From 649a919f3762bf60e80fbef9c5a9a193f12cd0b0 Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Thu, 5 Oct 2023 11:21:55 +0300
Subject: [PATCH 1/8] Remove effect constraint from TextMapPropagator, update
 packages, add tests

---
 .../scala/org/typelevel/otel4s/Otel4s.scala   |  3 +-
 .../typelevel/otel4s/ContextPropagators.scala | 39 --------
 .../propagation/ContextPropagators.scala      | 80 +++++++++++++++
 .../propagation}/TextMapGetter.scala          |  2 +-
 .../propagation}/TextMapPropagator.scala      | 99 ++++++++++++-------
 .../propagation}/TextMapSetter.scala          |  2 +-
 .../propagation}/TextMapUpdater.scala         |  2 +-
 .../propagation/ContextPropagatorsSuite.scala | 57 +++++++++++
 .../propagation}/TextMapGetterProps.scala     |  2 +-
 .../propagation}/TextMapGetterSuite.scala     |  4 +-
 .../propagation/TextMapPropagatorSuite.scala  | 69 +++++++++++++
 .../propagation}/TextMapSetterProps.scala     |  2 +-
 .../propagation}/TextMapSetterSuite.scala     |  2 +-
 .../propagation}/TextMapUpdaterProps.scala    |  2 +-
 .../propagation}/TextMapUpdaterSuite.scala    |  4 +-
 .../org/typelevel/otel4s/trace/Tracer.scala   |  4 +-
 .../org/typelevel/otel4s/java/OtelJava.scala  |  6 +-
 .../otel4s/java/ContextPropagatorsImpl.scala  |  9 +-
 .../typelevel/otel4s/java/Conversions.scala   |  2 +
 .../otel4s/java/TextMapPropagatorImpl.scala   | 24 ++---
 .../otel4s/java/trace/TracerBuilderImpl.scala |  4 +-
 .../otel4s/java/trace/TracerImpl.scala        | 10 +-
 .../java/trace/TracerProviderImpl.scala       |  6 +-
 .../typelevel/otel4s/java/trace/Traces.scala  |  5 +-
 .../otel4s/java/trace/TracerSuite.scala       |  2 +-
 25 files changed, 324 insertions(+), 117 deletions(-)
 delete mode 100644 core/common/src/main/scala/org/typelevel/otel4s/ContextPropagators.scala
 create mode 100644 core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
 rename core/common/src/main/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapGetter.scala (98%)
 rename core/common/src/main/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapPropagator.scala (53%)
 rename core/common/src/main/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapSetter.scala (97%)
 rename core/common/src/main/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapUpdater.scala (98%)
 create mode 100644 core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapGetterProps.scala (98%)
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapGetterSuite.scala (97%)
 create mode 100644 core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapSetterProps.scala (97%)
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapSetterSuite.scala (97%)
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapUpdaterProps.scala (97%)
 rename core/common/src/test/scala/org/typelevel/otel4s/{ => context/propagation}/TextMapUpdaterSuite.scala (96%)

diff --git a/core/all/src/main/scala/org/typelevel/otel4s/Otel4s.scala b/core/all/src/main/scala/org/typelevel/otel4s/Otel4s.scala
index bc19940b6..ff3583878 100644
--- a/core/all/src/main/scala/org/typelevel/otel4s/Otel4s.scala
+++ b/core/all/src/main/scala/org/typelevel/otel4s/Otel4s.scala
@@ -16,13 +16,14 @@
 
 package org.typelevel.otel4s
 
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.metrics.MeterProvider
 import org.typelevel.otel4s.trace.TracerProvider
 
 trait Otel4s[F[_]] {
   type Ctx
 
-  def propagators: ContextPropagators[F, Ctx]
+  def propagators: ContextPropagators[Ctx]
 
   /** A registry for creating named meters.
     */
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/ContextPropagators.scala b/core/common/src/main/scala/org/typelevel/otel4s/ContextPropagators.scala
deleted file mode 100644
index f1978ddce..000000000
--- a/core/common/src/main/scala/org/typelevel/otel4s/ContextPropagators.scala
+++ /dev/null
@@ -1,39 +0,0 @@
-/*
- * Copyright 2022 Typelevel
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.typelevel.otel4s
-
-import cats.Applicative
-
-trait ContextPropagators[F[_], Ctx] {
-  def textMapPropagator: TextMapPropagator[F, Ctx]
-}
-
-object ContextPropagators {
-
-  /** Creates a no-op implementation of the [[ContextPropagators]].
-    *
-    * A [[TextMapPropagator]] has no-op implementation too.
-    *
-    * @tparam F
-    *   the higher-kinded type of a polymorphic effect
-    */
-  def noop[F[_]: Applicative, Ctx]: ContextPropagators[F, Ctx] =
-    new ContextPropagators[F, Ctx] {
-      def textMapPropagator: TextMapPropagator[F, Ctx] =
-        TextMapPropagator.noop
-    }
-}
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
new file mode 100644
index 000000000..3caa320ca
--- /dev/null
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
@@ -0,0 +1,80 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.typelevel.otel4s.context.propagation
+
+/** A container of the registered propagators for every supported format.
+  *
+  * @tparam Ctx
+  *   the type of the Context
+  */
+trait ContextPropagators[Ctx] {
+
+  /** Returns a text map propagator to extract or inject data.
+    */
+  def textMapPropagator: TextMapPropagator[Ctx]
+}
+
+object ContextPropagators {
+
+  /** Creates a [[ContextPropagators]] which can be used to extract and inject
+    * context in text payloads with the given [[TextMapPropagator]].
+    *
+    * '''Hint''': use [[TextMapPropagator.composite]] to combine multiple text
+    * map propagators.
+    *
+    * @example
+    *   {{{
+    * val w3cPropagator: TextMapPropagator[Context] = ???
+    * val httpTracePropagator: TextMapPropagator[Context] = ???
+    * val textMapPropagator = TextMapPropagator.composite(w3cPropagator, httpTracePropagator)
+    * val contextPropagators = ContextPropagators.create(textMapPropagator)
+    *   }}}
+    *
+    * @param textMapPropagator
+    *   the text map propagator to extract or inject data
+    *
+    * @tparam Ctx
+    *   the context to use to extra or inject data
+    */
+  def create[Ctx](
+      textMapPropagator: TextMapPropagator[Ctx]
+  ): ContextPropagators[Ctx] =
+    new Default(textMapPropagator)
+
+  /** Creates a no-op implementation of the [[ContextPropagators]].
+    *
+    * A [[TextMapPropagator]] has no-op implementation too.
+    */
+  def noop[Ctx]: ContextPropagators[Ctx] =
+    new Noop
+
+  private final class Noop[Ctx] extends ContextPropagators[Ctx] {
+    val textMapPropagator: TextMapPropagator[Ctx] =
+      TextMapPropagator.noop
+
+    override def toString: String =
+      "ContextPropagators.Noop"
+  }
+
+  private final class Default[Ctx](
+      val textMapPropagator: TextMapPropagator[Ctx]
+  ) extends ContextPropagators[Ctx] {
+    override def toString: String =
+      s"ContextPropagators.Default{textMapPropagator=${textMapPropagator.toString}}"
+  }
+
+}
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/TextMapGetter.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapGetter.scala
similarity index 98%
rename from core/common/src/main/scala/org/typelevel/otel4s/TextMapGetter.scala
rename to core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapGetter.scala
index fb38f3f00..25e8347e4 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/TextMapGetter.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapGetter.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import cats.Contravariant
 
