Add ContravariantDerivation for cats.Show

2 files changed, 40 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[_]]
+}
diff --git a/tests/shared/src/main/scala/magnolia/cats.scala b/tests/shared/src/main/scala/magnolia/cats.scala
new file mode 100644
index 0000000..4a1c096
--- /dev/null
+++ b/tests/shared/src/main/scala/magnolia/cats.scala
@@ -0,0 +1,17 @@
+package magnolia
+
+import examples.{Address, Branch, Country, Entity, Leaf, Person}
+import cats.instances.all._
+import cats.syntax.all._
+import examples.cats.instances._
+import language.experimental.macros
+
+object CatsMain {
+
+  def main(args: Array[String]): Unit = {
+    println(Branch(Branch(Leaf(1, "a"), Leaf(2, "b")), Leaf(3, "c")).show)
+    println(List[Entity](Person("John Smith",
+      Address(List("1 High Street", "London", "SW1A 1AA"),
+        Country("UK", "GBR", false)))).show)
+  }
+}