is adding a function parameter with default value an ABI change?

tl;dr: Is there ordered queue with file-based storage for coroutines? I've got a Ktor socket server that captures log messages from a TCP port, and forwards them to a Kafka topic. However the connection to Kafka is unstable, so I need to store the logs locally, and then forward them to Kafka once the connection is up. I think the best solution is a local file-based ordered store that I can put messages into with a

MutableSharedFlow<String>

, and then another

SharedFlow<String>

that in another coroutine awaits new messages from the file-store and forwards them to the Kafka topic (I'm using Kotlin/JVM 1.7, Ktor 2.0, Kakfa 3.2)

Could anyone help me understand why the compiler complains here:

import com.fasterxml.jackson.core.type.TypeReference
import java.util.function.Supplier

class Bar(val x: Int)

fun foo(whatever: Supplier<TypeReference<out Bar>>) {}

fun main() {
    val typeRef = object : TypeReference<Bar>() {}
    val supplier = Supplier<TypeReference<Bar>> { typeRef }
    foo(supplier) // expected Supplier<TypeReference<out Bar>>, got Supplier<TypeReference<Bar>>
}

but it accepts this:

import java.util.function.Supplier

class Bar(val x: Int)

fun foo(whatever: Supplier<out Bar>) {}

fun main() {
    val supplier = Supplier<Bar> { Bar(42) }
    foo(supplier) // compiler happy
}

Is there something like filterNotNull for map keys? Such that a map with nullable key type is transformed to a map with not-nullable key type

is there some neat way to publicly expose a read-only view on a private, mutable collection? i always end up with something like

private val mutable = mutableListOf<Something>()
val readonly: List<Something> = mutable

which is kinda ugly

I wrote this convenience function for testing if a double is within a range.

fun assertWithin(expected: Double, actual: Double, tolerance: Double) {
    val message = "Difference of ${actual-expected} is out of range. " +
            "Expected $expected, got $actual"
    if(abs(expected-actual) > tolerance) println(message)
    assertTrue(abs(expected-actual) <= tolerance, message)
}

I see this message in the console, though

Difference of -6.004799503160669E14 is out of range. Expected 0.75, got -6.004799503160661E14

How could (-6.004799503160661E14)-(0.75) be equal to (-6.004799503160661E14)? Edit: misread the exponent. subtracting 0.75 from 6*10^14 is essentially 6*10^14

Does the concept of "an array of exactly 4 elements" exist in the kotlin type system? As an argument to my function.

This might come off as a bit crazy. But I inherited a project that has a mapper file that has like a 3,000 line when statement. There's only maybe about 300 when conditions, and each "mapper" is roughly 10 lines of code. My problem is that the IDE is suppppppper slow when this file is open. I can barely do anything. As a temporary measure I want to just split this file in half, and then half again. Which leads me to my question. Is there an easy/clever way to split a when statement across multiple files or do I have to refactor this, condition by condition. This wouldn't necessarily be a problem if the IDE could actually keep up with refactor tools. I just get the rainbow ball on macOS and have to wait like 3 minutes after every refactor. There has to be a better way. lol

I have a function that runs on main thread, which cannot be modified, like this:

fun calculate(): Int

And inside it I need to call a suspend function. How to do that?

runBlocking

?

given a map with possibly repeated values, is the following best way to reverse it (to a Map<V, List<K>>)

fun <K, V> Map<K, V>.reverse(): Map<V, List<K>> = entries.groupBy { it.value }.map { it.key to it.value.map { it.key } }.toMap()

Is there a library that implements the Debug / inspect functionality similar to the Rust

inspect

? Where you get a full dump of a class, not just the

toString

Would you recommend someone to do DSA interview in Kotlin? Or I should stick to traditional c#, Java, c/cpp or python

Is there a reliable way to determine the class of a reified generic, including typedefs? I tried a when(T::class), but I can’t use X::class when X is a typedef. I would like to distinguish X and Y, where X and Y are both typedefs for class Z.

I’m writing a block comment over a function and would like to use the nice formatting we get inside IntelliJ. I got a table right now which looks something like this

* | name | required | description | default value | example |
 * | :--- | :--- | :--- | :--- | :--- |
 * | name | ✅ | desc | ❌ | example |

It renders as I would expect, with the items left-aligned and everything, but everything is way too close to each other. Is there a way in this formatter to make it look like an actual table? Preferably keep some dividers vertically and horizontally across the items, but if not at least some way to force more space between the items.

Where do I put worksheets so I can use dependencies from the current project? When I put them into

src/main/java

, I get https://stackoverflow.com/questions/59366460/org-jetbrains-kotlin-resolve-lazy-nodescriptorfordeclarationexception-descripto

Hi all, just a general design choice question about value classes. If I create a value class, I am only allowed to have exactly 1 constructor parameter. If I then use this type, I still have to treat it as a class with a variable. If the compiler knows Foo only has 1 variable, and it knows its type. why does the compiler make me use myFoo.value instead of just myFoo?

