Hi all. I have the following two methods inside my...

# getting-started

[df]

Hi all. I have the following two methods inside my class Foo:

fun cancel(userId: String?, event: Cancel, options: EventOptions? = null, extra: MiddlewareExtra? = null)
fun ship(userId: String?, event: Ship, options: EventOptions? = null, extra: MiddlewareExtra? = null)

With Cancel and Ship defined as:

class Cancel : Event<Cancel> {}
class Ship : Event<Ship> {}
I'd like to create a map of events and their corresponding methods. I tried the following

val map = mapOf<KClass<out Event<*>>, (Foo, String?, Event<*>, EventOptions?, MiddlewareExtra?) -> Unit>(
Cancel::class to Foo::cancel,
Ship::class to Foo::ship
)

But this gives me the following error.

Required: (Foo, String?, Event<*>, EventOptions?, MiddlewareExtra?) → Unit
Found: KFunction5<Foo, String?, Cancel, EventOptions?, MiddlewareExtra?, Unit>

Any help is appreciated.

[Adam S]

can you use three backticks ``` to format your code blocks please? https://slack.com/help/articles/202288908-Format-your-messages

[Wout Werkman]

First of all, I'm gonna take the assumption that your Foo looks like this:

class Foo {
  fun cancel(userId: String?, event: Cancel, options: EventOptions? = null, extra: MiddlewareExtra? = null) {}
  fun ship(userId: String?, event: Ship, options: EventOptions? = null, extra: MiddlewareExtra? = null) {}

  val map = mapOf<KClass<out Event<*>>, (Foo, String?, Event<*>, EventOptions?, MiddlewareExtra?) -> Unit>(
    Cancel::class to Foo::cancel,
    Ship::class to Foo::ship
  )
}

You need to understand that function parameters are in when used as generics. (Cancel) -> Unit is not a subtype of (Event<*>) -> Unit.
Imagine the following scenario.

val ship: Ship = TODO()
val cancelProcessor: (Cancel) -> Unit = TODO()
val someProcessor: (Event<*>) -> Unit = cancelProcessor as (Event<*>) -> Unit
someProcessor(ship) // Will throw runtime exception

Now you can hack around this by just adding casts as follows:

val map = mapOf<KClass<out Event<*>>, (String?, Event<*>, EventOptions?, MiddlewareExtra?) -> Unit>(
  Cancel::class to Foo::cancel as (String?, Event<*>, EventOptions?, MiddlewareExtra?) -> Unit,
  Ship::class to Foo::ship as (String?, Event<*>, EventOptions?, MiddlewareExtra?) -> Unit,
)

Which will trick the compiler into allowing this.

[df]

Thanks for the explanation, not sure if I like the explicit casts though but it will do the trick for now 🙂

[Wout Werkman]

Yeah, it's verbose. But using such abstractions for these semantics can never be verified to be typesafe by the compiler. At some point you as the developer have to tell the compiler "I know what I'm doing, I'll take responsibility for the potential runtime errors". That's what casts are for 🙂.

[Wout Werkman]

I do agree that it's hard to comprehend the `in`/`out` semantics of generics sometimes