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/TextMapPropagator.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
similarity index 53%
rename from core/common/src/main/scala/org/typelevel/otel4s/TextMapPropagator.scala
rename to core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
index d09ad7c63..490c3a378 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/TextMapPropagator.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
@@ -14,9 +14,10 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
-import cats.Applicative
+import cats.data.NonEmptyList
+import cats.syntax.foldable._
 
 /** The process of propagating data across process boundaries involves injecting
   * and extracting values in the form of text into carriers that travel in-band.
@@ -30,10 +31,13 @@ import cats.Applicative
   * interceptors. On the client side, values are injected into the carriers,
   * while on the server side, values are extracted from them.
   *
-  * @tparam F
-  *   the higher-kinded type of a polymorphic effect
+  * @tparam Ctx
+  *   the context to use to extra or inject data
   */
-trait TextMapPropagator[F[_], Ctx] {
+trait TextMapPropagator[Ctx] {
+
+  /** The list of propagation fields. */
+  def fields: List[String]
 
   /** Extracts key-value pairs from the given `carrier` and adds them to the
     * given context.
@@ -52,20 +56,6 @@ trait TextMapPropagator[F[_], Ctx] {
     */
   def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx
 
-  /** Injects data from the context into the given mutable `carrier` for
-    * downstream consumers, for example as HTTP headers.
-    *
-    * @param ctx
-    *   the context containing the value to be injected
-    *
-    * @param carrier
-    *   holds propagation fields
-    *
-    * @tparam A
-    *   the type of the carrier, which is mutable
-    */
-  def inject[A: TextMapSetter](ctx: Ctx, carrier: A): F[Unit]
-
   /** Injects data from the context into a copy of the given immutable `carrier`
     * for downstream consumers, for example as HTTP headers.
     *
@@ -84,31 +74,70 @@ trait TextMapPropagator[F[_], Ctx] {
     * @return
     *   a copy of the carrier, with new fields injected
     */
-  def injected[A: TextMapUpdater](ctx: Ctx, carrier: A): A
+  def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A
 }
 
 object TextMapPropagator {
 
-  def apply[F[_], Ctx](implicit
-      ev: TextMapPropagator[F, Ctx]
-  ): TextMapPropagator[F, Ctx] = ev
+  /** Creates a [[TextMapPropagator]] which delegates injection and extraction
+    * to the provided propagators.
+    *
+    * @example
+    *   {{{
+    * val w3cPropagator: TextMapPropagator[Context] = ???
+    * val httpTracePropagator: TextMapPropagator[Context] = ???
+    * val textMapPropagator = TextMapPropagator.composite(w3cPropagator, httpTracePropagator)
+    *   }}}
+    * @tparam Ctx
+    *   the context to use to extra or inject data
+    */
+  def composite[Ctx](
+      propagators: TextMapPropagator[Ctx]*
+  ): TextMapPropagator[Ctx] =
+    propagators.toList match {
+      case Nil          => new Noop[Ctx]
+      case head :: Nil  => head
+      case head :: tail => new Multi(NonEmptyList(head, tail))
+    }
 
   /** Creates a no-op implementation of the [[TextMapPropagator]].
     *
     * All propagation operations are no-op.
-    *
-    * @tparam F
-    *   the higher-kinded type of a polymorphic effect
     */
-  def noop[F[_]: Applicative, Ctx]: TextMapPropagator[F, Ctx] =
-    new TextMapPropagator[F, Ctx] {
-      def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx =
-        ctx
+  def noop[Ctx]: TextMapPropagator[Ctx] =
+    new Noop
 
-      def inject[A: TextMapSetter](ctx: Ctx, carrier: A): F[Unit] =
-        Applicative[F].unit
+  private final class Noop[Ctx] extends TextMapPropagator[Ctx] {
+    def fields: List[String] =
+      Nil
+
+    def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx =
+      ctx
+
+    def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A =
+      carrier
+
+    override def toString: String = "TextMapPropagator.Noop"
+  }
+
+  private final class Multi[Ctx](
+      val propagators: NonEmptyList[TextMapPropagator[Ctx]]
+  ) extends TextMapPropagator[Ctx] {
+    val fields: List[String] =
+      propagators.toList.flatMap(_.fields)
+
+    def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx =
+      propagators.foldLeft(ctx) { (ctx, propagator) =>
+        propagator.extract(ctx, carrier)
+      }
+
+    def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A =
+      propagators.foldLeft(carrier) { (carrier, propagator) =>
+        propagator.inject(ctx, carrier)
+      }
+
+    override def toString: String =
+      s"TextMapPropagator.Multi(${propagators.map(_.toString).mkString_(", ")})"
+  }
 
-      def injected[A: TextMapUpdater](ctx: Ctx, carrier: A): A =
-        carrier
-    }
 }
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/TextMapSetter.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapSetter.scala
similarity index 97%
rename from core/common/src/main/scala/org/typelevel/otel4s/TextMapSetter.scala
rename to core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapSetter.scala
index c6adfcdd5..c79d9c216 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/TextMapSetter.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapSetter.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import scala.collection.mutable
 import scala.collection.mutable.Buffer
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/TextMapUpdater.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapUpdater.scala
similarity index 98%
rename from core/common/src/main/scala/org/typelevel/otel4s/TextMapUpdater.scala
rename to core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapUpdater.scala
index ed2fb6476..5bc179da5 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/TextMapUpdater.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapUpdater.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import cats.Invariant
 
