Trying to understand the Magnet pattern (as used in spray)

MagnetExample.scala

// Trying to understand the Magnet pattern (as used in spray)
// http://spray.io/blog/2012-12-13-the-magnet-pattern/

/**
 * A simple utility that writes things to strings.
 * We'd like it to be able to work for multiple different types
 */
object Writer {
  /**
   * We want this to work for multiple types that we don't know yet,
   * so we make it take a magnet. We'll define implicit converstions
   * for some of these types in the magnet's companion
   */
  def write(magnet: WriteMagnet): String = magnet()
}

sealed trait WriteMagnet {
  def apply(): String
}

/**
 * But some of these types we know, so we define the conversions in
 * the WriteMagnet companion object
 */
object WriteMagnet {

  implicit def fromInt(i: Int) = new WriteMagnet {
    override def apply = i.toString
  }

  // The next two functions couldn't have been defined if using
  // method overloading, because of type erasure

  implicit def fromStringList(list: List[String]) = new WriteMagnet {
    override def apply(): String = list.mkString("-")
  }

  implicit def fromList[T <% WriteMagnet](list: List[T]) = new WriteMagnet {
    override def apply(): String = list.map(Writer.write(_)).mkString(",")
  }

}

// And it works - for
Writer.write(10)
Writer.write(1 :: 34 :: 100 :: Nil)

Writer.write(List("A", "B", "C"))


// One of the types we'd like it to work for is not known in advance,
//so we couldn't pre-bake it in the magnet's companion object

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

// But we can put it here, in it's own companion object, and
// Writer.write will still work for it

object Person {
  implicit def toWriteMagnet(p: Person) = new WriteMagnet {
    override def apply(): String = s"(:${p.id},${p.name}:)"
  }
}

// Let's try if this works:

val adam = Person(1, "Adam")
val jeremy = Person(2, "Jeremy")

Writer.write(adam)

Writer.write(List(adam, jeremy))

// And the method can be lifted too - write becomes a function
// write: WriteMagnet => String = <function1>

val write = Writer.write _
write(1)
write(List(jeremy, adam))