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
: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