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
new file mode 100644
index 000000000..d5728e080
--- /dev/null
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
@@ -0,0 +1,57 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.typelevel.otel4s.context.propagation
+
+import munit._
+
+class ContextPropagatorsSuite extends FunSuite {
+
+  test("create a no-op instance") {
+    val propagators = ContextPropagators.noop
+
+    assertEquals(propagators.textMapPropagator.fields, Nil)
+    assertEquals(
+      propagators.textMapPropagator.toString,
+      "TextMapPropagator.Noop"
+    )
+    assertEquals(propagators.toString, "ContextPropagators.Noop")
+  }
+
+  test("create (noop propagator) - use noop") {
+    val fields = List("a", "b", "c")
+    val propagator = new TestPropagator[String](fields, "TestPropagator")
+    val propagators = ContextPropagators.create(propagator)
+
+    assertEquals(propagators.textMapPropagator.fields, fields)
+    assertEquals(
+      propagators.toString,
+      "ContextPropagators.Default{textMapPropagator=TestPropagator}"
+    )
+  }
+
+  private final class TestPropagator[Ctx](
+      val fields: List[String],
+      name: String
+  ) extends TextMapPropagator[Ctx] {
+    def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx = ctx
+
+    def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A = carrier
+
+    override def toString: String = name
+  }
+
+}
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterProps.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterProps.scala
similarity index 98%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterProps.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterProps.scala
index 34e1733ab..0fbbf9b61 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterProps.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterProps.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import lgbt.princess.platform.Platform
 import munit.ScalaCheckSuite
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterSuite.scala
similarity index 97%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterSuite.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterSuite.scala
index c1e92e6c5..72f64a59e 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapGetterSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapGetterSuite.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import cats.Eq
 import cats.laws.discipline._
@@ -23,6 +23,8 @@ import munit.DisciplineSuite
 import munit.FunSuite
 import org.scalacheck.Arbitrary
 import org.scalacheck.Gen
+import org.typelevel.otel4s.Box
+import org.typelevel.otel4s.NotQuiteExhaustiveChecks
 
 import scala.collection.immutable
 import scala.collection.mutable
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
new file mode 100644
index 000000000..b25788677
--- /dev/null
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.typelevel.otel4s.context.propagation
+
+import munit._
+
+class TextMapPropagatorSuite extends FunSuite {
+
+  test("create a no-op instance") {
+    val propagator = TextMapPropagator.noop
+
+    assertEquals(propagator.fields, Nil)
+    assertEquals(propagator.toString, "TextMapPropagator.Noop")
+  }
+
+  test("composite (empty input) - use noop") {
+    val composite = TextMapPropagator.composite()
+
+    assertEquals(composite.fields, Nil)
+    assertEquals(composite.toString, "TextMapPropagator.Noop")
+  }
+
+  test("composite (single input) - use this input") {
+    val fields = List("a", "b", "c")
+    val propagator = new TestPropagator[String](fields, "TestPropagator")
+    val composite = TextMapPropagator.composite(propagator)
+
+    assertEquals(composite.fields, fields)
+    assertEquals(composite.toString, "TestPropagator")
+  }
+
+  test("composite (multiple) - create a multi instance") {
+    val fieldsA = List("a", "b")
+    val fieldsB = List("c", "d")
+
+    val propagatorA = new TestPropagator[String](fieldsA, "PropagatorA")
+    val propagatorB = new TestPropagator[String](fieldsB, "PropagatorB")
+    val composite = TextMapPropagator.composite(propagatorA, propagatorB)
+
+    assertEquals(composite.fields, fieldsA ++ fieldsB)
+    assertEquals(
+      composite.toString,
+      "TextMapPropagator.Multi(PropagatorA, PropagatorB)"
+    )
+  }
+
+  private final class TestPropagator[Ctx](
+      val fields: List[String],
+      name: String
+  ) extends TextMapPropagator[Ctx] {
+    def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx = ctx
+    def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A = carrier
+    override def toString: String = name
+  }
+}
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterProps.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterProps.scala
similarity index 97%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterProps.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterProps.scala
index c12e0ff35..df2cacf08 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterProps.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterProps.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import lgbt.princess.platform.Platform
 import munit.ScalaCheckSuite
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterSuite.scala
similarity index 97%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterSuite.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterSuite.scala
index 7b162c1fd..349679361 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapSetterSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapSetterSuite.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import munit.FunSuite
 
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterProps.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterProps.scala
similarity index 97%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterProps.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterProps.scala
index 5f301592b..89521c22b 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterProps.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterProps.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import munit.ScalaCheckSuite
 import org.scalacheck.Arbitrary
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterSuite.scala
similarity index 96%
rename from core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterSuite.scala
rename to core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterSuite.scala
index d0a2bb35f..4b07ceed4 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/TextMapUpdaterSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapUpdaterSuite.scala
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import cats.Eq
 import cats.laws.discipline._
@@ -23,6 +23,8 @@ import munit.DisciplineSuite
 import munit.FunSuite
 import org.scalacheck.Arbitrary
 import org.scalacheck.Gen
+import org.typelevel.otel4s.Box
+import org.typelevel.otel4s.NotQuiteExhaustiveChecks
 
 import scala.collection.immutable
 
diff --git a/core/trace/src/main/scala/org/typelevel/otel4s/trace/Tracer.scala b/core/trace/src/main/scala/org/typelevel/otel4s/trace/Tracer.scala
index cf42bec8c..0dd3aa0c2 100644
--- a/core/trace/src/main/scala/org/typelevel/otel4s/trace/Tracer.scala
+++ b/core/trace/src/main/scala/org/typelevel/otel4s/trace/Tracer.scala
@@ -20,6 +20,8 @@ package trace
 import cats.Applicative
 import cats.effect.kernel.MonadCancelThrow
 import cats.~>
+import org.typelevel.otel4s.context.propagation.TextMapGetter
+import org.typelevel.otel4s.context.propagation.TextMapUpdater
 import org.typelevel.otel4s.meta.InstrumentMeta
 
 @annotation.implicitNotFound("""
@@ -182,7 +184,7 @@ trait Tracer[F[_]] extends TracerMacro[F] {
     *   a copy of the immutable carrier with this tracer's context appended to
     *   it
     * @see
-    *   [[org.typelevel.otel4s.TextMapPropagator.injected TextMapPropagator#injected]]
+    *   [[org.typelevel.otel4s.context.propagation.TextMapPropagator.inject TextMapPropagator#inject]]
     */
   def propagate[C: TextMapUpdater](carrier: C): F[C]
 
diff --git a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
index 45218687a..0b12ed21c 100644
--- a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
+++ b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
@@ -25,8 +25,8 @@ import cats.syntax.all._
 import io.opentelemetry.api.{OpenTelemetry => JOpenTelemetry}
 import io.opentelemetry.api.GlobalOpenTelemetry
 import io.opentelemetry.sdk.{OpenTelemetrySdk => JOpenTelemetrySdk}
-import org.typelevel.otel4s.ContextPropagators
 import org.typelevel.otel4s.Otel4s
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.java.Conversions.asyncFromCompletableResultCode
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.context.LocalContext
@@ -37,7 +37,7 @@ import org.typelevel.otel4s.metrics.MeterProvider
 import org.typelevel.otel4s.trace.TracerProvider
 
 sealed class OtelJava[F[_]] private (
-    val propagators: ContextPropagators[F, Context],
+    val propagators: ContextPropagators[Context],
     val meterProvider: MeterProvider[F],
     val tracerProvider: TracerProvider[F],
 ) extends Otel4s[F] {
@@ -66,7 +66,7 @@ object OtelJava {
   def local[F[_]: Async: LocalContext](
       jOtel: JOpenTelemetry
   ): OtelJava[F] = {
-    val contextPropagators = new ContextPropagatorsImpl[F](jOtel.getPropagators)
+    val contextPropagators = new ContextPropagatorsImpl(jOtel.getPropagators)
 
     val metrics = Metrics.forAsync(jOtel)
     val traces = Traces.local(jOtel, contextPropagators)
diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala
index 196ae90da..3e554da0d 100644
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala
+++ b/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala
@@ -17,15 +17,16 @@
 package org.typelevel.otel4s
 package java
 
