PlayControllersWithImplicits.md

Had a WTF moment figuring out how Actions in Play Framework could be defined with and without an implicit request.

package controllers

import play.api.mvc._

object ApplicationController extends Controller {
  def index = Action {
    Ok("Without implicit request")
  }

  def indexWithImplicit = Action { implicit request => //WTF is this magic?
    Ok("With implicit request")
  }
}

Figured out it was calling two seperate apply methods. Simplified example:

object Foo {
  def apply(f: Int => String) : String = {  //accept a function as a parameter
    println("apply with function")
    val num = 42
    f(num)
  }
  
  def apply(input: String) : String = {     //accepts a string as a parameter
    println("apply with String")
    println(s"input: $input")
    input
  }
}

def withImplicit = Foo { implicit num =>    //anonymous function returning a string
  println(s"With implicit num = $num")      //`implicit num` is the same as
  num.toString                              //`num => implicit val numImp = num;`
}

def withoutImplicit = Foo {                 //block of type string
  println("Without implicit num")
  "0"
}

withImplicit      //42
withoutImplicit   //0

Yeah, total pros and cons of DSLs in action here.