How to write parametric/generic functions in kotlin - kotlin

Question

I'm trying to find a workaround to make a Spring Reactive Webclient work with JDBC.
Here is were I got the idea from: https://gitorko.github.io/2019/04/02/Spring-Webflux-Reactive-JDBC/.
I'm writing a service that calls the jdbc repository interface and instead of returning the type of my domain object MyClass returns a Mono<MyClass> like this:
//other specific imports here
import org.springframework.stereotype.Service
import reactor.core.publisher.Mono
import reactor.core.scheduler.Scheduler
import reactor.core.scheduler.Schedulers
import java.util.concurrent.Callable
#Service
class MyClassService(val repo: MyClassRepository, val jdbcScheduler: Scheduler){
fun save(obj: MyClass?): Mono<MyClass?>? {
return asyncCallable { repo.save(obj) }
}
protected fun <S> asyncCallable(callable: Callable<S>?): Mono<S>? {
return Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler)
}
}
//this is a jdbc repository
interface MyClassRepository : CrudRepository<MyClass, UUID> {}
Now the problem is that calling asyncCallable { repo.save(obj) } returns the compile error inferred type is MyClass? but TypeVariable(S) was expected and Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler) returns the compile error inferred type is Callable<S>? but Callable<out TypeVariable(T)!> was expected.
I understand by reading about kotlin generics that this has to do with the variance. If I'm not wrong the function asyncCallableis invariant on the generic type Sand in this case covariance is required?

Answer 1

I think the syntax you need is asyncCallable(Callable { repo.save(obj) }).
Complete example:
#Service
class MyClassService(val repo: MyClassRepository, val jdbcScheduler: Scheduler){
fun save(obj: MyClass): Mono<MyClass?>? {
return asyncCallable(Callable { repo.save(obj) })
}
protected fun <S> asyncCallable(callable: Callable<S>): Mono<S>? {
return Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler)
}
}
I'd also remove the ?s, but I left them to keep it as close to your code as possible.