Result Builders

those buildEither() methods also enable switch statements to be used within result builder contexts, without requiring any additional build methods. 👉 Sundell

👉paiza.io

@resultBuilder
struct StringBuilder {

    // ⭐️ (mandantory)
    static func buildBlock(_ parts: String...) -> String {
        parts.joined(separator: "\n")
    }
    
    // ⭐️ support `if...else...` statement
    static func buildEither(first part: String) -> String {
        return part
    }

    static func buildEither(second part: String) -> String {
        return part
    }
    
    // ⭐️ support `for...in` loop
    static func buildArray(_ parts: [String]) -> String {
        parts.joined(separator: ", ")
    }
}

// ⭐️ applied on `func`
@StringBuilder func s1() -> String {
    "Why settle for a Duke"
    "when you can have"
    "a Prince?"
}

// ⭐️ applied on `var`
@StringBuilder var s2: String {
    "Why settle for a Duke"
    "when you can have"
    "a Prince?"
}

// ⭐️ translated by complier to:
// ------------------------------
// StringBuilder.buildBlock(
//     "Why settle for a Duke",
//     "when you can have",
//     "a Prince?"
// )
// ------------------------------

print(s1())
print(s2)
// Why settle for a Duke
// when you can have
// a Prince?

@StringBuilder var s3: String {

    "Why settle for a Duke"
    "when you can have"

    // ⭐️ supported by `buildEither(first:)` / `buildEither(second:)`
    if .random() { "a Frog?" } 
    else { "a King?" }
}

print(s3)

@StringBuilder var countDown: String {

    // ⭐️ supported by `buildArray(_:)`
    for i in (0...10).reversed() { "\(i)…" }

    "Lift off!"
}

print(countDown)
// 10…, 9…, 8…, 7…, 6…, 5…, 4…, 3…, 2…, 1…, 0…
// Lift off!

👉 paiza.io

// before using @SettingsBuilder
[
    Setting("Offline mode", .bool(false)),
    Setting("Search page size", .int(25)),
    Setting("Experimental", .group([
        Setting("Default name", .string("Untitled")),
        Setting("Fluid animations", .bool(true))
    ]))
]

// ⭐️ after using @SettingsBuilder
Settings {                                 // ⭐️ 1
    ("Offline mode", false)
    ("Search page size", 25)
    Settings.Group("Experimental") {       // ⭐️ 2
        ("Default name", "Untitled")
        ("Fluid animations", true)
    }
}

評論：這個例子用 Result Builder 似乎沒討到什麼好處。

// ---------------
//     Setting
// ---------------

struct Setting {
    var name: String
    var value: Value
}

extension Setting {

    enum Value {
        case bool(Bool)
        case int(Int)
        case string(String)
        case group([Setting])
    }
    
    init(_ name: String, _ value: Value){
        self.init(name: name, value: value)
    }
    
    init(_ tuple: Tuple){
        self.init(name: tuple.name, value: tuple.value)
    }
    
    typealias Tuple = (name: String, value: Value)
}

// -----------------------
//     SettingsBuilder
// -----------------------

@resultBuilder
struct SettingsBuilder {

    typealias Component = Settings
    
    static func buildBlock(_ components: Component...) -> Component {
        Array(components.joined())
    }
    
    // -------------------------
    //     ⭐️ buildExpression
    // -------------------------
    
    // for tuple
    static func buildExpression(_ tuple: Setting.Tuple) -> Component {
        [Setting(tuple)]
    }
    
    // ⭐️ case .bool
    static func buildExpression(_ tuple: (name: String, bool: Bool)) -> Component {
        [Setting(tuple.name, .bool(tuple.bool))]
    }
    
    // ⭐️ case .int
    static func buildExpression(_ tuple: (name: String, int: Int)) -> Component {
        [Setting(tuple.name, .int(tuple.int))]
    }
    
    // ⭐️ case .string
    static func buildExpression(_ tuple: (name: String, string: String)) -> Component {
        [Setting(tuple.name, .string(tuple.string))]
    }
    
