Swift Language

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

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