kotlinlang #announcements

It seems like there's still an outstanding issue in Kotlin, with regards to data classes with private constructors. The basic problem being that the copy method allows you to still set the members individually, and is always public, and cannot be deleted. This means that currently in Kotlin it doesn't really seem possible to write a data class in Kotlin that has any invariants amongst members which are part of its representation, which is annoying if you primarily want the data class for hashing or equality, for example. What's the recommended practice around this for now, and what direction is it headed in the future?

dmcg

12/21/2020, 5:54 PM

https://stackoverflow.com/questions/65397653/is-there-a-standard-kotlin-function-for-splitting-a-sequence-into-a-head-and-a-t

Łukasz Bednarczyk

12/21/2020, 7:15 PM

Hi 🙂 is here anyone who knows spring? I need advice

andylamax

12/22/2020, 1:46 AM

I am getting error: The feature "unit conversion" is disabled. What does this mean? and how do I enable it? IDEA suggested to set languageVersion to "1.4", I am still getting the error though

Nir

12/22/2020, 10:15 PM

This is the stdlib implementation of MutableMap.getOrPut

public inline fun <K, V> MutableMap<K, V>.getOrPut(key: K, defaultValue: () -> V): V {
    val value = get(key)
    return if (value == null) {
        val answer = defaultValue()
        put(key, answer)
        answer
    } else {
        value
    }
}

It seems like either the implementation or the description is wrong:

Returns the value for the given key. If the key is not found in the map, calls the defaultValue function, puts its result into the map under the given key and returns it.

println(mutableMapOf<Int, Int?>(5 to null).getOrPut(5) { 10 })

Prints 10. However, 5 is "in the map" (

println(5 in mutableMapOf<Int, Int?>(5 to null)

prints true). kotlin nulls are very tricky in generic code.

12/23/2020, 12:22 PM

why is transform in Map.mapValues defined as

transform: (Map.Entry<K, V>) -> R

instead of just

transform: (V) -> R

Robert Jaros

12/23/2020, 2:31 PM

Can we use current versions of coroutines and serialization with 1.4.30-M1?

Eric Lekwa

12/23/2020, 4:46 PM

Hey guys currently out of a Job 😩, but good news is I am willing to volunteer with any who needs help on a project 🙏.

zain

12/24/2020, 7:03 AM

Hey what does the colors signify in IDE for example Android Studio: For yellow and white over here

hisham bin awiad

12/24/2020, 8:01 AM

Hello every one can anyone help me with How to get list of videos from specific folder in android ?

Alexandru Hadăr

12/24/2020, 8:12 AM

In the coroutines documentation, it's stated about

join

(link) that if called on a coroutine that's cancelled, it will throw an

CancellationException

. However, in this example from the official docs,

job.join()

is called right after

job.cancel

, yet it does not throw anything. Why not ?

lawlorslaw

12/24/2020, 3:40 PM

Is there a better name for a class that has these properties?

Untitled

melatonina

12/24/2020, 5:45 PM

Hello. Are there

StateFlow

builders similar to

callbackFlow

? I'm trying to transform this code to use a

StateFlow

instead of a `Flow`:

fun <T> ObservableValue<T>.values(): Flow<T> = callbackFlow {
    val listener = javafx.beans.value.ChangeListener<T> { _, _, newValue ->
        offer(newValue)
    }

    addListener(listener)

    awaitClose { removeListener(listener) }
}

Edoardo Luppi

12/24/2020, 6:07 PM

I'm trying to import the atomicfu plugin using Gradle, but the dependency is unresolved. Do I need a particular repository?

Hamza GATTAL

12/24/2020, 8:12 PM

Hi everyone The wibinar of today is not available in the YouTube channel what's happened

CircuitRCAY

12/25/2020, 1:37 AM

Would union types in Kotlin, instead of overloads be possible in the future?

CircuitRCAY

12/25/2020, 1:40 AM

an example of union types in kotlin would be

fun union(message: String | ByteArray)

Andrew

12/25/2020, 3:19 AM

May not be specific to kotlin but I am trying to add some unit tests that run independent of regular tests, ideally the "test" gradle task would skip these but another task would run only those. How can I do this aside from adding another project just for testing?

kartikpatodi

12/25/2020, 7:29 AM

How do I add multiple -Xopt-in flags in gradle?

