What is the syntax meaning of “`class declaration head` { val_name : Type => `class body` }”

Question

When reading some articles about Scala, I found some examples with a curious syntax, which I might understand incorrectly

class Child[C <: Child[C]]         


        

Answer 1

It is a self type annotation.

This means that class Child must be of type C, i.e., creates inheritance dependencies which must satisfied for a given class.

A small example:

scala> trait Baz
defined trait Baz


scala> class Foo {
     | self:Baz => 
     | }
defined class Foo


scala> val a = new Foo
<console>:9: error: class Foo cannot be instantiated because it does not conform to its self-type Foo with Baz
       val a = new Foo
               ^

scala> val a = new Foo with Baz
a: Foo with Baz = $anon$1@199de181


scala> class Bar extends Foo with Baz
defined class Bar

In this case Foo is required to also be a Baz. Satisfying that requirement, a Foo instance can be created. Also, defining a new class (in this case Bar) there is also the requirement of it being Baz as well.

See: http://www.scala-lang.org/node/124

Answer 2

One very useful application of self types is a less verbose implementation of the CRTP ( http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern ), e.g.

abstract class Base[Sub] {
  self:Sub =>
  def add(s:Sub) : Sub
}

case class Vec3(x:Double,y:Double,z:Double) extends Base[Vec3] {
  def add(that:Vec3) = Vec3(this.x+that.x, this.y+that.y, this.z+that.z)
}

Attempts to "cheat" with the inheritance won't work:

class Foo extends Base[Vec3] {
  add(v:Vec3) = v
}

//error: illegal inheritance;
//self-type Foo does not conform to Base[Vec3]'s selftype Base[Vec3] with Vec3
//      class Foo extends Base[Vec3] {

Answer 3

In addition to the inheritance requirement in JaimeJorge's response, self types can be used to give the outer instance a name if you want to refer to it from an inner class:

scala> class Company(name: String) {
     |   company =>
     |   class Department(name: String) {
     |     override def toString = "Department "+ name +" of "+ company.name
     |   }
     | }
defined class Company

scala> val c = new Company("ACME")
c: Company = Company@56a57bb2

scala> val d = new c.Department("Marketing")
d: c.Department = Department Marketing of ACME