    // for a normal Setting
    static func buildExpression(_ setting: Setting) -> Component {
        [setting]
    }
}

// -----------------
//     Settings
// -----------------

typealias Settings = [Setting]

// typealias can be extended ⭐️
extension Settings {

    // Settings { ... }
    init(
        @SettingsBuilder settings: () -> [Setting]      // ⭐️ 1
    ) {
        self = settings()
    }
    
    // Settings.Group { ... }
    static func Group(
        _ name: String, 
        @SettingsBuilder settings: () -> [Setting]      // ⭐️ 2
    ) -> Setting {
        Setting(name: name, value: .group(settings()))
    }
}

👉 paiza.io

此範例建立一個 Drawable 協定來規範所有「用於組合的型別」。
Line 是 Drawable 這類型別的「最終組合目標」。
DrawingBuilder 用來實現「如何組合」這些型別。

// ---------------
//    Drawable
// ---------------
//
// • Types for drawing on a single line 
//   using stars ("*") and text (String)

// 🔸 Drawable
// defines the requirement for something that can be drawn (as a String)
protocol Drawable: CustomStringConvertible {
    func draw() -> String
}

extension Drawable {
    var description: String { draw() }      // CustomStringConvertible
}

// 🔸 Line
// represents a single-line drawing.
// serves the top-level container for most drawings.
struct Line: Drawable {

    var drawables: [Drawable]
    init(_ drawables: [Drawable]) { self.drawables = drawables }
    
    // ⭐ 5. closure parameter (@DrawingBuilder)
    init(@DrawingBuilder content: () -> Drawable) {
        self.drawables = [content()]
    }
    
    func draw() -> String {
        drawables
            .map { $0.draw() }
            .joined(separator: "")
    }
}

// 🔸 Text
// wraps a string to make it part of a drawing
struct Text: Drawable {
    var content: String
    init(_ content: String) { self.content = content }
    func draw() -> String { content }
}

// 🔸 Space
struct Space: Drawable {
    func draw() -> String { " " }
}

// 🔸 Stars
struct Stars: Drawable {
    var length: Int
    init(_ length: Int) { self.length = length }
    func draw() -> String { String(repeating: "🌟", count: length) }
}

// 🔸 AllCaps
// converts text in the drawing to uppercase
struct AllCaps: Drawable {
    var content: Drawable
    
    // ⭐ 3. closure parameter (@DrawingBuilder)
    init(@DrawingBuilder content: () -> Drawable) {
        self.content = content()
    }
    
    func draw() -> String { content.draw().uppercased() }
}

// -----------------------
//    ⭐ DrawingBuilder
// -----------------------

// 🔸 DrawingBuilder
@resultBuilder
struct DrawingBuilder {

    /// ⭐ 1. asupport a block of code 
    ///   每個 @DrawingBuilder closure 的最外層都是由此方法處理。
    static func buildBlock(_ drawables: Drawable...) -> Drawable {
        Line(drawables)
    }
    
    /// ⭐ 2. support `if-else`, `if-let-else` , `switch`
    static func buildEither(first: Drawable) -> Drawable {
        return first
    }
    static func buildEither(second: Drawable) -> Drawable {
        return second
    }
}

// -----------------------
//    ⭐ Use Cases
// -----------------------

// 🔸 greeting(for:)
@DrawingBuilder
func greeting(for name: String? = nil) -> Drawable {// ⭐ @DrawingBuilder

    Stars(3)
    Space()
    Text("Hello")
    Space()
    
    AllCaps {                                       // ⭐ 3. init (@DrawingBuilder)
        if let name = name { Text(name + "!") }     // ╮ 
        else { Text("World!") }                     // ╯ ⭐ 2. buildEither
    }
    
    Space()
    Stars(2)
}


print(greeting())
print(greeting(for: "swift taylor"))
// 🌟🌟🌟 Hello WORLD! 🌟🌟
// 🌟🌟🌟 Hello SWIFT TAYLOR! 🌟🌟

