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Claude frequently generates outdated Swift patterns from its training data.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:

• 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