Simple type class for formatting objects as JSON or XML

gistfile1.scala

package com.olchovy.format

import scala.xml._
import net.liftweb.json.JsonAST._

trait Formatter[-A, B] {
  def format(a: A): B
}

object Formatters {
  abstract class JsonFormatter[-A] extends Formatter[A, JValue]
  
  implicit def a2fooJson[A](implicit ev: JsonFormatter[A]) = new FooJsonFormatter[A]

  implicit def a2seqJson[A](implicit ev: JsonFormatter[A]) = new SeqJsonFormatter[A]

  implicit object StringJsonFormatter extends JsonFormatter[String] {
    def format(a: String) = JString(a)
  }

  implicit object IntJsonFormatter extends JsonFormatter[Int] {
    def format(a: Int) = JInt(a)
  }

  implicit object DoubleJsonFormatter extends JsonFormatter[Double] {
    def format(a: Double) = JDouble(a)
  }

  class SeqJsonFormatter[A](implicit ev: JsonFormatter[A]) extends JsonFormatter[Seq[A]] {
    def format(a: Seq[A]) = JArray(a.map(ev.format(_)).toList)
  }

  class FooJsonFormatter[A](implicit ev: JsonFormatter[A]) extends JsonFormatter[Foo[A]] {
    def format(a: Foo[A]) = JField(a.descriptor, ev.format(a.underlying))
  }
  
  abstract class XmlFormatter[-A] extends Format[A, NodeSeq]

  implicit def a2fooXml[A](implicit ev: XmlFormatter[A]) = new FooXmlFormatter[A]
  
  implicit def a2seqXml[A](implicit ev: XmlFormatter[A]) = new SeqXmlFormatter[A]

  implicit object StringXmlFormatter extends XmlFormatter[String] {
    def format(a: String) = Text(a)
  }

  implicit object IntXmlFormatter extends XmlFormatter[Int] {
    def format(a: Int) = Text(a.toString)
  }

  implicit object DoubleXmlFormatter extends XmlFormatter[Double] {
    def format(a: Double) = Text(a.toString)
  }

  class SeqXmlFormatter[A](implicit ev: XmlFormatter[A]) extends XmlFormatter[Seq[A]] {
    def format(a: Seq[A]) = new NodeSeq {
      def theSeq = a.map {
        a => Elem(null, "item", Null, TopScope, ev.format(a).theSeq: _*)
      }.flatMap(a => a)
    }
  }

  class FooXmlFormatter[A](implicit ev: XmlFormatter[A]) extends XmlFormatter[Foo[A]] {
    def format(a: Foo[A]) = {
      val label = a.descriptor
      val child = ev.format(a.underlying).theSeq

      Elem(null, label, Null, TopScope, child: _*)
    }
  }
}

abstract class Foo[+A] {
  val descriptor: String
  val underlying: A 

  def format[B](implicit ev: Formatter[Foo[A], B]) = ev.format(this)
}