Esa

12/25/2020, 9:07 PM

Hi, I'm experiencing an issue with the official code style in IntelliJ. Some times it seems to override my preferred code style with this one. And the two styles are quite different, so it is very jarring when it happens. Is there any foolproof way to make sure it does not randomly revert back to the stock code style?

buszi0809

12/26/2020, 7:25 PM

Hi, I'm looking for a good explanation of cold and hot Flows in Kotlin, i feel like I don't really understand the concept and implementation from the ones I've found, can anyone provide good one?

Ahmed Mourad

12/27/2020, 12:14 PM

For a class `X`:

data class X(val v: String)

If this works

X::v

shouldn't this also work

X?::v

hisham bin awiad

12/27/2020, 4:43 PM

how can show list of videos by

kotlin

any one faced this before Plz help me

❓ 4

Poohrang

12/27/2020, 5:01 PM

Is there channel related with exposed for database?

🤦 1

caelum19

12/27/2020, 10:07 PM

I've an

object

that is quite useful to access statically and dynamically. It has some nested properties and classes, and I am currently serializing it with kotlinx.serialization to save it when the application closes. However for loading, Kotlin does not allow me to assign the object with =, is there a way to assign it anyway or another way to do this? The object again has nested objects inside it, so manually setting each property is a solution I would rather avoid 🙂

Peter Ertl

12/28/2020, 3:35 PM

what would be the proper way with kotlin coroutines to signal events (like wait / notify in java)?

Animesh Sahu

12/29/2020, 3:56 AM

Not really specific to Kotlin, but to JVM (and indirectly linked :p). Can somebody help? https://stackoverflow.com/q/65486924/11377112

Mark Fisher

12/29/2020, 10:21 AM

Hi Kotliners, A post xmas question for anyone alive in this time of limbo before the new year! I've got a generics question. I can't understand why there's an error in the following:

interface HexData
object EmptyHexData : HexData
data class Hex(val q: Int, val r: Int, val s: Int)

interface HexDataStorage<T: HexData> {
    fun addHex(hex: Hex)
    fun addHex(hex: Hex, data: T): Boolean
}

class DefaultHexDataStorage<T: HexData> : HexDataStorage<T> {
    private val storage = LinkedHashMap<Hex, T>()

    override fun addHex(hex: Hex) {
        storage[hex] = EmptyHexData // <-- ERROR
    }

    override fun addHex(hex: Hex, data: T): Boolean {
        val previous = storage.put(hex, data)
        return previous != null
    }
}

In the function addHex(hex: Hex), I cannot use "EmptyHexData", and I don't understand why. AFAIU It's a sub-type of HexData, so I thought it would comply with the requirement of being a

in this case. What's wrong, and how do I fix it? I tried using a class too, e.g.

class EmptyHexDataClass: HexData

but that didn't work either.

Title

Mark Fisher

12/29/2020, 10:21 AM

Hi Kotliners, A post xmas question for anyone alive in this time of limbo before the new year! I've got a generics question. I can't understand why there's an error in the following:

interface HexData
object EmptyHexData : HexData
data class Hex(val q: Int, val r: Int, val s: Int)

interface HexDataStorage<T: HexData> {
    fun addHex(hex: Hex)
    fun addHex(hex: Hex, data: T): Boolean
}

class DefaultHexDataStorage<T: HexData> : HexDataStorage<T> {
    private val storage = LinkedHashMap<Hex, T>()

    override fun addHex(hex: Hex) {
        storage[hex] = EmptyHexData // <-- ERROR
    }

    override fun addHex(hex: Hex, data: T): Boolean {
        val previous = storage.put(hex, data)
        return previous != null
    }
}

In the function addHex(hex: Hex), I cannot use "EmptyHexData", and I don't understand why. AFAIU It's a sub-type of HexData, so I thought it would comply with the requirement of being a

in this case. What's wrong, and how do I fix it? I tried using a class too, e.g.

class EmptyHexDataClass: HexData

but that didn't work either.

