Swift Language

Swift Language Patterns

Core Swift language features and modern syntax patterns targeting Swift 6.3. Covers language constructs, type system features, basic Codable, string and collection APIs, basic formatting, C interop (@c), module disambiguation (ModuleName::symbol), and performance attributes (@specialized, @inline(always)). For @c corrections, enumerate invalid Swift-only signature types: String, Array, UnsafeBufferPointer, closures, and generic placeholders. Route deeper Codable/API decoding to swift-codable, detailed formatting/localization to swift-formatstyle, API naming to swift-api-design-guidelines, concurrency to swift-concurrency, and SwiftUI state/view work to swiftui-patterns.

Contents

• If/Switch Expressions
• Typed Throws
• Result Builders
• Property Wrappers
• Opaque and Existential Types
• Guard Patterns
• Never Type
• Regex Builders
• Codable Best Practices
• Modern Collection APIs
• FormatStyle
• String Interpolation
• Common Mistakes
• Review Checklist
• References

If/Switch Expressions

Swift 5.9+ allows if and switch as expressions that return values. Use them to assign, return, or initialize directly.

// Assign from if expression
let icon = if isComplete { "checkmark.circle.fill" } else { "circle" }

// Assign from switch expression
let label = switch status {
case .draft: "Draft"
case .published: "Published"
case .archived: "Archived"
}

// Works in return position
func color(for priority: Priority) -> Color {
    switch priority {
    case .high: .red
    case .medium: .orange
    case .low: .green
    }
}

Rules:

• Every branch must produce a value of the same type.
• Multi-statement branches are not allowed -- each branch is a single expression.
• Wrap in parentheses when used as a function argument to avoid ambiguity.

Typed Throws

Swift 6+ allows specifying the error type a function throws.

enum ValidationError: Error {
    case tooShort, invalidCharacters, alreadyTaken
}

func validate(username: String) throws(ValidationError) -> String {
    guard username.count >= 3 else { throw .tooShort }
    guard username.allSatisfy(\.isLetterOrDigit) else { throw .invalidCharacters }
    return username.lowercased()
}

// Caller gets typed error -- no cast needed
do {
    let name = try validate(username: input)
} catch {
    // error is ValidationError, not any Error
    switch error {
    case .tooShort: print("Too short")
    case .invalidCharacters: print("Invalid characters")
    case .alreadyTaken: print("Taken")
    }
}

Rules:

• Use throws(SomeError) only when callers benefit from exhaustive error

handling. For mixed error sources, use untyped throws.

• throws(Never) marks a function that syntactically throws but never actually

does -- useful in generic contexts.

• Typed throws propagate: a function calling throws(A) and throws(B) must

itself throw a type that covers both (or use untyped throws).

Result Builders

@resultBuilder enables DSL-style syntax. SwiftUI's @ViewBuilder is the most common example, but you can create custom builders for any domain.

@resultBuilder
struct ArrayBuilder<Element> {
    static func buildBlock(_ components: [Element]...) -> [Element] {
        components.flatMap { $0 }
    }
    static func buildExpression(_ expression: Element) -> [Element] { [expression] }
    static func buildOptional(_ component: [Element]?) -> [Element] { component ?? [] }
    static func buildEither(first component: [Element]) -> [Element] { component }
    static func buildEither(second component: [Element]) -> [Element] { component }
    static func buildArray(_ components: [[Element]]) -> [Element] { components.flatMap { $0 } }
}

func makeItems(@ArrayBuilder<String> content: () -> [String]) -> [String] { content() }

let items = makeItems {
    "Always included"
    if showExtra { "Conditional" }
    for name in names { name.uppercased() }
}

Builder methods: buildBlock (combine statements), buildExpression (single value), buildOptional (if without else), buildEither (if/else), buildArray (for..in), buildFinalResult (optional post-processing).

Property Wrappers

Custom @propertyWrapper types encapsulate storage and access patterns.

