inceptzws/dsh-emil-skills/write-swift

Write Swift

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I'm ready to help you write modern Swift, the way the language wants to be written.

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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:

1. Model data with value types

Value types are the default in Swift, not a special case.

• Default to `struct` and `enum`. Use `class` only for identity, shared mutable state, inheritance, or resource lifetime. A window, a database connection, an entity stored in a rendering engine — those have identity. A Point, a Drink, a Material does not.
• `let` by default; `var` only when you mutate. This is the same discipline as some before any and value before reference: start narrow, widen with cause.
• A struct with a mutable reference-type property is neither a value nor a reference. Copies share the object; mutations leak across copies. Either keep the referenced type immutable, expose only computed properties that forward to it, or make it a private stored property behind copy-on-write.
• Copy-on-write is how you get out-of-line storage _and_ value semantics. Wrap a final class in a struct and check isKnownUniquelyReferenced(&storage) before mutating; copy first if it isn't. This is exactly how Array, String, and Dictionary work.
• Enums are the tool for "a fixed set of things" and for mutually exclusive state. Replacing a pile of optional stored properties (isSharing, selectedRows, shareTarget) with one enum State makes invalid combinations unrepresentable and makes state change atomic instead of a sequence of property writes you can forget to finish.
• Composing values yields a value. A struct whose stored properties are all value types has value semantics for free — which is what makes undo, diffing, and state restoration a single code path instead of one per property.

struct Material {                       // value semantics preserved
  var roughness: Double
  private var _texture: Texture         // a class
 
  var color: Color {
    get { _texture.color }
    set {
      if !isKnownUniquelyReferenced(&_texture) { _texture = Texture(copying: _texture) }
      _texture.color = newValue
    }
  }
}

Noncopyable types (~Copyable) express unique ownership: a file descriptor, a bank transfer, an open resource. Suppressing the copy turns "you must not run this twice" from an assertion into a compile error, and makes deinit on a struct meaningful. Mark the finishing method consuming so the compiler proves it's the last use. Parameter ownership becomes explicit: borrowing (read-only, the default), consuming (takes it away), inout/mutating (temporary write access).

2. Errors and optionals — make the failure paths visible

Swift error handling rests on three points: sources of error are marked so they can't surprise you; errors carry enough context to act on; and recoverable errors are different from programmer mistakes.

• Recoverable → `throw`. Programmer mistake → `precondition`/`fatalError`. A failed network call keeps the program running. An out-of-bounds index means the code is wrong and must halt before the bug becomes a security issue.
• Enums with associated values make the best error types. case duplicateFriend(String) beats case duplicateFriend — the context is the whole point.
• `guard` for error conditions, because it forces the exit path. if let for the ordinary unwrap.
• Typed throws (`throws(MyError)`) are for internal functions, error-forwarding generic code, and constrained environments where boxing any Error is too costly. For public API, untyped throws preserves your freedom to change the error type later. Note the unification: throws is throws(any Error), and non-throwing is throws(Never) — which is what lets map abstract over both.
• Force-unwrap only where you can state the invariant, and prefer a failing #require/precondition with a message over a bare !.

3. Concurrency: stay single-threaded until profiling says otherwise

This is the section agents get wrong most often, because the model changed in Swift 6.2.

Start every app entirely on the main thread. Single-threaded code goes a long way, and most apps never need to leave it.

The progression, in order. Do not skip steps.

1. Single-threaded on the main actor. No concurrency at all. Fine for most apps.
2. `async`/`await` to hide latency (network, disk). Still no concurrency of your own — SDK APIs like URLSession.data(from:) offload on your behalf.
3. `@concurrent` to move _your_ expensive work off the main thread — only after Instruments shows a hang.
4. `actor` to move _state_ off the main actor — only when too much main-actor state is forcing tasks to hop back constantly.

Turn on the right build settings first. Enable Approachable Concurrency in every project. For app modules and UI-facing modules, also set Default Actor Isolation to MainActor — it's the default for new app projects in Xcode 26, and it deletes most of your @MainActor annotations. In a package: swiftSettings: [.defaultIsolation(MainActor.self)]. Do not set main-actor-by-default for a general-purpose library — libraries should ship nonisolated APIs and let clients decide where work runs.