@JvmInline
value class Foo(val value : Int) {
}

//This doesnt work
fun bar(myFoo : Foo) : Int{
   return myFoo + 1
}

//This works
fun bar(myFoo : Foo) : Int{
    return myFoo.value + 1
}

Can I express this code without having to have the

TLV<T>

signature, and just use

TLV

to represent it, if I'm using the interface to designate the type?

sealed interface TLV<T> {
    val value: T
    val type: Byte
}
object TLVNULL: TLV<Unit> {
    override val type: Byte = 0x00
    override val value: Unit = Unit
}
data class TLVNDEF(override val value: NdefMessage): TLV<NdefMessage> {
    override val type: Byte = 0x03
}
data class TLVProp(override val value: ByteArray): TLV<ByteArray> {
    override val type: Byte = 0xFD.toByte()
}
object TLVTerminator: TLV<Unit> {
    override val type: Byte = 0xFE.toByte()
    override val value: Unit = Unit
}

I have an enum defined, and I want to get the enum from the key that I get from another source (a network response to be exact). The manual mapping seems fine, but is there another way (thats idiomatic) to do this kind of thing?

enum class Foo(val key: String, val userName: String) {
  Bar("bar_item", "Bar For You"),
  Baz("baz_item", "Baz For Me"),
  OTHER("other", "Other"),
}

fun getTheFoo(key: String): Foo {
  return when (key) {
    "bar_item" -> Foo.Bar
    "baz_item" -> Foo.Baz
    "other" -> Foo.OTHER
    else -> Foo.OTHER
  }
}

Hi! I'm not sure of which channel is best for this, but I'm trying to get

provideDelegate

to work with generic output types. Basically, what I'm trying to do is the following setup:

class Producer {
    inline fun <reified T> create(): ReadOnlyProperty<Any, T> {
        println("Creating a property with create(), return type is ${T::class}")
        TODO()
    }
    
    inline operator fun <reified T> provideDelegate(thisRef: Any, prop: KProperty<*>): ReadOnlyProperty<Any, T> {
        println("Creating a property with provideDelegate, return type is ${T::class}")
        TODO()
    }
}

class MyClass(producer: Producer) {
    val one: String by producer.create()
    //val two: String by producer
}

While the

val one

works perfectly fine, if I uncomment

val two

, it no longer compiles. Is there any way to do this? Kotlin Playground link: https://pl.kotl.in/vXOAoeauS Compilation fails with:

Property delegate must have a 'provideDelegate(MyClass, KProperty<*>)' method. None of the following functions is suitable: public final inline operator fun <reified T> provideDelegate(thisRef: Any, prop: KProperty<*>): ReadOnlyProperty<Any, ???> defined in Producer

I want to build a simple web admin panel with kotlin. I suck at (web) design so I prefer to have a solid UI library. The web resources should ideally run from a static webspace (not an application server). (All backend api requests will be 3rd party APIs anyhow). What would you recommend?

Is there no smart cast in case of `OR`ed null checks inside of

if

?

val selectedScene1: String? = // ...
val selectedScene2: String? = // ...
if (selectedScene1 != null || selectedScene2 != null) {
    val sceneToFind: String = selectedScene1 ?: selectedScene2!! // The !! are necessary to make this work
}

is there a youtube channel I could check for more advanced kotlin?

why am I unable to import this extension function?

import io.github.gunpowder.api.config

doesn't resolve.

Is there an online tool to pretty print the

data class

toString()

output? I have a lot of nested data classes and when a test fails it's quite cumbersome to compare the linear outputs of the standard

toString()

implementation of a data class.

What is the idiomatic way of copying an immutable list with one element removed?

mapNotNull { if (it == removeMe) null else it }

doesn't support lists with nullable elements. In my case the elements have unique IDs, so I could also

filter {}

or

dropWhile {}

based on ID, but the names are not obviously suggesting that only one element gets removed. Do we have to do

toMutableList().remove(removeMe).toList()

?

anyone who would be able to help ? I think I have lost type safety using inline function with reified type, but not sure why

Why does this

if (true) 1 else if (false) { 2 } else { 3 }.also { println("hello") }
if (true) 1 else { 3 }.also { println("world") }

print only "world" and not "hello"? first one treats whole if/else as an expression why the latter one treats only else branch as an expression?

Is it possible to zip 3 lists? I am zipping 2 here, but I would like to throw a list of icons into the mix. How would it work in Kotlin?

@Composable
fun ShowDropDown(
    openDropDown: MutableState<Boolean>,
    text: List<String>,
    destination: List<DirectionDestination>,
    navigator: DestinationsNavigator,
) {
    DropdownMenu(
        expanded = openDropDown.value,
        onDismissRequest = { openDropDown.value = false },
    ) {
        text.zip(destination)
            .forEach {
                DropdownMenuItem(onClick = { navigator.navigate(it.second) },
                text = { Text(it.first) })
            }

    }
}