I keep running into weird things with Swift 3 Collections.
A) Collection has its count property defined as an Int via IndexDistance:
public protocol Collection : Indexable, Sequence {
associatedtype IndexDistance : SignedInteger = Int // line 182 in
Collection.swift
public var count: IndexDistance { get } // line 776
}
Given this, why can’t `count` be treated as an Int?
func intCount<C: Collection>(of c: C) -> Int {
return c.count // nope!
}
A numericCast is required here in order to “convert” what is known-to-be-an-Int
to an actual Int.
Why this is so? IndexDistance is defined such that it must be an integer (“A
type that represents the number of steps between a pair of indices”)...
What is gained by making it so generic that it can’t simply be an Int?
B) Collection.Indices is a Collection of Index, *but*
- Collection.Indices.Index is not the same type as Collection.IndexDistance
- Collection.Indices.Iterator.Element is somehow not the automatically the same
type as Collection.Index
(ugh)
The second seems like a conditional conformance issue, but the first one is
baffling.
I did find that with String.CharacterView:
String.CharacterView.Indices.Index != String.CharacterView.IndexDistance
although, as expected,
String.CharacterView.Indices.IndexDistance == String.CharacterView.IndexDistance
(which provides an clear workaround.)
***
I wanted to extend Collection with concurrentPerform; which seems simple on the
surface:
extension Collection {
public func concurrentPerform(task: @escaping (Self.Iterator.Element) ->
Void) {
DispatchQueue.concurrentPerform(iterations: count) {
iteration in
task(self[indices[iteration]])
}
}
}
… but that won’t do.
The closest thing I found that does work is this:
extension Collection {
public func concurrentPerform(task: @escaping (Self.Iterator.Element) ->
Void) {
let count: Int = numericCast(self.count)
let indexList = (0..<count).map { index(startIndex, offsetBy:
numericCast($0)) }
DispatchQueue.concurrentPerform(iterations: count) {
iteration in
task(self[indexList[iteration]])
}
}
}
Note the unfortunate creation of an array of Index.
Generic Collection code is exhausting!
Cheers,
Guillaume Lessard
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