// ---------------------------------
//    ⭐ DrawingBuilder extension
// ---------------------------------

extension DrawingBuilder {
    /// ⭐ 4. support `for-in` , `.map()`
    static func buildArray(_ components: [Drawable]) -> Drawable {
        Line(components)
    }
}

// -----------------------
//    ⭐ Use Cases
// -----------------------

let stars = Line {      // ⭐ 5. @DrawingBuilder

    Text("Stars:")
    
    // ⭐ 4. buildArray
    for i in 1...3 {        // ╮
        Space()             // ⭐️ 1. buildBlock
        Stars(i)            // │
    }                       // ╯
}

print(stars)
// Stars: 🌟 🌟🌟 🌟🌟🌟

此例說明 result building methods 到底是如何執行組合的任務。 👉 Result-Building Methods

👉 paiza.io

// --------------------
//     ArrayBuilder
// --------------------

@resultBuilder
struct ArrayBuilder {

    // log message index ⭐️
    static var index = 0

    typealias Component = [Int]
    typealias Expression = Int
    
    // helper function ⭐️
    static func reportAndReturn<Input, Result>(
        _ input:Input, _ result: Result, _ token: String
    ) -> Result 
    {
        index += 1
        print("\(index)] build \(token): \(input) -> \(result)")
        return result
    }
    
    // translate Expression into Component ⭐️
    static func buildExpression(_ element: Expression) -> Component {
        reportAndReturn(element, [element], "expression")
    }
    
    // support `if` ⭐️
    static func buildOptional(_ component: Component?) -> Component {
        reportAndReturn(component, component ?? [], "optional  ")
    }
    
    // support `if-else`, `if-let-else` ⭐️
    static func buildEither(first component: Component) -> Component {
        reportAndReturn(component, component, "1st       ")
    }
    static func buildEither(second component: Component) -> Component {
        reportAndReturn(component, component, "2nd       ")
    }
    
    // support `for-in` loop ⭐️
    static func buildArray(_ components: [Component]) -> Component {
        reportAndReturn(components, Array(components.joined()), "array     ")
    }
    
    // translate block statement into Component (required) ⭐️
    static func buildBlock(_ components: Component...) -> Component {
        reportAndReturn(components, Array(components.joined()), "block     ")
    }
}

// --------------------
//     test run
// --------------------

@ArrayBuilder var c1: [Int] {               // 3
    let _ = print("hi")                     // `print` in builder ⭐️
    10                                      // 2
}

var c2 = ArrayBuilder.buildExpression(5)    // 1
// 1] build expression: 5 -> [5]            // 這個竟然先執行 ⭐️
// hi
// 2] build expression: 10 -> [10]
// 3] build block     : [[10]] -> [10]

print(c1)   //  [10]
print(c2)   //  [5]

var n = 19
@ArrayBuilder var c3: [Int] {               // 7
    if n < 12 {
        21
    } else {                                // 5, 6
        22                                  // 4
    } 
}
// 4] build expression: 22 -> [22]
// 5] build block     : [[22]] -> [22]
// 6] build 2nd       : [22] -> [22]
// 7] build block     : [[22]] -> [22]

print(c3)   // [22]

n = 51
@ArrayBuilder var c4: [Int] {                   // 15 ({)      
    if n < 12 { 
        let _ = print("level: 1, 1st")
        31
    } else {                                    // 13 ({), 14 (else)
        let _ = print("level: 1, 2nd")
        if n == 19 {                        
            let _ = print("level: 2, 1st")
            32                              
        } else {                                // 11 ({), 12 (else)
            let _ = print("level: 2, 2nd")
            if n > 50 {                         // 9 ({), 10 (if)
                let _ = print("level: 3, 1st")
                33                              // 8
            } else {                        
                let _ = print("level: 3, 2nd")
                34                          
            }
        }
    }
}

print(c4)
// level: 1, 2nd
// level: 2, 2nd
// level: 3, 1st
//  8] build expression: 33 -> [33]
//  9] build block     : [[33]] -> [33]
// 10] build 1st       : [33] -> [33]
// 11] build block     : [[33]] -> [33]
// 12] build 2nd       : [33] -> [33]
// 13] build block     : [[33]] -> [33]
// 14] build 2nd       : [33] -> [33]
// 15] build block     : [[33]] -> [33]

