kotlinlang #getting-started

Hi guys, I tried getting started on kotlin native and dont understand whether or not we can use exceptions. What ever I do cannot import IllegalArgumentException in native code. Why is that? Tried searching but didn;t find anywhere stating that this shoudn;t work

Uberunix

04/13/2020, 5:05 PM

Hello! I'm beginning my first Kotlin course and I'm confused about the creation of objects. The instructor has created them in two different ways Method 1:

val myNewObject = NewObject(x,y,z)

Method 2:

val myNewObject: NewOject = NewObject(x,y,z)

Is there a practical reason that he declared the type of "myNewObject" in certain cases? It seems unnecessary.

Enock Kalya

04/14/2020, 6:00 AM

Hello everyone, can someone point to useful resources for kotlin like a book or practice exercise I can get my hands dirty with.

:google: 4

Sanket Mehta

04/14/2020, 10:26 AM

fun testWhenSyntaxDataProvider(): Array<Array<Any?>> {
        return arrayOf(
            arrayOf(null, null, 10),
            arrayOf(1, null, 10),
            arrayOf(null, 2, 20),
            arrayOf(10, 20, 30)
        )
    }

The above function is throwing below exception:

Kotlin: Type inference failed. Expected type mismatch: inferred type is Array<Array<out Any?>> but Array<Array<Any?>> was expected

How can I typecast Int to Any?

iseki

04/15/2020, 12:29 AM

Hi, I want to know the best practices to writing a codegen. KAPT only support JVM and it can't modify code. Compiler plugin is so difficult to write and looks it's only works on IR level? And both them make IDE hardly to use. So... What's the best practices? Thank you...

crummy

04/15/2020, 10:53 AM

historically it was common to run the oracle JDK in production. Nowadays I think a lot of folks just run openJDK everywhere.

Pedro González

04/15/2020, 8:48 PM

I'm used to working with tornadofx for java 8 but a newbie with the new way javafx is decoupled, and also starting with gradle

Keith

04/16/2020, 8:06 PM

val alsoEmpty = listOf<String>()

alsoEmpty.add("funny face")
error: unresolved reference: add
alsoEmpty.add("funny face")

Ananiya

04/17/2020, 4:25 PM

how do i correct this code i was trying to print "very strong" when the password is very strong but it prints both "very strong" and "strong" and "need improvement" if the password is weak it works fine

Slawomir Ziolkowski

04/18/2020, 11:18 AM

Hi, I'm new to Kotlin. Amazing language but sometimes it confuses me a lot 🙂 Maybe someone could help me there:

fun main(args: Array<String>) {
    val firstList = listOf<User>(
            User(name="P-0"), User(name="P-0"), User(name="P-0"),
            User(name="P-2"), User(name="P-2"), User(name="P-2"), User(name="P-2")
    )
    val secondList = listOf<User>(User(name="P-2"))
    print(firstList.minus(secondList))
}

data class User(val name: String)

I expect that only one P-2 element will be not in the result, but the result looks like this: [User(name=P-0), User(name=P-0), User(name=P-0)]

Raymond Gao

04/18/2020, 5:44 PM

Hi I'm playing around with some serialization using reflection. I would like to be able to serialize some simple classes into JSONObjects in the following way

val foo : Foo = Foo()
var jobj : JSONObject = serialize(foo)
var foo2 : Foo = construct(jobj, Foo::class)

The methods serialize and construct will inspect the object using reflection, find any fields that match a set of constraints and add their values to a json object or send them into the appropriate constructor of

Foo

to create a new object It works until I run into Generics

class Foo(val map : Map<String, Int>)
{
...
}

I'm having some difficulty getting the type parameters passed into

Map

at runtime. My friend has told me about the concept of type erasure. It seems that kotlin throws away these arguments at runtime. I have resorted to annotating the field with the appropriate types for now, but it causes some ugly duplication of information already typed down.

class Foo(@param:GenericsHint([String::class, Int::class]) val map : Map<String, Int>)

Does anyone have any ideas on how I can get around this problem? Thanks 🙂

fmcauley

04/19/2020, 4:55 PM

Disclaimer: not sure if this the correct channel for my question... I have an older android project with numerous dependencies that I, as of yet, have been able to get past Gradle 3.3. When I attempt to build a new class using Kotlin and Android Studio offers to update my Gradle dependencies I get an error that I need Gradle version 4.9 or higher. So, is there a way to add Kotlin support to my project independent of Gradle? Or is there another workaround? I would love to develop with Kotlin as I work on cleaning up the codes dependencies and navigate to newer versions of Android Studio and Gradle.