@propertyWrapper
struct Clamped<Value: Comparable> {
    private var value: Value
    let range: ClosedRange<Value>

    var wrappedValue: Value {
        get { value }
        set { value = min(max(newValue, range.lowerBound), range.upperBound) }
    }

    var projectedValue: ClosedRange<Value> { range }

    init(wrappedValue: Value, _ range: ClosedRange<Value>) {
        self.range = range
        self.value = min(max(wrappedValue, range.lowerBound), range.upperBound)
    }
}

// Usage
struct Volume {
    @Clamped(0...100) var level: Int = 50
}

var v = Volume()
v.level = 150   // clamped to 100
print(v.$level) // projected value: 0...100

Design rules:

• wrappedValue is the primary getter/setter.
• projectedValue (accessed via $property) provides metadata or bindings.
• Property wrappers can be composed: @A @B var x applies outer wrapper first.
• Do not use property wrappers when a simple computed property suffices.

Opaque and Existential Types

some Protocol (Opaque Type)

The caller does not know the concrete type, but the compiler does. The underlying type is fixed for a given scope.

func makeCollection() -> some Collection<Int> {
    [1, 2, 3]  // Always returns Array<Int> -- compiler knows the concrete type
}

Use some for:

• Return types when you want to hide implementation but preserve type identity.
• Parameter types (Swift 5.7+): some P is shorthand for an unnamed generic

parameter such as <T: P>.

any Protocol (Existential Type)

An existential box that can hold any conforming type at runtime. It uses dynamic dispatch and may allocate when the value does not fit in the inline buffer.

func process(items: [any StringProtocol]) {
    for item in items {
        print(item.uppercased())
    }
}

When to choose

Rule of thumb: Default to some. Use any only when you need a heterogeneous collection or runtime type flexibility.

Guard Patterns

guard enforces preconditions and enables early exit. It keeps the happy path left-aligned and reduces nesting.

func processOrder(_ order: Order?) throws -> Receipt {
    // Unwrap optionals
    guard let order else { throw OrderError.missing }

    // Validate conditions
    guard order.items.isEmpty == false else { throw OrderError.empty }
    guard order.total > 0 else { throw OrderError.invalidTotal }

    // Boolean checks
    guard order.isPaid else { throw OrderError.unpaid }

    // Pattern matching
    guard case .confirmed(let date) = order.status else {
        throw OrderError.notConfirmed
    }

    return Receipt(order: order, confirmedAt: date)
}

Best practices:

• Use guard for preconditions, if for branching logic.
• Combine related guards: guard let a, let b else { return }.
• The else block must exit scope: return, throw, continue, break, or

fatalError().

• Use shorthand unwrap: guard let value else { ... } (Swift 5.7+).

Never Type

Never is an uninhabited type for code paths that never produce a value. It works as Swift's bottom type in expression contexts, but it does not implicitly conform to arbitrary protocols or satisfy a generic T: SomeProtocol constraint. When recommending Result<T, Never> or throws(Never), explicitly state all three points: uninhabited, bottom-like, and no universal protocol conformance.

// Function that terminates the program
func crashWithDiagnostics(_ message: String) -> Never {
    let diagnostics = gatherDiagnostics()
    logger.critical("\(message): \(diagnostics)")
    fatalError(message)
}

enum Result<Success, Failure: Error> {
    case success(Success)
    case failure(Failure)
}
// Result<String, Never> -- a result that can never fail

// Exhaustive switch: no default needed since Never has no cases
func handle(_ result: Result<String, Never>) {
    switch result {
    case .success(let value): print(value)
    // No .failure case needed -- compiler knows it's impossible
    }
}

Regex Builders

Swift 5.7+ Regex builder DSL provides compile-time checked, readable patterns.

import Foundation
import RegexBuilder

// Parse "2024-03-15" into components
let dateRegex = Regex {
    Capture { /\d{4}/ }; "-"; Capture { /\d{2}/ }; "-"; Capture { /\d{2}/ }
}

if let match = "2024-03-15".firstMatch(of: dateRegex) {
    let (_, year, month, day) = match.output
    _ = (year, month, day)
}

// TryCapture with transform
let priceRegex = Regex {
    "$"
    TryCapture { OneOrMore(.digit); "."; Repeat(.digit, count: 2) }
        transform: { Decimal(string: String($0)) }
}

When to use builder vs. literal:

• Builder: complex patterns, reusable components, strong typing on captures.
• Literal (/pattern/): simple patterns, familiarity with regex syntax.
• Both can be mixed: embed /.../ literals inside builder blocks.

