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Claude frequently generates outdated Swift patterns from its training data.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