charleswiltgen/axiom-swift-modern

Modern Swift Idioms

Purpose

Claude frequently generates outdated Swift patterns from its training data. This skill corrects the most common ones — patterns that compile fine but use legacy APIs when modern equivalents are clearer, more efficient, or more correct.

Philosophy: "Don't repeat what LLMs already know — focus on edge cases, surprises, soft deprecations." (Paul Hudson)

Modern API Replacements

| Old Pattern | Modern Swift | Since | Why | |-------------|-------------|-------|-----| | Date() | Date.now | 5.6 | Clearer intent | | filter { }.count | count(where:) | 5.0 | Single pass, no intermediate allocation | | replacingOccurrences(of:with:) | replacing(_:with:) | 5.7 | Swift native, no Foundation bridge | | CGFloat | Double | 5.5 | Implicit bridging; exceptions: optionals, inout, ObjC-bridged APIs | | Task.sleep(nanoseconds:) | Task.sleep(for: .seconds(1)) | 5.7 | Type-safe Duration API | | DateFormatter() | .formatted() / FormatStyle | 5.5 | No instance management, localizable by default | | String(format: "%.2f", val) | val.formatted(.number.precision(.fractionLength(2))) | 5.5 | Type-safe, localized | | localizedCaseInsensitiveContains() | localizedStandardContains() | 5.0 | Handles diacritics, ligatures, width variants | | "\(firstName) \(lastName)" | PersonNameComponents with .formatted() | 5.5 | Respects locale name ordering | | "yyyy-MM-dd" with DateFormatter | try Date(string, strategy: .iso8601) | 5.6 | Modern parsing (throws); use "y" not "yyyy" for display | | contains() on user input | localizedStandardContains() | 5.0 | Required for correct text search/filtering |

Modern Syntax

| Old Pattern | Modern Swift | Since | |-------------|-------------|-------| | if let value = value { | if let value { | 5.7 | | Explicit return in single-expression | Omit return; if/switch are expressions | 5.9 | | Circle() in modifiers | .circle (static member lookup) | 5.5 | | import UIKit alongside import SwiftUI | Often not needed — SwiftUI re-exports most UIKit/AppKit types. Retain for UIKit-only APIs (UIApplication, etc.) | 5.5 |

Foundation Modernization

| Old Pattern | Modern Foundation | Since | |-------------|------------------|-------| | FileManager.default.urls(for: .documentDirectory, ...) | URL.documentsDirectory | 5.7 | | url.appendingPathComponent("file") | url.appending(path: "file") | 5.7 | | books.sorted { $0.author < $1.author } (repeated) | Conform to Comparable, call .sorted() | — | | "yyyy" in date format for display | "y" — correct in all calendar systems | — |

SwiftUI Convenience APIs Claude Misses

• ContentUnavailableView.search(text: searchText) (iOS 17+) automatically includes the search term — no need to compose a custom string
• LabeledContent in Forms (iOS 16+) provides consistent label alignment without manual HStack layout
• confirmationDialog() must attach to triggering UI — Liquid Glass morphing animations depend on the source element

Common Claude Hallucinations

These patterns appear frequently in Claude-generated code:

1. Creates DateFormatter instances inline — Use .formatted() or FormatStyle instead. If a formatter must exist, make it static let.
2. Uses DispatchQueue.main.async — Use @MainActor or MainActor.run. Never GCD. (See axiom-swift-concurrency for full guidance.)
3. Uses CGFloat for SwiftUI parameters — Double works everywhere since Swift 5.5 implicit bridging.
4. Generates guard let x = x else — Use guard let x else shorthand.
5. Returns explicitly in single-expression computed properties — Omit return.

Resources

Skills: axiom-swift-performance, axiom-swift-concurrency, axiom-swiftui-architecture