Codable Best Practices

Custom CodingKeys

Rename keys without writing a custom decoder:

struct User: Codable {
    let id: Int
    let displayName: String
    let avatarURL: URL

    enum CodingKeys: String, CodingKey {
        case id
        case displayName = "display_name"
        case avatarURL = "avatar_url"
    }
}

Custom Decoding

Handle mismatched types, defaults, and transformations:

struct Item: Decodable {
    let name: String
    let quantity: Int
    let isActive: Bool

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        quantity = try container.decodeIfPresent(Int.self, forKey: .quantity) ?? 0
        if let boolValue = try? container.decode(Bool.self, forKey: .isActive) {
            isActive = boolValue
        } else {
            isActive = (try container.decode(String.self, forKey: .isActive)).lowercased() == "true"
        }
    }
    enum CodingKeys: String, CodingKey { case name, quantity; case isActive = "is_active" }
}

Nested Containers

Flatten nested JSON into a flat Swift struct:

// JSON: { "id": 1, "metadata": { "created_at": "...", "tags": [...] } }
struct Record: Decodable {
    let id: Int
    let createdAt: String
    let tags: [String]

    enum CodingKeys: String, CodingKey {
        case id, metadata
    }

    enum MetadataKeys: String, CodingKey {
        case createdAt = "created_at"
        case tags
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        let metadata = try container.nestedContainer(
            keyedBy: MetadataKeys.self, forKey: .metadata)
        createdAt = try metadata.decode(String.self, forKey: .createdAt)
        tags = try metadata.decode([String].self, forKey: .tags)
    }
}

See references/swift-patterns-extended.md for additional Codable patterns (enums with associated values, date strategies, unkeyed containers).

Modern Collection APIs

Prefer these modern APIs over manual loops:

let numbers = [1, 2, 3, 4, 5, 6, 7, 8]

// count(where:) -- use instead of .filter { }.count
let evenCount = numbers.count(where: { $0.isMultiple(of: 2) })

// contains(where:) -- short-circuits on first match
let hasNegative = numbers.contains(where: { $0 < 0 })

// first(where:) / last(where:)
let firstEven = numbers.first(where: { $0.isMultiple(of: 2) })

// String replacing() -- Swift 5.7+, returns new string
let cleaned = rawText.replacing(/\s+/, with: " ")
let snakeCase = name.replacing("_", with: " ")

// compactMap -- unwrap optionals from a transform
let ids = strings.compactMap { Int($0) }

// flatMap -- flatten nested collections
let allTags = articles.flatMap(\.tags)

// Dictionary(grouping:by:)
let byCategory = Dictionary(grouping: items, by: \.category)

// reduce(into:) -- efficient accumulation
let freq = words.reduce(into: [:]) { counts, word in
    counts[word, default: 0] += 1
}

FormatStyle

Use .formatted() instead of DateFormatter/NumberFormatter. It is type-safe, localized, and concise.

// Dates
let now = Date.now
now.formatted()                                       // "3/15/2024, 2:30 PM"
now.formatted(date: .abbreviated, time: .shortened)   // "Mar 15, 2024, 2:30 PM"
now.formatted(.dateTime.year().month().day())          // "Mar 15, 2024"
now.formatted(.relative(presentation: .named))        // "yesterday"

// Numbers
let price = 42.5
price.formatted(.currency(code: "USD"))               // "$42.50"
price.formatted(.percent)                             // "4,250%"
(1_000_000).formatted(.number.notation(.compactName)) // "1M"

// Measurements
let distance = Measurement(value: 5, unit: UnitLength.kilometers)
distance.formatted(.measurement(width: .abbreviated)) // "5 km"

// Duration (Swift 5.7+)
let duration = Duration.seconds(3661)
duration.formatted(.time(pattern: .hourMinuteSecond)) // "1:01:01"

// Byte counts
Int64(1_500_000).formatted(.byteCount(style: .file)) // "1.5 MB"

// Lists
["Alice", "Bob", "Carol"].formatted(.list(type: .and)) // "Alice, Bob, and Carol"

Parsing: FormatStyle also supports parsing:

let value = try Decimal("$42.50", format: .currency(code: "USD"))
let date = try Date("Mar 15, 2024", strategy: .dateTime.month().day().year())

String Interpolation

Extend DefaultStringInterpolation for domain-specific formatting. Use """ for multi-line strings (indentation is relative to the closing """). See references/swift-patterns-extended.md for custom interpolation examples.