-import cats.effect.kernel.Sync
 import io.opentelemetry.context.propagation.{
   ContextPropagators => JContextPropagators
 }
+import org.typelevel.otel4s.context.propagation.ContextPropagators
+import org.typelevel.otel4s.context.propagation.TextMapPropagator
 import org.typelevel.otel4s.java.context.Context
 
-private[java] class ContextPropagatorsImpl[F[_]: Sync](
+private[java] class ContextPropagatorsImpl(
     propagators: JContextPropagators
-) extends ContextPropagators[F, Context] {
-  val textMapPropagator: TextMapPropagator[F, Context] =
+) extends ContextPropagators[Context] {
+  val textMapPropagator: TextMapPropagator[Context] =
     new TextMapPropagatorImpl(propagators.getTextMapPropagator)
 }
diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/Conversions.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/Conversions.scala
index 43ac18fcf..0d9c12e79 100644
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/Conversions.scala
+++ b/java/common/src/main/scala/org/typelevel/otel4s/java/Conversions.scala
@@ -24,6 +24,8 @@ import io.opentelemetry.api.common.{Attributes => JAttributes}
 import io.opentelemetry.context.propagation.{TextMapGetter => JTextMapGetter}
 import io.opentelemetry.context.propagation.{TextMapSetter => JTextMapSetter}
 import io.opentelemetry.sdk.common.CompletableResultCode
+import org.typelevel.otel4s.context.propagation.TextMapGetter
+import org.typelevel.otel4s.context.propagation.TextMapSetter
 
 import scala.jdk.CollectionConverters._
 
diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
index 6f54c8996..8b1656dfa 100644
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
+++ b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
@@ -17,33 +17,33 @@
 package org.typelevel.otel4s
 package java
 
-import cats.effect.Sync
 import io.opentelemetry.context.propagation.{
   TextMapPropagator => JTextMapPropagator
 }
+import org.typelevel.otel4s.context.propagation.TextMapGetter
+import org.typelevel.otel4s.context.propagation.TextMapPropagator
+import org.typelevel.otel4s.context.propagation.TextMapUpdater
 import org.typelevel.otel4s.java.Conversions._
 import org.typelevel.otel4s.java.context.Context
 
-private[java] class TextMapPropagatorImpl[F[_]: Sync](
+import scala.jdk.CollectionConverters._
+
+private[java] class TextMapPropagatorImpl(
     jPropagator: JTextMapPropagator
-) extends TextMapPropagator[F, Context] {
+) extends TextMapPropagator[Context] {
+  lazy val fields: List[String] =
+    jPropagator.fields().asScala.toList
+
   def extract[A: TextMapGetter](ctx: Context, carrier: A): Context =
     ctx.map(jPropagator.extract(_, carrier, fromTextMapGetter))
 
-  def inject[A: TextMapSetter](ctx: Context, carrier: A): F[Unit] =
-    Sync[F].delay(
-      jPropagator.inject(ctx.underlying, carrier, fromTextMapSetter)
-    )
-
-  def injected[A](ctx: Context, carrier: A)(implicit
-      injector: TextMapUpdater[A]
-  ): A = {
+  def inject[A: TextMapUpdater](ctx: Context, carrier: A): A = {
     var injectedCarrier = carrier
     jPropagator.inject[Null](
       ctx.underlying,
       null, // explicitly allowed per opentelemetry-java, so our setter can be a lambda!
       (_, key, value) => {
-        injectedCarrier = injector.updated(injectedCarrier, key, value)
+        injectedCarrier = TextMapUpdater[A].updated(injectedCarrier, key, value)
       }
     )
     injectedCarrier
diff --git a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerBuilderImpl.scala b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerBuilderImpl.scala
index bfd2b4f56..26efc7c75 100644
--- a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerBuilderImpl.scala
+++ b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerBuilderImpl.scala
@@ -18,14 +18,14 @@ package org.typelevel.otel4s.java.trace
 
 import cats.effect.Sync
 import io.opentelemetry.api.trace.{TracerProvider => JTracerProvider}
-import org.typelevel.otel4s.ContextPropagators
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.context.LocalContext
 import org.typelevel.otel4s.trace._
 
 private[java] final case class TracerBuilderImpl[F[_]: Sync: LocalContext](
     jTracerProvider: JTracerProvider,
-    propagators: ContextPropagators[F, Context],
+    propagators: ContextPropagators[Context],
     name: String,
     version: Option[String] = None,
     schemaUrl: Option[String] = None
diff --git a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerImpl.scala b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerImpl.scala
index f737c1e8d..c50f092bb 100644
--- a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerImpl.scala
+++ b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerImpl.scala
@@ -19,9 +19,9 @@ package org.typelevel.otel4s.java.trace
 import cats.effect.Sync
 import io.opentelemetry.api.trace.{Span => JSpan}
 import io.opentelemetry.api.trace.{Tracer => JTracer}
-import org.typelevel.otel4s.ContextPropagators
-import org.typelevel.otel4s.TextMapGetter
-import org.typelevel.otel4s.TextMapUpdater
+import org.typelevel.otel4s.context.propagation.ContextPropagators
+import org.typelevel.otel4s.context.propagation.TextMapGetter
+import org.typelevel.otel4s.context.propagation.TextMapUpdater
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.context.LocalContext
 import org.typelevel.otel4s.trace.SpanBuilder
@@ -30,7 +30,7 @@ import org.typelevel.otel4s.trace.Tracer
 
 private[java] class TracerImpl[F[_]: Sync](
     jTracer: JTracer,
-    propagators: ContextPropagators[F, Context]
+    propagators: ContextPropagators[Context]
 )(implicit L: LocalContext[F])
     extends Tracer[F] {
 
