What play's Action really is?

user_7254635 0
Viewed 267

In

val echo = Action { request =>
  Ok("Got request [" + request + "]")
}

It seems Action is a function, have one function type parameter, it's type is Request[A] => Result

In the doc:https://www.playframework.com/documentation/2.7.x/api/scala/play/api/mvc/Action.html

It tell me Action is a trait:

trait Action[A] extends EssentialAction

"An action is essentially a (Request[A] => Result) function that handles a request and generates a result to be sent to the client."

so what the Action really is? a function, or a trait?

playframework scala
1 Answers

In Scala, a function is indeed defined using a trait, for example

object foo extends (Int => String) {
  def apply(i: Int): String = s"hello $i"
}

or 

val foo: Int => String = i => s"hello $i"

or

val foo = new Function1[Int, String] {
  override def apply(i: Int): String = s"hello $i"
}

all define a function we can call with foo(42), which desugars to foo.apply(42).

An action is

trait Action[A] extends EssentialAction

where EssentialAction is

trait EssentialAction extends (RequestHeader) => Accumulator[...]

where we see the extends (RequestHeader) => Accumulator syntax. Note that A => B is syntactic sugar for Function1 trait, so we could write

trait EssentialAction extends Function1[RequestHeader, Accumulator[...]]`

Now an Action trait also has an Action companion object which takes a function argument block and constructs an Action with default request body:

Action.apply(block: (Request[AnyContent]) => Result): Action[AnyContent]

and this is in fact what is used when we write

Action { request =>
  Ok("Got request [" + request + "]")
}
user_52050220
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