Common Mistakes

1. Using `any` when `some` works. Default to some for return types and parameters. any has runtime overhead and loses type information. 2. Manual loops instead of collection APIs. Use count(where:), contains(where:), compactMap, flatMap instead of manual iteration. 3. `DateFormatter` instead of FormatStyle. .formatted() is simpler, type-safe, and handles localization automatically. 4. Force-unwrapping Codable decodes. Use decodeIfPresent with defaults for optional or missing keys. 5. Nested if-let chains. Use guard let for preconditions to keep the happy path at the top level. 6. Invalid `@c` signatures. Name valid C types and explicitly reject: String, Array, UnsafeBufferPointer, closures, generic placeholders. 7. Ignoring typed throws. When a function has a single, clear error type, typed throws give callers exhaustive switch without casting. 8. Overusing property wrappers. A computed property is simpler when there is no reuse or projected value needed. 9. Underspecifying `Never`. For Result<T, Never> or throws(Never), say: uninhabited, bottom-like, and not arbitrary T: P protocol conformance. 10. Owning deep formatting/localization. Use swift-formatstyle for detailed formatting and ios-localization for market/localized-display QA.

Review Checklist

• [ ] some used for opaque returns and Swift 5.7+ generic-parameter shorthand
• [ ] guard for preconditions; collection APIs instead of manual loops
• [ ] .formatted() used instead of DateFormatter/NumberFormatter
• [ ] Codable types use CodingKeys for API mapping; decodeIfPresent with defaults for optional fields
• [ ] if/switch expressions for conditional assignment; property wrappers have clear reuse justification
• [ ] Regex builder used for complex patterns (literal OK for simple ones)
• [ ] Typed throws used when callers benefit from exhaustive error handling
• [ ] @c corrections enumerate rejected Swift-only types by name
• [ ] Never guidance says uninhabited, bottom-like, and not arbitrary T: P protocol conformance
• [ ] deep Codable, formatting/localization, naming, concurrency, and SwiftUI work routed to sibling skills

References

• Extended patterns and Codable examples: references/swift-patterns-extended.md
• Attributes and C interop: references/swift-attributes-interop.md

Swift Attributes and C Interoperability

Attributes and interoperability features for Swift. Covers C-calling-convention export, module disambiguation, performance annotations, and symbol visibility control.

Contents

• C Interoperability — `@c` Attribute
• Module Selectors
• Performance Annotations
• Symbol Visibility and Layout

C Interoperability — @c Attribute

The @c attribute (SE-0495) marks a Swift function for direct C-calling-convention export. The function becomes callable from C, C++, and Objective-C without bridging headers or @_cdecl.

@c(MyLib_processBuffer)
public func processBuffer(_ buffer: UnsafePointer<UInt8>?, _ count: Int32) -> Int32 {
    guard let buffer else { return 0 }
    return buffer.pointee == 0 ? 0 : count
}

Requirements:

• Parameters and return types must be C-compatible (primitives, pointers, tuples of C-compatible types)
• No Swift-only types (String, Array, UnsafeBufferPointer, closures, generic placeholders, etc.) in the signature
• The function must be a module-level free function (not a method)
• Use @c(CustomName) when the C symbol should differ from the Swift function name

Module Selectors

SE-0491 adds ModuleName::symbolName syntax to disambiguate identically named symbols from different modules without import aliasing.

import NetworkingA
import NetworkingB

// Both modules export a top-level `configure()` function
func setup() {
    NetworkingA::configure()
    NetworkingB::configure()
}

// Works with types too
let client: NetworkingA::Client = .init()

Performance Annotations

@specialized

SE-0460 makes @specialized an official attribute (previously underscored as @_specialize). Forces the compiler to emit a specialized version of a generic function for specific concrete types.

@specialized(where T == Int)
@specialized(where T == Double)
func sum<T: Numeric>(_ values: [T]) -> T {
    values.reduce(.zero, +)
}

@inline(always) Guarantee

SE-0496 guarantees @inline(always) will inline the function at every call site. Previously it was a hint the compiler could ignore. A compilation error is now emitted if inlining is impossible (e.g., recursive calls).