/*
    規則：由最內層往外回溯 ⭐️
    ------------------------------
    • 最內層   ：build Expression
    • 遇到 {   ：build Block
    • 遇到 if  ：build 1st / Optional
    • 遇到 else：build 2nd
    • 遇到 for ：build Array
*/

n = 51
@ArrayBuilder var c5: [Int] {           // 20 ({)
    if n < 12 { 
        let _ = print("level: 1, 1st")
        41
    } else if n == 19 {
        let _ = print("level: 2, 1st")
        42                              
    } else if n > 50 {                  // 17 ({), 18 (if), 19 (else)
        let _ = print("level: 3, 1st")
        43                              // 16
    } else {                        
        let _ = print("level: 3, 2nd")
        44                          
    }
}

// level: 3, 1st
// 16] build expression: 43 -> [43]
// 17] build block     : [[43]] -> [43]
// 18] build 1st       : [43] -> [43]
// 19] build 2nd       : [43] -> [43]
// 20] build block     : [[43]] -> [43]

print(c5)   // [43]

@ArrayBuilder var c6: [Int] {       // 24 ({)
    if (n % 2) == 1 {               // 22 ({), 23 (if)
        51                          // 21
    }
}
// 21] build expression: 51 -> [51]
// 22] build block     : [[51]] -> [51]
// 23] build optional  : Optional([51]) -> [51]
// 24] build block     : [[51]] -> [51]

print(c6)   // [51]

@ArrayBuilder var c7: [Int] {   // ({) 32
    for i in 1...3 {            // ({) 26, 28, 30 | 31 (for)
        100 + i                 // (E) 25, 27, 29
    }
}
// 25] build expression: 101 -> [101]
// 26] build block     : [[101]] -> [101]
// 27] build expression: 102 -> [102]
// 28] build block     : [[102]] -> [102]
// 29] build expression: 103 -> [103]
// 30] build block     : [[103]] -> [103]
// 31] build array     : [[101], [102], [103]] -> [101, 102, 103]
// 32] build block     : [[101, 102, 103]] -> [101, 102, 103]

print(c7)   // [101, 102, 103]

// --------------- // Setting // --------------- struct Setting { var name: String var value: Value } extension Setting { enum Value { case bool(Bool) case int(Int) case string(String) case group([Setting]) } init(_ name: String, _ value: Value){ self.init(name: name, value: value) } init(_ tuple: Tuple){ self.init(name: tuple.name, value: tuple.value) } typealias Tuple = (name: String, value: Value) } // ----------------------- // SettingsBuilder // ----------------------- @resultBuilder struct SettingsBuilder { typealias Component = Settings static func buildBlock(_ components: Component...) -> Component { Array(components.joined()) } // ------------------------- // ⭐️ buildExpression // ------------------------- // for tuple static func buildExpression(_ tuple: Setting.Tuple) -> Component { [Setting(tuple)] } // ⭐️ case .bool static func buildExpression(_ tuple: (name: String, bool: Bool)) -> Component { [Setting(tuple.name, .bool(tuple.bool))] } // ⭐️ case .int static func buildExpression(_ tuple: (name: String, int: Int)) -> Component { [Setting(tuple.name, .int(tuple.int))] } // ⭐️ case .string static func buildExpression(_ tuple: (name: String, string: String)) -> Component { [Setting(tuple.name, .string(tuple.string))] } // for a normal Setting static func buildExpression(_ setting: Setting) -> Component { [setting] } } // ----------------- // Settings // ----------------- typealias Settings = [Setting] // typealias can be extended ⭐️ extension Settings { // Settings { ... } init( @SettingsBuilder settings: () -> [Setting] // ⭐️ 1 ) { self = settings() } // Settings.Group { ... } static func Group( _ name: String, @SettingsBuilder settings: () -> [Setting] // ⭐️ 2 ) -> Setting { Setting(name: name, value: .group(settings())) } }

