kotlinlang #announcements

I have an abstraction which amounts to Pair<Int,(Int)->Any?> this gets my needs met, however, there appears to be a JVM Integer boxing detour when the abstraction starts to nest; so the questions i have: 1. is this due to type erasure and/or generics forcing an object base class? 2. does th llvm compilation do a better job with LTO to eliminate boxing?

Nikky

07/21/2020, 7:49 AM

how does intellij idea know that the string in

".*".toRegex()

is regex and marks the string as such so that highlighting works? is this something i can define on a function wit the right annotations? ps: lees like

@Language("LANGUAGE_ID")

works on preoperties.. sadly not on extension properties or extension functions it seems either way jsonpath is not in the list of supported things but this should still be very useful later

Rob Murdock

07/21/2020, 2:00 PM

I guess this is an IDEA question - in a kotlin live templates, how does one include an import that references a receiver function? Normally the rule is use the fully qualified classname in the code and the editor converts that to imports automatically, but for the life of me I can’t figure out how to fully qualify a receiver function

pp.amorim

07/21/2020, 5:13 PM

Hi, I am getting this error:

e: org.jetbrains.kotlin.codegen.CompilationException: Back-end (JVM) Internal error: Failed to generate expression: KtProperty

In the code below:

private val foo: Foo = object: Foo() {

    override fun bar(results: Results) {
        val results: List<Zed>? = safeCast(results.values)
    }

    private inline fun<reified T> safeCast(values: Any): List<T>? =
        (values as? List<*>)?.filterIsInstance<T>()
}

Is that because the

inline fun

inside the

object

Foo

is a Java

abstract class

Mina Makram

07/21/2020, 8:45 PM

Hello 😁😁 I have a question concerning the kotlin multiplatform, should I post it here or which channel?

z3ntu

07/22/2020, 2:03 PM

Hi, am I doing something wrong with this or is this a Kotlin bug?

abstract class TestClass(open val value: Any)
class TestClass2(override val value: UInt) : TestClass(value)

val instance = TestClass2(0x1234u)
println("value: ${instance.value}")

I'm getting this exception:

java.lang.ClassCastException: java.lang.Integer cannot be cast to kotlin.UInt
	at Scratch_1$TestClass2.getValue(scratch_1.kts:2)
	at Scratch_1.<init>(scratch_1.kts:5)

Sean Najera

07/22/2020, 4:19 PM

👋 does anyone know of a linter that would fail the build if classes/methods/fields were not documented with Kdoc comments?

Marc Knaup

07/22/2020, 4:54 PM

What Kotlin technologies and frameworks is JetBrains Space based on? Is it really already possible to build a large web project in Kotlin without going nuts? 😁 (The more I use JS/TS & React/Vue/Angular the more I’m going crazy…)

pp.amorim

07/22/2020, 5:59 PM

I am trying to test a function where it uses

List.take(n)

and I setup the

iterator

of the mocked list and populated it. I thought it could be enough, but I am getting NPE.

`when`(mockedList.iterator()).thenReturn(list.iterator())

I noticed that

take

creates a new arraylist by doing

val list = ArrayList<T>(n)

. could be the problem here?

TwoClocks

07/22/2020, 6:04 PM

I want a function to take a

KClass

that extends a sealed class. Is there anyway to specify this at compile-time, or is this a run-time check?

pp.amorim

07/22/2020, 8:10 PM

Hi, how this answer could work with Kotlin? I tried with Consumer but it’s not being called. https://stackoverflow.com/a/49407513/2430555

Marshall

07/23/2020, 4:27 PM

Any suggestions on how to apply a set of checks to a data model? My first thoughts are to define various checks like this:

interface ErrorCheck<T> {
    fun isValid(item: T): Boolean
}

class PropertyRangeCheck<T, R : Comparable<R>>(val prop: KProperty1<T, R>, val range: ClosedRange<R>) : ErrorCheck<T> {
    override fun isValid(item: T) = range.contains(prop.get(item))
}

class PropertyStringLengthCheck<T>(val prop: KProperty1<T, String>, val range: IntRange) : ErrorCheck<T>{
    override fun isValid(item: T) = range.contains(prop.get(item).length)
}

Alberto

07/23/2020, 4:33 PM

what is the recommended project setup for a Kotlin Multiplatform library using Android Studio IDE?

