<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">I was perusing the library for array ideas, and noticed that several types had methods in common, but without a grouping protocol. Shouldn’t that be fixed?</div><div class=""><br class=""></div><div class="">(Oh, if multiple protocols have an associated-type with the same name, is that a problem? Is it a problem if they resolve differently for each protocol attached to a given type? I’m asking because these protocols reuse Sequence’s Element for their own purpose.)</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><h1 id="toc_0" style="-webkit-print-color-adjust: exact; margin-right: 0px; margin-bottom: 10px; margin-left: 0px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 28px; font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255); margin-top: 0px !important;" class="">Formal Protocol for Contiguous Storage Visitation</h1><ul style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding-left: 30px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Proposal: <a href="file:///Users/daryle/Documents/NNNN-filename.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SE-NNNN</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Authors: <a href="https://github.com/CTMacUser" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">Daryle Walker</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Review Manager: TBD</li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Status: <strong style="-webkit-print-color-adjust: exact; margin-top: 0px;" class="">Awaiting review</strong></li></ul><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><em style="-webkit-print-color-adjust: exact;" class="">During the review process, add the following fields as needed:</em></p><ul style="-webkit-print-color-adjust: exact; margin: 15px 0px; padding-left: 30px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Decision Notes: <a href="https://lists.swift.org/pipermail/swift-evolution/" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">Rationale</a>, <a href="https://lists.swift.org/pipermail/swift-evolution/" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">Additional Commentary</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Bugs: <a href="https://bugs.swift.org/browse/SR-NNNN" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SR-NNNN</a>, <a href="https://bugs.swift.org/browse/SR-MMMM" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">SR-MMMM</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Previous Revision: <a href="https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">1</a></li><li style="-webkit-print-color-adjust: exact; margin: 0px;" class="">Previous Proposal: <a href="file:///Users/daryle/Documents/XXXX-filename.md" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196); margin-top: 0px;" class="">SE-XXXX</a></li></ul><h2 id="toc_1" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Introduction</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The standard library types <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">Array</code>, <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ArraySlice</code>, and <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ContiguousArray</code> have an interface for visiting their elements as a contiguous block of memory (arranging said elements to that configuration first if necessary). These methods are all the same, but not under a common protocol (i.e. seeming to match by "coincidence").</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">This proposal seeks to correct that with two new protocols that these types will implement.</p><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Swift-evolution thread: <a href="https://lists.swift.org/pipermail/swift-evolution/" style="-webkit-print-color-adjust: exact; color: rgb(65, 131, 196);" class="">Discussion thread topic for that proposal</a></p><h2 id="toc_2" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Motivation</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">Just adding these protocols for consistency is relatively minor, but they may be used for other types. Particularly, they would be needed if fixed-sized arrays are added to the language.</p><h2 id="toc_3" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Proposed solution</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The library array types will follow the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">MutableContiguousBlock</code>protocol, which inherits from the <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ContiguousBlock</code> protocol.</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">extension Array: MutableContiguousBlock {
}
extension ArraySlice: MutableContiguousBlock {
}
extension ContiguousArray: MutableContiguousBlock {
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">For example, a repackaging of the library's example code:</p><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">func change<T: MutableContiguousBlock>(object: inout T) -> T.Element where T.Element: Integer {
let sum = object.withUnsafeBufferPointer { (buffer) -> T.Element in
var result: T.Element = 0
for i in stride(from: buffer.startIndex, to: buffer.endIndex, by: 2) {
result += buffer[i]
}
return result
}
object.withUnsafeMutableBufferPointer { buffer in
for j in stride(from: buffer.startIndex, to: buffer.endIndex - 1, by: 2) {
swap(&buffer[j], &buffer[j + 1])
}
}
return sum
}
var numbers = [1, 2, 3, 4, 5]
print(change(object: &numbers)) // 9
print(numbers) // [2, 1, 4, 3, 5]</code></pre></div><h2 id="toc_4" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Detailed design</h2><div style="-webkit-print-color-adjust: exact; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class=""><pre style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-bottom: 15px; background-color: rgb(248, 248, 248); border: 1px solid rgb(204, 204, 204); font-size: 13px; line-height: 19px; overflow: auto; padding: 6px 10px; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class=""><code class="language-none" style="-webkit-print-color-adjust: exact; margin: 0px; padding: 0px; border: none; background-color: transparent; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">/**
Visitation protocol of the receiver's contiguous storage of immutable elements.
*/
protocol ContiguousBlock {
/// Inferred alias to the element type to visit
associatedtype Element
/**
Calls a closure with a pointer to the receiver's contiguous storage. If no such storage exists, it is first created.
