Partition: Make mutating functions return `@discardableResult`

[mdznr]

Using the mutating partitioning functions are useful even when the returned index isn’t used.

The Embracing Algorithms (WWDC 2018) session implements a bringForward(elementsSatisfying:) function on MutableCollection.¹ It uses stablePartition(by:) in its implementation, but doesn’t need its return value, resulting in a warning.

Actual behavior

extension MutableCollection {
    mutating func bringForward(elementsSatisfying predicate: (Element) -> Bool) {
        if let predecessor = indexBeforeFirst(where: predicate) {
            self[predecessor...].stablePartition(by: { !predicate($0) }) // ⚠️ Result of call to 'stablePartition(by:)' is unused
        }
    }
}

Expected behavior

extension MutableCollection {
    mutating func bringForward(elementsSatisfying predicate: (Element) -> Bool) {
        if let predecessor = indexBeforeFirst(where: predicate) {
            self[predecessor...].stablePartition(by: { !predicate($0) })
        }
    }
}

While it could be argued that bringForward(elementsSatisfying:) should return an Index as well, even that return value isn’t always needed to be used and should be marked as @discardableResult.

Checklist

• [x] If possible, I've reproduced the issue using the main branch of this package
• [x] I've searched for existing GitHub issues

---

1. This implementation function can be seen on page 218 of the presentation slides PDF.

[kylemacomber]

It's a good question.

I can't recall, but I imagine we just followed the precedent set by partition(by:) from the standard library which does not mark its result as a @discardableResult.

While it could be argued that bringForward(elementsSatisfying:) should return an Index as well, even that return value isn’t always needed to be used and should be marked as @discardableResult.

I do think bringForward(elementsSatisfying:) should probably return an Index (one of Alexander Stepanov's API design principles is "don't throw away useful information"), but I agree that should arguably be marked as @discardableResult.

[kocsma]

@discardableResult #67

[Saklad5]

The standard library can also be changed. If not now, then in Swift 6.