Play Action Composition - hardcoding parameterized parser - scala

Question

I would like to create a custom action which takes away the boilerplate of writing actions like this:
Action[MyClass](BodyParsers.parse.json[MyClass]) { req => ...
However, I keep running into class definition errors. Here has been my most successful attempt:
class JsonAction[A: Reads] extends ActionBuilder[Request] {
def hardcodedJson[A: Reads](action: Action[A]) =
Action.async(BodyParsers.parse.json[A]) { request => action(request) }
def invokeBlock[A: Reads](request: Request[A], block: (Request[A]) => Future[Result]) = {
block(request)
}
override def composeAction[A: Reads](action: Action[A]) = hardcodedJson(action)
}
but I get the following error: method composeAction overrides nothing.
If I change composeAction[A: Reads] to composeAction[A] it tells me there isn't a Json Serializer for type A.
Is there some other way to define this custom action?

Answer 1

Yep, I tried to get this to work with the official ActionBuilder way-of-doing-it and just could not get the types to line up.
Here's something that worked for me though:
object JsonActionHelper {
def withReads[A](act: Request[A] => Future[Result])(implicit reads:Reads[A]) =
Action.async(BodyParsers.parse.json(reads))(act)
}
Usage in your controller (FooJson is an object containing an implicit Reads[Foo]):
import models.FooJson._
import JsonActionHelper._
def handleIncomingFoo(fooId:String) = withReads[Foo] { req =>
val foo:Foo = req.body
...
Ok(...)
}

Answer 2

ActionBuilder is not generic enough for your use-case; there's nowhere for you to pass in your Reads[T].
There's nothing special about ActionBuilder though. It's a collection of apply and async factory methods. You can define your own Action type with whatever factory methods you need:
object JsonAction {
def apply[A : Reads](request: Request[A] => Result) = Action(BodyParsers.parse.json[A])(request)
}
// these are equivalent:
Action[MyClass](BodyParsers.parse.json[MyClass]) { req => ??? }
JsonAction[MyClass] { req => ??? }