@@ -70,5 +70,5 @@ private[java] class TracerImpl[F[_]: Sync](
   }
 
   def propagate[C: TextMapUpdater](carrier: C): F[C] =
-    L.reader(propagators.textMapPropagator.injected(_, carrier))
+    L.reader(propagators.textMapPropagator.inject(_, carrier))
 }
diff --git a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerProviderImpl.scala b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerProviderImpl.scala
index 25d4d7f7e..841d55c63 100644
--- a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerProviderImpl.scala
+++ b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/TracerProviderImpl.scala
@@ -18,7 +18,7 @@ package org.typelevel.otel4s.java.trace
 
 import cats.effect.Sync
 import io.opentelemetry.api.trace.{TracerProvider => JTracerProvider}
-import org.typelevel.otel4s.ContextPropagators
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.context.LocalContext
 import org.typelevel.otel4s.trace.TracerBuilder
@@ -26,7 +26,7 @@ import org.typelevel.otel4s.trace.TracerProvider
 
 private[java] class TracerProviderImpl[F[_]: Sync: LocalContext] private (
     jTracerProvider: JTracerProvider,
-    propagators: ContextPropagators[F, Context]
+    propagators: ContextPropagators[Context]
 ) extends TracerProvider[F] {
   def tracer(name: String): TracerBuilder[F] =
     TracerBuilderImpl(jTracerProvider, propagators, name)
@@ -36,7 +36,7 @@ private[java] object TracerProviderImpl {
 
   def local[F[_]: Sync: LocalContext](
       jTracerProvider: JTracerProvider,
-      propagators: ContextPropagators[F, Context]
+      propagators: ContextPropagators[Context]
   ): TracerProvider[F] =
     new TracerProviderImpl(jTracerProvider, propagators)
 }
diff --git a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/Traces.scala b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/Traces.scala
index 0c0155561..f6e676d29 100644
--- a/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/Traces.scala
+++ b/java/trace/src/main/scala/org/typelevel/otel4s/java/trace/Traces.scala
@@ -21,6 +21,7 @@ import cats.effect.IOLocal
 import cats.effect.LiftIO
 import cats.effect.Sync
 import io.opentelemetry.api.{OpenTelemetry => JOpenTelemetry}
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.context.LocalContext
 import org.typelevel.otel4s.java.instances._
@@ -34,7 +35,7 @@ object Traces {
 
   def local[F[_]: Sync: LocalContext](
       jOtel: JOpenTelemetry,
-      propagators: ContextPropagators[F, Context]
+      propagators: ContextPropagators[Context]
   ): Traces[F] = {
     val provider =
       TracerProviderImpl.local(jOtel.getTracerProvider, propagators)
@@ -45,7 +46,7 @@ object Traces {
 
   def ioLocal[F[_]: LiftIO: Sync](
       jOtel: JOpenTelemetry,
-      propagators: ContextPropagators[F, Context]
+      propagators: ContextPropagators[Context]
   ): F[Traces[F]] =
     IOLocal(Context.root)
       .map { implicit ioLocal: IOLocal[Context] =>
diff --git a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
index b08122320..683857969 100644
--- a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
+++ b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
@@ -1123,7 +1123,7 @@ class TracerSuite extends CatsEffectSuite {
       val textMapPropagators =
         W3CTraceContextPropagator.getInstance() +: additionalPropagators
 
-      val propagators = new ContextPropagatorsImpl[IO](
+      val propagators = new ContextPropagatorsImpl(
         JContextPropagators.create(
           JTextMapPropagator.composite(textMapPropagators.asJava)
         )

From 5691fa342f659cff1a6d4de855a41ad85db21b5b Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Thu, 5 Oct 2023 19:19:29 +0300
Subject: [PATCH 2/8] Make `ContextPropagators` sealed

---
 .../propagation/ContextPropagators.scala      |  2 +-
 .../org/typelevel/otel4s/java/OtelJava.scala  |  4 ++-
 .../otel4s/java/ContextPropagatorsImpl.scala  | 32 -------------------
 .../otel4s/java/trace/TracerSuite.scala       | 10 +++---
 4 files changed, 8 insertions(+), 40 deletions(-)
 delete mode 100644 java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala

diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
index 3caa320ca..841136dcd 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
@@ -21,7 +21,7 @@ package org.typelevel.otel4s.context.propagation
   * @tparam Ctx
   *   the type of the Context
   */
-trait ContextPropagators[Ctx] {
+sealed trait ContextPropagators[Ctx] {
 
   /** Returns a text map propagator to extract or inject data.
     */
diff --git a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
index 0b12ed21c..ca985c8f1 100644
--- a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
+++ b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
@@ -66,7 +66,9 @@ object OtelJava {
   def local[F[_]: Async: LocalContext](
       jOtel: JOpenTelemetry
   ): OtelJava[F] = {
-    val contextPropagators = new ContextPropagatorsImpl(jOtel.getPropagators)
+    val contextPropagators = ContextPropagators.create(
+      new TextMapPropagatorImpl(jOtel.getPropagators.getTextMapPropagator)
+    )
 
     val metrics = Metrics.forAsync(jOtel)
     val traces = Traces.local(jOtel, contextPropagators)
diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala
deleted file mode 100644
index 3e554da0d..000000000
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/ContextPropagatorsImpl.scala
+++ /dev/null
@@ -1,32 +0,0 @@
-/*
- * Copyright 2022 Typelevel
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.typelevel.otel4s
-package java
-
-import io.opentelemetry.context.propagation.{
-  ContextPropagators => JContextPropagators
-}
-import org.typelevel.otel4s.context.propagation.ContextPropagators
-import org.typelevel.otel4s.context.propagation.TextMapPropagator
-import org.typelevel.otel4s.java.context.Context
-
-private[java] class ContextPropagatorsImpl(
-    propagators: JContextPropagators
-) extends ContextPropagators[Context] {
-  val textMapPropagator: TextMapPropagator[Context] =
-    new TextMapPropagatorImpl(propagators.getTextMapPropagator)
-}
diff --git a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
index 683857969..246a24ebc 100644
--- a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
+++ b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
@@ -32,9 +32,6 @@ import io.opentelemetry.api.common.{AttributeKey => JAttributeKey}
 import io.opentelemetry.api.common.Attributes
 import io.opentelemetry.api.trace.StatusCode
 import io.opentelemetry.api.trace.propagation.W3CTraceContextPropagator
-import io.opentelemetry.context.propagation.{
-  ContextPropagators => JContextPropagators
-}
 import io.opentelemetry.context.propagation.{
   TextMapPropagator => JTextMapPropagator
 }
@@ -51,7 +48,8 @@ import io.opentelemetry.sdk.trace.`export`.SimpleSpanProcessor
 import io.opentelemetry.sdk.trace.internal.data.ExceptionEventData
 import munit.CatsEffectSuite
 import org.typelevel.otel4s.Attribute
-import org.typelevel.otel4s.java.ContextPropagatorsImpl
+import org.typelevel.otel4s.context.propagation.ContextPropagators
+import org.typelevel.otel4s.java.TextMapPropagatorImpl
 import org.typelevel.otel4s.java.context.Context
 import org.typelevel.otel4s.java.instances._
 import org.typelevel.otel4s.trace.Span
@@ -1123,8 +1121,8 @@ class TracerSuite extends CatsEffectSuite {
       val textMapPropagators =
         W3CTraceContextPropagator.getInstance() +: additionalPropagators
 
