kotlinlang #announcements

Hello, I’m new in kotlin. Is there anyone who knows Scala language? I wanna convert from a code written Scala to kotlin. I don’t have knowledges related bounded type. For example this is a code written Scala language.

// scala
sealed trait Test[+A] {
   …
   def test[B >: A]: B
   …
}

// kotlin
Sealed class Test<out A> {
   …
   fun <A : B> test(): B.   // <— this is problem
   …
}

I want to make type B is a super type of A. Anyone who knows this problem??

edwardwongtl

02/26/2019, 9:22 AM

try

fun <A, B> test(): B where A: B

Jeremy

02/26/2019, 9:34 AM

So.. is there any other solution ?

natpryce

02/26/2019, 11:16 AM

However, I still cannot get that example to compile when converted to Kotlin syntax. Seems to be a limitation of either the Kotlin type inference algorithm, or (more likely) my ability to bend it to my will.

Rob

02/26/2019, 2:13 PM

is there a better way to have each subclass call the superclasses toString method without having

override fun toString(): String {
    return super.toString()
}

in each of the subclasses?

karelpeeters

02/26/2019, 3:46 PM

There's

Pair

and

Triple

, but I'm curious about what you mean by "bulky".

data class Point(val x: Int, val y: Int)

isn't a lot of code.

➕ 3

Joe

02/26/2019, 5:38 PM

I have a few generic `Map`s of String->Predicate functions of different types. eg, a

Map<String, (Int)->Boolean>

and a

Map<String, (String)->Boolean>

. I'd like to combine them to get a

Map<String, (Any)->Boolean>

, but can't quite figure out how to do so safely. I can safely cast to

Map<String, *>

Map<String, Any?>

without an unchecked cast warning, but I can't safely cast to `Map<String, (*)->Boolean>`(compile error) or

Map<String, (Any?)->Boolean>

(unchecked cast)

awzurn

02/27/2019, 12:18 AM

Has anyone else run into a use-case of wanting to be able to copy a data class with a generic field, where the new value being provided (for the generic field) is a different type? I recently ran into when I was using a data class as a message wrapper for a parsed message (which included both the parsed typed entity and the payload it was parsed from). Scala support something like this with their `case class`:

scala> case class Something[T](something: T)
defined class Something

scala> Something(1)
res0: Something[Int] = Something(1)

scala> res0.copy(something = "something")
res1: Something[String] = Something(something)

Kotlin, on the other hand, won’t compile something like this:

data class Something<T>(val value: T)
Something(1).copy("something")

I’m wondering if it was an explicit design decisions, or maybe something that just hasn’t been implemented yet?

David Cuesta

02/27/2019, 8:25 AM

Hi guys, question about coroutines. Anyone knows why testList works and testMap launches an error when calling the suspend function?

import kotlinx.coroutines.delay

suspend fun first(a: Int): Int {
    delay(100)
    return a + 1
}

suspend fun testMap(a: Map<String, Int>): Int {

    a.forEach { key, value ->
        first(value)
        println(value)
    }

    return 0
}

suspend fun testList(a: List<Int>): Int {

    a.forEach {
        first(it)
        println(it)
    }

    return 0
}

Edgars

02/27/2019, 11:04 AM

Pardon if there's a better-suited channel for this, but I've finally thought of a decent project that I could do. I want to make it multi-platform (OS-wise). 1) What would be better for this task - Kotlin/JVM or Kotlin/Native? 2) How do I go about doing the GUI so that it works on all platforms? Swing (if JVM)? I'm not really experienced with making desktop applications. I have a couple of years of professional experience with Kotlin/JVM, so it'd be easier for me to do it that way + whatever GUI framework, but perhaps there's an advantage doing it the native way?

louiscad

02/27/2019, 1:58 PM

Has anyone encountered that error regarding

jvmTarget

> Task :some-android-lib-module:compileDebugKotlin FAILED
e: Unknown JVM target version: 1.7
Supported versions: 1.6, 1.8

David R.

02/27/2019, 2:31 PM

Is there a way in Kotlin to create an empty EnumSet from a reflected enum Class? It seems like I can do it with Java, but not with Kotlin based on the enum type constraints and inference (and if I take Java code and paste it into Kotlin and have IntelliJ do the auto convert, it fails)

Lamberto Basti

02/27/2019, 3:00 PM

Someone knows how to locally add a multiplatform project as a dependecy for a JVM project? I've posted my usecase here: https://discuss.kotlinlang.org/t/how-do-you-import-a-module-of-a-local-multiplatform-library-using-gradle/11752

LeoColman

02/27/2019, 4:04 PM

Guys, in a week from now I'll be organizing a Kotlin Dojo. The idea is bringing the Java-people to start playing around with Kotlin, and maybe use it in their projects. Do you have any tips for this? Maybe a dojo-like problem?

hudsonb

02/27/2019, 11:51 PM

I have a a Polygon class. I'm adding several operators and useful functions. Would you add these as methods in the class, or as extension functions? Not concerned with Java interop.

