GH-205 Added initial Kotlin support documentation

polished tests and test configuration

Resolves #205

docs/src/main/asciidoc/spring-cloud-function.adoc

@@ -21,14 +21,21 @@ include::getting-started.adoc[]
 
 == Function Catalog and Flexible Function Signatures
 
-One of the main features of Spring Cloud Function is to adapt and
+One of the main features of Spring Cloud Function is to adapt and support a range of type signatures for user-defined functions,
-support a range of type signatures for user-defined functions. So
+while providing a consistent execution model.
-users can supply a bean of type `Function<String,String>`, for
+That's why all user defined functions are transformed into a canonical representation by `FunctionCatalog`, using primitives
-instance, and the `FunctionCatalog` will wrap it into a
+defined by the https://projectreactor.io/[Project Reactor] (i.e., `Flux<T>` and `Mono<T>`).
-`Function<Flux<String>,Flux<String>>`. Users don't normally have to
+Users can supply a bean of type `Function<String,String>`, for instance, and the `FunctionCatalog` will wrap it into a
-care about the `FunctionCatalog` at all, but it is useful to know what
+`Function<Flux<String>,Flux<String>>`.
+
+Using Reactor based primitives not only helps with the canonical representation of user defined functions, but it also
+facilitates a more robust and flexible(reactive) execution model.
+
+While users don't normally have to care about the `FunctionCatalog` at all, it is useful to know what
 kind of functions are supported in user code.
 
+=== Java 8 function support
+
 Generally speaking users can expect that if they write a function for
 a plain old Java type (or primitive wrapper), then the function
 catalog will wrap it to a `Flux` of the same type. If the user writes
@@ -58,13 +65,33 @@ of a non publisher type (which is normal), it will be converted to a
 function that returns a publisher, so that it can be subscribed to in
 a controlled way.
 
-A function catalog can contain a `Supplier` and a `Function` (or
+=== Kotlin Lambda support
-`Consumer`) with the same name (like a GET and a POST to the same
+
-resource). It can even contain a `Consumer<Flux<>>` with the same name
+We also provide support for Kotlin lambdas (since v2.0).
-as a `Function`, but it cannot contain a `Consumer<T>` and a
+Consider the following:
-`Function<T,S>` with the same name when `T` is not a `Publisher`
+
-because the consumer would be converted to a `Function` and only one
+[source, java]
-of them can be registered.
+----
+@Bean
+open fun kotlinSupplier(): () -> String {
+    return  { "Hello from Kotlin" }
+}
+
+@Bean
+open fun kotlinFunction(): (String) -> String {
+    return  { it.toUpperCase() }
+}
+
+@Bean
+open fun kotlinConsumer(): (String) -> Unit {
+    return  { println(it) }
+}
+
+----
+The above represents Kotlin lambdas configured as Spring beans. The signature of each maps to a Java equivalent of
+`Supplier`, `Function` and `Consumer`, and thus supported/recognized signatures by the framework.
+While mechanics of Kotlin-to-Java mapping are outside of the scope of this documentation, it is important to understand that the
+same rules for signature transformation outlined in "Java 8 function support" section are applied here as well.
 
 == Standalone Web Applications
 

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/ContextFunctionCatalogAutoConfigurationTests.java

@@ -397,6 +397,8 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 
 		assertThat(context.getBean("kotlinSupplier")).isInstanceOf(Supplier.class);
 		assertThat(context.getBean("kotlinSupplier")).isInstanceOf(Function0.class);
+		Supplier<Flux<String>> supplier = catalog.lookup(Supplier.class, "kotlinSupplier");
+		supplier.get().subscribe(System.out::println);
 		assertThat(catalog.lookup(Supplier.class, "kotlinSupplier"))
 				.isInstanceOf(Supplier.class);
 		assertThat(inspector.getOutputType(catalog.lookup(Supplier.class, "kotlinSupplier")))

spring-cloud-function-context/src/test/kotlin/org/springframework/cloud/function/kotlin/KotlinLambdasConfiguration.kt

@@ -34,7 +34,7 @@ open class KotlinLambdasConfiguration {
 
     @Bean
     open fun kotlinConsumer(): (String) -> Unit {
-        return  { }
+        return  { println(it) }
     }
 
     @Bean