Chris Cordero

04/20/2020, 1:15 PM

Are Kotlin `MutableList`s implemented like a linked list under the hood? Can I pop/append from both ends or from the middle cheaply? Or is this an expensive calculation

Madalin Valceleanu

04/21/2020, 7:53 AM

📚 Android Sample App using modular, clean, scalable, testable Architecture written in Kotlin that presents modern approach to Android application development, latest tech-stack and best practices. Application is based, apply and strictly complies with each of the following 5 points: • A single-activity architecture, using the Navigation component to manage fragment operations. • Android architecture components, part of Android Jetpack give to project a robust design, testable and maintainable. • Pattern Model-View-ViewModel (MVVM) facilitating separation of development of the graphical user interface. • S.O.L.I.D design principles intended to make software designs more understandable, flexible and maintainable. • Modular app architecture allows being developed features in isolation, independently from other features. Project link: https://github.com/VMadalin/kotlin-sample-app

iseki

04/21/2020, 4:14 PM

In Kotlin. I need a lambda:

(T)->Unit

`<T: Any`> but allow null when T is nullable type... How to do that? Thank you

Daniel Lukic

04/22/2020, 12:01 PM

could someone point me to where i can ask some - most probably stupid - ktor client related beginner questions? 😳 🙈 like.. how to read a json response if 200 and handle non-200 otherwise explicitly.. 🤔 right now it seems this should be handled via try catch.. which seems a bit weird.. but maybe it isn't.. 🤔

Alexander Finn

04/22/2020, 4:29 PM

I have a newbie generics question here. I'm using a library (KMongo) that exposes an inline method with reified parameter:

inline fun <reified TDocument : Any> getCollection(
    collectionName: String = KMongoUtil.defaultCollectionName(TDocument::class)
): CoroutineCollection<TDocument>

In my code I want to create an interface with a default method that would return a collection for specified type:

interface MongoDBRepository<T: Any> {

    val client: CoroutineClient

    fun collection(): CoroutineCollection<T> {
        return client.getDatabase(accountId).getCollection<T>()
    }

}

However, the compiler complains that 'T' can not be used as a reified type parameter here. And defining a reified type parameter would require me to change collection() to an inline function which is something I can not do in an interface. Any thoughts on how to change the code to make an interface functional? Or am I doing something completely wrong?

Maciek

04/22/2020, 4:52 PM

is there a way to do multiple class generic constraints and interface delegation? Because of delegation it doesn't compile 🤔

Untitled

Kenneth

04/23/2020, 8:52 AM

I have a java class without a constructor that I want to initialize with a value, is there a shortcut to do this in kotlin or do I have to initialize and use the setter?

Ananiya

04/23/2020, 11:57 AM

i was trying to change the value externally here is my code

class SettingMyValue{           var setLength = 9 }

and from external kt file i put this code

SettingMyValue().setLength = 7

to reassign the value of

setLength

externally but it return 9 but all i want was to print the reassigned value (7) and if there is no external change returns 9

Kenneth

04/23/2020, 2:28 PM

Is using

!!

something that should be avoided?

:yes: 13

🇳🇴 2

Gilberto Diaz

04/23/2020, 4:35 PM

it is a way to get the parameter’s name?

Erik Dreyer

04/24/2020, 8:29 PM

What’s the correct way to convert an

Array<Int?>

Array<Int>

Gilberto Diaz

04/25/2020, 2:29 AM

I want to make sure I understand the docs. Here says that Map preserves the order of elements insertion. What I understand is that when I use

mapOf(…)

mutableMapOf(…)

, the order in which elements are inserted are preserved. Is that a correct statement?

