Hi all, I have no idea how to do the following : I...

# announcements

[Gilles Barbier]

Hi all, I have no idea how to do the following :
I have a few classes A, B, C, ... (for our use-case, they are describing different type of Message)
I have a function f defined by f(msg: A) : AP, f(msg: B) : BP, f(msg: C) : CP ... that convert those message into another format (Avro classes)
My issue is that I have no way to apply this function to an arbitrary collection of A, B, C, ..
setOf(msgA, msgB, msgA, msgC, msgB..).forEach { f(it) } because it is then of type Any
(despite each f(it) individually is valid, the syntax is invalid)
Would be happy to receive suggestion 🙂 about how to handle this

[Animesh Sahu]

Do a when check (just a suggesstion), it is more like boilerplate ;(

[Gilles Barbier]

Yes thx. Actually I wanted to avoid that. Not just because it's a lot of boilerplate, but also because I want to avoid to forget some cases..)

[Sandesh Baliga]

Why not make a sealed class and make all the message types as subclasses for it. Then make each f(x) a member function of respective classes, you can directly call then right?

[Matteo Mirk]

Yes, I support Sandesh solution: either make A,B,C… into a sealed class hierarchy or implement a common interface with the f() function. In the end polymorphism is what will save you from ifs/when

[Gilles Barbier]

Thx. I did that and it works well. Still, it's strange to be forced to write such code

fun convertToAvro(message: Message): SpecificRecordBase = when (message) {
    is CancelJob -> convertToAvro(message)
    is DispatchJob -> convertToAvro(message)
    is JobAttemptCompleted -> convertToAvro(message)
    is JobAttemptDispatched -> convertToAvro(message)
    is JobAttemptFailed -> convertToAvro(message)
    is JobAttemptStarted -> convertToAvro(message)
    is JobCanceled -> convertToAvro(message)
    is JobCompleted -> convertToAvro(message)
    is JobCreated -> convertToAvro(message)
    is JobDispatched -> convertToAvro(message)
    is JobStatusUpdated -> convertToAvro(message)
    is RetryJob -> convertToAvro(message)
    is RetryJobAttempt -> convertToAvro(message)
    is RunJob -> convertToAvro(message)
}

[Gilles Barbier]

(Message is a sealed class here)

[Sandesh Baliga]

Oh I see lot of message types.. in that case no idea :(

[Sandesh Baliga]

Why not make an abstract class MessageType with an abstract method convertToAvro, and instead of when statement, use

(message as MessageType).convertToAvro

[Gilles Barbier]

@Sandesh Baliga sorry, I do not get it.

[Animesh Sahu]

Make an abstract function in Message sealed class, and then implement it on every subclass

[Animesh Sahu]

And this function will call that

[Gilles Barbier]

ok. thx. Actually it gives the idea to use extension:

fun convertToAvro(message: Message): SpecificRecordBase = message.test()

    fun Message.test() : SpecificRecordBase = convertToAvro(this)

[Gilles Barbier]

apparently it's enough 🙂

[Gilles Barbier]

Sorry the extension is not valid - i though it was but not. It leads to a stackoverflow due to recursive calls

[Matteo Mirk]

Yeah it’s an infinite recursion…it won’t get you anywhere. It seems you’re not familiar with polymorphism, so I’ll show you a couple of solutions you can employ to eliminate that ugly when, which is hardly maintainable.
Use a common interface

interface Message {
    fun convertToAvro(): SpecificRecordBase
}

class CancelJob : Message {
    override fun convertToAvro(): SpecificRecordBase {
        TODO("create avro record")
    }
}

class DispatchJob : Message {
    override fun convertToAvro(): SpecificRecordBase {
        TODO("create avro record")
    }
}

// and so on...

or
Use a sealed class

sealed class Message {
    abstract fun convertToAvro(): SpecificRecordBase

    class CancelJob : Message() {
        override fun convertToAvro(): SpecificRecordBase {
            TODO("create avro record")
        }
    }

    class DispatchJob : Message() {
        override fun convertToAvro(): SpecificRecordBase {
            TODO("create avro record")
        }
    }
    // and so on...
}

then in any case you can convert various message types in the same collection:

setOf<Message>(...).forEach { it.convertToAvro() } // polymorphic call is dynamically dispatched to runtime type

In this way, whenever you add a new Message subtype the client code above won’t break.

[Gilles Barbier]

Thx a lot @Matteo Mirk . What if Message is not under my control (from another library) but convertToAvro is ?

[Matteo Mirk]

Oh I see… so the problem is a little harder. You could try to solve it with an extension function but unfortunately they’re resolved statically on the extension type, so they’re not polymorphic: https://kotlinlang.org/docs/reference/extensions.html#extensions-are-resolved-statically
Right now I can’t find a solution from the top of my head…

[Gilles Barbier]

ok thx anyway @Matteo Mirk it was helpful