swift-language

Swift Language Patterns Core Swift language features and modern syntax patterns targeting Swift 5.9+ through Swift 6. Covers language constructs, type system features, Codable, string and collection APIs, and formatting. For concurrency (actors, async/await, Sendable), see the swift-concurrency skill. For SwiftUI views and state management, see 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 -- compiler knows the concrete type } Use some for: Return types when you want to hide implementation but preserve type identity. Parameter types (Swift 5.9+): func process(_ items: some Collection) -- equivalent to a generic > . any Protocol (Existential Type) An existential box that can hold any conforming type at runtime. Has overhead from dynamic dispatch and heap allocation. 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 indicates a function that never returns. It conforms to all protocols since Swift 5.5+ (bottom type). // Function that terminates the program func crashWithDiagnostics ( _ message : String ) -> Never { let diagnostics = gatherDiagnostics ( ) logger . critical ( " ( message ) : ( diagnostics ) " ) fatalError ( message ) } // Useful in generic contexts enum Result < Success , Failure : Error

{ case success ( Success ) case failure ( Failure ) } // Result -- a result that can never fail // Result -- a result that can never succeed // 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 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 } // 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:) -- Swift 5.0+, 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 Using any when some works. Default to some for return types and parameters. any has runtime overhead and loses type information. Manual loops instead of collection APIs. Use count(where:) , contains(where:) , compactMap , flatMap instead of manual iteration. 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. Nested if-let chains. Use guard let for preconditions to keep the happy path at the top level. String regex for simple operations. Use .replacing() and .contains() before reaching for Regex. 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. Building collections with var + append in a loop. Prefer map , filter , compactMap , or reduce(into:) . Not using if/switch expressions. When assigning from a condition, use an expression instead of declaring var and mutating it. Review Checklist some used over any where possible guard used for preconditions, not nested if let Collection APIs used instead of manual loops .formatted() used instead of DateFormatter / NumberFormatter Codable types use CodingKeys for API key mapping decodeIfPresent with defaults for optional JSON fields if/switch expressions used for simple conditional assignment Property wrappers have clear reuse justification Regex builder used for complex patterns (literal OK for simple ones) String interpolation is clean -- no unnecessary String(describing:) Typed throws used when callers benefit from exhaustive error handling Never used appropriately in generic contexts References Extended patterns and Codable examples: references/swift-patterns-extended.md