1 files changed, 35 insertions, 0 deletions
diff --git a/examples/shared/src/main/scala/default.scala b/examples/shared/src/main/scala/default.scala
new file mode 100644
index 0000000..4c1b634
--- /dev/null
+++ b/examples/shared/src/main/scala/default.scala
@@ -0,0 +1,35 @@
+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]
+
+  /** 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]): 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 }
+
+  /** generates default instances of [[Default]] for case classes and sealed traits */
+  implicit def gen[T]: Default[T] = macro Magnolia.gen[T]
+}