The rule that changed

In Swift 6.2, marking a function `async` does _not_ move it off the current actor. It runs where it was called from. This is what makes "the most natural code to write" data-race free by default.

• `@concurrent` — always switches to the concurrent thread pool. Use it on _your_ CPU-heavy work.
• `nonisolated` — runs wherever it's called from. This is the right default for library APIs, because the caller decides. nonisolated on a type makes all its members nonisolated (Swift 6.1+).
• Neither one — stays on the caller's actor.

nonisolated struct PhotoProcessor {          // decoupled from the main actor
  @concurrent                                // guaranteed to run in the background
  func process(_ data: Data) async -> ProcessedPhoto {
    async let sticker = extractSticker(data)  // two independent jobs, in parallel
    async let colors  = extractColors(data)
    return await ProcessedPhoto(sticker: sticker, colors: colors)
  }
}

• Profile before you offload. Use Instruments (Time Profiler, hangs). If the code can be made faster without concurrency, always do that first. Concurrency has real cost — task allocation, scheduling, and reasoning.
• Don't spawn a task for trivial work. A child task to read a UserDefaults value costs more than it saves.
• One task per end-to-end operation. Work that must happen in order goes in _one_ task; independent operations get separate tasks so the runtime can interleave them.
• `await` is a suspension point, and it breaks atomicity. State can change while you're suspended, and you may resume on a different thread. Re-check assumptions after every await. Never hold a lock across one. Never rely on thread-local storage across one.

Actor reentrancy

Actors guarantee mutual exclusion, not transactions. Between two awaits on the same actor, other work runs.

• Mutate actor state in synchronous methods. Synchronous code on an actor runs to completion uninterrupted — that's your transaction boundary.
• Keep async actor methods thin, composed of synchronous transactional operations, and leave the actor in a consistent state at every await.
• The classic bug: check cache → await download → write cache. Two tasks both miss, both download, the second clobbers the first. Re-check after the await, or dedupe the in-flight work.
• Actors are not FIFO. They run highest-priority work first, precisely to avoid priority inversion. If you need ordering, use a task (which runs start to finish) or an AsyncStream, not an actor.

4. Sendable and sharing data

Sendable marks a type safe to share across isolation domains. The compiler checks it at every task and actor boundary.

• Value types are `Sendable` when their storage is — inferred automatically for non-public types. Public types never get inferred sendability: marking a public type Sendable is a promise to your clients, so Swift makes you write it.
• Actors and `@MainActor` classes are implicitly `Sendable`, because their state is isolated.
• Most model classes should be neither `@MainActor` nor `Sendable`. Keep them non-Sendable on purpose — it prevents half the model being mutated on the main thread while the other half is mutated in the background. If they need to leave the main actor, make them nonisolated, not Sendable.
• You can still _send_ a non-`Sendable` object between domains as long as the sender stops using it. Make all your mutations _before_ handing it off; touching it afterward is the error.
• Closures capture state too. Only mark a function type @Sendable if it genuinely crosses domains.
• `@unchecked Sendable` is a promise the compiler can't check. Reserve it for types with real internal synchronization (a Mutex, a lock). Same for nonisolated(unsafe) on a global — last resort, not a warning silencer.

When you hit a data-race error, work down this list:

1. Don't share it. Move the shared object into a local so each concurrent job gets its own instance. (This is the fix for the overwhelming majority of real errors.)
2. Make it a `Sendable` value type, so "sharing" is really copying.
3. Isolate it to an actor — the main actor, or your own.
4. Only then reach for Mutex/Atomic from the Synchronization module (store them in let properties), or @unchecked Sendable.

Global and static variables are the most common source of errors. In order of preference: make it a let; put it on @MainActor; wrap it in a Mutex; nonisolated(unsafe). Note globals in Swift are initialized lazily _and_ atomically — unlike C.