-      val propagators = new ContextPropagatorsImpl(
-        JContextPropagators.create(
+      val propagators = ContextPropagators.create(
+        new TextMapPropagatorImpl(
           JTextMapPropagator.composite(textMapPropagators.asJava)
         )
       )

From b767cf6bb9922914d9a1fdcf14d756d0fdf01b2c Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Fri, 6 Oct 2023 09:44:12 +0300
Subject: [PATCH 3/8] Rename `create` -> `of`, add
 `Monoid[TextMapPropagator[Ctx]]`

---
 .../propagation/ContextPropagators.scala      | 27 +++++----
 .../propagation/TextMapPropagator.scala       | 58 ++++++++++++++-----
 .../propagation/ContextPropagatorsSuite.scala | 20 ++++++-
 .../TextMapPropagatorLawTests.scala           | 52 +++++++++++++++++
 .../propagation/TextMapPropagatorSuite.scala  | 36 +++++++++---
 .../org/typelevel/otel4s/java/OtelJava.scala  |  2 +-
 .../otel4s/java/trace/TracerSuite.scala       |  2 +-
 7 files changed, 159 insertions(+), 38 deletions(-)
 create mode 100644 core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala

diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
index 841136dcd..f23db2635 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala
@@ -33,27 +33,34 @@ object ContextPropagators {
   /** Creates a [[ContextPropagators]] which can be used to extract and inject
     * context in text payloads with the given [[TextMapPropagator]].
     *
-    * '''Hint''': use [[TextMapPropagator.composite]] to combine multiple text
-    * map propagators.
+    * If multiple text map propagators are passed, the combined (composite)
+    * TextMapPropagator instance will be created.
+    *
+    * It's a shortcut for:
+    * {{{
+    * ContextPropagators.of(TextMapPropagator.of(w3cPropagator, httpTracePropagator))
+    * }}}
     *
     * @example
     *   {{{
     * val w3cPropagator: TextMapPropagator[Context] = ???
     * val httpTracePropagator: TextMapPropagator[Context] = ???
-    * val textMapPropagator = TextMapPropagator.composite(w3cPropagator, httpTracePropagator)
-    * val contextPropagators = ContextPropagators.create(textMapPropagator)
+    * val contextPropagators = ContextPropagators.of(w3cPropagator, httpTracePropagator)
     *   }}}
     *
-    * @param textMapPropagator
-    *   the text map propagator to extract or inject data
+    * @see
+    *   [[TextMapPropagator.of]]
+    *
+    * @param textMapPropagators
+    *   the propagators to use for injection and extraction
     *
     * @tparam Ctx
-    *   the context to use to extra or inject data
+    *   the context to use to extract or inject data
     */
-  def create[Ctx](
-      textMapPropagator: TextMapPropagator[Ctx]
+  def of[Ctx](
+      textMapPropagators: TextMapPropagator[Ctx]*
   ): ContextPropagators[Ctx] =
-    new Default(textMapPropagator)
+    new Default(TextMapPropagator.of(textMapPropagators: _*))
 
   /** Creates a no-op implementation of the [[ContextPropagators]].
     *
diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
index 490c3a378..ffc13b18f 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
@@ -16,7 +16,7 @@
 
 package org.typelevel.otel4s.context.propagation
 
-import cats.data.NonEmptyList
+import cats.Monoid
 import cats.syntax.foldable._
 
 /** The process of propagating data across process boundaries involves injecting
@@ -32,7 +32,7 @@ import cats.syntax.foldable._
   * while on the server side, values are extracted from them.
   *
   * @tparam Ctx
-  *   the context to use to extra or inject data
+  *   the context to use to extract or inject data
   */
 trait TextMapPropagator[Ctx] {
 
@@ -86,19 +86,19 @@ object TextMapPropagator {
     *   {{{
     * val w3cPropagator: TextMapPropagator[Context] = ???
     * val httpTracePropagator: TextMapPropagator[Context] = ???
-    * val textMapPropagator = TextMapPropagator.composite(w3cPropagator, httpTracePropagator)
+    * val textMapPropagator = TextMapPropagator.of(w3cPropagator, httpTracePropagator)
     *   }}}
+    *
+    * @param propagators
+    *   the propagators to use for injection and extraction
+    *
     * @tparam Ctx
-    *   the context to use to extra or inject data
+    *   the context to use to extract or inject data
     */
-  def composite[Ctx](
+  def of[Ctx](
       propagators: TextMapPropagator[Ctx]*
   ): TextMapPropagator[Ctx] =
-    propagators.toList match {
-      case Nil          => new Noop[Ctx]
-      case head :: Nil  => head
-      case head :: tail => new Multi(NonEmptyList(head, tail))
-    }
+    propagators.combineAll
 
   /** Creates a no-op implementation of the [[TextMapPropagator]].
     *
@@ -107,6 +107,34 @@ object TextMapPropagator {
   def noop[Ctx]: TextMapPropagator[Ctx] =
     new Noop
 
+  implicit def textMapPropagatorMonoid[Ctx]: Monoid[TextMapPropagator[Ctx]] =
+    new Monoid[TextMapPropagator[Ctx]] {
+      val empty: TextMapPropagator[Ctx] =
+        noop[Ctx]
+
+      def combine(
+          x: TextMapPropagator[Ctx],
+          y: TextMapPropagator[Ctx]
+      ): TextMapPropagator[Ctx] =
+        (x, y) match {
+          case (that, _: Noop[Ctx]) =>
+            that
+          case (_: Noop[Ctx], other) =>
+            other
+          case (that: Multi[Ctx], other: Multi[Ctx]) =>
+            multi(that.propagators ++ other.propagators)
+          case (that: Multi[Ctx], other) =>
+            multi(that.propagators :+ other)
+          case (that, other: Multi[Ctx]) =>
+            multi(that +: other.propagators)
+          case (that, other) =>
+            multi(List(that, other))
+        }
+
+      private def multi(propagators: List[TextMapPropagator[Ctx]]): Multi[Ctx] =
+        Multi(propagators, propagators.flatMap(_.fields))
+    }
+
   private final class Noop[Ctx] extends TextMapPropagator[Ctx] {
     def fields: List[String] =
       Nil
@@ -120,12 +148,10 @@ object TextMapPropagator {
     override def toString: String = "TextMapPropagator.Noop"
   }
 
-  private final class Multi[Ctx](
-      val propagators: NonEmptyList[TextMapPropagator[Ctx]]
+  private final case class Multi[Ctx](
+      propagators: List[TextMapPropagator[Ctx]],
+      fields: List[String]
   ) extends TextMapPropagator[Ctx] {
-    val fields: List[String] =
-      propagators.toList.flatMap(_.fields)
-
     def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx =
       propagators.foldLeft(ctx) { (ctx, propagator) =>
         propagator.extract(ctx, carrier)
@@ -137,7 +163,7 @@ object TextMapPropagator {
       }
 
     override def toString: String =
-      s"TextMapPropagator.Multi(${propagators.map(_.toString).mkString_(", ")})"
+      s"TextMapPropagator.Multi(${propagators.map(_.toString).mkString(", ")})"
   }
 
