basically: ``` val list: List<Pair<String, ...

# announcements

[Bernhard]

basically:

val list: List<Pair<String, String>> = listOf(
            "a" to null,
            "b" to "hi"
        )
            .filter {it.second != null}

[Alowaniak]

Not sure if mapNotNull works?
listOf("a" to "b", "c" to null).mapNotNull { if(it.second == null) null else it }
Maybe you would need a it as Pair<String, String> but idk

[Luca Nicoletti]

if (it.second == null) null else it is useless, just use .mapNotNull { it.second }

[Bernhard]

right, I need it.first as well though

[Luca Nicoletti]

@Alowaniak the check is doing exactly this 🙂

[Alowaniak]

.mapNotNull { it.second } would map to the second element, he would indeed lose the it.first

[Bernhard]

@Alowaniak your example just asserts that the pair is not null

[Bernhard]

it still returns a Pair<String, String?>

[Alowaniak]

even when adding as Pair<String, String> ?

[Bernhard]

nah, but I suppose I can just cast it anyways

[Bernhard]

shouldn’t be any different than using !! in that case

[Bernhard]

which basically just circumvents the typechecker to error at runtime

[Bernhard]

which itself is fine if your function is generic enough that you have to review it only once

[Alowaniak]

Well you could make a function of it and then do:

fun nonNullPair(from: Pair<String, String?>): Pair<String, String>? {
    return from.first to (from.second ?: return null)
}

fun main() {
    val xs: List<Pair<String, String>> = listOf("a" to "b", "c" to null).mapNotNull(::nonNullPair)
    println(xs)
}

[karelpeeters]

Does anyone know a language where stuff like this works? What does the type system have to look like? I know the Kotlin team is thinking about this (https://github.com/Kotlin/KEEP/blob/master/proposals/kotlin-contracts.md#open-questions), but I'm curious if there's a precedent.

[Alowaniak]

Typescript supports it when the method for filter used has a typeguard, which I think are sort of similar to contracts?