| name | axiom-swift-modern |
| description | Use when reviewing or generating Swift code for modern idiom correctness — catches outdated APIs, pre-Swift 5.5 patterns, and Foundation legacy usage that Claude defaults to |
| license | MIT |
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Claude frequently generates outdated Swift patterns from its training data.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 stringLabeledContent in Forms (iOS 16+) provides consistent label alignment without manual HStack layoutconfirmationDialog() 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:
- Creates
DateFormatter instances inline — Use .formatted() or FormatStyle instead. If a formatter must exist, make it static let.
- Uses
DispatchQueue.main.async — Use @MainActor or MainActor.run. Never GCD. (See axiom-swift-concurrency for full guidance.)
- Uses
CGFloat for SwiftUI parameters — Double works everywhere since Swift 5.5 implicit bridging.
- Generates
guard let x = x else — Use guard let x else shorthand.
- Returns explicitly in single-expression computed properties — Omit
return.
Resources
Skills: axiom-swift-performance, axiom-swift-concurrency, axiom-swiftui-architecture
