)
}

class MyClass extends MyTrait {
  def f(i: Int) {
    println(i + " was an Int")
  }
  def f(s: String) {
    println(s + " was a String")
  }
}

// this allows:
val c = new MyClass()
c.f("hello")
c.f(42)

来源：https://stackoverflow.com/questions/13630366/scala-macros-add-function-to-class

标签

scala

reflection

metaprogramming

scala-2.10

scala-macros