@inline(always)
func fastPath(_ x: Int) -> Int {
    x &+ 1  // Guaranteed to be inlined at every call site
}

Symbol Visibility and Layout

@export

SE-0497 gives explicit control over symbol visibility and definition availability:

• @export(interface) — ensures a callable symbol exists in the binary but hides the definition from clients (no inlining/specialization by external callers). Replaces @_neverEmitIntoClient.
• @export(implementation) — makes the definition available for inlining/specialization but does not guarantee a callable symbol. Replaces @_alwaysEmitIntoClient.

@export(interface)
public func stableAPI() -> Int {
    // Callable symbol guaranteed; definition hidden from clients
    return computeValue()
}

@section and @used

SE-0492 places global variables into named binary sections and prevents dead-stripping. Primarily for Embedded Swift and systems programming.

@section(".mydata") @used
var configFlag: Int32 = 1

Swift Patterns Extended Reference

Additional patterns and examples that extend the core SKILL.md. Refer to this file for deeper Codable patterns, advanced result builder techniques, and supplementary collection/formatting recipes.

Contents

• Codable: Enums with Associated Values
• Codable: Date Decoding Strategies
• Codable: Unkeyed Containers (Arrays)
• Codable: Wrapper for Lossy Array Decoding
• Codable: `@dynamicMemberLookup` Wrapper
• Result Builder: HTML Builder
• Result Builder: buildFinalResult
• Property Wrapper: UserDefaults-Backed
• Property Wrapper: Validated
• Advanced Regex Builder Patterns
• FormatStyle: Custom FormatStyle
• Collection Patterns: Chunking and Windows
• Guard: Complex Pattern Matching
• Typed Throws: Protocol with Typed Errors
• String Interpolation: Custom appendInterpolation
• Never: Advanced Usage

Codable: Enums with Associated Values

enum Shape: Codable {
    case circle(radius: Double)
    case rectangle(width: Double, height: Double)

    enum CodingKeys: String, CodingKey {
        case type, radius, width, height
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
        case .circle(let radius):
            try container.encode("circle", forKey: .type)
            try container.encode(radius, forKey: .radius)
        case .rectangle(let width, let height):
            try container.encode("rectangle", forKey: .type)
            try container.encode(width, forKey: .width)
            try container.encode(height, forKey: .height)
        }
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try container.decode(String.self, forKey: .type)
        switch type {
        case "circle":
            let radius = try container.decode(Double.self, forKey: .radius)
            self = .circle(radius: radius)
        case "rectangle":
            let width = try container.decode(Double.self, forKey: .width)
            let height = try container.decode(Double.self, forKey: .height)
            self = .rectangle(width: width, height: height)
        default:
            throw DecodingError.dataCorruptedError(
                forKey: .type, in: container,
                debugDescription: "Unknown shape type: \(type)")
        }
    }
}

Codable: Date Decoding Strategies

// Configure decoder for specific date formats
let decoder = JSONDecoder()

// ISO 8601 (most common for APIs)
decoder.dateDecodingStrategy = .iso8601

// Unix timestamp (seconds since epoch)
decoder.dateDecodingStrategy = .secondsSince1970

// Custom format
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
formatter.locale = Locale(identifier: "en_US_POSIX")
decoder.dateDecodingStrategy = .formatted(formatter)

// Multiple formats in one payload
decoder.dateDecodingStrategy = .custom { decoder in
    let container = try decoder.singleValueContainer()
    let string = try container.decode(String.self)

    let iso = ISO8601DateFormatter()
    if let date = iso.date(from: string) { return date }

    let fallback = DateFormatter()
    fallback.dateFormat = "yyyy-MM-dd"
    fallback.locale = Locale(identifier: "en_US_POSIX")
    if let date = fallback.date(from: string) { return date }

    throw DecodingError.dataCorruptedError(
        in: container, debugDescription: "Cannot decode date: \(string)")
}

Codable: Unkeyed Containers (Arrays)

// JSON: { "coordinates": [37.7749, -122.4194] }
struct Location: Decodable {
    let latitude: Double
    let longitude: Double

    enum CodingKeys: String, CodingKey {
        case coordinates
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        var coords = try container.nestedUnkeyedContainer(forKey: .coordinates)
        latitude = try coords.decode(Double.self)
        longitude = try coords.decode(Double.self)
    }
}

