swift-api-design-guidelines

Swift API Design Guidelines

Apply the Swift API Design Guidelines when naming types, methods, properties, parameters, and argument labels. Targets Swift 6.3. For language features and syntax, see swift-language. For concurrency patterns, see swift-concurrency. For mixed requests, answer the API naming portion briefly, then route Swift type-system details to swift-language and lint configuration to swiftlint instead of implementing those sibling domains here.

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 these rules in order.

When to omit the first argument label

Grammatical phrase rule. When the first argument forms a grammatical phrase with the base name, omit the label. Move any leading words from what would be the label into the base name instead.

// GOOD — reads as "add subview y"
view.addSubview(y)

// BAD — redundant label breaks the phrase
view.add(subview: y)

Value-preserving type conversions. When an initializer performs a value-preserving (widening) conversion, omit the first argument label.

// GOOD — widening conversion, no label
let value = Int64(someUInt32)
let str = String(someCharacter)

// Narrowing or lossy conversions keep a label
let approx = Int64(truncating: someDecimal)
let str = String(describing: someObject)

Indistinguishable arguments. When all arguments cannot be usefully distinguished, omit all labels.

// GOOD — arguments are peers
let smaller = min(x, y)
zip(sequence1, sequence2)

When to use a prepositional label

Prepositional phrase rule. When the first argument completes a prepositional phrase with the base name, label it with the preposition.

// GOOD — "remove boxes having length 12"
x.removeBoxes(havingLength: 12)

// GOOD — "fade from red"
view.fade(from: red)

// GOOD — "relative path from root"
path.relativePath(from: root)

Exception — abstraction boundary. When the first two arguments represent parts of a single abstraction, fold the preposition into the base name so each component gets its own label.

// GOOD — x and y are parts of a single abstraction (a point)
a.moveTo(x: b, y: c)

// BAD — preposition attaches to first arg, leaving y unlabeled
a.move(toX: b, y: c)

Default: label everything else

When no special rule above applies, label the argument.

// GOOD
array.split(maxSplits: 2)
button.setTitle("OK", for: .normal)
controller.dismiss(animated: true)
array.sorted(by: >)

Argument label decision table

For extended examples and edge cases, see references/argument-labels-and-parameters.md.

Side-Effect Naming

Name functions and methods by their side effects.

Functions with side effects — imperative verbs

When a function mutates state, name it as an imperative verb phrase.

// Mutates — imperative verb
array.sort()
array.append(newElement)
list.remove(at: index)
timer.invalidate()

Functions without side effects — nouns or adjective phrases

When a function returns a result without mutating anything, name it as a noun phrase, adjective phrase, or read as a description of what it returns.

// Pure — noun/description
let d = point.distance(to: origin)
let area = rect.intersection(other)
let line = text.trimmingCharacters(in: .whitespaces)

Boolean properties and methods

Boolean properties and methods read as assertions about the receiver.

// GOOD — reads as "line is empty"
line.isEmpty
set.contains(element)
url.isFileURL

// BAD — not an assertion
line.empty       // verb? adjective?
set.includes     // incomplete phrase

For more examples, see references/side-effects-and-mutating-pairs.md.

Mutating and Nonmutating Pairs

When an operation has both mutating and nonmutating variants, name them as a pair.

Verb-described operations — -ed/-ing suffix

When the operation is naturally described by a verb:

• Mutating: imperative verb (sort, append, reverse)
• Nonmutating: past participle -ed or present participle -ing

Default to -ed (past participle) when the phrase naturally describes the returned value. Use -ing (present participle) only when the -ed form is ungrammatical or describes the direct object rather than the returned receiver or result. A direct object is a clue to check the grammar, not the rule by itself.

Noun-described operations — form- prefix

When the operation is naturally described by a noun:

• Nonmutating: the noun itself (union, intersection)
• Mutating: form prefix (formUnion, formIntersection)

// Nonmutating — returns new value
let combined = a.union(b)

// Mutating — modifies in place
a.formUnion(b)

Factory methods — make- prefix

Factory methods that create a new value start with make.

let iterator = collection.makeIterator()
let buffer = parser.makeBuffer()

In mixed routing answers, briefly validate existing make... factory names before handing off unrelated type-system or linting details to sibling skills.

Pair decision table

For the full -ed/-ing decision tree and expanded naming patterns, see references/side-effects-and-mutating-pairs.md.

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

1. Omitting needed argument labels. Using remove(position) instead of remove(at: position) when the role of the argument is ambiguous without the label.

1. Using -ed when -ing is correct. Applying stripped() when the past participle is ungrammatical — use stripping() instead. Test: does "a [verb]-ed [noun]" read naturally?

1. Using verb names for side-effect-free operations. Naming a nonmutating method sort() that returns a new collection — use sorted() to signal no mutation.

1. Naming by type instead of role. Using string instead of greeting, or array instead of elements, when the role would be more informative.

1. Missing documentation comments. Leaving public declarations undocumented, or writing summaries that describe the implementation rather than the purpose.

1. Not documenting non-O(1) computed properties. Exposing a linear-time computed property without a Complexity: note, causing callers to assume O(1) and use it in loops.

1. Applying form- prefix to verb-based operations. Writing formSort() instead of just sort() — the form prefix is only for noun-based operations (formUnion).

1. Factory methods without make- prefix. Naming factory methods as createIterator() or buildBuffer() instead of makeIterator() and makeBuffer().

1. Repeating type information in names. Writing removeElement(cancelButton) or stringValue: String when the type is already evident from context.

1. Return-type-only overloads. Defining overloads that differ only in return type, creating ambiguity when the compiler cannot infer the expected type.

1. Unlabeled tuple members and closure parameters. Exposing tuples or closures in public API without naming their components, forcing callers to use positional access.

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