Hey Guys, I was trying to solve a bug in a application here and I noticed something that seems weird...

# stdlib

[Yacov Rosenberg]

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...

[ephemient]

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

[ephemient]

similarly, emptyList().any(predicate) == false and emptyList().none(predicate) == true

[ephemient]

previously: https://kotlinlang.slack.com/archives/C0B8MA7FA/p1661874970967069?thread_ts=1661870292.432869&cid=C0B8MA7FA

[Yacov Rosenberg]

Thank you @ephemient 🙂

[ilya.gorbunov]

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