version: "1.0.2" name: swift-api-design-guidelines description: "Apply Swift API Design Guidelines to name, label, and document Swift APIs. Covers argument label rules (prepositional phrase rule, grammatical phrase rule, first-label omission), mutating/nonmutating pair naming (-ed/-ing participle pattern, form- prefix, sort/sorted, formUnion/union), side-effect naming (noun for pure, verb for mutating), documentation comment structure (summary by declaration kind, O(1) complexity rule), clarity at call site, role-based naming, protocol naming (-able/-ible/-ing), default arguments over method families, casing conventions, and terminology. Use when designing new Swift APIs, reviewing naming and argument labels, writing documentation comments, or refactoring for call site clarity."

Swift API Design Guidelines

Apply the Swift API Design Guidelines to naming, labels, documentation, and call-site clarity. For mixed requests, handle the API-design portion here and route language/type-system work to swift-language, concurrency to swift-concurrency, and lint configuration to swiftlint.

Contents

• Argument Label Rules
• Side-Effect Naming
• Mutating and Nonmutating Pairs
• Documentation Comments
• Clarity and Naming
• Fluent Usage and Protocols
• General Conventions
• Common Mistakes
• Review Checklist
• References

Argument Label Rules

Argument labels determine how a call site reads. Apply the first matching row:

Load Argument Labels and Parameters when resolving abstraction boundaries, multiple prepositions, conversion initializers, indistinguishable peers, parameter naming, or default arguments.

Side-Effect Naming

Use imperative verbs for operations with side effects, result-describing noun or adjective phrases for operations without side effects, and assertion-style names for Boolean APIs.

array.sort()
array.append(newElement)
let d = point.distance(to: origin)
line.isEmpty
set.contains(element)

Load Side Effects and Mutating Pairs when reviewing extended pure/mutating examples or Boolean naming.

Mutating and Nonmutating Pairs

Name mutating/nonmutating pairs from the operation's natural description:

• For verb operations, use the imperative for mutation and a result-describing

participle for the copy: sort()/sorted() or append(_:)/appending(_:). Prefer -ed; use -ing only when -ed is ungrammatical or describes the direct object instead of the returned result.

• For noun operations, use the noun for the copy and form + noun for

mutation: union(_:) / formUnion(_:).

• Prefix factories that create new values with make.

Load the `-ed`/`-ing` Decision Tree when the returned-result grammar is unclear. The same reference contains expanded form-prefix, Boolean, and factory patterns.

Documentation Comments

Every public declaration must have a documentation comment.

Summary rules by declaration kind

Write summaries as a single sentence fragment, beginning with a verb (for actions) or a noun phrase (for entities), ending in a period.

/// Returns the element at the specified index.
func element(at index: Int) -> Element { ... }
 
/// The number of elements in the collection.
var count: Int { ... }
 
/// Creates a new array with the given elements.
init(_ elements: some Sequence<Element>) { ... }
 
/// Accesses the element at the specified position.
subscript(index: Int) -> Element { ... }

Symbol markup

Use standard symbol markup after the summary when relevant:

• - Parameter name: for individual parameters
• - Parameters: block for multiple parameters
• - Returns: for the return value
• - Throws: for errors thrown
• - Complexity: for algorithmic complexity

/// Removes and returns the element at the specified position.
///
/// - Parameter index: The position of the element to remove.
/// - Returns: The removed element.
/// - Complexity: O(*n*), where *n* is the length of the collection.
mutating func remove(at index: Int) -> Element { ... }

O(1) complexity rule

Document the complexity of any computed property that is not O(1). Callers assume properties are O(1) by default. If a property does more than constant-time work, state the complexity explicitly.

/// The total weight of all items.
///
/// - Complexity: O(*n*), where *n* is the number of items.
var totalWeight: Double {
    items.reduce(0) { $0 + $1.weight }
}

For documentation patterns and examples, see references/conventions-and-special-rules.md.

Clarity and Naming

Clarity at the point of use is the most important goal. Every design decision serves the person reading a call site.

Clarity over brevity. Longer names are acceptable when they remove ambiguity. Do not abbreviate.

// GOOD
employees.remove(at: position)
 
// BAD — ambiguous: remove the element? remove at position?
employees.remove(position)

Include words needed to avoid ambiguity. If omitting a word makes the call site unclear, keep it.

// GOOD — "at" clarifies the argument's role
friends.remove(at: index)
 
// BAD — is "index" the element to remove or the position?
friends.remove(index)

Omit needless words. Do not repeat type information already available from the context.

