write-swift

Work down a priority list: first stop sharing the object by giving each concurrent job its own instance, then make it a Sendable value type, then isolate it to an actor, and only as a last resort use Mutex, Atomics, or @unchecked Sendable. Most real errors are fixed at step one.

In Swift 6.2, marking a function async no longer moves it off the current actor; it runs where it was called from. Use @concurrent to guarantee background execution for CPU-heavy work and nonisolated for library APIs where the caller decides isolation.

Write some P by default because it preserves associated-type relationships and lets the compiler specialize. Switch to any P only when you need heterogeneous storage, optionality of the underlying type, or full type erasure, accepting the runtime cost.

Use Swift Testing for all new tests since @Test, #expect, and parameterized arguments cover most needs. XCTest remains required only for UI automation with XCUIApplication, performance metrics with XCTMetric, and Objective-C exception testing.

Actors guarantee mutual exclusion, not transactions, so state can change between awaits on the same actor. Mutate actor state in synchronous methods, keep async methods thin, and re-check assumptions after every await to avoid check-then-act bugs.

Start single-threaded on the main actor and add concurrency only after profiling shows a hang. Spawning tasks for trivial work costs more than it saves, and async functions carry real scheduling overhead, so synchronous code is the correct default.