Add Identity type alias & monad implementation

core/src/main/scala/sttp/monad/MonadError.scala

@@ -1,5 +1,7 @@
 package sttp.monad
 
+import sttp.shared.Identity
+
 import scala.concurrent.{ExecutionContext, Future, Promise}
 import scala.util.{Failure, Success, Try}
 
@@ -169,3 +171,17 @@ class FutureMonad(implicit ec: ExecutionContext) extends MonadAsyncError[Future]
 
   override def blocking[T](t: => T): Future[T] = Future(scala.concurrent.blocking(t))
 }
+
+object IdentityMonad extends MonadError[Identity] {
+  override def unit[T](t: T): Identity[T] = t
+  override def map[T, T2](fa: Identity[T])(f: T => T2): Identity[T2] = f(fa)
+  override def flatMap[T, T2](fa: Identity[T])(f: T => Identity[T2]): Identity[T2] = f(fa)
+  override def error[T](t: Throwable): Identity[T] = throw t
+  override protected def handleWrappedError[T](rt: Identity[T])(
+      h: PartialFunction[Throwable, Identity[T]]
+  ): Identity[T] = rt
+  override def eval[T](t: => T): Identity[T] = t
+  override def ensure[T](f: Identity[T], e: => Identity[Unit]): Identity[T] =
+    try f
+    finally e
+}

core/src/main/scala/sttp/shared/package.scala

@@ -0,0 +1,9 @@
+package sttp
+
+package object shared {
+
+  /** The `Identity` type constructor can be used where an "effect" or wrapper (usually called `F[_]`) is expected. It
+    * represents direct-style / synchronous programming, and allows passing in code written in this style.
+    */
+  type Identity[X] = X
+}