Let’s say I want to define a data class over a generic type, as part of a sealed class so that I can use it like this

when (result) {
    is SuccessDataClassWithGenericTypeOf<MyGenericType> -> doThings(it)
    is Failed -> doFailedThings(it.failureMessage)
    is Loading -> doLoadingThings
}

Why does the type get erased in this case:

sealed class MyResultType {
    data class SuccessDataClassWithGenericTypeOf<T>(val result: T) : MyResultType()
    object Loading : MyResultType()
    data class Failed(val failureMessage: String) : MyResultType()
}

But not in this case:

sealed class MyResultType<out T> {
    data class SuccessDataClassWithGenericTypeOf<T>(val result: T) : MyResultType<T>()
    object Loading : MyResultType<Nothing>()
    data class Failed(val failureMessage: String) : MyResultType<Nothing>()
}

Hi! I am trying to implement some sort of event store / redux store using corutines in android application Here is the pseudocode:

class Store {
  private val channel = Channel<Action>()
  
  suspend func dispatch(action: Action) {
    channel.send(action)
  }

  init {
    GlobalScope.launch {
      for (action in channel) {
        reduce(action)
        notifyObservers()
      }
    }
  }
}

Sadly, this code doesn't work - actions are not reducer. Small detail: If i will explicitly set Dispatchers.Main in init and in dispatch - it work, but defeats my purpose. My goal is to move action processing of main thread.

notifyObservers

will build

Props

for each active

ViewModel

and publish.

Hi! I'm learning Coroutines and trying to wrap my head around how coroutines are run on a thread. So if I run large number of work-heavy coroutines on a thread, it will block that thread but if I run handful of them, there's a very small probability that the thread will be blocked. I'm trying to understand, how and why does coroutines block the thread in the former case and not in the latter one?

is it possible to configure an object in a sealed class to execute some behavior any time it is accessed? e.g.

sealed class ErrorStates {
    object ThingsAreBadNow {
        onAccess {
            Logger.log("A bad thing happened")
        }
    }
}

The

init

block can be used for data classes. Is there something I can do to get a similar behavior for objects?

Hi, is anyone able to help me with the correct syntax here please? I have the following class, which basically gets a JSON string from AWS, then converts it to an instance of a data class...

class SecretsManager(region: String) {
    private val gson = Gson()
    private val smClient = AWSSecretsManagerClientBuilder.standard().withRegion(region).build()

    fun <T> getSecret(id: String): T {
        val req = GetSecretValueRequest().withSecretId(id)
        val json = smClient.getSecretValue(req).getSecretString()
        return gson.fromJson(json, T::class.java)
    }
}

To be used like this...

val myInstance = SecretsManager("eu-west-2").getSecret<MyDataClass>("myId")

Currently, I get an error

Cannot use 'T' as reified type parameter

. I can get around this by marking the function as

inline

and T as

reified

, but then I can't access the private attributes from within the function. What's the best way to do this in Kotlin?

Simple question about nullable objects. is there a simple way to map them to other objects. Something similar to javas

Optional<String>.map(...).orElse(null)

I'm currently writing this

if (source.mpi.dsEndProtocolVersion!= null) Version(source.mpi.dsEndProtocolVersion!!) else null

but that doesn't feel right to me. Nullable String to nullable Version

Hi everyone, I just joined this Slack and wow its a hidden gem. I had a few questions. Most others say you need to learn Java before you try to learn Kotlin, is this true? My other question is, I have never used Kotlin before, just briefly watched some YouTube videos about it. Does anybody have any suggestions for resources for learning the language or would you guys just build projects to learn? Thanks in advance! 🙂

Hi! I'm new to Kotlin. Does Kotlin support having more than one src folder?

I can't get my Gradle DSL integration to work, IDEA doesn't understand that I want to have several src-filders. Any ideas? This is how my gradle.build.kts looks like, that lives on the root of my project:

plugins {
    kotlin("jvm") version "1.3.72"
}

buildscript {
    repositories {
        mavenCentral()
    }

    dependencies {
        classpath(kotlin("gradle-plugin", version = "1.3.72"))
    }
}

sourceSets.main {
    java.srcDirs("components/logger/src",
                 "components/user/src")
}

Is there anyone here that knows Clojure?

