Explain request passing in play framework

Question

I was trying this in playframework. When I came through this part, at first it seemed pretty straightforward but while trying to trace it I'm not able to understand how this works. I know what kind of parameter is accepted by apply method of Action but I couldn't get how request is available and how it can be passed to Ok. Can anyone show analogous example with simple definition in scala.

@Singleton
class HomeController @Inject()(configuration: Configuration, cc: ControllerComponents)(implicit assetsFinder: AssetsFinder)
  extends AbstractController(cc) {
  def tweets = Action { implicit request =>
    Ok(s"request is $request")
  }
}

Thanks in advance

Have a look at the scaladoc. https://www.playframework.com/documentation/2.6.x/api/scala/index.html#play.api.mvc.Action — mfirry, Apr 09 '18 at 09:51

Dmytro Mitin · Answer 1 · 2018-04-09T13:15:58.920

Object Action extends trait DefaultActionBuilder extending trait ActionBuilder. The latter has method

apply(block: R[B] => Result): Action[B]

In your case request is of type Request[AnyContent] i.e. R is Request and B is AnyContent, Ok(s"request is $request") is of type Result,

Action { implicit request =>
  Ok(s"request is $request")
}

is of type Action[B] i.e. Action[AnyContent]. So the syntax is just apply method of an object and a lambda as an argument of the method.

What is the apply function in Scala?

foo(implicit x => ???) is the same as foo(x => { implicit val x_ = x; ??? }).

Implicit keyword before a parameter in anonymous function in Scala

Ok is just new Status(OK) i.e. new Status(200) and class Status has method

def apply[C](content: C)(implicit writeable: Writeable[C]): Result

i.e. C is now String and content is s"request is $request" (i.e. string "request is " + request.toString).

If you use IDE you can investigate similar inheritance hierarchies and types yourself.

Thank you for the answer. The real thing I need to know is according to which scala construct are we able to pass request to Ok? — pramesh, Apr 09 '18 at 15:52
`Ok(s"request is $request")` is the same as `Ok.apply("request is" + request.toString)`. It's just function syntax in Scala (`f(x)` is the same as `f.apply(x)`) and string interpolation. — Dmytro Mitin, Apr 09 '18 at 15:55

score 1 · Accepted Answer · answered Apr 09 '18 at 12:52

I'll try and simplify a bit:

trait A extends (String => String) { self => def apply() = this }

A effectively extends a function from String to String.

object A { def apply(f: String => String): A = new A { def apply(x: String) = f(x) } }

A's companion object actually implements A.

So you can now do this:

val f: String => String = _.toLowerCase
A(f)

This is valid Scala code.

You define f (toLowerCase on strings) and you pass it to the apply method of A.

This can also be written like this:

A { s => s.toLowerCase } // this way of putting it should remind you of Action { request => ... }

This is exactly how Play Action and EssentialAction works.

The other thing you ask is about Ok.

Ok is defined as a short version of Status with a set status code (200) and the given body (so something like Ok("Hello world!") will work.

On top of this there's the usual string interpolation you should know about.

Explain request passing in play framework

2 Answers2