Add ContravariantDerivation for cats.Show

1 files changed, 23 insertions, 0 deletions

diff --git a/examples/src/main/scala/cats.scala b/examples/src/main/scala/cats.scala
new file mode 100644
index 0000000..08690c2
--- /dev/null
+++ b/examples/src/main/scala/cats.scala
@@ -0,0 +1,23 @@
+package magnolia.examples.cats
+
+import scala.collection.immutable.ListMap
+import scala.language.experimental.macros
+import scala.language.higherKinds
+
+import cats.Show
+import magnolia.ContravariantDerivation
+import magnolia.Macros
+
+object instances extends instances1 {
+
+  implicit val showDerivation = new ContravariantDerivation[Show] {
+    type Return = String
+    def call[T](show: Show[T], value: T): String = show.show(value)
+    def construct[T](body: T => String): Show[T] = body(_)
+    def join(xs: ListMap[String, String]): String = xs.map { case (k, v) => s"$k=$v" }.mkString("{", ", ", "}")
+  }
+}
+
+trait instances1 {
+  implicit def genericShow[T]: Show[T] = macro Macros.magnolia[T, Show[_]]
+}