Add labels to the tuples produced by chunked(on:)

Label the tuple produced by `chunked(on:)` with `subject` and
`chunk`. This will improve the legibility of code downstream of
the said function. For example, when providing an identifier
for SwiftUI's `ForEach`:

```
-ForEach(chunkedByDate, id: \.0) { date, items in
+ForEach(chunkedByDate, id: \.subject) { date, items in
```

This aligns `ChunkedOnCollection` with the `IndexedCollection`,
which specifies `index` and `element` as well as the built-in
`EnumeratedSequence` (specifying `offset` and `element`).

The introduction of the label will have no effect on existing code,
because the index-based address of the tuple component will remain
unchanged and absent labels will be inferred.

Author: cweider

Sources/Algorithms/Chunked.swift

@@ -162,6 +162,7 @@ public struct ChunkedOnCollection<Base: Collection, Subject: Equatable> {
 
 extension ChunkedOnCollection: Collection {
   public typealias Index = ChunkedByCollection<Base, Subject>.Index
+  public typealias Element = (subject: Subject, chunk: Base.SubSequence)
 
   @inlinable
   public var startIndex: Index {
@@ -174,7 +175,7 @@ extension ChunkedOnCollection: Collection {
   }
 
   @inlinable
-  public subscript(position: Index) -> (Subject, Base.SubSequence) {
+  public subscript(position: Index) -> Element {
     let subsequence = chunked[position]
     // swift-format-ignore: NeverForceUnwrap
     // Chunks are never empty, so `.first!` is safe.
@@ -509,7 +510,7 @@ extension Collection {
   @inlinable
   public func chunked<Subject: Equatable>(
     on projection: (Element) throws -> Subject
-  ) rethrows -> [(Subject, SubSequence)] {
+  ) rethrows -> [ChunkedOnCollection<Self, Subject>.Element] {
     guard !isEmpty else { return [] }
     var result: [(Subject, SubSequence)] = []
 

Tests/SwiftAlgorithmsTests/ChunkedTests.swift

@@ -71,6 +71,16 @@ final class ChunkedTests: XCTestCase {
     IndexValidator().validate(lazyChunks)
   }
 
+  func testChunkedOnLabels() {
+    let arrayChunks: Array = fruits.chunked(on: { $0.first })
+    XCTAssert(arrayChunks.first!.0 == arrayChunks.first!.subject)
+    XCTAssert(arrayChunks.first!.1 == arrayChunks.first!.chunk)
+
+    let lazyChunks = fruits.lazy.chunked(on: { $0.first })
+    XCTAssert(lazyChunks.first!.0 == lazyChunks.first!.subject)
+    XCTAssert(lazyChunks.first!.1 == lazyChunks.first!.chunk)
+  }
+
   func testChunkedBy() {
     validateFruitChunks(fruits.chunked(by: { $0.first == $1.first }))