Nir

07/23/2020, 6:09 PM

I noticed that Kotlin zip only really operates on two sequences, compared to say python's zip which conveniently works on more than 2. How would you zip multiple sequences without nesting the result deeper and deeper? This sort of thing makes me scratch my head a bit on why tuples were removed from the language.

Jim

07/23/2020, 9:21 PM

Hey there, I've got a kotlin library that I'd like to use from a java project. Is there a way I can prevent needing to import kotlin-bom and apply the kotlin plugin to the java project's gradle file?

TwoClocks

07/23/2020, 11:34 PM

For Enum definition : Is there a way to specify the ordinal value of the types? Or do you just get what the compiler decides?

TwoClocks

07/24/2020, 12:09 AM

why does this compile :

listOf(1,2,3,4).fold("") { acc, i -> acc }

but this doesn't

listOf(1,2,3,4).fold("") { acc, i -> { acc } }

Or put another way : How do I write fold that is multi-statements and has some variables?

TwoClocks

07/24/2020, 5:17 AM

how do I get gradle to depend on a local project that isn't in a subdirectory of the current project? I keep getting the error "could not be found in root project of...."

Andrew Gazelka

07/24/2020, 6:30 AM

This isn’t a Kotlin-specific question per-say, but I am doing this in kotlin. I’m trying to get away from extending classes because of intertwining behavior. I am abstracting my question, so if you want more specific details, ask me. I have a

GUI

which groups together `GUIComponent`s. Each

GUI

has a

onExit()

method that is called when the user tries to exit the

GUI

. • suppose I have

GUI

with a

counter

and every time a user

GUIComponent

is (A) clicked or the (B) user exits a
GUI
the

counter

should be increased. Once

counter

reached a

threshold

AnEvent

occurs how should I structure this? WAY 1 Right now I almost might think of having

GUIWithCounter(val counter: Int) : BaseGUI(...)

and each time (A | B) occurs we open

GUIWithCounter(counter+1)

• this hard because there is a lot of logic which goes into the creation of

BaseGUI

… therefore, when creating it we’d want to use a factory method instead. In Kotlin, we can’t extend a class by using factory methods WAY 2 We could have

GUIWithCounter : GUI

(here

GUI

is an interface) … we’d create an instance of

BaseGUI

with factory methods with desired arguments… and then use strategy pattern (delegate

GUI

methods to the instance of

BaseGUI

) .. this seems nice and dandy but here we’d need

BaseGUI

to be a concrete class. Therefore, we’d need a

onExit()

function in

BaseGUI

that does nothing/throws

UnimplementedException

(probably not an issue… but this is obv bad code. Perhaps

BaseGUI

would instead take a

GUIEventHandler

interface in the constructor and delegate

onExit()

to implementation defined there. Maybe just me but handlers seem like they could get messy with a bunch of

object: …

in code.) WAY 3 I don’t think this is a good idea???… but we could also have a

GUICounterState

and have a function which creates a

BaseGUI

with a handler and components that increment the

GUICounterState

. The

GUICounterState

would then have a detection method to see when

counter

has gone over a specific threshold. Thoughts?

Alberto

07/24/2020, 3:38 PM

Is reflection handled differently between Android and iOS with Kotlin Multiplatform? Accessing the 'constructors' val from KClass returns "Unresolved reference: constructors" when building the iOS variant, but doesn't complain for the Android variant. I've tried also adding the reflect dependency in case.

Pacane

07/24/2020, 4:06 PM

Does anyone know of a kotlin library that lets you do interactive CLI apps? I'd like something that wraps the logic of asking questions / parsing stdin, etc.

bbaldino

07/24/2020, 5:35 PM

I have a method with a generic type

bounded by

Any

and call a method which returns a

, but in this case

is a java type (an enum) and the method (defined in java) returns null. I expected an exception to be thrown here, but instead the value of null trickles up just fine, and a (kotlin) method which is set to return a value

T : Any

ends up happily returning null. Is it expected that this won't throw somewhere? Is this just a limitation of interacting with Java types from kotlin?

