Hey Guys, I was trying to solve a bug in a application here and I noticed something that seems weird to me. Basically I had this code in my application:

fun DownloadManager.isAudiobookCompleted(isbn: String): Boolean {
    return currentDownloads
        .filter { it.audiobookIsbn == isbn }
        .all { it.state == Download.STATE_COMPLETED }
}

Very intuitive where I want to return true, but was buggy. So I went to see why and just realized how the .all works:

/**
 * Returns `true` if all elements match the given [predicate].
 * 
 * @sample samples.collections.Collections.Aggregates.all
 */
public inline fun <T> Iterable<T>.all(predicate: (T) -> Boolean): Boolean {
    if (this is Collection && isEmpty()) return true
    for (element in this) if (!predicate(element)) return false
    return true
}

Basically when the list isEmpty, it returns true. Is't it wrong? I mean, for me it should return true only match the predicate, and if the list is empty for sure don't match the predicate...

for all

list

and

x

,

(list + listOf(x)).all(predicate) == (list.all(predicate) && predicate(x))

and of course

listOf(x).all(predicate) == predicate(x)

now, if

list == emptyList()

, the only reasonable value for

.all(predicate)

is

true

Thank you @ephemient 🙂

Sometimes I wonder if they should just leave a link to this wiki article in the .all() docstring

It is already there https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/all.html