swift-language-pro

Swift Language Pro

Write idiomatic, safe, modern Swift. Follow Apple's API Design Guidelines.

When to use

• Writing or reviewing non-UI Swift (models, services, utilities).
• Designing public APIs, naming, and type choices.
• Cleaning up optional handling, error handling, or generics.

Trigger: /swift-language-pro.

Core principles

• Prefer value types (struct/enum) by default; use class only for identity or reference semantics.
• Make illegal states unrepresentable with enums and non-optional types.
• Throw typed errors; don't return sentinel values or booleans for failure.
• Name for clarity at the call site, not the definition.

Value vs reference

❌

class Point { var x = 0.0; var y = 0.0 }   // accidental shared mutation

✅

struct Point { var x = 0.0; var y = 0.0 }

Model with enums, kill optionals

❌ Two bools that allow impossible states

struct State { var isLoading: Bool; var error: Error? }   // loading + error?

✅

enum LoadState<Value> { case idle, loading, loaded(Value), failed(Error) }

Optionals

• Unwrap with if let / guard let; avoid force-unwrap ! outside tests.
• guard for early exit, keeping the happy path unindented.

❌

func name(_ u: User?) -> String { return u!.name }

✅

func name(_ u: User?) -> String {
    guard let u else { return "Guest" }
    return u.name
}

Error handling

❌ Boolean failure

func save() -> Bool

✅

enum SaveError: Error { case diskFull, notAuthorized }
func save() throws    // call sites use try/catch; errors carry meaning

API design (naming)

• Read at the call site like a phrase. Include the noun a method acts on.
• Omit needless words; drop type names from labels.

❌

func insertObject(_ obj: Element, atIndex i: Int)
list.insertObject(x, atIndex: 0)

✅

func insert(_ element: Element, at index: Int)
list.insert(x, at: 0)

Generics & protocols

• Constrain generics with where; prefer protocols with associated types over Any.
• Use protocol extensions for default behavior; don't reach for class inheritance.

✅

func max<C: Collection>(of items: C) -> C.Element? where C.Element: Comparable {
    items.max()
}

Common mistakes checklist

• class where a struct would do.
• Optional/bool combos that allow impossible states (use an enum).
• Force-unwrap ! in app code.
• Returning Bool/nil for failures instead of throws.
• Method names that repeat type info or don't read at the call site.
• Any/type-erasure where a generic constraint fits.

Output format (when reviewing)

Per issue: file:line, the guideline violated, before/after. Lead with safety (force-unwraps, impossible states) before style.