Giorgos Neokleous

12/29/2020, 10:32 AM

Do you need generics at all in your example? Seems like you can just get by without them

interface HexDataStorage {
    fun addHex(hex: Hex)
    fun addHex(hex: Hex, data: HexData): Boolean
}
class DefaultHexDataStorage : HexDataStorage {
    private val storage = LinkedHashMap<Hex, HexData>()
    override fun addHex(hex: Hex) {
        storage[hex] = EmptyHexData
    }
    override fun addHex(hex: Hex, data: HexData): Boolean {
        val previous = storage.put(hex, data)
        return previous != null
    }
}

Yev Kanivets

12/29/2020, 10:33 AM

addHex(hex: Hex)

takes

Hex

, not

, and

EmptyHexData

doesn’t inherit from

Hex

, but from

HexData

😅 1

👎 2

Vampire

12/29/2020, 10:34 AM

@Yev Kanivets I think you misread the example

Mark Fisher

12/29/2020, 10:34 AM

@Yev Kanivets that's not right here, the type is in the storage.

Vampire

12/29/2020, 10:35 AM

@Mark Fisher

HexData

or a subtype of it

So if you have

interface MyHexData : HexData

and then have a

DefaultHexDataStorage<MyHexData>

that would be legal

But then you couldn't assign

EmptyHexData

as it is a

HexData

, but not a

MyHexData

That's why it says "you have

EmptyHexData

but

was expected"

Mark Fisher

12/29/2020, 10:37 AM

@Giorgos Neokleous I did consider a non-generics version, and instead have a Maybe<T> there, but still would like to get this working without the Maybe class which I think adds noise. @Vampire isn't EmptyHexData a subtype of HexData?

Vampire

12/29/2020, 10:37 AM

So it's back to the question of Girogos, why generics at all

Of course it is, but not of

MyHexData

as I just explained

If you create a

DefaultHexDataStorage<MyHexData>

your map will have runtime type

LinkedHashMap<Hex, MyHexData>

and

EmptyHexData

is not a

MyHexData

but a sibling

Mark Fisher

12/29/2020, 10:39 AM

ah, that explains it better, when considering the actual implementation, i need a full type.

Vampire

12/29/2020, 10:40 AM

So again the question of Giorgos, why do you need generics at all? Or a

Maybe

whatever that should be.

Why wouldn't his suggestion work for you?

Mark Fisher

12/29/2020, 10:41 AM

It is good enough to not use Generics, but I couldn't understand why the posted version wasn't working. You've explained that well enough now for me to understand, so thanks! 🙂

👌 1

Rob Elliot

12/29/2020, 11:11 AM

In terms of what you’re trying to achieve with the generic type, you’re probably better off using

null

to represent `EmptyHexData`:

private val storage = LinkedHashMap<Hex, T?>()
    override fun addHex(hex: Hex) {
        storage[hex] = null
    }

Mark Fisher

12/29/2020, 2:52 PM

@Rob Elliot That's why I used Maybe<T>, to avoid the use of nulls:

private val storage = LinkedHashMap<Hex, Maybe<T>>()

    override fun addHex(hex: Hex) {
        storage[hex] = Maybe.empty()
    }

    override fun addHex(hex: Hex, data: T): Boolean {
        val previous = storage.put(hex, Maybe.of(data))
        return previous != null
    }

Although my current implementation doesn't need generics, so I've stripped it back to simple HexData types directly.

Vampire

12/29/2020, 2:56 PM

And where is the advantage to wrap the entries instead of using a nullable type?

Mark Fisher

12/29/2020, 2:58 PM

I'm averse to using nulls and nullable types.

Youssef Shoaib [MOD]

12/29/2020, 3:37 PM

Using null and nullable types is idiomatic in Kotlin instead of using a maybe since the language has complete support for it. This isn't only my opinion, but also the opinion of actual fp experts like the whole Arrow team who decided to deprecate Maybe in favour of the native Kotlin nullables since the compiler tracks the nullable types and checks any calls that you try to do on them automatically (similar to how they decided to go with suspend instead of an

IO

wrapper since the compiler automatically tracks effects with suspend)

Nir

12/29/2020, 3:40 PM

I'd agree that nullable types are idiomatic in Kotlin broadly speaking but nullables can be a bit of a pain in generic contexts

and in particular Map<Hex, HexData?> can be pretty annoying

a lot of the extension functions for Map (and MutableMap) don't do what you'd expect, and basically act as though an entry to null isn't actually in the map

I actually agree with the approach here, it leads to less confusing code

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