Rough draft of an MTL-based logger

Author: rossabaker

build.sbt

@@ -28,6 +28,7 @@ ThisBuild / tlVersionIntroduced := Map("3" -> "2.1.1")
 
 val catsV = "2.11.0"
 val catsEffectV = "3.5.7"
+val catsMtlV = "1.4.0"
 val slf4jV = "1.7.36"
 val munitCatsEffectV = "2.0.0"
 val logbackClassicV = "1.2.13"
@@ -47,7 +48,8 @@ lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
     name := "log4cats-core",
     libraryDependencies ++= Seq(
       "org.typelevel" %%% "cats-core"       % catsV,
-      "org.typelevel" %%% "cats-effect-std" % catsEffectV
+      "org.typelevel" %%% "cats-effect-std" % catsEffectV,
+      "org.typelevel" %%% "cats-mtl"        % catsMtlV
     ),
     libraryDependencies ++= {
       if (tlIsScala3.value) Seq.empty

core/shared/src/main/scala/org/typelevel/log4cats/AskStructuredLogger.scala

@@ -0,0 +1,71 @@
+package org.typelevel.log4cats
+
+import cats.Monad
+import cats.mtl.Ask
+
+private[log4cats] class AskStructuredLogger[F[_]](logger: StructuredLogger[F])(implicit
+    F: Monad[F],
+    ask: Ask[F, Map[String, String]]
+) extends StructuredLogger[F] {
+
+  override def error(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.error(askCtx)(message))
+
+  override def warn(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.warn(askCtx)(message))
+
+  override def info(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.info(askCtx)(message))
+
+  override def debug(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.debug(askCtx)(message))
+
+  override def trace(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.trace(askCtx)(message))
+
+  override def error(t: Throwable)(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.error(askCtx, t)(message))
+
+  override def warn(t: Throwable)(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.warn(askCtx, t)(message))
+
+  override def info(t: Throwable)(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.info(askCtx, t)(message))
+
+  override def debug(t: Throwable)(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.debug(askCtx, t)(message))
+
+  override def trace(t: Throwable)(message: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.trace(askCtx, t)(message))
+
+  override def trace(ctx: Map[String, String])(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.trace(askCtx ++ ctx)(msg))
+
+  override def trace(ctx: Map[String, String], t: Throwable)(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.trace(askCtx ++ ctx, t)(msg))
+
+  override def debug(ctx: Map[String, String])(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.debug(askCtx ++ ctx)(msg))
+
+  override def debug(ctx: Map[String, String], t: Throwable)(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.debug(askCtx ++ ctx, t)(msg))
+
+  override def info(ctx: Map[String, String])(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.info(askCtx ++ ctx)(msg))
+
+  override def info(ctx: Map[String, String], t: Throwable)(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.info(askCtx ++ ctx, t)(msg))
+
+  override def warn(ctx: Map[String, String])(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.warn(askCtx ++ ctx)(msg))
+
+  override def warn(ctx: Map[String, String], t: Throwable)(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.warn(askCtx ++ ctx, t)(msg))
+
+  override def error(ctx: Map[String, String])(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.error(askCtx ++ ctx)(msg))
+
+  override def error(ctx: Map[String, String], t: Throwable)(msg: => String): F[Unit] =
+    F.flatMap(ask.ask)(askCtx => logger.error(askCtx ++ ctx, t)(msg))
+
+}

core/shared/src/main/scala/org/typelevel/log4cats/StructuredLogger.scala

@@ -18,6 +18,7 @@ package org.typelevel.log4cats
 
 import cats.*
 import cats.Show.Shown
+import cats.mtl.Ask
 
 trait StructuredLogger[F[_]] extends Logger[F] {
   def trace(ctx: Map[String, String])(msg: => String): F[Unit]
@@ -58,6 +59,11 @@ object StructuredLogger {
   )(modifyCtx: Map[String, String] => Map[String, String]): StructuredLogger[F] =
     new ModifiedContextStructuredLogger[F](sl)(modifyCtx)
 
+  def withAskContext[F[_]](
+      sl: StructuredLogger[F]
+  )(implicit F: Monad[F], ask: Ask[F, Map[String, String]]) =
+    new AskStructuredLogger[F](sl)
+
   private class ModifiedContextStructuredLogger[F[_]](sl: StructuredLogger[F])(
       modify: Map[String, String] => Map[String, String]
   ) extends StructuredLogger[F] {