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package magnolia.examples
import magnolia._
import scala.language.experimental.macros
/** typeclass for providing a default value for a particular type */
trait Default[T] { def default: T }
/** companion object and derivation object for [[Default]] */
object Default {
type Typeclass[T] = Default[T]
type ParamType[T, P] = Param[Default, T] { type PType = P }
/** constructs a default for each parameter, using the constructor default (if provided),
* otherwise using a typeclass-provided default */
def combine[T](ctx: CaseClass[Default, T, Param[Default, T]]): Default[T] = new Default[T] {
def default = ctx.construct { param =>
param.default.getOrElse(param.typeclass.default)
}
}
/** chooses which subtype to delegate to */
def dispatch[T](ctx: SealedTrait[Default, T])(): Default[T] = new Default[T] {
def default: T = ctx.subtypes.head.typeclass.default
}
/** default value for a string; the empty string */
implicit val string: Default[String] = new Default[String] { def default = "" }
/** default value for ints; 0 */
implicit val int: Default[Int] = new Default[Int] { def default = 0 }
/** default value for sequences; the empty sequence */
implicit def seq[A]: Default[Seq[A]] = new Typeclass[Seq[A]] { def default = Seq.empty }
/** generates default instances of [[Default]] for case classes and sealed traits */
implicit def gen[T]: Default[T] = macro Magnolia.gen[T]
}
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