scala - Explain "this.type" as a return type of various API methods -

i looking though source code of promise[t] trait scala.concurrent package. here method declaration need in 1 place:

trait promise[t] {  ....  def complete(result: try[t]): this.type =  if (trycomplete(result)) else throw new illegalstateexception("promise completed.")  .... } 

i not understand how interpret this.type complete method return type. why not simple return type promise[t] ?

sorry if question seem simple someone, learning stuff.

this.type necessary in path dependent context:

scala> class { class b; def f(b: b): = } defined class  scala> val a1 = new a; val b1 = new a1.b; val a2 = new a1: = a@721f1edb b1: a1.b = a$b@5922f665 a2: = a@65e8e9b  scala> a1.f(b1) res6: = a@721f1edb  scala> a2.f(b1) <console>:12: error: type mismatch;  found   : a1.b  required: a2.b               a2.f(b1)                    ^ 

path dependent means compiler knows types belong together. in above example 1 can see new a1.b produces value of type a1.b , not b. however, not work:

scala> a1.f(b1).f(b1) <console>:11: error: type mismatch;  found   : a1.b  required: _2.b val _2:               a1.f(b1).f(b1)                          ^ 

the problem return type of f a, has no information anymore path dependent relationship. returning this.type instead tells compiler return type fulfills relationship:

scala> class { class b; def f(b: b): this.type = } defined class  scala> val a1 = new a; val b1 = new a1.b; val a2 = new a1: = a@60c40d9c b1: a1.b = a$b@6759ae65 a2: = a@30c89de5  scala> a1.f(b1).f(b1) res10: a1.type = a@60c40d9c