Swift Conventions and Foundation Formatting

Use this skill for Swift coding conventions, numeric formatting, and modern Foundation usage.

Swift Conventions

• •Prefer let over var.
• •Prefer private by default; widen only when needed.
• •Use final unless subclassing is intended.
• •Use typed errors (enum) for domain failures; do not silently swallow errors.
• •Prefer Swift Concurrency (async/await) for new code.

Number and Value Formatting

• •Never use C-style formatting such as String(format:) for numbers.
• •Always use locale-aware Foundation formatting.

Preferred approaches:

• •FormatStyle:
  swift

  let text = value.formatted(.number.precision(.fractionLength(2)))
  

• •NumberFormatter for reusable or more complex formatting needs.

Notes:

• •This rule applies to UIKit, AppKit, and shared Core framework code.
• •SwiftUI formatting examples may appear in external docs, but SwiftUI itself is forbidden in this repo.

Concurrency and Timing

• •Never use Task.sleep(nanoseconds:).
• •Always use Task.sleep(for:) with Duration.

Example:

try await Task.sleep(for: .seconds(1))

Modern Swift and Foundation Usage

Concurrency assumptions:

• •Assume strict Swift concurrency rules are enabled.
• •Respect actor isolation and Sendable requirements.
• •Do not silence concurrency warnings without a clear, documented reason.

Prefer Swift-native APIs when they exist:

let result = string.replacing("hello", with: "world")

instead of:

let result = string.replacingOccurrences(of: "hello", with: "world")

Prefer modern Foundation APIs:

• •URL.documentsDirectory instead of manual directory lookups
• •url.appending(path:) instead of string-based path concatenation
• •Avoid legacy APIs that rely on stringly-typed paths or implicit assumptions