scala self-types

2010-07-15 comments | programming, scala

instead of multiple inheritance, scala provide a mixin-based composition mechanism. this is usually done with traits, which is excellent for decoupling behaviours.

but sometimes you want to restrict these behaviours to a special domain. this can be done with self-types.

self-types are written as follows:

trait Foo { self: SomeType =>
  ...
}

self here refers to this, which can be very usefull in situations, where you can not easily access the particular this of the current context.
also self can be extended with type bounds. in this example, the trait Foo can only be mixed in a class which also implements SomeType. you can use this feature to restrict behaviours to a special domain.

I think the following examples are self-explanatory...

object SelfTypes {

  trait MustHave1
  trait MustHave2
  trait Independent

  // constrain the traits A, B and C
  trait A { self: MustHave1 => }
  trait B { self: MustHave2 => }
  trait C { self: MustHave1 with MustHave2 => }

  // correct usage
  class Test1 extends MustHave1 with A
  class Test2 extends MustHave2 with B
  class Test3 extends MustHave1 with MustHave2 with C
  class Test4 extends MustHave1 with Independent with A
  class Test5 extends Test1 with A
  class Test6 extends Test3 with A with B

// -- these examples do not type check
//  class Wrong1 extends A
//  class Wrong2 extends A with Independent
//  class Wrong3 extends MustHave2 with A
//  class Wrong4 extends MustHave1 with C

  // mixing the behaviours Resize and ColorCount in our domain Image
  case class Image(width: Int, height: Int, buf: Array[Byte])

  trait Resize { self: Image =>
    def resize(newWidth: Int, newHeight: Int): Image = {
      // a really stupid implementation
      Image(newWidth, newHeight, buf.slice(0, newWidth * newHeight))
    }
  }

  trait ColorCount { self: Image =>
    def count(color: Byte) = buf.foldLeft(0) {(a, v) => if(v == color) a + 1 else a }
  }

  def main(args: Array[String]) = {
    val i = new Image(5, 3, Array(1,2,2,2,2,1,3,3,3,3,1,2,2,2,2))
                  with Resize
                  with ColorCount
    val n = i.resize(3, 1)
    assert(n.width == 3)
    assert(n.height == 1)
    assert(n.buf.size == (n.width * n.height))
    assert(i.count(1) == 3)
    assert(i.count(2) == 8)
    assert(i.count(3) == 4)
  }
}

hth