Why cannot this.type be used for new instances

Question

I\'d like to be able to use this.type to define a method that makes new instances of an immutable case class. Something like this:

trait Expression
{
  def          


        

Answer 1

this.type is the unique type of this particular instance. It's a singleton type - a distinct type from any other instance of the same class. This works

class Foo { def f : this.type = this}

But this does not

class Foo { def f : this.type = new Foo}

this.type isn't needed that often, but it can be used to express some constraints that can't be expressed otherwise

For instance, here the Inner class says each instance's outer method will return the specific Outer instance from which it came.

scala> class Outer{ class Inner { def outer : Outer.this.type = Outer.this}; def f(x : Inner) = println("ok")}
defined class Outer

scala> val o1 = new Outer
o1: Outer = Outer@13c1b69

scala> val o2 = new Outer
o2: Outer = Outer@1a3f178


scala> val in1 = new o1.Inner
in1: o1.Inner = Outer$Inner@627b5c

scala> val in2 = new o2.Inner
in2: o2.Inner = Outer$Inner@158c3b7

scala> val o3 = in1.outer
o3: o1.type = Outer@13c1b69

scala> o1.f(new o3.Inner)  
ok

scala> o1.f(new o2.Inner)
:8: error: type mismatch;
 found   : o2.Inner
 required: o1.Inner
       o1.f(new o2.Inner)

This article has another nice example of using this.type to enable method chaining across subclass boundaries: http://scalada.blogspot.com/2008/02/thistype-for-chaining-method-calls.html

scala>   class A { def method1: this.type = this }
defined class A

scala>   class B extends A { def method2: this.type = this }
defined class B

scala> val b = new B
b: B = B@15cb235

scala> b.method1.method2
res3: b.type = B@15cb235