Manuel Pérez Alcolea

07/24/2020, 6:06 PM

I'm trying to do something with Kotlin just for the sake of learning (and not for practical reasons). What I thought was: I want to see if I can have some sort of class built with mixins in Kotlin. That's not possible per-se since there's no multiple inheritance or any specific tool that allows it, but I tried to have a similar effect with delegates, knowing that

by

exists. So, my idea was to have, for example, something like this...

Joshy Josh

07/24/2020, 10:17 PM

I’m still learning generics in general, trying to implement a generic builder pattern for the type safe builder approach. I currently have… `

fun <T> screen(init: T.() -> Unit): T = init as T

Then eventually extend it have bounds, but for now. I keep getting a casting error. I’m assuming because it’s not invoking init, but I can’t seem to figure that part out.

Marc Knaup

07/25/2020, 9:56 AM

Why is this internal?

someLong.toIntOrNull()

would be nice if the value is out of range

ahmad abas

07/25/2020, 10:32 AM

What is the

swiftUI

of kotlin for android

07/25/2020, 6:07 PM

data class Dummy(val namespace: String, val key: String, val value: String)
val dummies = listOf(...)

// do sth. with dummies

// expected: Map<String (namespace), Map<String (key), String (value)>>

jeggy

07/25/2020, 8:11 PM

val data: Map<Int, Pair<String, String>> = mapOf(
    25 to ("Hello" to "World")
)

data.map { (num: Int, (hello: String, world: String)) ->
    // ...
}

How come something like this is not valid code?

andylamax

07/26/2020, 6:09 AM

I don't know if this is the right place to ask this but here we go. Can someone deeply explain to me the difference between

listOf<T>()

and

arrayListOf<T>

. And while we are at it I would like to know more about the differences of 1.

setOf<T>()

and

hashSetOf<T>()

and

linkedSetOf<T>()

mapOf<T>()

and

hashMapOf<T>()

and

linkedMapOf<T>()

Also why,

List<T>

has no factory methods called

linkedListOf<T>()

Mehdi Haghgoo

07/26/2020, 10:44 AM

In a kotlin code for Compose I saw a usage like this:

val (a, b) = fun{}

Could someone tell me please what this usage is? Is this an assignment? Why is this crazy usage even legal? Sorry I know Kotlin is great but it sometimes make me feel stupid 😞

Title

Mehdi Haghgoo

07/26/2020, 10:44 AM

In a kotlin code for Compose I saw a usage like this:

val (a, b) = fun{}

Could someone tell me please what this usage is? Is this an assignment? Why is this crazy usage even legal? Sorry I know Kotlin is great but it sometimes make me feel stupid 😞

Milan Hruban

07/26/2020, 10:50 AM

it's descructuring variable declaration, you can read about it here - https://kotlinlang.org/docs/reference/multi-declarations.html

❤️ 2

David Hernando

07/26/2020, 11:01 AM

as Milan said it is destructuring the result of

state { DrawerState.Closed }

see https://developer.android.com/reference/kotlin/androidx/compose/package-summary#state it grabs the type of the return in the init lambda

❤️ 1

here you can see that MutableState has 2 components which end up being the destructured variables https://developer.android.com/reference/kotlin/androidx/compose/MutableState

❤️ 1

andylamax

07/26/2020, 11:05 AM

The function does return a variable that can be destructured, and the coder destructures it right away without extracting the returned variable into another variable. It makes the code concise, but I do agree with you, It can sometimes be hard to read.

❤️ 1

Mehdi Haghgoo

07/26/2020, 1:16 PM

Does this have anything to do with inline functions? I have a hard time understanding Kotlin docs on inline functions.

David Hernando

07/26/2020, 2:39 PM

no, the inline keyword is used to tell the kotlin compiler to embed the code of the function instead of creating an object for it

❤️ 1

so if the function is used multiple times, the code is magically copypasted by the compiler, avoiding stack allocations and everything needed to call an extra function

❤️ 1

brandonmcansh

07/26/2020, 9:08 PM

Just to reiterate what Milan and David said, as long as the target implements componentN() interfaces you can destructure it.

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