lundi 20 avril 2015

Shapeless: case classes with attributes and typeclasses

I am currently implementing a library to serialize and deserialize to and from XML-RPC messages. It's almost done but now I am trying to remove the boilerplate of my current asProduct method using Shapeless. My current code:

trait Serializer[T] {
  def serialize(value: T): NodeSeq
} 

trait Deserializer[T] {
  type Deserialized[T] = Validation[AnyErrors, T]
  type AnyErrors = NonEmptyList[AnyError]
  def deserialize(from: NodeSeq): Deserialized[T]
}

trait Datatype[T] extends Serializer[T] with Deserializer[T]

// Example of asProduct, there are 20 more methods like this, from arity 1 to 22
def asProduct2[S, T1: Datatype, T2: Datatype](apply: (T1, T2) => S)(unapply: S => Product2[T1, T2]) = new Datatype[S] {
  override def serialize(value: S): NodeSeq = {
    val params = unapply(value)
    val b = toXmlrpc(params._1) ++ toXmlrpc(params._2)
    b.theSeq
  }

  // Using scalaz
  override def deserialize(from: NodeSeq): Deserialized[S] = (
      fromXmlrpc[T1](from(0)) |@| fromXmlrpc[T2](from(1))
    ) {apply}
}

My goal is to allow the user of my library to serialize/deserialize case classes without force him to write boilerplate code. Currently, you have to declare the case class and an implicit val using the aforementioned asProduct method to have a Datatype instance in context. This implicit is used in the following code:

def toXmlrpc[T](datatype: T)(implicit serializer: Serializer[T]): NodeSeq =
  serializer.serialize(datatype)

def fromXmlrpc[T](value: NodeSeq)(implicit deserializer: Deserializer[T]): Deserialized[T] =
  deserializer.deserialize(value)

This is the classic strategy of serializing and deserializing using type classes. I have grasped how to convert from case classes to HList via Generic or LabelledGeneric. The problem is once I have this conversion done how I can call the methods fromXmlrpc and toXmlrpc as in the asProduct2 example. I don't have any information about the types of the attributes in the case class and, therefore, the compiler cannot find any implicit that satisfy fromXmlrpc and toXmlrpc.

As I am a beginner with Shapeless, I would like to know what's the best way of get this functionality. I have some insights but I definitely have no idea of how to get it done using Shapeless. The ideal would be to have a way to get the type from a given attribute of the case class and pass this type explicitly to fromXmlrpc and toXmlrpc. I imagine that this is not how it can be done.

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