Kotlin - How to create an alias function of RxJava flatmap()?

Question

I tried to create an alias function to Flowable.flatmap() as follow, but compile error.

fun <T, R> Flowable<T>.then(mapper: Function<T, Publisher<R>>): Flowable<R> {
  return flatMap(mapper)
}

The error is : One type argument expected for interface Function<out R> defined in kotlin

Have any idea? Thanks!

Answer 1

The flatMap takes a java.util.function.Function, the actually error is you didn't import the java.util.function.Function in your Kotlin file, but I don't suggest you use the java-8 functions because you can't take advantage of the SAM Conversions to use the lambda directly from the Kotlin code which defined with java-8 functional interface as parameter type.

You should replace Function with Function1, since the Function interface is a Kotlin marker interface only. for example:

//                                  v--- use the `Function1<T,R>` here
fun <T, R> Flowable<T>.then(mapper: Function1<T, Publisher<R>>): Flowable<R> {
    return flatMap(mapper)
}

OR use the Kotlin function type as below, for example:

//                                      v--- use the Kotlin function type here  
fun <T, R> Flowable<T>.then(mapper: (T) -> Publisher<R>): Flowable<R> {
    return flatMap(mapper)
}