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Implement next(isolation:) in AsyncAdjacentPairsSequence #390

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Implement next(isolation:) in AsyncAdjacentPairsSequence #390

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0f896c5
Implement next(isolation:) in AsyncAdjacentPairsSequence
alex-waldron Dec 26, 2025
ec84893
Merge branch 'apple:main' into implement-isolated-next-adjacent-pairs
alex-waldron Jan 19, 2026
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14 changes: 14 additions & 0 deletions Sources/AsyncAlgorithms/AsyncAdjacentPairsSequence.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,20 @@ public struct AsyncAdjacentPairsSequence<Base: AsyncSequence>: AsyncSequence {
self.base = base
}

@available(AsyncAlgorithms 1.1, *)
mutating public func next(isolation actor: isolated (any Actor)?) async throws(Base.Failure) -> (Base.Element, Base.Element)? {
if previousElement == nil {
previousElement = try await base.next(isolation: actor)
}

guard let previous = previousElement, let next = try await base.next(isolation: actor) else {
return nil
}

previousElement = next
return (previous, next)
}

@inlinable
public mutating func next() async rethrows -> (Base.Element, Base.Element)? {
if previousElement == nil {
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26 changes: 26 additions & 0 deletions Tests/AsyncAlgorithmsTests/TestAdjacentPairs.swift
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Expand Up @@ -11,6 +11,7 @@

import XCTest
import AsyncAlgorithms
import Observation
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@phausler phausler Jan 26, 2026

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I think the test does not really need Observations to work - we have other things that dont have this added dependency.


final class TestAdjacentPairs: XCTestCase {
func test_adjacentPairs_produces_tuples_of_adjacent_values_of_original_element() async {
Expand Down Expand Up @@ -95,4 +96,29 @@ final class TestAdjacentPairs: XCTestCase {
task.cancel()
await fulfillment(of: [finished], timeout: 1.0)
}

@available(macOS 26.0, iOS 26.0, tvOS 26.0, watchOS 26.0, visionOS 26.0, *)
@MainActor func test_adjacentPairs_respects_immediate() async {
let testObservable = TestObservable()
let observations = Observations { testObservable.prop }

let iterated = expectation(description: "iterates once")

// with `Task.immediate`, the first element in the adjacent pair should be populated immediately
let t = Task.immediate {
for await (previous, current) in observations.adjacentPairs() {
XCTAssertEqual(previous, 1)
XCTAssertEqual(current, 2)
iterated.fulfill()
}
}
testObservable.prop = 2
await fulfillment(of: [iterated], timeout: 1.0)
t.cancel()
}
}

@available(macOS 26.0, iOS 26.0, tvOS 26.0, watchOS 26.0, visionOS 26.0, *)
@MainActor @Observable private final class TestObservable {
var prop = 1
}
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