LiftTrace concept

build.sbt

@@ -298,6 +298,7 @@ lazy val mtlJS = mtl.js.dependsOn(coreJS)
     scalaJSLinkerConfig ~= (_.withModuleKind(ModuleKind.CommonJSModule)),
   )
 
+// Noop srcs haved move to core, but keeping artifact for compatibility
 lazy val noop = crossProject(JSPlatform, JVMPlatform)
   .in(file("modules/noop"))
   .dependsOn(core)

modules/core/shared/src/main/scala/LiftTrace.scala

@@ -0,0 +1,39 @@
+// Copyright (c) 2019-2020 by Rob Norris and Contributors
+// This software is licensed under the MIT License (MIT).
+// For more information see LICENSE or https://opensource.org/licenses/MIT
+
+package natchez
+
+import cats.arrow.FunctionK
+import cats.data.Kleisli
+import cats.effect.IO
+import cats.effect.kernel.MonadCancelThrow
+import cats.~>
+import natchez.noop.NoopSpan
+
+trait LiftTrace[F[_], G[_]] {
+  def run[A](span: Span[F])(f: Trace[G] => G[A]): F[A]
+  def liftK: F ~> G
+  def lowerK: G ~> F
+}
+
+object LiftTrace extends LiftTraceLowPriority {
+  implicit val ioInstance: LiftTrace[IO, IO] = new LiftTrace[IO, IO] {
+    def run[A](span: Span[IO])(f: Trace[IO] => IO[A]): IO[A] =
+      Trace.ioTrace(span).flatMap(f)
+    def liftK: IO ~> IO = FunctionK.id
+    def lowerK: IO ~> IO = FunctionK.id
+  }
+}
+
+private[natchez] trait LiftTraceLowPriority {
+  implicit def kleisliInstance[F[_]: MonadCancelThrow]: LiftTrace[F, Kleisli[F, Span[F], *]] =
+    new LiftTrace[F, Kleisli[F, Span[F], *]] {
+      def run[A](span: Span[F])(f: Trace[Kleisli[F, Span[F], *]] => Kleisli[F, Span[F], A]): F[A] =
+        f(Trace.kleisliInstance).run(span)
+      def liftK: F ~> Kleisli[F, Span[F], *] = Kleisli.liftK
+      def lowerK: Kleisli[F, Span[F], *] ~> F = new (Kleisli[F, Span[F], *] ~> F) {
+        def apply[A](ga: Kleisli[F, Span[F], A]): F[A] = ga(NoopSpan[F]())
+      }
+    }
+}

modules/noop/shared/src/main/scala/NoopSpan.scala → modules/core/shared/src/main/scala/noop/NoopSpan.scala

@@ -19,7 +19,7 @@ final case class NoopSpan[F[_]: Applicative]() extends Span[F] {
     Applicative[F].pure(Kernel(Map.empty))
 
   override def span(name: String): Resource[F, Span[F]] =
-    Resource.eval(NoopSpan[F]().pure[F])
+    Resource.pure(this)
 
   // TODO
   def traceId: F[Option[String]] = none.pure[F]