Hi all, I’m trying  to add on my build.gradle.kts this command

buildConfigField("String", apiKey, "123abc")

but I received this error: “Unresolved reference: buildConfigField ” All the example that I found put the command  in Android{ … } but I’m not doing an android app, only a simple web service. Do you have any idea ho I could resolve the problem? thanks

Hi everyone! I want to build a Kotlin Multiplatform App so I figured getting my feet wet in open source would be great for learning. Any open source multiplatform Kotlin apps you all know of?

Hi, thanks for the invite! Brief introduction: I have completed a two year diploma, mostly did C# and Java webapp stuff. I just started learning Kotlin a month ago, I'm almost done the Android Kotlin Fundamentals Codelabs. First thing, I'm still a bit confused with the Kotlin syntax... I can comment my tutorial code and know what's happening overall, experience with Java helps here, I understand the idea of Entity classes and data binding etc. But the details of each part of the code still confuse me. This is bad and I want to fix ASAP, would you guys suggest Kotlin koans for this, or maybe the Kotlin bootcamp Codelabs instead? Second, at school we did everything with local databases. I'm trying to build an Android app that saves data online and a webapp that can access this data. On the app side of things, it looks like the Room persistence library sorts out most of my issues, but I could use some more reading material and/or tutorials regarding using it with an online database instead of a local database. Last, where would I be able to find more info on frameworks used for Kotlin web app development? I've been trying to research but I'm pretty unfamiliar with the subject so it's hard for me to find useful information. It looks like ktor or spring boot would be worth considering, am I on the right path here? If someone could point me in the right direction that would be awesome. Apologies if I'm ranting a bit here or if I missed any important information, I can clarify or add information if needed. Thanks for any input :)

What options exist for converting JSON schemas (not payloads) into Kotlin data classes? I have tested the JsonToKotlinClass plugin via IntelliJ which comes close, but it does not seem to interpret required / optional directives correctly. E.g. given a schema with two properties, one required and the other optional -

{
  "$schema": "<http://json-schema.org/draft-07/schema#>",
  "$id": "<https://abc.com/request.schema.json>",
  "title": "Request record",
  "type": "object",
  "properties": {
    "id": {
      "description": "The unique ID of a request",
      "type": "string"
    },
    "name": {
      "description": "The name of a request",
      "type": "string"
    }
  },
  "required": [
    "id"
  ]
}

I would expect to receive output like -

data class Request(
    val id: String,
    val name: String?
)

but instead, this is generated (both fields are marked as required) -

data class Request(
    val id: String,
    val name: String
)

Ideally I'd like to find a solution that can be implemented programatically too, rather than requiring a manual plugin (though that's not as important as getting the nullable part right).

Is there any Protobuf libraries that create kotlin data classes directly from the .proto files?

Hey everyone! Long time iOS developer here looking to build my first Android app. Are there any tutorials or lessons you can recommend to get my feet wet? I'm looking to recreate an app I built with SwiftUI. Thanks and nice to meet everyone!

Hey, could anyone help me with something? with the data class below:

data class Item(val completed: Boolean, val details: String? = null)

and a list:

val p = listOf(
        Item(true, "one"),
        Item(false, "two"),
        Item(true, "three")
	)

I need to transform this data (truncate the second value in the Item to return something like:

listOf(
        Item(true, "o"),
        Item(false, "t"),
        Item(true, "t")
	)

Im trying:

val p = listOf(
        Item(true, "one"),
        Item(false, "two"),
        Item(true, "three")
	)
    
    println(p.map { (_,details) -> details?.take(1) })

but it returns: [o, t, t] What’s the best way to do this in Kotlin? Thank you in advance

HI, I am quite new to kotlin. I have currently an generated Java interface containing spring Requestmapping annotations and extend this in my kotlin project. This works fine and the annotations are picked up and processed. When I generate the same interface as a kotlin interface though, these annotations seem to be ignored. Do I have to configure something to make this work?

