Swift forEach vs for-in

This repository is made to show some edge cases where forEach is better than for ... in in terms of performance.

To check performance, try the test. In the test code, there are two simple tests;

func testPerformance_forEach() throws {
    let random = RandomUInt8Sequence(count: 1_000_000)
    var count = 0
    measure {
        random.forEach { _ in
            count += 1
        }
    }
}

func testPerformance_forIn() throws {
    let random = RandomUInt8Sequence(count: 1_000_000)
    var count = 0
    measure {
        for _ in random {
            count += 1
        }
    }
}

For normal sequences these two codes works mostly in the same speed. However, because the implementation of forEach in RandomUInt8Sequence is special, forEach performs 5x faster than for ... in in the test as follows.

$ swift test -c release

Building for production...
[5/5] Linking swift-foreachPackageTests
Build complete! (0.75s)
Test Suite 'All tests' started at 2024-06-01 00:40:02.816.
Test Suite 'swift-foreachPackageTests.xctest' started at 2024-06-01 00:40:02.817.
Test Suite 'swift_foreachTests' started at 2024-06-01 00:40:02.817.
Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forEach]' started.
swift-foreach/Tests/swift-foreachTests/swift_foreachTests.swift:8: Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forEach]' measured [Time, seconds] average: 0.008, relative standard deviation: 62.348%, values: [0.023342, 0.011465, 0.008703, 0.005896, 0.006538, 0.005232, 0.005412, 0.005375, 0.006408, 0.006325], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , polarity: prefers smaller, maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forEach]' passed (0.381 seconds).
Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forIn]' started.
swift-foreach/Tests/swift-foreachTests/swift_foreachTests.swift:18: Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forIn]' measured [Time, seconds] average: 0.041, relative standard deviation: 20.214%, values: [0.062800, 0.038211, 0.036936, 0.036743, 0.036901, 0.036774, 0.036894, 0.036328, 0.051231, 0.039921], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , polarity: prefers smaller, maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[swift_foreachTests.swift_foreachTests testPerformance_forIn]' passed (0.666 seconds).
Test Suite 'swift_foreachTests' passed at 2024-06-01 00:40:03.864.
	 Executed 2 tests, with 0 failures (0 unexpected) in 1.047 (1.047) seconds
Test Suite 'swift-foreachPackageTests.xctest' passed at 2024-06-01 00:40:03.864.
	 Executed 2 tests, with 0 failures (0 unexpected) in 1.047 (1.047) seconds
Test Suite 'All tests' passed at 2024-06-01 00:40:03.864.
	 Executed 2 tests, with 0 failures (0 unexpected) in 1.047 (1.049) seconds