// --------------- // Drawable // --------------- // // • Types for drawing on a single line // using stars ("*") and text (String) // 🔸 Drawable // defines the requirement for something that can be drawn (as a String) protocol Drawable: CustomStringConvertible { func draw() -> String } extension Drawable { var description: String { draw() } // CustomStringConvertible } // 🔸 Line // represents a single-line drawing. // serves the top-level container for most drawings. struct Line: Drawable { var drawables: [Drawable] init(_ drawables: [Drawable]) { self.drawables = drawables } // ⭐ 5. closure parameter (@DrawingBuilder) init(@DrawingBuilder content: () -> Drawable) { self.drawables = [content()] } func draw() -> String { drawables .map { $0.draw() } .joined(separator: "") } } // 🔸 Text // wraps a string to make it part of a drawing struct Text: Drawable { var content: String init(_ content: String) { self.content = content } func draw() -> String { content } } // 🔸 Space struct Space: Drawable { func draw() -> String { " " } } // 🔸 Stars struct Stars: Drawable { var length: Int init(_ length: Int) { self.length = length } func draw() -> String { String(repeating: "🌟", count: length) } } // 🔸 AllCaps // converts text in the drawing to uppercase struct AllCaps: Drawable { var content: Drawable // ⭐ 3. closure parameter (@DrawingBuilder) init(@DrawingBuilder content: () -> Drawable) { self.content = content() } func draw() -> String { content.draw().uppercased() } } // ----------------------- // ⭐ DrawingBuilder // ----------------------- // 🔸 DrawingBuilder @resultBuilder struct DrawingBuilder { /// ⭐ 1. asupport a block of code /// 每個 @DrawingBuilder closure 的最外層都是由此方法處理。 static func buildBlock(_ drawables: Drawable...) -> Drawable { Line(drawables) } /// ⭐ 2. support `if-else`, `if-let-else` , `switch` static func buildEither(first: Drawable) -> Drawable { return first } static func buildEither(second: Drawable) -> Drawable { return second } } // ----------------------- // ⭐ Use Cases // ----------------------- // 🔸 greeting(for:) @DrawingBuilder func greeting(for name: String? = nil) -> Drawable {// ⭐ @DrawingBuilder Stars(3) Space() Text("Hello") Space() AllCaps { // ⭐ 3. init (@DrawingBuilder) if let name = name { Text(name + "!") } // ╮ else { Text("World!") } // ╯ ⭐ 2. buildEither } Space() Stars(2) } print(greeting()) print(greeting(for: "swift taylor")) // 🌟🌟🌟 Hello WORLD! 🌟🌟 // 🌟🌟🌟 Hello SWIFT TAYLOR! 🌟🌟 // --------------------------------- // ⭐ DrawingBuilder extension // --------------------------------- extension DrawingBuilder { /// ⭐ 4. support `for-in` , `.map()` static func buildArray(_ components: [Drawable]) -> Drawable { Line(components) } } // ----------------------- // ⭐ Use Cases // ----------------------- let stars = Line { // ⭐ 5. @DrawingBuilder Text("Stars:") // ⭐ 4. buildArray for i in 1...3 { // ╮ Space() // ⭐️ 1. buildBlock Stars(i) // │ } // ╯ } print(stars) // Stars: 🌟 🌟🌟 🌟🌟🌟