Hi, What's the best way to abstract out the fromJson functionality so that it can be re-used? There doesn't seem to be a way to do it using interfaces that I can find...

data class MyClass(val type: String) {
    companion object {
        fun fromJson(j: String) = Gson().fromJson(j, MyClass::class.java)
    }
}

data class MyOtherClass(val type: String) {
    companion object {
        fun fromJson(j: String) = Gson().fromJson(j, MyOtherClass::class.java)
    }
}

Hi! I wonder why List<T>, where T is @Serializable is not @Serializable itself? There's a

T.serializer().list

serializer, that does exactly what I expect from List<T>.serializer(), but with this approach it seems like I have to write a custom serializer for any class that contains List in it

can someone explain me enum classes. I didn't get what are they and it's my first time using enum classes

👋 I’m trying to understand what this error message is meaning:

ERROR - pplication.impl.LaterInvocator - loader constraint violation: when resolving method 'com.intellij.ui.layout.CellBuilder com.intellij.ui.layout.Row.intTextField(kotlin.reflect.KMutableProperty0, java.lang.Integer, kotlin.ranges.IntRange)' the class loader com.intellij.ide.plugins.cl.PluginClassLoader @6ad38805 of the current class, com/github/intheclouddan/intellijpluginld/LaunchDarklyConfigurable$createPanel$$inlined$panel$lambda$4, and the class loader com.intellij.util.lang.UrlClassLoader @215be6bb for the method's defining class, com/intellij/ui/layout/Cell, have different Class objects for the type kotlin/ranges/IntRange used in the signature (com.github.intheclouddan.intellijpluginld.LaunchDarklyConfigurable$createPanel$$inlined$panel$lambda$4 is in unnamed module of loader com.intellij.ide.plugins.cl.PluginClassLoader @6ad38805, parent loader 'bootstrap'; com.intellij.ui.layout.Cell is in unnamed module of loader com.intellij.util.lang.UrlClassLoader @215be6bb, parent loader 'platform')

I’m trying to use an intellij

intTextField

and just assign a var as

refreshRate: Int

I have Android Interview day after tomorrow, so how should I prepare for that?

I have an extension function declared within a

Fragment

subclass:

inline fun <T> Flow<T>.collectInViewScope(crossinline action: suspend (value: T) -> Unit) {
    viewLifecycleOwner.lifecycleScope.launch { 
        collect(action)
    }
}

But is there a way to declare this outside of the class (so it can be used from any

Fragment

subclass, without having to pass in the

Fragment

instance as an arg? A kind of extension function with added context of the call site. One could imagine

inline <http://Fragment.fun|Fragment.fun> <T> Flow<T>.collectInViewScope(…)

How would i translate 2 bytes from and to a UShort... I'm sure I'm missing something but functions dealing with Unsigned Integers and multiple bytes seems to be lacking from the standard library.

Hi. I'm writing a parser for a format that contains heterogeneous values that don't share a common class (other than

Object

). What's the best / most idiomatic way to represent that collection? In TypeScript I'd do something like

Array<Type1 | Type2>

, but I don't know how to do something like that on Kotlin/Java/JVM.

Is there any way to access a property delegate via an extension property? For example:

class Settings(val sharedPreferences: SharedPreferences) {
    val appVersion: Int by <http://sharedPreferences.int|sharedPreferences.int>(APP_VERSION)
}

but I would like to do:

val Settings.appVersion: Int by <http://sharedPreferences.int|sharedPreferences.int>(APP_VERSION)

Been using Kotlin for 3, 4 yrs now. I recently took “Kotlin for Java Developers” on Coursera, and it was very interesting and there were a lot to learn. Wonder if there are more courses like this. I’m looking for a course for more experienced user, because the Coursera course took me 2 days to complete it. (and although I learned a lot there were lots of things I already knew.) Or a book like effective java of java. Any recommendations are welcome! Thanks.

Hi there, is Kotlin in Action still up-to-date? I had done some Kotlin 3 years ago. I would like to to refresh my memory now. Has anything of importance changed in Kotlin or was added to the language since then which is not covered in Kotlin in Action. Coroutines come to mind. Are there any other gotchas? Like major stdlib changes/additions?   Thanks!