Add more Publisher extensions.

src/main/kotlin/reactor/kotlin/core/publisher/MonoExtensions.kt

@@ -18,6 +18,7 @@ package reactor.kotlin.core.publisher
 
 import org.reactivestreams.Publisher
 import reactor.core.publisher.Mono
+import java.util.Optional
 import java.util.concurrent.Callable
 import java.util.concurrent.CompletableFuture
 import java.util.function.Function
@@ -68,6 +69,20 @@ fun <T> CompletableFuture<out T?>.toMono(): Mono<T> = Mono.fromFuture(this)
  */
 fun <T> Callable<T?>.toMono(): Mono<T> = Mono.fromCallable(this::call)
 
+/**
+ * Extension for transforming a [Runnable] to a [Mono].
+ *
+ * @author Cyril Delmas
+ */
+fun <T> Runnable.toMono(): Mono<T> = Mono.fromRunnable(this)
+
+/**
+ * Extension for transforming an [Optional] to a [Mono].
+ *
+ * @author Cyril Delmas
+ */
+fun <T> Optional<out T?>.toMono(): Mono<T> = Mono.justOrEmpty(this)
+
 /**
  * Extension for transforming an exception to a [Mono] that completes with the specified error.
  *

src/test/kotlin/reactor/kotlin/core/publisher/MonoExtensionsTests.kt

@@ -28,6 +28,7 @@ import reactor.kotlin.test.verifyError
 import reactor.test.StepVerifier
 import reactor.test.publisher.TestPublisher
 import java.io.IOException
+import java.util.*
 import java.util.concurrent.Callable
 import java.util.concurrent.CompletableFuture
 
@@ -164,6 +165,25 @@ class MonoExtensionsTests {
                 .verifyError(IllegalStateException::class)
     }
 
+    @Test
+    fun optionalToMono() {
+        StepVerifier.create(Optional.of(42).toMono())
+                .expectNext(42)
+                .verifyComplete()
+    }
+
+    @Test
+    fun optionalEmptyToMono() {
+        StepVerifier.create(Optional.empty<Any>().toMono())
+                .verifyComplete()
+    }
+
+    @Test
+    fun runnableToMono() {
+        StepVerifier.create(Runnable { }.toMono<Any>())
+                .verifyComplete()
+    }
+
     @Test
     fun `cast() with generic parameter`() {
         val monoOfAny: Mono<Any> = Mono.just("foo")