What does this Scala type-parameter declaration mean? [A:B]

Question

Recently I've seen a trait that resembles this definition:

trait Server[T, Reader[_], Writer[_]] {
  def read[Result:Reader](t: T): Result
  def write[Result:Writer](r: Result): T
}

(The original definition is taken from the "autowire" project )

What does the [Result:Reader] and [Result:Writer] parts mean, exactly?

How is it to be interpreted? What is the relationship between Reader, Writer and Result?

Answer 1

Result - type parametrization for read and write methods. It can be T, but T is already used in trait definition, so author picked a bit longer name for type parameter.

:Reader and :Writer - means that it should Reader and Writer type classes in scope for type Result

More about context bounds can be found here: http://docs.scala-lang.org/tutorials/FAQ/context-and-view-bounds.html

This can be rewritten in this way:

trait Server[T, Reader[_], Writer[_]] {
  def read[Result](t: T)(implicit reader: Reader[Result]): Result
  def write[Result](r: Result)(implicit writer: Writer[Result]): T
}

However it's a little strange that Result is not used at all in trait definition. But totally ok.