Merge pull request #318 from commercetools/JSON_instance

find JSON instance from FromJSON and ToJSON

json/json-core/src/main/scala/io/sphere/json/JSON.scala

@@ -1,14 +1,24 @@
 package io.sphere.json
 
+import org.json4s.JsonAST.JValue
+
 import scala.annotation.implicitNotFound
 
 @implicitNotFound("Could not find an instance of JSON for ${A}")
 trait JSON[A] extends FromJSON[A] with ToJSON[A]
 
-object JSON {
+object JSON extends JSONLowPriorityImplicits {
   @inline def apply[A](implicit instance: JSON[A]): JSON[A] = instance
 }
 
+trait JSONLowPriorityImplicits {
+  implicit def fromJSONAndToJSON[A](implicit fromJSON: FromJSON[A], toJSON: ToJSON[A]): JSON[A] =
+    new JSON[A] {
+      override def read(jval: JValue): JValidation[A] = fromJSON.read(jval)
+      override def write(value: A): JValue = toJSON.write(value)
+    }
+}
+
 class JSONException(msg: String) extends RuntimeException(msg)
 
 sealed abstract class JSONError

json/json-core/src/test/scala/io/sphere/json/JSONSpec.scala

@@ -0,0 +1,27 @@
+package io.sphere.json
+
+import org.json4s.JsonAST.JValue
+import org.scalatest.matchers.must.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+class JSONSpec extends AnyWordSpec with Matchers {
+  import JSONSpec._
+  "JSON.apply" must {
+    "find possible JSON instance" in {
+      implicit val testJson: JSON[Test] = new JSON[Test] {
+        override def read(jval: JValue): JValidation[Test] = ???
+        override def write(value: Test): JValue = ???
+      }
+      JSON[Test] must be(testJson)
+    }
+    "create instance from FromJSON and ToJSON" in {
+      JSON[Int]
+      JSON[List[Double]]
+      JSON[Map[String, Int]]
+    }
+  }
+}
+
+object JSONSpec {
+  case class Test(a: String)
+}