i feel like this should work, but i get “Type infe...

# announcements

[gjesse]

i feel like this should work, but i get “Type inference failed - required List<List<String>>, found List<Any>

kotlin
    val hmm: List<List<String>> = listOf(
            listOf("a"),
            listOf("b")
    )
    val hmm2: List<List<String>> = hmm + listOf("c")

[edwardwongtl]

hmm is of type List<List<String>> while listOf("c") is List<String>, so the end result become List<Any>

[edwardwongtl]

Instead you should do val hmm2: List<List<String>> = hmm + listOf(listOf("c"))

[gjesse]

duh yeah .. just figured it out

[gjesse]

for some reason was thinking it was + the element value

[gjesse]

but it’s concatenating 2 lists

[gjesse]

not appending an element

[edwardwongtl]

In some language the + does means append, but not the case of Kotlin

[karelpeeters]

It does also mean append though:

val list = listOf("a")
println(list + "b")

prints ["a", "b"].

[edwardwongtl]

Well, when I take a look in the plus operator, it will auto flatten the listOf("c"), since it is a collection

[edwardwongtl]

public operator fun <T> Collection<T>.plus(elements: Iterable<T>): List<T> {
    if (elements is Collection) {
        val result = ArrayList<T>(this.size + elements.size)
        result.addAll(this)
        result.addAll(elements)
        return result
    } else {
        val result = ArrayList<T>(this)
        result.addAll(elements)
        return result
    }
}

[karelpeeters]

Sure, + is both append and merge, and the latter has precedence because the types are more specific.

[gjesse]

thanks y’all. that was surprising