swift-style

SKILL.md

Swift Style Guide

Code style conventions for clean, readable Swift code.

Core Principles

Clarity > Brevity > Consistency

Code should compile without warnings.

Naming

UpperCamelCase — Types, protocols

lowerCamelCase — Everything else

Clarity at call site

No abbreviations except universal (URL, ID)

// Preferred
let maximumWidgetCount = 100
func fetchUser(byID id: String) -> User

Golden Path

Left-hand margin is the happy path. Don't nest if statements.

// Preferred
func process(value: Int?) throws -> Result {
    guard let value = value else {
        throw ProcessError.nilValue
    }
    guard value > 0 else {
        throw ProcessError.invalidValue
    }
    return compute(value)
}

Code Organization

Use extensions and MARK comments:

class MyViewController: UIViewController {
    // Core implementation
}

// MARK: - UITableViewDataSource
extension MyViewController: UITableViewDataSource { }

Spacing

Braces open on same line, close on new line

One blank line between methods

Colon: no space before, one space after

Self

Avoid self unless required by compiler.

// Preferred
func configure() {
    backgroundColor = .systemBackground
}

Computed Properties

Omit get for read-only:

var diameter: Double {
    radius * 2
}

Closures

Trailing closure only for single closure parameter.

Type Inference

Let compiler infer when clear. For empty collections, use type annotation:

var names: [String] = []

Syntactic Sugar

// Preferred
var items: [String]
var cache: [String: Int]
var name: String?

Access Control

private over fileprivate

Don't add internal (it's the default)

Access control as leading specifier

Memory Management

resource.request().onComplete { [weak self] response in
    guard let self else { return }
    self.updateModel(response)
}

Comments

Explain why, not what

Use // or ///, avoid /* */

Keep up-to-date or delete

Constants

Use case-less enum for namespacing:

enum Math {
    static let pi = 3.14159
}

Common Mistakes

Abbreviations beyond URL, ID, UUID — Abbreviations like cfg, mgr, ctx, desc hurt readability. Spell them out: configuration, manager, context, description. The three exceptions are URL, ID, UUID.

Nested guard/if statements — Deep nesting makes code hard to follow. Use early returns and guards to keep the happy path left-aligned.

Inconsistent self usage — Either always omit self (preferred) or always use it. Mixing makes code scanning harder and confuses capture semantics.

Overly generic type names — Manager, Handler, Helper, Coordinator are too vague. Names should explain responsibility: PaymentProcessor, EventDispatcher, ImageCache, NavigationCoordinator.

Implied access control — Don't skip access control. Explicit private, public helps future maintainers understand module boundaries. internal is default, so omit it.