// GOOD
allViews.remove(cancelButton)
 
// BAD — "Element" repeats the type
allViews.removeElement(cancelButton)

Name variables and parameters by role, not type. Use the entity's role in the current context, not its type name.

// GOOD — describes the role
var greeting: String
func add(_ observer: NSObject, for keyPath: String)
 
// BAD — names the type
var string: String
func add(_ object: NSObject, for string: String)

Compensate for weak type information. When a parameter type is Any, AnyObject, or a fundamental type like Int or String, add role-clarifying words to the name.

// GOOD — role is clear despite weak types
func addObserver(_ observer: NSObject, forKeyPath path: String)
 
// BAD — what does "string" mean here?
func add(_ object: NSObject, for string: String)

For extended naming examples and patterns, see references/naming-and-clarity.md.

Fluent Usage and Protocols

Call sites read as grammatical English. Prefer names that form grammatical phrases at the point of use.

// GOOD — reads fluently
x.insert(y, at: z)          // "x, insert y at z"
x.subviews.remove(at: i)    // "x's subviews, remove at i"
x.makeIterator()             // "x, make iterator"
 
// BAD — ungrammatical
x.insert(y, position: z)
x.subviews.remove(i)

Initializer first argument. The first argument to an initializer should not form a phrase continuing the type name.

// GOOD
let foreground = Color(red: 32, green: 64, blue: 128)
 
// BAD — "Color with red" reads awkwardly
let foreground = Color(havingRGBValuesRed: 32, green: 64, blue: 128)

Protocol naming conventions:

General Conventions

Casing. Types and protocols use UpperCamelCase. Everything else uses lowerCamelCase. Acronyms that are commonly all-caps in American English appear uniformly upper- or lower-cased based on position.

var utf8Bytes: [UTF8.CodeUnit]
var isRepresentableAsASCII = true
var userSMTPServer: SMTPServer

Methods and properties over free functions. Prefer methods and properties. Use free functions only when:

1. There is no obvious self — min(x, y)
2. The function is an unconstrained generic — print(value)
3. The function syntax is established domain notation — sin(x)

Default arguments over method families. Prefer a single method with default parameters over a family of methods that differ only in which parameters they accept. Place defaulted parameters at the end. Parameters with default values should always have argument labels — defaulted parameters are usually omitted at call sites, so their labels must be clear when they do appear.

// GOOD — labeled with defaults
func decode(_ data: Data, encoding: String.Encoding = .utf8) -> String?
 
// BAD — method family
func decode(_ data: Data) -> String?
func decode(_ data: Data, encoding: String.Encoding) -> String?

Overload safety. Methods may share a base name when they operate in different type domains or when their meaning is clear from context. Avoid return-type-only overloads that cause ambiguity at the call site.

For casing edge cases, overload patterns, and tuple/closure naming, see references/conventions-and-special-rules.md.

Common Mistakes

Review Checklist

Argument Labels

• [ ] First argument follows the correct label rule (grammatical phrase, prepositional, conversion, or labeled)
• [ ] Prepositional labels do not incorrectly group independent arguments
• [ ] Value-preserving conversion initializers omit the first label
• [ ] All non-special-case arguments have labels

Naming Semantics

• [ ] Mutating methods use imperative verb form
• [ ] Nonmutating methods use -ed/-ing or noun form
• [ ] Mutating/nonmutating pairs follow the correct pattern (verb pair or noun/form-noun pair)
• [ ] Boolean properties read as assertions (isEmpty, isValid, contains)
• [ ] Variables and parameters are named by role, not type

Documentation

• [ ] Every public declaration has a doc comment
• [ ] Summaries are single sentence fragments ending in a period
• [ ] Summaries describe the correct thing per declaration kind (action, access, creation, entity)
• [ ] Non-O(1) computed properties document their complexity
• [ ] Parameters, return values, and thrown errors are documented with symbol markup

Conventions

• [ ] Types and protocols use UpperCamelCase; everything else uses lowerCamelCase
• [ ] Acronyms are uniformly cased based on position
• [ ] Default arguments are preferred over method families
• [ ] Overloads do not differ only in return type
• [ ] Protocol names follow the noun (is-a) or suffix (capability) convention

References

• Naming clarity, role-based naming, weak-type compensation, and terminology: references/naming-and-clarity.md
• Argument label edge cases, parameter naming, and default argument strategy: references/argument-labels-and-parameters.md
• Side-effect naming examples, -ed/-ing decision tree, form- prefix patterns, and factory methods: references/side-effects-and-mutating-pairs.md
• Casing edge cases, complexity documentation, overload safety, tuple/closure naming, and free function exceptions: references/conventions-and-special-rules.md