JSON Macro Inception

请注意本文最初由Pascal Voitot (@mandubian ) 发表在mandubian.com

这个特性仍处理实验中，因为Scala 宏（Macros）在Scala 2.10中仍是实验性的。如果你不想使用Scala中的实验特性, 请手写 Reads/Writes/Format，可有一样的效果。

Writing a default case class Reads/Writes/Format is so boring!

Remember how you write a Reads[T] for a case class.

import play.api.libs.json._
import play.api.libs.functional.syntax._

case class Person(name: String, age: Int, lovesChocolate: Boolean)

implicit val personReads = (
  (__ \ 'name).read[String] and
  (__ \ 'age).read[Int] and
  (__ \ 'lovesChocolate).read[Boolean]
)(Person)

So you write 4 lines for this case class. You know what? We have had a few complaints from some people who think it’s not cool to write a Reads[TheirClass] because usually Java JSON frameworks like Jackson or Gson do it behind the curtain without writing anything. We argued that Play2.1 JSON serializers/deserializers are:

  • completely typesafe,
  • fully compiled,
  • nothing was performed using introspection/reflection at runtime.

But for some, this didn’t justify the extra lines of code for case classes.

We believe this is a really good approach so we persisted and proposed:

  • JSON simplified syntax
  • JSON combinators
  • JSON transformers

Added power, but nothing changed for the additional 4 lines.

Let’s be minimalist

As we are perfectionist, now we propose a new way of writing the same code:

import play.api.libs.json._
import play.api.libs.functional.syntax._

case class Person(name: String, age: Int, lovesChocolate: Boolean)

implicit val personReads = Json.reads[Person]

1 line only. Questions you may ask immediately:

Does it use runtime bytecode enhancement? -> NO

Does it use runtime introspection? -> NO

Does it break type-safety? -> NO

So what?

After creating buzzword JSON coast-to-coast design, let’s call it JSON INCEPTION.

JSON Inception

Code Equivalence

As explained just before:

import play.api.libs.json._
// please note we don't import functional.syntax._ as it is managed by the macro itself

implicit val personReads = Json.reads[Person]

// IS STRICTLY EQUIVALENT TO writing

implicit val personReads = (
  (__ \ 'name).read[String] and
  (__ \ 'age).read[Int] and
  (__ \ 'lovesChocolate).read[Boolean]
)(Person)

Inception equation

Here is the equation describing the windy Inception concept:

(Case Class INSPECTION) + (Code INJECTION) + (COMPILE Time) = INCEPTION

Case Class Inspection

As you may deduce by yourself, in order to ensure preceding code equivalence, we need :

  • to inspect Person case class,
  • to extract the 3 fields name, age, lovesChocolate and their types,
  • to resolve typeclasses implicits,
  • to find Person.apply.

INJECTION?

No I stop you immediately…

Code injection is not dependency injection… No Spring behind inception… No IOC, No DI… No No No ;)

I used this term on purpose because I know that injection is now linked immediately to IOC and Spring. But I’d like to re-establish this word with its real meaning. Here code injection just means that we inject code at compile-time into the compiled scala AST (Abstract Syntax Tree).

So Json.reads[Person] is compiled and replaced in the compile AST by:

(
  (__ \ 'name).read[String] and
  (__ \ 'age).read[Int] and
  (__ \ 'lovesChocolate).read[Boolean]
)(Person)

Nothing less, nothing more…

COMPILE-TIME

Yes everything is performed at compile-time. No runtime bytecode enhancement. No runtime introspection.

As everything is resolved at compile-time, you will have a compile error if you did not import the required implicits for all the types of the fields.

Json inception is Scala 2.10 Macros

We needed a Scala feature enabling:

  • compile-time code enhancement
  • compile-time class/implicits inspection
  • compile-time code injection

This is enabled by a new experimental feature introduced in Scala 2.10: Scala Macros

Scala macros is a new feature (still experimental) with a huge potential. You can :

  • introspect code at compile-time based on Scala reflection API,
  • access all imports, implicits in the current compile context
  • create new code expressions, generate compiling errors and inject them into compile chain.

Please note that:

  • We use Scala Macros because it corresponds exactly to our requirements.
  • We use Scala macros as an enabler, not as an end in itself.
  • The macro is a helper that generates the code you could write by yourself.
  • It doesn’t add, hide unexpected code behind the curtain.
  • We follow the no-surprise principle

As you may discover, writing a macro is not a trivial process since your macro code executes in the compiler runtime (or universe).

So you write macro code 
  that is compiled and executed 
  in a runtime that manipulates your code 
     to be compiled and executed 
     in a future runtime…

That’s also certainly why I called it Inception ;)

So it requires some mental exercises to follow exactly what you do. The API is also quite complex and not fully documented yet. Therefore, you must persevere when you begin using macros.

I’ll certainly write other articles about Scala macros because there are lots of things to say. This article is also meant to begin the reflection about the right way to use Scala Macros. Great power means greater responsability so it’s better to discuss all together and establish a few good manners…

Writes[T] & Format[T]

Please remark that JSON inception just works for structures having unapply/apply functions with corresponding input/output types.

Naturally, you can also incept Writes[T] and Format[T].

Writes[T]

import play.api.libs.json._
implicit val personWrites = Json.writes[Person]

Format[T]

import play.api.libs.json._
implicit val personWrites = Json.format[Person]

Special patterns

  • You can define your Reads/Writes in your companion object This is useful because then the implicit Reads/Writes is implicitly infered as soon as you manipulate an instance of your class.
import play.api.libs.json._

case class Person(name: String, age: Int)

object Person{
  implicit val personFmt = Json.format[Person]
}
  • You can now define Reads/Writes for single-field case class (known limitation until 2.1-RC2)
import play.api.libs.json._

case class Person(names: List[String])

object Person{
  implicit val personFmt = Json.format[Person]
}

Known limitations

  • Don’t override apply function in companion object because then the Macro will have several apply functions and won’t choose.
  • Json Macros only work when apply and unapply have corresponding input/output types: This is naturally the case for case classes. But if you want to the same with a trait, you must implement the same apply/unapply you would have in a case class.
  • Json Macros are known to accept Option/Seq/List/Set & Map[String, _]. For other generic types, test and if not working, use traditional way of writing Reads/Writes manually.