(JS) Expose handler function type to allow manual exporting of Lambda…

… handler

Related to #246

lambda/js/src/main/scala/feral/lambda/IOLambdaPlatform.scala

@@ -26,12 +26,23 @@ import scala.scalajs.js.|
 private[lambda] trait IOLambdaPlatform[Event, Result] {
   this: IOLambda[Event, Result] =>
 
+  /**
+   * Lambda handler. Implement this type with a val and a call to `handlerFn` to export your
+   * handler.
+   *
+   * @example
+   *   {{{
+   *  val handler: HandlerFn = handlerFn
+   *   }}}
+   */
+  final type HandlerFn = js.Function2[js.Any, facade.Context, js.Promise[js.Any | Unit]]
+
   final def main(args: Array[String]): Unit =
     js.Dynamic.global.exports.updateDynamic(handlerName)(handlerFn)
 
   protected def handlerName: String = getClass.getSimpleName.init
 
-  private lazy val handlerFn
+  protected lazy val handlerFn
       : js.Function2[js.Any, facade.Context, js.Promise[js.Any | Unit]] = {
     (event: js.Any, context: facade.Context) =>
       (for {

lambda/js/src/test/scala/feral/lambda/ExportedLambdaSuite.scala

@@ -0,0 +1,60 @@
+package feral.lambda
+
+import cats.effect.IO
+import cats.effect.Ref
+import cats.effect.kernel.Resource
+import cats.syntax.all._
+import io.circe.scalajs._
+import munit.CatsEffectSuite
+
+import scala.scalajs.js
+import scala.scalajs.js.|
+
+class ExportedLambdaSuite extends CatsEffectSuite {
+  test("exported lambda") {
+    val context = DummyContext.asInstanceOf[facade.Context]
+
+    for {
+      allocationCounter <- IO.ref(0)
+      invokeCounter <- IO.ref(0)
+      lambda = new CountingIOLambda(allocationCounter, invokeCounter)
+
+      _ <- ('0' to 'z')
+        .map(_.toString)
+        .toList
+        .traverse(x =>
+          IO.fromPromise(IO(lambda.impl(x, context)))
+            .assertEquals(x.asJsAny.asInstanceOf[js.Any | Unit]))
+
+      _ <- allocationCounter.get.assertEquals(1)
+      _ <- invokeCounter.get.assertEquals(75)
+    } yield ()
+
+  }
+}
+
+class CountingIOLambda(allocationCounter: Ref[IO, Int], invokeCounter: Ref[IO, Int])
+    extends IOLambda[String, String] {
+
+  override def handler: Resource[IO, LambdaEnv[IO, String] => IO[Option[String]]] =
+    Resource
+      .eval(allocationCounter.getAndUpdate(_ + 1))
+      .as(_.event.map(Some(_)) <* invokeCounter.getAndUpdate(_ + 1))
+
+  // This would be exported with `@JSExportTopLevel("handler")`
+  def impl: HandlerFn = handlerFn
+}
+
+object DummyContext extends js.Object {
+  def callbackWaitsForEmptyEventLoop: Boolean = true
+  def functionName: String = ""
+  def functionVersion: String = ""
+  def invokedFunctionArn: String = ""
+  def memoryLimitInMB: String = "512"
+  def awsRequestId: String = ""
+  def logGroupName: String = ""
+  def logStreamName: String = ""
+  def identity: js.UndefOr[CognitoIdentity] = js.undefined
+  def clientContext: js.UndefOr[ClientContext] = js.undefined
+  def getRemainingTimeInMillis(): Double = 0
+}