Add Cats Monoid and Eq instances for Kernel

build.sbt

@@ -59,7 +59,10 @@ lazy val commonSettings = Seq(
     "org.scalameta" %%% "munit" % "1.0.0" % Test,
     "org.scalameta" %%% "munit-scalacheck" % "1.0.0-M11" % Test,
     "org.typelevel" %%% "munit-cats-effect" % "2.0.0" % Test,
-    "org.typelevel" %%% "scalacheck-effect-munit" % "2.0.0-M2" % Test
+    "org.typelevel" %%% "scalacheck-effect-munit" % "2.0.0-M2" % Test,
+    "org.typelevel" %%% "cats-kernel-laws" % "2.11.0" % Test,
+    "org.typelevel" %%% "cats-laws" % "2.11.0" % Test,
+    "org.typelevel" %%% "discipline-munit" % "2.0.0-M3" % Test
   )
 )
 

modules/core-tests/shared/src/test/scala/KernelInstancesLawSuite.scala

@@ -0,0 +1,13 @@
+// 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.kernel.laws.discipline.*
+import munit.DisciplineSuite
+
+class KernelInstancesLawSuite extends DisciplineSuite with Arbitraries {
+  checkAll("Kernel.MonoidLaws", MonoidTests[Kernel].monoid)
+  checkAll("Kernel.EqLaws", EqTests[Kernel].eqv)
+}

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

@@ -4,10 +4,12 @@
 
 package natchez
 
-import org.typelevel.ci._
+import cats.syntax.all.*
+import cats.{Eq, Monoid}
+import org.typelevel.ci.*
 
 import java.util
-import scala.jdk.CollectionConverters._
+import scala.jdk.CollectionConverters.*
 
 /** An opaque hunk of data that we can hand off to another system (in the form of HTTP headers),
   * which can then create new spans as children of this one. By this mechanism we allow our trace
@@ -21,4 +23,10 @@ final case class Kernel(toHeaders: Map[CIString, String]) {
 object Kernel {
   private[natchez] def fromJava(headers: util.Map[String, String]): Kernel =
     apply(headers.asScala.map { case (k, v) => CIString(k) -> v }.toMap)
+
+  implicit val kernelMonoid: Monoid[Kernel] =
+    Monoid.instance(Kernel(Map.empty), (a, b) => Kernel(a.toHeaders |+| b.toHeaders))
+
+  implicit val kernelEq: Eq[Kernel] =
+    Eq[Map[CIString, String]].contramap(_.toHeaders)
 }