Codable: Wrapper for Lossy Array Decoding

Skip invalid elements instead of failing the entire array:

struct LossyArray<Element: Decodable>: Decodable {
    let elements: [Element]

    init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        var result: [Element] = []
        while !container.isAtEnd {
            if let element = try? container.decode(Element.self) {
                result.append(element)
            } else {
                _ = try? container.decode(AnyCodable.self) // skip invalid
            }
        }
        elements = result
    }
}

private struct AnyCodable: Decodable {}

Codable: @dynamicMemberLookup Wrapper

Type-safe access to arbitrary JSON:

@dynamicMemberLookup
struct JSONValue: Decodable {
    private let storage: [String: Any]

    subscript(dynamicMember key: String) -> JSONValue? {
        guard let value = storage[key] as? [String: Any] else { return nil }
        return JSONValue(storage: value)
    }

    func string(for key: String) -> String? {
        storage[key] as? String
    }

    func int(for key: String) -> Int? {
        storage[key] as? Int
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        guard let dict = try container.decode([String: AnyCodableValue].self)
            .mapValues({ $0.value }) as? [String: Any] else {
            throw DecodingError.typeMismatch(
                [String: Any].self,
                .init(codingPath: decoder.codingPath,
                      debugDescription: "Expected dictionary"))
        }
        storage = dict
    }
}

Result Builder: HTML Builder

A practical example of a custom result builder:

@resultBuilder
struct HTMLBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: "\n")
    }

    static func buildOptional(_ component: String?) -> String {
        component ?? ""
    }

    static func buildEither(first component: String) -> String {
        component
    }

    static func buildEither(second component: String) -> String {
        component
    }

    static func buildArray(_ components: [String]) -> String {
        components.joined(separator: "\n")
    }
}

func div(@HTMLBuilder content: () -> String) -> String {
    "<div>\n\(content())\n</div>"
}

func p(_ text: String) -> String { "<p>\(text)</p>" }
func h1(_ text: String) -> String { "<h1>\(text)</h1>" }

let html = div {
    h1("Welcome")
    p("Hello, world!")
    if showDetails {
        p("Details here")
    }
}

Result Builder: buildFinalResult

Transform the accumulated result at the end:

@resultBuilder
struct AttributedStringBuilder {
    static func buildBlock(_ components: AttributedString...) -> AttributedString {
        components.reduce(into: AttributedString()) { $0.append($1) }
    }

    static func buildFinalResult(_ component: AttributedString) -> Text {
        Text(component)
    }
}

Property Wrapper: UserDefaults-Backed

@propertyWrapper
struct AppStorage<Value> {
    let key: String
    let defaultValue: Value
    let store: UserDefaults

    var wrappedValue: Value {
        get { store.object(forKey: key) as? Value ?? defaultValue }
        set { store.set(newValue, forKey: key) }
    }

    init(wrappedValue: Value, _ key: String, store: UserDefaults = .standard) {
        self.key = key
        self.defaultValue = wrappedValue
        self.store = store
    }
}

// Usage
struct Settings {
    @AppStorage("onboarding_complete") var onboardingComplete = false
    @AppStorage("preferred_theme") var theme = "system"
}

Property Wrapper: Validated

@propertyWrapper
struct Validated<Value> {
    private var value: Value
    private let validator: (Value) -> Bool
    private(set) var isValid: Bool

    var wrappedValue: Value {
        get { value }
        set {
            value = newValue
            isValid = validator(newValue)
        }
    }

    var projectedValue: Bool { isValid }

    init(wrappedValue: Value, _ validator: @escaping (Value) -> Bool) {
        self.value = wrappedValue
        self.validator = validator
        self.isValid = validator(wrappedValue)
    }
}

// Usage
struct SignUpForm {
    @Validated({ $0.count >= 3 }) var username = ""
    @Validated({ $0.contains("@") && $0.contains(".") }) var email = ""

    var canSubmit: Bool { $username && $email }
}

Advanced Regex Builder Patterns

Reference captures with strong typing

import RegexBuilder

struct LogEntry {
    let timestamp: String
    let level: String
    let message: String
}

let timestampRef = Reference(Substring.self)
let levelRef = Reference(Substring.self)
let messageRef = Reference(Substring.self)

let logRegex = Regex {
    Capture(as: timestampRef) { /\[.+?\]/ }
    " "
    Capture(as: levelRef) {
        ChoiceOf { "INFO"; "WARN"; "ERROR"; "DEBUG" }
    }
    ": "
    Capture(as: messageRef) { OneOrMore(.any) }
}

if let match = "[2026-05-28] INFO: Started".firstMatch(of: logRegex) {
    let entry = LogEntry(
        timestamp: String(match[timestampRef]),
        level: String(match[levelRef]),
        message: String(match[messageRef])
    )
    _ = entry
}