Often, the optimizer can eliminate bounds checks within an array algorithm, but when that fails, invoking the same algorithm on the buffer pointer passed into your closure lets you trade safety for speed.
The following example shows how you can iterate over the contents of the buffer pointer:
let numbers = [1, 2, 3, 4, 5]
let sum = numbers.withUnsafeBufferPointer { buffer -> Int in
var result = 0
for i in stride(from: buffer.startIndex, to: buffer.endIndex, by: 2) {
result += buffer[i]
}
return result
}
// 'sum' == 9
- Parameter body: A closure with an `UnsafeBufferPointer` parameter that points to the contiguous storage for the receiver. If `body` has a return value, it is used as the return value for this method. The pointer argument is valid only for the duration of the closure's execution.
- Returns: The return value of the `body` closure parameter, if any.
- SeeAlso: Swift.UnsafeBufferPointer
*/
func withUnsafeBufferPointer<R>(_ body: (UnsafeBufferPointer<Element>) throws -> R) rethrows -> R
}
/**
Visitation protocol of the receiver's contiguous storage of elements, allowing mutation.
*/
protocol MutableContiguousBlock: ContiguousBlock {
/**
Calls the given closure with a pointer to the receiver's mutable contiguous storage. If no such storage exists, it is first created.
Often, the optimizer can eliminate bounds checks within an array algorithm, but when that fails, invoking the same algorithm on the buffer pointer passed into your closure lets you trade safety for speed.
The following example shows modifying the contents of the `UnsafeMutableBufferPointer` argument to `body` alters the contents of the receiver:
var numbers = [1, 2, 3, 4, 5]
numbers.withUnsafeMutableBufferPointer { buffer in
for i in stride(from: buffer.startIndex, to: buffer.endIndex - 1, by: 2) {
swap(&buffer[i], &buffer[i + 1])
}
}
print(numbers)
// Prints "[2, 1, 4, 3, 5]"
- Warning: Do not rely on anything about `self` (the receiver of this method) during the execution of the `body` closure: It may not appear to have its correct value. Instead, use only the `UnsafeMutableBufferPointer` argument to `body`.
- Parameter body: A closure with an `UnsafeMutableBufferPointer` parameter that points to the contiguous storage for the receiver. If `body` has a return value, it is used as the return value for this method. The pointer argument is valid only for the duration of the closure's execution.
- Returns: The return value of the `body` closure parameter, if any.
- SeeAlso: withUnsafeBufferPointer, Swift.UnsafeMutableBufferPointer
*/
mutating func withUnsafeMutableBufferPointer<R>(_ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R) rethrows -> R
}</code></pre></div><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">I don't know how the 3 library types implement this methods, but we know it can be done. If added as an implicit interface to built-in fixed-sized arrays, the implementers can use similar techniques or compiler magic. I don't know the difficulty to adapt these interfaces with third-party code.</p><h2 id="toc_5" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Source compatibility</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The changes are strictly additive, and at the library level, so there should be no impact to existing code.</p><h2 id="toc_6" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Effect on ABI stability</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">These changes should not affect ABI stability.</p><h2 id="toc_7" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Effect on API resilience</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">As just stated, the addition of two protocol names to the API shouldn't affect the ABI.</p><h2 id="toc_8" style="-webkit-print-color-adjust: exact; margin: 20px 0px 10px; padding: 0px; -webkit-font-smoothing: antialiased; cursor: text; position: relative; font-size: 24px; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); font-family: Helvetica, arial, sans-serif; background-color: rgb(255, 255, 255);" class="">Alternatives considered</h2><p style="-webkit-print-color-adjust: exact; margin: 15px 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255);" class="">The main alternative is to do nothing. If fixed-sized arrays are later added, they could have the same methods too, but there would still be no common link.</p><p style="-webkit-print-color-adjust: exact; margin-top: 15px; margin-right: 0px; margin-left: 0px; font-family: Helvetica, arial, sans-serif; font-size: 14px; background-color: rgb(255, 255, 255); margin-bottom: 0px !important;" class="">Another alternative would make <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">MutableContiguousBlock</code> not inherit from <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">ContiguousBlock</code>, meaning the former would need to define an <code style="-webkit-print-color-adjust: exact; margin: 0px 2px; padding: 0px 5px; white-space: nowrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">Element</code> associated type. But don't see a need for a protocol with a read-write method without its read-only counterpart available.</p></div><div class=""><br class=""></div><br class=""><div class="">
<div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">— </div><div class="">Daryle Walker<br class="">Mac, Internet, and Video Game Junkie<br class="">darylew AT mac DOT com </div></div>
</div>
<br class=""></body></html>