swift-language

SKILL.md

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.

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