Swift API Design Guidelines

Follow Apple's Swift API Design Guidelines so code feels consistent with the Swift ecosystem. For full detail and examples, see reference.md.

When to Apply

• Writing or refactoring Swift types, functions, methods, properties, or parameters
• Reviewing Swift code for API clarity and naming
• User asks for Swift style, naming conventions, or API design

Fundamentals

• Clarity at the point of use is the top goal. Design so call sites read clearly, not just the declaration.
• Clarity over brevity. Avoid minimizing character count for its own sake.
• Document every declaration with a doc comment. Use Swift's Markdown dialect. Start with a one-line summary (sentence fragment, period). If the API is hard to describe simply, reconsider the design.
• Document non-O(1) computed properties so callers don't assume cheap access.

Naming Checklist

Promote clear usage

• Include words needed to avoid ambiguity at the call site (e.g. remove(at: x) not remove(x)).
• Omit needless words; avoid repeating type information (e.g. remove(_ member:) not removeElement(_ member:)).
• Name variables, parameters, and associated types by role, not type (e.g. ContentView, supplier, not ViewType, widgetFactory).
• For weak types (Any, NSObject, Int, String), add a noun that clarifies role (e.g. addObserver(_:forKeyPath:)).

Fluent usage

• Method names should form grammatical phrases at use: e.g. x.insert(y, at: z), x.capitalizingNouns().
• Factory methods start with make: e.g. x.makeIterator().
• Initializer/factory first argument should not continue the base name as a phrase (e.g. Color(red: 32, green: 64, blue: 128); avoid Color(havingRGBValuesRed: 32, ...)).
• Side-effect-free → noun phrase: x.distance(to: y). With side effects → imperative verb: x.sort(), x.append(y).
• Mutating/nonmutating pairs: verb imperative for mutating, -ed or -ing for nonmutating (e.g. reverse() / reversed(); stripNewlines() / strippingNewlines()). Noun-based: nonmutating = noun, mutating = form prefix (e.g. union(_:) / formUnion(_:)).
• Boolean uses read as assertions: x.isEmpty, line1.intersects(line2).
• Types/protocols (what something is) → nouns (e.g. Collection). Capability protocols → -able, -ible, or -ing (e.g. Equatable, ProgressReporting).

Terminology

• Prefer common words unless a term of art is needed; use terms in their established meaning.
• Avoid abbreviations unless meaning is easy to find (e.g. web search). Prefer precedent (e.g. Array, sin(x)).

Conventions Checklist

General

• Prefer methods and properties over free functions unless: no obvious self, unconstrained generic, or domain notation (e.g. min(x,y,z), print(x), sin(x)).
• Case: types and protocols UpperCamelCase; everything else lowerCamelCase. Acronyms follow case: utf8Bytes, userSMTPServer; others as words: radarDetector.
• Methods can share a base name only when meaning is the same or domains are distinct. Avoid overloading on return type.

Parameters

• Parameter names should read well in documentation (e.g. filter(_ predicate:), replaceRange(_ subRange:, with newElements:)).
• Use defaulted parameters to simplify common uses; prefer one method with defaults over method families.
• Put parameters with defaults at the end.
• Production APIs: prefer #fileID; use #filePath only for dev-only helpers; use #file for Swift 5.2 compatibility.

Argument labels

• Omit all labels when arguments can't be distinguished: min(a, b), zip(s1, s2).
• Value-preserving type conversion initializers: omit first label, e.g. Int64(someUInt32), String(veryLargeNumber, radix: 16). Narrowing conversions: use a descriptive label (e.g. init(truncating:), init(saturating:)).
• First argument in a prepositional phrase → label starting at preposition: x.removeBoxes(havingLength: 12). Single abstraction (e.g. coordinates) → label after preposition: a.moveTo(x:b, y:c).
• First argument that completes a grammatical phrase → omit label, e.g. x.addSubview(y). Otherwise use a label: view.dismiss(animated: false), words.split(maxSplits: 12).
• All other arguments have labels.

Special instructions

• Name tuple members and closure parameters where they appear in the API.
• With unconstrained polymorphism (Any, AnyObject, unconstrained generics), avoid overload ambiguity (e.g. name append(contentsOf:) distinctly from append(_:) when Element could be Any).

Doc comment summary rules

• Function/method: what it does and what it returns; omit void and no-op.
• Subscript: what it accesses.
• Initializer: what instance it creates.
• Other declarations: what the entity is.

Use symbol command bullets when useful: Parameter, Returns, Throws, Note, SeeAlso, etc.

Additional resources

• Full guidelines and code samples: reference.md
• Official source: Swift API Design Guidelines