#794: Derivation tools for external newtypes

core-tests/shared/src/test/scala/zio/prelude/newtypes/DerivationSpec.scala

@@ -0,0 +1,43 @@
+package zio.prelude.newtypes
+
+import zio.prelude.newtypes.derivation.derive
+import zio.prelude.{Newtype, Subtype}
+import zio.test._
+
+object DerivationSpec extends DefaultRunnableSpec {
+  trait Show[A] {
+    def show(a: A): String
+  }
+
+  implicit val showString: Show[String] = (a: String) => a
+
+  // these are imported from an external library - we have no control over them
+  object External {
+    object Secret extends Newtype[String]
+    type Secret = Secret.Type
+
+    object Password extends Subtype[String]
+    type Password = Password.Type
+  }
+
+  import External._
+
+  override def spec: ZSpec[Environment, Failure] = suite("external derivation for newtypes")(
+    test("works for Newtype") {
+      val showSecret: Show[Secret] = derive
+      assertTrue(showSecret.show(Secret("foo")) == "foo")
+    },
+    test("works for Subtype") {
+      val showPassword: Show[Password] = derive
+      assertTrue(showPassword.show(Password("foo")) == "foo")
+    },
+    testM("works with auto derivation") {
+      typeCheck("""
+                  |import zio.prelude.newtypes.derivation.auto._
+                  |implicitly[Show[Secret]]
+                  |implicitly[Show[Password]]
+                  |""".stripMargin).absolve
+        .as(assertCompletes)
+    }
+  )
+}

core/shared/src/main/scala/zio/prelude/newtypes/derivation.scala

@@ -0,0 +1,16 @@
+package zio.prelude.newtypes
+
+import zio.prelude.Newtype
+
+object derivation {
+  def derive[A, STA <: Newtype[A], F[_]](implicit ev: STA <:< Newtype[A], typeClass: F[A]): F[STA#Type] =
+    typeClass.asInstanceOf[F[STA#Type]]
+
+  trait Auto {
+    implicit def auto[A, STA <: Newtype[A], F[_]](implicit ev: STA <:< Newtype[A], typeClass: F[A]): F[STA#Type] = {
+      // the type parameters and implicits are needed for 2.11 compatibility
+      derive[A, STA, F](ev, typeClass)
+    }
+  }
+  object auto extends Auto
+}