Programming Swift

Overview

Use for Swift language implementation and review across packages, services, and apps. Keep platform UI, persistence, and distribution decisions in their platform context; focus here on values, APIs, concurrency, and explicit failures.

Mental model

Value semantics control mutation; actor isolation controls access; task structure controls lifetime. None substitutes for the others. Make these contracts visible instead of quieting compiler diagnostics with unchecked annotations or detached tasks.

Version and isolation gate

Verified 2026-09-05: Swift 6.3 is released; Apple's Xcode 27 beta lists Swift 6.4. Check the repository's toolchain, swift-tools-version, language mode, upcoming-feature flags, default isolation, and deployment targets separately. A Swift 6 compiler can compile in Swift 5 language mode.

• Swift 6 language mode enables data-race safety checks. Migrate boundaries deliberately; don't assume a toolchain upgrade changes the package's language mode.
• Swift 6.2 adds opt-in default MainActor isolation and NonisolatedNonsendingByDefault, under which nonisolated async functions inherit caller isolation. Read actual settings before inferring where a function runs. async does not mean background execution.
• @concurrent on supported toolchains explicitly moves eligible async work to the concurrent executor. Use it for computation that must leave an actor, not as an annotation on every async function.
• Swift 6.2 also introduces InlineArray and Span; use them for demonstrated fixed-storage or borrowing requirements. Swift 6.3 adds @c interoperability; prefer supported declarations over underscored attributes when the compiler and C contract permit them.

Values and API design

• Prefer structs for independent values and enums with associated values for distinct states. Use classes for identity or shared lifecycle; don't impose reference semantics merely to enable inheritance.
• A struct containing a mutable class reference is not deeply independent. Keep mutation private, and validate invariants at creation rather than distributing checks through callers.
• Read APIs at their call sites. Labels should distinguish meaning (remove(at:) versus remove(_:)); expose minimal access and document non-obvious ownership, failure, or complexity contracts.
• Use generics or some Protocol when one concrete type remains statically determined. Use any Protocol for a heterogeneous existential contract; neither guarantees value semantics.
• Add protocols where substitution or a boundary is useful, not for every concrete type. Keep computed properties predictable; expensive or failing work usually deserves an explicit method.
• Model valid absence with Optional. Use throws for failures and typed throws only when a stable error set benefits callers; don't replace informative failures with try?, sentinel values, or empty collections.

Concurrency and resource lifetime

• Actors protect isolated mutable state. Actor references can cross isolation boundaries; accessing isolated members requires the appropriate isolation, often await. Transfer data using Sendable values or valid ownership transfer, not unsafe shared references.
• An actor method can interleave with other work at each await. Recheck state after suspension when correctness depends on it; serial access does not make a multi-await operation atomic.
• Prefer async let and task groups for child work that belongs to the current operation. Bound large fan-out. A Task {} is unstructured and may inherit actor context; retain its handle when explicit cancellation or result observation is needed.
• Task.detached loses inherited context and structured cancellation. Use only for a deliberate independent boundary, with explicit lifetime and failure handling.
• Cancellation is cooperative. Propagate cancellation rather than displaying it as failure; check it in long computations and before publishing stale results. Don't assume cancellation guarantees an underlying callback or I/O operation stopped.
• Checked continuations must resume exactly once on every path; bridge cancellation and callback lifetimes explicitly. Use defer for synchronous cleanup and deliberate asynchronous cleanup where necessary.
• @unchecked Sendable, nonisolated(unsafe), and force operations require a documented invariant; don't use them to bypass an unresolved ownership problem.

Packages and checks

• Preserve the dependency manager and target layout. Create an SPM target only for a real reusable or independently testable boundary; macros require their supported compiler-plugin setup.
• Follow existing formatters and tests. Swift Testing supports modern unit tests; XCTest remains appropriate for existing suites, UI automation, and XCTest-specific facilities. Don't migrate tests merely for syntax.
• Test actor reentrancy, cancellation, and invalid input when affected. Distinguish source review from checks actually executed under the supported toolchain.

Example

Keep an actor's invariant inside a non-suspending operation:

actor Inventory {
    private var available = 10

    func reserve(_ quantity: Int) -> Bool {
        guard quantity > 0, quantity <= available else { return false }
        available -= quantity
        return true
    }
}

A caller uses await inventory.reserve(2) across isolation. If remote authorization is added before mutation, state must be revalidated after that await or reserved with an explicit rollback protocol.

Checklist

• Compiler, language mode, isolation flags, and deployment target are known.
• Value ownership and actor access remain correct across suspension.
• Every task and callback bridge has a lifetime and failure path.
• Absence, cancellation, and failure remain distinct.
• Relevant checks were run when authorized; skipped checks are stated.

References

• Swift 6.3 and Swift 6.2
• Compiler and SDK matrix
• Concurrency
• Migration guide
• API design guidelines