Swift Modern Patterns (6.0–6.2)

Overview

Use this skill to ensure Swift code uses current language features and avoids deprecated or outdated patterns. It covers Swift 6.0, 6.1, and 6.2 with SE-proposal-level traceability. This skill focuses on language-level features and best practices without enforcing specific architectures.

Workflow Decision Tree

1) Generate new Swift code

• Determine the target Swift version and language mode (5 vs 6)
• Use modern concurrency patterns by default (see Concurrency Guidelines)
• Apply typed throws where the error set is fixed and internal (see references/SWIFT_6_0.md)
• Use @Observable + Observations for reactive state outside SwiftUI (SE-0475, see references/SWIFT_6_2.md)
• Use trailing commas in multi-line parameter lists (SE-0439)
• Use raw identifiers for test names (SE-0451, see references/SWIFT_6_2.md)
• Name all tasks for debuggability (SE-0469)
• Prefer InlineArray for fixed-size collections (SE-0453, see references/SWIFT_6_2.md)

2) Review existing Swift code

• Check for deprecated or outdated patterns against the Quick Reference table below
• Audit concurrency safety: global variables, actor isolation, Sendable conformance (see references/SWIFT_6_0.md)
• Verify @MainActor annotations are explicit where needed post-SE-0401
• Check import visibility: transitive imports should be explicit (SE-0444, see references/SWIFT_6_1.md)
• Look for unnecessary @unchecked Sendable — compiler region analysis (SE-0414) may make them redundant
• Verify test code uses Swift Testing patterns, not legacy XCTest (see Testing)
• Run through the Review Checklist systematically

3) Refactor or migrate Swift 5.x → 6.x

• Read references/MIGRATION.md for comprehensive old → new mappings with before/after code
• Start with concurrency: enable strict checking, resolve global variable warnings (SE-0412)
• Add explicit @MainActor to views using @StateObject/@ObservedObject (SE-0401)
• Consider enabling -default-isolation MainActor for UI-focused modules (SE-0466, Swift 6.2+)
• Replace implicit imports with explicit per-file imports (SE-0444)
• Upgrade test suites: #expect(throws:) return values (ST-0006), exit tests (ST-0008)
• For version-specific details, see references/SWIFT_6_0.md, references/SWIFT_6_1.md, references/SWIFT_6_2.md

4) Debug concurrency warnings or errors

• "Sending value of non-Sendable type..." → Check if compiler region analysis (SE-0414) resolves it; if not, see references/SWIFT_6_0.md for sending keyword (SE-0430)
• "Main actor-isolated ... cannot be used from nonisolated context" → Verify @MainActor annotations; consider nonisolated at type level (SE-0449, see references/SWIFT_6_1.md)
• "Global variable ... is not concurrency-safe" → Make it let, isolate to @MainActor, or use nonisolated(unsafe) as last resort (SE-0412, see references/SWIFT_6_0.md)
• Nonisolated async running on wrong actor → In Swift 6.2+, nonisolated async inherits caller's actor (SE-0461); use @concurrent to opt out (see references/SWIFT_6_2.md)
• Deinit cannot access actor-isolated state → Use isolated deinit (SE-0371, Swift 6.2+, see references/SWIFT_6_2.md)

5) Answer questions about Swift features

• Identify the Swift version the question relates to
• Route to the appropriate reference file for syntax and examples
• Always cite SE proposal numbers when explaining features

Core Guidelines

Concurrency

• Default to MainActor isolation when appropriate. For app/UI modules in Swift 6.2+, prefer -default-isolation MainActor and only opt out with nonisolated or @concurrent where background work is genuinely needed (SE-0466).
• Trust the compiler's isolation region analysis. Do not add unnecessary @Sendable conformances or @unchecked Sendable when the compiler can prove safety through region isolation (SE-0414).
• Use Mutex for shared mutable state — prefer Mutex from the Synchronization library over NSLock or os_unfair_lock. Use Atomic for simple lock-free flags and counters (SE-0433, SE-0410).
• Use nonisolated on types to opt out of inherited actor isolation at the type level rather than annotating every member (SE-0449, Swift 6.1+).
• Prefer Task.immediate when the task must begin executing synchronously from the caller's context before any suspension point (SE-0472, Swift 6.2+).
• Name tasks for debuggability. Always pass a name: parameter to Task.init(), Task.detached(), and addTask() in task groups (SE-0469, Swift 6.2+).
• Understand nonisolated async changes. Nonisolated async functions now run on the caller's actor by default. Use @concurrent to opt into the old hop-off behavior (SE-0461, Swift 6.2+).
• Use isolated deinit for actor-isolated classes that need safe cleanup of non-Sendable state (SE-0371, Swift 6.2+).
• Use global-actor isolated conformances when protocol conformance requires access to actor-isolated state, e.g., @MainActor Equatable (SE-0470, Swift 6.2+).

Error Handling

• Prefer typed throws for constrained domains where the error set is fixed and known (internal modules, embedded Swift). Use untyped throws for public library APIs to preserve flexibility (SE-0413).
• Leverage typed throws in generic contexts — throws(E) enables throws(Never) inference for non-throwing closures, eliminating unnecessary do/catch.

Types and Data

• Use InlineArray for fixed-size collections where the count is known at compile time. Prefer [N of T] sugar syntax (SE-0483). Note: does not conform to Sequence/Collection — iterate via .indices (SE-0453, Swift 6.2+).
• Use weak let for immutable weak references, especially in Sendable types where weak var would prevent conformance (SE-0481, Swift 6.2+).
• Prefer noncopyable types (~Copyable) for unique-ownership semantics. Optional and Result now support noncopyable wrapped types (SE-0427, SE-0437, Swift 6.0+).

