Add AsyncSequence.flatMapLatest operator#395
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This introduces a simplified version of the flatMapLatest operator for AsyncSequence, along with a basic unit test. Note that this implementation uses unstructured concurrency and may have race conditions regarding task cancellation.
- Replaces naive implementation with a thread-safe approach using ManagedCriticalState. - Introduces generation tracking to prevent race conditions where cancelled inner sequences could yield stale values. - Adds test_interleaving_race_condition to verify correctness under concurrent load. - Ensures Swift 6 Sendable compliance.
- Replaces AsyncThrowingStream implementation with a custom AsyncSequence, Storage, and StateMachine. - Implements explicit state management using Lock for thread safety. - Handles concurrency between outer and inner sequences robustly. - Ensures correct cancellation propagation and error handling. - Verified with test_interleaving_race_condition.
- Moves AsyncFlatMapLatestSequence.swift to Sources/AsyncAlgorithms/FlatMapLatest/ - Extracts FlatMapLatestStateMachine and FlatMapLatestStorage into their own files. - Aligns project structure with other complex operators like CombineLatest and Debounce.
- Added test_outer_throwing to verify outer sequence error propagation - Added test_inner_throwing to verify inner sequence error propagation - Added test_cancellation to verify proper cancellation handling - Added test_empty_outer to verify empty outer sequence handling - Added test_empty_inner to verify empty inner sequence handling - Fixed test_simple_sequence to be more robust against timing issues
- Changed from synchronous map with throwIf to AsyncThrowingStream - Added delay to ensure proper error propagation timing - Fixes intermittent test failures
- Removed LLM-style thinking comments - Kept only essential, professional explanations - No functional changes, all tests pass
- Added FlatMapLatest.md documentation guide - Authored by Peter Friese - Includes introduction, code samples, and use cases - Covers search-as-you-type, location-based data, and dynamic config examples - Compares with similar operators in ReactiveX and Combine
- Created SAA-00nn proposal for flatMapLatest operator - Includes motivation, detailed design, and examples - Covers implementation strategy with state machine - Compares with ReactiveX switchMap and Combine switchToLatest
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This refines #382 a bit to be at a pitchable state.
It refines the signature to be opaque and use typed throws, plus it adds a few annotations with alternatives considered such that review can discuss the merits of those alternatives in addition to the original ones @peterfriese proposed.