Implicit parameter for literal function

Question

While reading Play! Framework documentation, I came across this snippet:

def index = Action { implicit request =>
  session.get(\"connected\").map { user          


        

Answer 1

Consider the following code:

class MyImplicitClass(val session: Int)
object Tester {
  def apply(fun: MyImplicitClass => Int): Int = ???
  def apply(fun: => Int): Int = ???
}
Tester { implicit myImplicitClass => session * 20}

If this function:

def session(implicit myImplicitClass: MyImplicitClass): Int = myImplicitClass.session

is in scope, then the first code snippet will compile, because clearly the implicit parameter myImplicitClass will be passed to the function session in order to access the field myImplicitClass.session, allowing you to omit the field access. This is exactly the trick the Play! Framework is using, check Controller to find the session function.

As a side note, the above closure is not stating that it takes an implicit parameter, it's a language feature to avoid having to do the following:

Tester { myImplicitClass => 
  implicit val x = myImplicitClass
  session * 20
}

when one wants to use a closure parameter as an implicit value in the body of the closure. Also note that as of Scala 2.9, you are limited to closures with exactly 1 parameter for this trick.