Observation

• Use Observations for programmatic observation of @Observable changes outside SwiftUI — replaces manual KVO or Combine-based observation (SE-0475, Swift 6.2+).

Debugging

• Use @DebugDescription on types with CustomDebugStringConvertible to enable custom formatting in LLDB's p command and Xcode's variables view (SE-0440, Swift 6.0+).

Enums and API Design

• Leave public package enums extensible — do not add @frozen unless the case list is genuinely complete and will never grow. This allows adding cases in minor versions without breaking consumers (SE-0487, Swift 6.2.3+).
• Add @unknown default to switches over public enums from external packages to future-proof against new cases.
• Add @retroactive when conforming external types to external protocols to acknowledge the retroactive conformance and silence the compiler warning (SE-0364, Swift 6.0+).

Code Style

• Use trailing commas in multi-line parameter lists, arrays, and generics. Improves diff quality and commenting convenience (SE-0439, Swift 6.1+).
• Use raw identifiers for human-readable test names and enum cases starting with numbers (SE-0451, Swift 6.2+).
• Use default values in string interpolation — \(optional, default: "fallback") supports cross-type defaults (SE-0477, Swift 6.2+).

Imports and Modules

• Use access-level import modifiers — internal import or private import to prevent accidental API leakage (SE-0409, Swift 6.0+).
• Enable MemberImportVisibility flag to require explicit imports per file (SE-0444, Swift 6.1+).

Testing

• Prefer Swift Testing over XCTest for new test targets. Use @Test, #expect, #require.
• Use range-based confirmations — confirmation(expectedCount: 5...10) (ST-0005, Swift 6.1+).
• Use returned errors from #expect(throws:) instead of the deprecated dual-trailing-closure form (ST-0006, Swift 6.1+).
• Use test scoping traits for concurrency-safe shared test configuration via TestScoping protocol (ST-0007, Swift 6.1+).
• Use exit tests — #expect(processExitsWith: .failure) for testing precondition/fatalError paths (ST-0008, Swift 6.2+).
• Use test attachments — Attachment.record(value, named:) for debugging failing tests (ST-0009, Swift 6.2+).

Quick Reference: Old → New

Review Checklist

Concurrency Safety

• No bare mutable global variables (must be let, actor-isolated, or nonisolated(unsafe))
• Views using @StateObject/@ObservedObject have explicit @MainActor
• No unnecessary @unchecked Sendable — check if region analysis handles it
• Shared mutable state uses Mutex or Atomic, not NSLock/os_unfair_lock
• Tasks are named with name: parameter
• nonisolated async behavior is intentional (caller-inheriting in 6.2+ vs hop-off in <6.2)
• isolated deinit used where actor-isolated class accesses non-Sendable state in cleanup

Error Handling

• Typed throws used for internal/fixed error sets; untyped throws for public APIs
• No unnecessary catch-all blocks when typed throws covers all cases

Modern Patterns

• InlineArray uses [N of T] sugar syntax where available
• weak let used instead of weak var where immutability suffices (enables Sendable)
• Observations used instead of Combine/KVO for @Observable outside SwiftUI
• String interpolation uses default: parameter for optional cross-type fallbacks
• Trailing commas in multi-line lists
• @DebugDescription used on types with CustomDebugStringConvertible
• String(validating:as:) used instead of String(bytes:encoding:) for safe encoding

Imports

• No transitive import dependencies (MemberImportVisibility enabled)
• Access-level imports used to prevent API leakage (internal import, private import)

API Design

• Public package enums: extensible by default, @frozen only when case list is truly fixed
• Consumer switch over external enums includes @unknown default
• Retroactive conformances annotated with @retroactive

Testing

• Swift Testing preferred over XCTest for new tests
• #expect(throws:) returns error for separate validation (not dual-closure form)
• Exit tests used for precondition/fatalError paths
• Test scoping traits used for shared configuration (not mutable shared state)
• Raw identifiers used for readable test names

Version Coverage

References

• references/SWIFT_6_0.md — Concurrency defaults, Mutex/Atomics, typed throws, @DebugDescription, noncopyable upgrades, AsyncSequence generics, RangeSet, Int128, BitwiseCopyable
• references/SWIFT_6_1.md — Trailing commas, metatype key paths, TaskGroup inference, nonisolated types, import visibility, diagnostic groups, Swift Testing improvements
• references/SWIFT_6_2.md — Default MainActor isolation, raw identifiers, InlineArray + [N of T] sugar, immediate tasks, weak let, Observations, Span/MutableSpan, method key paths, exit tests
• references/SWIFT_6_2_3.md — Extensible enums for non-resilient modules (SE-0487)
• references/MIGRATION.md — Comprehensive old → new pattern mappings with before/after code for all versions

Philosophy

This skill focuses on language-level facts and best practices, not opinions:

• No architecture enforcement. We don't prescribe MVVM, TCA, VIPER, or any pattern. We suggest modern language features; how you structure your app is your decision.
• No formatting or linting rules. Property ordering, brace style, and line length are out of scope.
• SE-proposal grounded. Every recommendation traces to a Swift Evolution proposal. If it's not in an SE, it's not in this skill.
• Version-aware. Every feature is tagged with the Swift version that introduced it. The agent should not suggest Swift 6.2 features to a project targeting Swift 6.0.
• Practical over comprehensive. The agent's context window is finite. These files contain what matters for day-to-day code generation and review, not exhaustive API documentation.