/swift-language

name: swift-language
description: "Apply modern Swift language patterns and idioms for non-concurrency, non-SwiftUI code. Covers if/switch expressions (Swift 5.9+), typed throws (Swift 6+), result builders, property wrappers, opaque and existential types (some vs any), guard patterns, Never type, Regex builders (Swift 5.7+), basic Codable shaping (CodingKeys, custom decoding, nested containers), modern collection APIs (count(where:), contains(where:), replacing()), basic FormatStyle usage, and string interpolation patterns. Use when writing core Swift code involving generics, protocols, enums, closures, or modern language features; route deep Codable to swift-codable, detailed formatting/localization to swift-formatstyle, and API naming to swift-api-design-guidelines."

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](#ifswitch-expressions)
• [Typed Throws](#typed-throws)
• [Result Builders](#result-builders)
• [Property Wrappers](#property-wrappers)
• [Opaque and Existential Types](#opaque-and-existential-types)
• [Guard Patterns](#guard-patterns)
• [Never Type](#never-type)
• [Regex Builders](#regex-builders)
• [Codable Best Practices](#codable-best-practices)
• [Modern Collection APIs](#modern-collection-apis)
• [FormatStyle](#formatstyle)
• [String Interpolation](#string-interpolation)
• [Common Mistakes](#common-mistakes)
• [Review Checklist](#review-checklist)
• [References](#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