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package magnolia.examples
import magnolia._
import scala.language.experimental.macros
/** shows one type as another, often as a string
*
* Note that this is a more general form of `Show` than is usual, as it permits the return type to
* be something other than a string. */
trait Show[Out, T] { def show(value: T): Out }
/** companion object to [[Show]] */
object Show {
/** the type constructor for new [[Show]] instances
*
* The first parameter is fixed as `String`, and the second parameter varies generically. */
type Typeclass[T] = Show[String, T]
/** creates a new [[Show]] instance by labelling and joining (with `mkString`) the result of
* showing each parameter, and prefixing it with the class name */
def combine[T](ctx: CaseClass[Typeclass, T]): Show[String, T] = new Show[String, T] {
def show(value: T) = {
val paramStrings = ctx.parameters.map { param =>
s"${param.label}=${param.typeclass.show(param.dereference(value))}"
}
paramStrings.mkString(s"${ctx.typeName.split("\\.").last}(", ",", ")")
}
}
/** choose which typeclass to use based on the subtype of the sealed trait */
def dispatch[T](ctx: SealedTrait[Typeclass, T]): Show[String, T] = new Show[String, T] {
def show(value: T): String = ctx.dispatch(value) { sub =>
sub.typeclass.show(sub.cast(value))
}
}
/** show typeclass for strings */
implicit val string: Show[String, String] = new Show[String, String] {
def show(s: String): String = s
}
/** show typeclass for integers */
implicit val int: Show[String, Int] = new Show[String, Int] {
def show(s: Int): String = s.toString
}
/** bind the Magnolia macro to this derivation object */
implicit def gen[T]: Show[String, T] = macro Magnolia.gen[T]
}
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