Reusing regex components

let ipOctet = Regex {
    ChoiceOf {
        Regex { "25"; ("0"..."5") }
        Regex { "2"; ("0"..."4"); .digit }
        Regex { Optionally { ("0"..."1") }; .digit; Optionally { .digit } }
    }
}

let ipAddress = Regex {
    ipOctet; "."; ipOctet; "."; ipOctet; "."; ipOctet
}

FormatStyle: Custom FormatStyle

Create reusable format styles for domain types:

struct FileSize {
    let bytes: Int64
}

struct FileSizeFormatStyle: FormatStyle {
    typealias FormatInput = FileSize
    typealias FormatOutput = String

    func format(_ value: FileSize) -> String {
        ByteCountFormatter.string(fromByteCount: value.bytes, countStyle: .file)
    }
}

extension FormatStyle where Self == FileSizeFormatStyle {
    static var fileSize: FileSizeFormatStyle { .init() }
}

// Usage
let size = FileSize(bytes: 1_500_000)
size.formatted(.fileSize) // "1.5 MB"

Collection Patterns: Chunking and Windows

// chunks(ofCount:) -- Swift Algorithms package
import Algorithms

let batches = items.chunks(ofCount: 10)
for batch in batches {
    try await upload(batch)
}

// windows(ofCount:) -- sliding window
let movingAverages = values.windows(ofCount: 3).map { window in
    window.reduce(0, +) / Double(window.count)
}

// adjacentPairs() -- process consecutive elements
for (previous, current) in values.adjacentPairs() {
    if current > previous * 2 {
        print("Spike detected")
    }
}

Guard: Complex Pattern Matching

func processResponse(_ data: Data) throws -> User {
    guard let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
          let userData = json["user"] as? [String: Any],
          let name = userData["name"] as? String,
          let id = userData["id"] as? Int
    else {
        throw ParseError.invalidFormat
    }

    // Use Codable instead when practical -- this is for mixed/dynamic JSON
    return User(id: id, name: name)
}

// Guard with where clause
func processItems(_ items: [Item]) {
    for item in items {
        guard case .active(let config) = item.status,
              config.isEnabled,
              !config.isExpired
        else { continue }

        activate(item, with: config)
    }
}

Typed Throws: Protocol with Typed Errors

protocol DataStore {
    associatedtype StoreError: Error
    func save(_ data: Data) throws(StoreError)
    func load(id: String) throws(StoreError) -> Data
}

struct FileStore: DataStore {
    enum StoreError: Error {
        case notFound, permissionDenied, diskFull
    }

    func save(_ data: Data) throws(StoreError) {
        // ...
    }

    func load(id: String) throws(StoreError) -> Data {
        guard fileExists(id) else { throw .notFound }
        // ...
    }
}

String Interpolation: Custom appendInterpolation

Apple documents DefaultStringInterpolation as the type used while building interpolated strings, and supports extending it with custom appendInterpolation(...) overloads.

extension DefaultStringInterpolation {
    mutating func appendInterpolation(json value: some Encodable) {
        let encoder = JSONEncoder()
        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
        if let data = try? encoder.encode(value),
           let string = String(data: data, encoding: .utf8) {
            appendLiteral(string)
        }
    }

    mutating func appendInterpolation(ordinal value: Int) {
        let formatter = NumberFormatter()
        formatter.numberStyle = .ordinal
        if let result = formatter.string(from: value as NSNumber) {
            appendLiteral(result)
        }
    }
}

print("Config: \(json: settings)")
print("You placed \(ordinal: 3)")

Never: Advanced Usage

// Publisher that never fails
let publisher: AnyPublisher<String, Never> = Just("hello").eraseToAnyPublisher()

// Phantom type preventing construction
enum Locked {}
enum Unlocked {}

struct Door<State> {
    private init() {}
}

extension Door where State == Unlocked {
    func open() { /* ... */ }
}

// Generic constraint meaning "this case cannot happen"
func absurd<T>(_ never: Never) -> T {
    // No body needed -- Never has no values, so this is never called
}