Modern Swift Idioms

Corrects the outdated Swift that AI assistants tend to write: legacy APIs, older syntax, and Foundation calls that now have modern replacements.

When to Use

Use this skill when:

• Reviewing generated Swift code for modern idioms
• You notice your assistant using Date(), CGFloat, DateFormatter(), or other legacy patterns
• Asking "is there a more modern way to do this?"
• Reviewing code for newer Swift APIs, from Swift 5.5 through Swift 6.4

Example Prompts

Questions you can ask your assistant that will draw from this skill:

• "Review my Swift code for outdated patterns."
• "Is there a more modern way to write this?"
• "Should I use FormatStyle instead of DateFormatter?"
• "Check whether my Foundation usage is current."
• "How do I write availability for all Apple platforms with anyAppleOS?" (Swift 6.4)
• "How do I silence a deprecation warning in just one function?" (Swift 6.4)
• "When should I use isTriviallyIdentical(to:) instead of ==?" (Swift 6.4)

What This Skill Provides

• Modern API replacements – 11 common outdated patterns, each with its modern equivalent and the Swift version that introduced it
• Modern syntax – Shorthand optional binding, static member lookup, if and switch expressions, and why a SwiftUI file doesn't need import UIKit
• Foundation modernization – URL APIs, Comparable sorting, and the "y" date format
• SwiftUI conveniences – ContentUnavailableView.search, LabeledContent, and where to attach confirmationDialog()
• Swift 6.4 language features (Xcode 27) – anyAppleOS availability, weak let, ~Sendable, the second memberwise initializer, @diagnose for one declaration's warnings, and the isTriviallyIdentical(to:) O(1) equality fast path (SE-0494), with guidance on when it helps and when it backfires
• Swift 6.4 concurrency defaults – The posture new code should start from: main actor isolation by default, handled Task errors, structured concurrency, and escape hatches only as temporary bridges
• Common AI mistakes – 14 patterns AI assistants get wrong, such as inline DateFormatter instances, DispatchQueue.main.async, and Task.detached used to reach a background thread

Philosophy

The skill follows Paul Hudson's principle: "Don't repeat what LLMs already know — focus on edge cases, surprises, soft deprecations."

It is intentionally small, covering only the patterns AI assistants consistently get wrong. General Swift knowledge doesn't need a skill.

Documentation Scope

This page documents the swift-modern skill in the axiom-swift suite. It sits in this section because its concurrency defaults overlap with the concurrency skills.

Related

• Swift Concurrency – The full async/await, actor, and Sendable guidance behind this skill's concurrency defaults
• Swift Performance – Low-level optimization (copy-on-write, ARC, generics), as opposed to idiom correctness
• Modernization Helper – An agent that scans for legacy SwiftUI patterns, such as ObservableObject where @Observable would do