write-swiftlisted

# Write Swift How to write Swift the way the language wants to be written, current through Swift 6.4. **Toolchain baseline: Swift 6.3** (current release as of August 2026). Everything here compiles on 6.3 unless marked ⚠, which flags unreleased Swift 6.4 features. Concurrency guidance assumes the Swift 6.2 model — if the project is on 6.1 or earlier, §3's rules about `async` and `@concurrent` do not apply. The through-line: **Swift is a progressive-disclosure language. Start with the simplest, most static, most single-threaded thing that works, and buy dynamism — concurrency, reference semantics, existentials, unsafe pointers — only where you can point at the reason.** Every rule below is an application of that. Model this hierarchy of defaults. Move down a level only with a reason you can state: | Need | Reach for | Move down only when | | ------------ | ----------------------- | ---------------------------------------------------------- | | Data | `struct` / `enum` | you need identity, sharing, or inheritance | | Abstraction | concrete type | you have repeated code across types | | Polymorphism | `some P` (generic) | you need heterogeneous storage → `any P` | | Execution | main actor, synchronous | profiling shows a hang → `async` → `@concurrent` → `actor` | | Memory | `Array`, `String` | profiling shows the