cats.mtl.Local[IO, E] from IOLocal[E]

build.sbt

@@ -292,6 +292,7 @@ ThisBuild / apiURL := Some(url("https://typelevel.org/cats-effect/api/3.x/"))
 ThisBuild / autoAPIMappings := true
 
 val CatsVersion = "2.9.0"
+val CatsMtlVersion = "1.3.0"
 val Specs2Version = "4.19.2"
 val ScalaCheckVersion = "1.17.0"
 val DisciplineVersion = "1.4.0"
@@ -445,6 +446,7 @@ lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
   .settings(
     name := "cats-effect",
     libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats-mtl" % CatsMtlVersion,
       "org.typelevel" %% "scalac-compat-annotation" % ScalacCompatVersion % CompileTime
     ),
     mimaBinaryIssueFilters ++= Seq(
@@ -833,7 +835,8 @@ lazy val tests: CrossProject = crossProject(JSPlatform, JVMPlatform, NativePlatf
       "org.scalacheck" %%% "scalacheck" % ScalaCheckVersion,
       "org.specs2" %%% "specs2-scalacheck" % Specs2Version % Test,
       "org.typelevel" %%% "discipline-specs2" % DisciplineVersion % Test,
-      "org.typelevel" %%% "cats-kernel-laws" % CatsVersion % Test
+      "org.typelevel" %%% "cats-kernel-laws" % CatsVersion % Test,
+      "org.typelevel" %% "cats-mtl-laws" % CatsMtlVersion % Test
     ),
     buildInfoPackage := "catseffect"
   )

core/shared/src/main/scala/cats/effect/IOLocal.scala

@@ -16,7 +16,9 @@
 
 package cats.effect
 
+import cats.Applicative
 import cats.data.AndThen
+import cats.mtl.Local
 
 /**
  * [[IOLocal]] provides a handy way of manipulating a context on different scopes.
@@ -243,6 +245,17 @@ sealed trait IOLocal[A] { self =>
     }
   }
 
+  final def toLocal: Local[IO, A] =
+    new Local[IO, A] {
+      def applicative: Applicative[IO] =
+        IO.asyncForIO
+
+      def ask[A2 >: A] =
+        self.get
+
+      def local[B](iob: IO[B])(f: A => A): IO[B] =
+        self.modify(e => f(e) -> e).bracket(Function.const(iob))(self.set)
+    }
 }
 
 object IOLocal {
@@ -319,4 +332,6 @@ object IOLocal {
       underlying.get.flatMap(s => underlying.reset.as(getter(s)))
   }
 
+  def local[E](e: E): IO[Local[IO, E]] =
+    IOLocal(e).map(_.toLocal)
 }

tests/shared/src/test/scala/cats/effect/IOLocalLocalSpec.scala

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2020-2022 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 cats
+package effect
+
+import cats.mtl.Local
+import cats.mtl.laws.discipline._
+
+import org.typelevel.discipline.specs2.mutable.Discipline
+
+import java.util.concurrent.CancellationException
+
+class IOLocalLocalSpec extends BaseSpec with Discipline {
+  sequential
+
+  implicit val ticker = Ticker()
+
+  implicit val local: Local[IO, Int] =
+    // Don't try this at home
+    unsafeRun(IOLocal.local(0)).fold(
+      throw new CancellationException("canceled"),
+      throw _,
+      _.get
+    )
+
+  checkAll("Local[IO, Int]", LocalTests[IO, Int].local[Int, Int])
+}