Joshua

04/25/2020, 1:30 PM

So I changed the parent's layout from

layout

androidx.drawerlayout.widget.DrawerLayout

and now I get the following error...

AAPT: error: 'ferm.jonny.willow.models.ToolbarViewConfig' is incompatible with attribute type (attr) enum [linear=0, radial=1, sweep=2].

Any clue what this is about?

keishi kubo

04/25/2020, 6:14 PM

Does kotlin has string lietral type as typescript? Such as type Role = “admin” | “user” | “root”

04/26/2020, 5:37 AM

apologies in advance if this is not the correct place to ask this question

Chetan Sachdeva

04/27/2020, 1:13 PM

Hello all! What is the way to setup CI/CD for a kotlin multiplatform project?

Alex Wilson

04/28/2020, 4:08 PM

I am working through The Joy of Kotlin and came to this earlier today. Pages 61 and 62.

fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = { f(g(it)) }

I understand this example. The next step was to make the compose function polymorphic by using type parameters. I'm having a hard time making sense of this. Since they are all the same type of Int, I thought it would be just this:

fun <T> compose23Example(f: (T) -> T, g: (T) -> T): (T) -> T = { f(g(it)) }

The solution however is this:

fun <T, U, V> compose(f: (U) -> V, g: (T) -> U): (T) -> V = { f(g(it)) }

I've been looking at it and am having difficulty understanding exactly what I'm looking at. Can anyone help?

Title

Alex Wilson

04/28/2020, 4:08 PM

I am working through The Joy of Kotlin and came to this earlier today. Pages 61 and 62.

fun compose(f: (Int) -> Int, g: (Int) -> Int): (Int) -> Int = { f(g(it)) }

fun <T> compose23Example(f: (T) -> T, g: (T) -> T): (T) -> T = { f(g(it)) }

The solution however is this:

fun <T, U, V> compose(f: (U) -> V, g: (T) -> U): (T) -> V = { f(g(it)) }

I've been looking at it and am having difficulty understanding exactly what I'm looking at. Can anyone help?

Dominik wuttke

04/28/2020, 4:14 PM

Your example would mean, that you can only enter 1 type, when you have an Int everything had to be an Int. What really happen is, that you can enter any type for the function

g()

and return any type you want. You can for example have an`Int` as variable for

g()

and return a

String

. The returntype of

g()

is the type which

f()

needs.

f()

can now return any type you want. Your case is only correct when everything has the same type of variable, which is not always the case.

molikuner

04/28/2020, 4:16 PM

Theoretically your solution would work as well, as long as you only compose functions with the same type. So you could use your function to do the same thing as the example does with Int, but the actual “solution” is way more agile, as you can use multiple types there. Let me give an example:

val myWeiredLength: (String) -> Int = compose<String, Int, Int>(f = { it + 1 }, g = { it.length })

That wouldn’t be possible with your solution.

Casey Brooks

04/28/2020, 4:18 PM

Your solution is technically valid, but it only works for composing functions of a single type. There is no way for it to convert from one type to another. This example might help clear up the differences https://pl.kotl.in/fXK4lgQz4

Alex Wilson

04/28/2020, 4:40 PM

Thank you! I can tell that I'm on the verge of understanding it, having moments of clarity, just waiting for the final picture. I'll chew on the examples you've all given. I appreciate you taking the time to respond!

👍 2

This make any sense?

fun <T, U, V> compose(f: (U) -> V, g: (T) -> U): (T) -> V = { f(g(it)) }

A function called "compose" that returns a function that satisfies the signature

(T) -> V

. We're setting that return to a lambda of

f(g(it))

. Our method signature takes two parameters:

f:

a function with a parameter of type

that transforms into a result into type

g:

a function with a parameter of type

that transforms into a result into into type

represents

f(g(it))

represents

it

transforms

👍🏻 1

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