Kotlin lambda with explicit return type

This worked! Someone else gave me the answer in another forum, and if they respond here, I'll mark their answer correct.

fun handle(single: Single<String>): Single<Result> {
  return single
    .map<Result> { Single.Success() }
    .onError<Result> { Single.Error() }
}

You actually cannot explicitly declare the return type in a Kotlin lambda:

One thing missing from the lambda expression syntax presented above is the ability to specify the return type of the function.

https://kotlinlang.org/docs/reference/lambdas.html#anonymous-functions