 }
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
index d5728e080..32fc7e6c7 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/ContextPropagatorsSuite.scala
@@ -31,10 +31,10 @@ class ContextPropagatorsSuite extends FunSuite {
     assertEquals(propagators.toString, "ContextPropagators.Noop")
   }
 
-  test("create (noop propagator) - use noop") {
+  test("of (single input) - use this input") {
     val fields = List("a", "b", "c")
     val propagator = new TestPropagator[String](fields, "TestPropagator")
-    val propagators = ContextPropagators.create(propagator)
+    val propagators = ContextPropagators.of(propagator)
 
     assertEquals(propagators.textMapPropagator.fields, fields)
     assertEquals(
@@ -43,6 +43,22 @@ class ContextPropagatorsSuite extends FunSuite {
     )
   }
 
+  test("of (multiple inputs) - create a multi text map propagator instance") {
+    val fieldsA = List("a", "b")
+    val fieldsB = List("c", "d")
+
+    val propagatorA = new TestPropagator[String](fieldsA, "TestPropagatorA")
+    val propagatorB = new TestPropagator[String](fieldsB, "TestPropagatorB")
+
+    val propagators = ContextPropagators.of(propagatorA, propagatorB)
+
+    assertEquals(propagators.textMapPropagator.fields, fieldsA ++ fieldsB)
+    assertEquals(
+      propagators.toString,
+      "ContextPropagators.Default{textMapPropagator=TextMapPropagator.Multi(TestPropagatorA, TestPropagatorB)}"
+    )
+  }
+
   private final class TestPropagator[Ctx](
       val fields: List[String],
       name: String
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala
new file mode 100644
index 000000000..a927bf61f
--- /dev/null
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.typelevel.otel4s.context.propagation
+
+import cats.Eq
+import cats.kernel.laws.discipline.MonoidTests
+import munit._
+import org.scalacheck.{Arbitrary, Gen}
+
+class TextMapPropagatorLawTests extends DisciplineSuite {
+
+  implicit val textMapPropagatorEq: Eq[TextMapPropagator[String]] =
+    Eq.fromUniversalEquals
+
+  implicit val textMapPropagatorArb: Arbitrary[TextMapPropagator[String]] =
+    Arbitrary(
+      for {
+        fields <- Gen.listOf(Gen.alphaNumStr)
+        name <- Gen.alphaNumStr
+      } yield TestPropagator(fields, name)
+    )
+
+  checkAll(
+    "TextMapPropagator.MonoidLaws",
+    MonoidTests[TextMapPropagator[String]].monoid
+  )
+
+  private case class TestPropagator[Ctx](
+      fields: List[String],
+      name: String
+  ) extends TextMapPropagator[Ctx] {
+    def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx = ctx
+
+    def inject[A: TextMapUpdater](ctx: Ctx, carrier: A): A = carrier
+
+    override def toString: String = name
+  }
+}
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
index b25788677..567245664 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
@@ -27,37 +27,57 @@ class TextMapPropagatorSuite extends FunSuite {
     assertEquals(propagator.toString, "TextMapPropagator.Noop")
   }
 
-  test("composite (empty input) - use noop") {
-    val composite = TextMapPropagator.composite()
+  test("of (empty input) - use noop") {
+    val composite = TextMapPropagator.of()
 
     assertEquals(composite.fields, Nil)
     assertEquals(composite.toString, "TextMapPropagator.Noop")
   }
 
-  test("composite (single input) - use this input") {
+  test("of (single input) - use this input") {
     val fields = List("a", "b", "c")
     val propagator = new TestPropagator[String](fields, "TestPropagator")
-    val composite = TextMapPropagator.composite(propagator)
+    val composite = TextMapPropagator.of(propagator)
 
     assertEquals(composite.fields, fields)
     assertEquals(composite.toString, "TestPropagator")
   }
 
-  test("composite (multiple) - create a multi instance") {
+  test("of (multiple) - create a multi instance") {
     val fieldsA = List("a", "b")
     val fieldsB = List("c", "d")
 
     val propagatorA = new TestPropagator[String](fieldsA, "PropagatorA")
     val propagatorB = new TestPropagator[String](fieldsB, "PropagatorB")
-    val composite = TextMapPropagator.composite(propagatorA, propagatorB)
 
-    assertEquals(composite.fields, fieldsA ++ fieldsB)
+    val propagator = TextMapPropagator.of(propagatorA, propagatorB)
+
+    assertEquals(propagator.fields, fieldsA ++ fieldsB)
     assertEquals(
-      composite.toString,
+      propagator.toString,
       "TextMapPropagator.Multi(PropagatorA, PropagatorB)"
     )
   }
 
+  test("of (multiple) - flatten out nested multi instances") {
+    val fieldsA = List("a", "b")
+    val fieldsB = List("c", "d")
+
+    val propagatorA = new TestPropagator[String](fieldsA, "PropagatorA")
+    val propagatorB = new TestPropagator[String](fieldsB, "PropagatorB")
+
+    val multi1 = TextMapPropagator.of(propagatorA, propagatorB)
+    val multi2 = TextMapPropagator.of(propagatorA, propagatorB)
+
+    val propagator = TextMapPropagator.of(multi1, multi2)
+
+    assertEquals(propagator.fields, fieldsA ++ fieldsB ++ fieldsA ++ fieldsB)
+    assertEquals(
+      propagator.toString,
+      "TextMapPropagator.Multi(PropagatorA, PropagatorB, PropagatorA, PropagatorB)"
+    )
+  }
+
   private final class TestPropagator[Ctx](
       val fields: List[String],
       name: String
diff --git a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
index ca985c8f1..baa63ec82 100644
--- a/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
+++ b/java/all/src/main/scala/org/typelevel/otel4s/java/OtelJava.scala
@@ -66,7 +66,7 @@ object OtelJava {
   def local[F[_]: Async: LocalContext](
       jOtel: JOpenTelemetry
   ): OtelJava[F] = {
-    val contextPropagators = ContextPropagators.create(
+    val contextPropagators = ContextPropagators.of(
       new TextMapPropagatorImpl(jOtel.getPropagators.getTextMapPropagator)
     )
 
diff --git a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
index 246a24ebc..c5646bc7a 100644
--- a/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
+++ b/java/trace/src/test/scala/org/typelevel/otel4s/java/trace/TracerSuite.scala
@@ -1121,7 +1121,7 @@ class TracerSuite extends CatsEffectSuite {
       val textMapPropagators =
         W3CTraceContextPropagator.getInstance() +: additionalPropagators
 