Nikolai

02/28/2019, 4:53 AM

Hi, everyone Sorry if I am writing to wrong channel. I used Kotlin for previous multiplatform project. I am using IDEA, so I used: Kotlin (Mobile Android/iOS) project type. Now I want to develop another project for both mobile platforms but in addition - develop a server on Kotlin as well and want to share same model classes or common code betwee those parts. What is the best project type/setup/structure (preferably in IDEA) for this kind of projects?

Daniel Eke

02/28/2019, 8:27 AM

Hi! Is there a Slack group or a discussion somewhere about

SQLDelight

? I have a bunch of questions but I don’t want to bombard them with issues. 😄

JoakimForslund

02/28/2019, 8:35 AM

Is there a deep diff util already existing for checking differences in objects within a collection usable in a kotlin multiplatform scenario?

Julien Genoud

02/28/2019, 11:51 AM

Hi who can explain me or give me an useful tutorial/usages on

inline

i still don't understand it ... thanks

s1m0nw1

02/28/2019, 1:52 PM

When I see two instances of the same

object

in my application, what might be the issue? Seems to be possible

Andrew Gazelka

02/28/2019, 3:30 PM

I am solving problems on https://projecteuler.net and want to employ TDD. There are often utility methods closely coupled to each problem I am solving, so much so that it might not be appropriate to make a utility class to include top-level functions. How do you recommend me best unit testing these top-level functions? ... is it a bad practice to include the specific top-level functions in the same class as my problem?

Andrew Gazelka

02/28/2019, 4:34 PM

have https://gist.github.com/andrewgazelka/63b8665928ae68dd00bdaed40161e9c2 and getting when testing (

1.3.21

)

CLOVIS

02/28/2019, 5:31 PM

Is there a server for multiplatform Kotlin? I have a few questions but don't know where to ask

Andrew Gazelka

03/01/2019, 12:21 AM

iyo is using HOF with builder pattern sometimes bad because people can execute arbitrary code in them?

tipsy

03/01/2019, 6:56 AM

i'm seeing

java.lang.IllegalStateException: Resource not found in classpath: kotlin/coroutines/coroutines.kotlin_builtins

intermittently in a tests that use to work fine. it only seems to happen on windows (build seems to work on linux consistently, and it's never failed on travis). has anyone run into this?

mbonnin

03/01/2019, 10:36 AM

Hi! Are there any recommendations on how to use exceptions in kotlin ? With the lack of checked exceptions, we've been hit a couple of times by missing

catch

clauses. For an example IOExceptions on files/connections that made the app crash while we should have just shown an error screen. We were considering removing exceptions altogether from our codebase and just rely on return values. Legacy java dependencies like

File

would still need

catch

blocks and that's okay. But then we realized that even new libs like retrofit-coroutines throw exceptions. How are you handling this ? Do you have a lot of exception handling in your codebases or do you use return values ? Is it okay for kotlin libs to throw non-fatal exceptions ?

mmaillot

03/01/2019, 12:50 PM

Hi! Is it possible to create an extension on enum to get a random value. I have severals enum and for my test, I want to get a random value of this enum. Is it possible?

Title

mmaillot

03/01/2019, 12:50 PM

Hi! Is it possible to create an extension on enum to get a random value. I have severals enum and for my test, I want to get a random value of this enum. Is it possible?

diesieben07

03/01/2019, 12:53 PM

inline fun <reified E : Enum<E>> randomValue(): E {
    return randomValue(E::class.java)
}

fun <E : Enum<E>> randomValue(cls: Class<E>): E {
    return cls.enumConstants.random()
}

Usage:

val rnd = randomValue<Foo>()
// or
val rnd = randonValue(Foo::class.java)

mmaillot

03/01/2019, 12:56 PM

Exactly what I want! thanks!

Mark McCormick

03/01/2019, 1:14 PM

YourEnumClass.values().withIndex().shuffled().first().value

diesieben07

03/01/2019, 1:15 PM

That is way slower.

Dico

03/02/2019, 1:23 PM

enumValues<E>().let { it[Random.nextInt(size)] }

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