Swift Language Agent Skill

Swift Language Patterns

Apply current Swift language syntax without changing behavior or evaluation order.

Route deep decoding to swift-codable, formatting to swift-formatstyle, 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

For modernization, pin current behavior and evaluation order, make one semantic

rewrite, compile the affected module, and run focused fixtures/tests. Fix any

change before continuing; repeat until behavior is preserved.

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 badgeText(for priority: Priority) -> String {
    switch priority {
    case .high: "High"
    case .medium: "Medium"
    case .low: "Low"
    }
}

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.

• When modernizing a helper with one local error enum, prefer throws(ErrorEnum) and note Swift 6+.
• 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. A -> some P

return has one fixed underlying concrete type across all return branches.

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

| Use some | Use any |

|---|---|

| Return type hiding concrete type | Heterogeneous collections |

| Function parameters (replaces simple generics) | Dynamic type erasure needed |

| Better performance (static dispatch) | Protocol has Self or associated type requirements you need to erase |

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

behaves like Swift's bottom type only where a value expression can be used or

inferred; it is not a universal type witness, does not implicitly conform to

arbitrary protocols, and cannot satisfy generic constraints such as T: P

unless the constraint is otherwise valid for Never.

// 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

Use CodingKeys for simple renames and custom decoding only for real payload

shape or transformation mismatches. Load

extended Swift patterns for a compact

language example; use swift-codable for implementation and verification.

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() and Text(_:format:) for basic display. Route style

selection, parsing, localization testing, and reusable formatter design to

swift-formatstyle.

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

• Using any when some works. Default to some for return types and

parameters, but every -> some P branch must return the same concrete type.

• Manual loops or .filter { }.count instead of collection APIs. Use

count(where:) for conditional counts, plus contains(where:),

compactMap, and flatMap instead of extra iteration or arrays.

• DateFormatter instead of FormatStyle. .formatted() is simpler,

type-safe, and handles localization automatically.

• Force-unwrapping Codable decodes. Use decodeIfPresent with defaults

for optional or missing keys.

• Reordering preconditions during modernization. Use guard without moving

normalization or transformations before validation.

• Invalid @c signatures. Say UnsafeBufferPointer is a Swift struct/value

wrapper, then reject String, Array, closures, and generic placeholders.

• Ignoring typed throws. When a function has a single, clear error type,

typed throws give callers exhaustive switch without casting.

• Overusing property wrappers. A computed property is simpler when there

is no reuse or projected value needed.

• Underspecifying Never. For Result<T, Never> or throws(Never), write the caveat explicitly: Never does not implicitly conform to arbitrary protocols, cannot satisfy arbitrary T: P constraints, and is bottom-like only in valid expression/inference contexts.

10. Owning sibling implementation. Name the owner skill and stop. Avoid

snippets for CodingKeys, decoders, formatters, SwiftUI, or concurrency.

Review Checklist

• [ ] some used only when every opaque-return branch has one concrete type
• [ ] guard for preconditions; count(where:) instead of manual counting or .filter { }.count
• [ ] .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 for single local error domains, with Swift 6+ compatibility noted
• [ ] @c corrections call UnsafeBufferPointer a Swift struct/value wrapper and enumerate rejected Swift-only types by name
• [ ] Never guidance uses uninhabited and bottom-like, and says no implicit arbitrary protocol/generic conformance
• [ ] deep Codable to swift-codable; FormatStyle APIs to swift-formatstyle; market/localized-display QA to ios-localization; naming/concurrency/SwiftUI routed to sibling skills

References

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