-      val propagators = ContextPropagators.create(
+      val propagators = ContextPropagators.of(
         new TextMapPropagatorImpl(
           JTextMapPropagator.composite(textMapPropagators.asJava)
         )

From 367062f3983fbf6cebc828c17fe7dc9bdc961fb7 Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Fri, 6 Oct 2023 09:45:28 +0300
Subject: [PATCH 4/8] TextMapPropagator - `fields: List[String]` -> `fields:
 Iterable[String]`

---
 .../otel4s/context/propagation/TextMapPropagator.scala      | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
index ffc13b18f..f024c388e 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
@@ -36,8 +36,8 @@ import cats.syntax.foldable._
   */
 trait TextMapPropagator[Ctx] {
 
-  /** The list of propagation fields. */
-  def fields: List[String]
+  /** The collection of propagation fields. */
+  def fields: Iterable[String]
 
   /** Extracts key-value pairs from the given `carrier` and adds them to the
     * given context.
@@ -136,7 +136,7 @@ object TextMapPropagator {
     }
 
   private final class Noop[Ctx] extends TextMapPropagator[Ctx] {
-    def fields: List[String] =
+    def fields: Iterable[String] =
       Nil
 
     def extract[A: TextMapGetter](ctx: Ctx, carrier: A): Ctx =

From 2fbfecc90962c4f4f33ea6847c7c6d2d6c4ee11b Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Fri, 6 Oct 2023 09:46:28 +0300
Subject: [PATCH 5/8] Run `scalafixAll`

---
 .../otel4s/context/propagation/TextMapPropagatorLawTests.scala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala
index a927bf61f..53bd3877c 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorLawTests.scala
@@ -19,7 +19,8 @@ package org.typelevel.otel4s.context.propagation
 import cats.Eq
 import cats.kernel.laws.discipline.MonoidTests
 import munit._
-import org.scalacheck.{Arbitrary, Gen}
+import org.scalacheck.Arbitrary
+import org.scalacheck.Gen
 
 class TextMapPropagatorLawTests extends DisciplineSuite {
 

From 63ef5df6553db71c93dfe1af1708bfe71a2fb41d Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Tue, 17 Oct 2023 14:07:20 +0300
Subject: [PATCH 6/8] Update
 java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala

Co-authored-by: Marissa | April <7505383+NthPortal@users.noreply.github.com>
---
 .../org/typelevel/otel4s/java/TextMapPropagatorImpl.scala     | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
index 8b1656dfa..e87805fed 100644
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
+++ b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
@@ -31,8 +31,8 @@ import scala.jdk.CollectionConverters._
 private[java] class TextMapPropagatorImpl(
     jPropagator: JTextMapPropagator
 ) extends TextMapPropagator[Context] {
-  lazy val fields: List[String] =
-    jPropagator.fields().asScala.toList
+  lazy val fields: Iterable[String] =
+    jPropagator.fields().asScala
 
   def extract[A: TextMapGetter](ctx: Context, carrier: A): Context =
     ctx.map(jPropagator.extract(_, carrier, fromTextMapGetter))

From 03ef5907c0cb3dc9307906f016b1e6a1db1f0a7c Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Tue, 17 Oct 2023 14:15:38 +0300
Subject: [PATCH 7/8] TextMapPropagatorImpl: access implicit `TextMapUpdater`
 directly

---
 .../org/typelevel/otel4s/java/TextMapPropagatorImpl.scala   | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
index e87805fed..19d20bda5 100644
--- a/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
+++ b/java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala
@@ -37,13 +37,15 @@ private[java] class TextMapPropagatorImpl(
   def extract[A: TextMapGetter](ctx: Context, carrier: A): Context =
     ctx.map(jPropagator.extract(_, carrier, fromTextMapGetter))
 
-  def inject[A: TextMapUpdater](ctx: Context, carrier: A): A = {
+  def inject[A](ctx: Context, carrier: A)(implicit
+      injector: TextMapUpdater[A]
+  ): A = {
     var injectedCarrier = carrier
     jPropagator.inject[Null](
       ctx.underlying,
       null, // explicitly allowed per opentelemetry-java, so our setter can be a lambda!
       (_, key, value) => {
-        injectedCarrier = TextMapUpdater[A].updated(injectedCarrier, key, value)
+        injectedCarrier = injector.updated(injectedCarrier, key, value)
       }
     )
     injectedCarrier

From d747bd6f6dcd220022187c028844ec0400977c26 Mon Sep 17 00:00:00 2001
From: Maksym Ochenashko <maxochenashko@gmail.com>
Date: Tue, 17 Oct 2023 14:24:35 +0300
Subject: [PATCH 8/8] TextMapPropagator: keep unique fields when making a multi
 instance

---
 .../propagation/TextMapPropagator.scala       |  2 +-
 .../propagation/TextMapPropagatorSuite.scala  | 21 ++++++++++++++++++-
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
index f024c388e..7952de835 100644
--- a/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
+++ b/core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapPropagator.scala
@@ -132,7 +132,7 @@ object TextMapPropagator {
         }
 
       private def multi(propagators: List[TextMapPropagator[Ctx]]): Multi[Ctx] =
-        Multi(propagators, propagators.flatMap(_.fields))
+        Multi(propagators, propagators.flatMap(_.fields).distinct)
     }
 
   private final class Noop[Ctx] extends TextMapPropagator[Ctx] {
diff --git a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
index 567245664..7bdce9367 100644
--- a/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
+++ b/core/common/src/test/scala/org/typelevel/otel4s/context/propagation/TextMapPropagatorSuite.scala
@@ -71,7 +71,26 @@ class TextMapPropagatorSuite extends FunSuite {
 
     val propagator = TextMapPropagator.of(multi1, multi2)
 
-    assertEquals(propagator.fields, fieldsA ++ fieldsB ++ fieldsA ++ fieldsB)
+    assertEquals(propagator.fields, fieldsA ++ fieldsB)
+    assertEquals(
+      propagator.toString,
+      "TextMapPropagator.Multi(PropagatorA, PropagatorB, PropagatorA, PropagatorB)"
+    )
+  }
+
+  test("of (multiple) - keep unique fields") {
+    val fieldsA = List("a", "b", "c", "d")
+    val fieldsB = List("c", "d", "e", "f")
+
+    val propagatorA = new TestPropagator[String](fieldsA, "PropagatorA")
+    val propagatorB = new TestPropagator[String](fieldsB, "PropagatorB")
+
+    val multi1 = TextMapPropagator.of(propagatorA, propagatorB)
+    val multi2 = TextMapPropagator.of(propagatorA, propagatorB)
+
+    val propagator = TextMapPropagator.of(multi1, multi2)
+
+    assertEquals(propagator.fields, List("a", "b", "c", "d", "e", "f"))
     assertEquals(
       propagator.toString,
       "TextMapPropagator.Multi(PropagatorA, PropagatorB, PropagatorA, PropagatorB)"