// -------------------- // ArrayBuilder // -------------------- @resultBuilder struct ArrayBuilder { // log message index ⭐️ static var index = 0 typealias Component = [Int] typealias Expression = Int // helper function ⭐️ static func reportAndReturn<Input, Result>( _ input:Input, _ result: Result, _ token: String ) -> Result { index += 1 print("\(index)] build \(token): \(input) -> \(result)") return result } // translate Expression into Component ⭐️ static func buildExpression(_ element: Expression) -> Component { reportAndReturn(element, [element], "expression") } // support `if` ⭐️ static func buildOptional(_ component: Component?) -> Component { reportAndReturn(component, component ?? [], "optional ") } // support `if-else`, `if-let-else` ⭐️ static func buildEither(first component: Component) -> Component { reportAndReturn(component, component, "1st ") } static func buildEither(second component: Component) -> Component { reportAndReturn(component, component, "2nd ") } // support `for-in` loop ⭐️ static func buildArray(_ components: [Component]) -> Component { reportAndReturn(components, Array(components.joined()), "array ") } // translate block statement into Component (required) ⭐️ static func buildBlock(_ components: Component...) -> Component { reportAndReturn(components, Array(components.joined()), "block ") } } // -------------------- // test run // -------------------- @ArrayBuilder var c1: [Int] { // 3 let _ = print("hi") // `print` in builder ⭐️ 10 // 2 } var c2 = ArrayBuilder.buildExpression(5) // 1 // 1] build expression: 5 -> [5] // 這個竟然先執行 ⭐️ // hi // 2] build expression: 10 -> [10] // 3] build block : [[10]] -> [10] print(c1) // [10] print(c2) // [5] var n = 19 @ArrayBuilder var c3: [Int] { // 7 if n < 12 { 21 } else { // 5, 6 22 // 4 } } // 4] build expression: 22 -> [22] // 5] build block : [[22]] -> [22] // 6] build 2nd : [22] -> [22] // 7] build block : [[22]] -> [22] print(c3) // [22] n = 51 @ArrayBuilder var c4: [Int] { // 15 ({) if n < 12 { let _ = print("level: 1, 1st") 31 } else { // 13 ({), 14 (else) let _ = print("level: 1, 2nd") if n == 19 { let _ = print("level: 2, 1st") 32 } else { // 11 ({), 12 (else) let _ = print("level: 2, 2nd") if n > 50 { // 9 ({), 10 (if) let _ = print("level: 3, 1st") 33 // 8 } else { let _ = print("level: 3, 2nd") 34 } } } } print(c4) // level: 1, 2nd // level: 2, 2nd // level: 3, 1st // 8] build expression: 33 -> [33] // 9] build block : [[33]] -> [33] // 10] build 1st : [33] -> [33] // 11] build block : [[33]] -> [33] // 12] build 2nd : [33] -> [33] // 13] build block : [[33]] -> [33] // 14] build 2nd : [33] -> [33] // 15] build block : [[33]] -> [33] /* 規則：由最內層往外回溯 ⭐️ ------------------------------ • 最內層：build Expression • 遇到 { ：build Block • 遇到 if ：build 1st / Optional • 遇到 else：build 2nd • 遇到 for ：build Array */ n = 51 @ArrayBuilder var c5: [Int] { // 20 ({) if n < 12 { let _ = print("level: 1, 1st") 41 } else if n == 19 { let _ = print("level: 2, 1st") 42 } else if n > 50 { // 17 ({), 18 (if), 19 (else) let _ = print("level: 3, 1st") 43 // 16 } else { let _ = print("level: 3, 2nd") 44 } } // level: 3, 1st // 16] build expression: 43 -> [43] // 17] build block : [[43]] -> [43] // 18] build 1st : [43] -> [43] // 19] build 2nd : [43] -> [43] // 20] build block : [[43]] -> [43] print(c5) // [43] @ArrayBuilder var c6: [Int] { // 24 ({) if (n % 2) == 1 { // 22 ({), 23 (if) 51 // 21 } } // 21] build expression: 51 -> [51] // 22] build block : [[51]] -> [51] // 23] build optional : Optional([51]) -> [51] // 24] build block : [[51]] -> [51] print(c6) // [51] @ArrayBuilder var c7: [Int] { // ({) 32 for i in 1...3 { // ({) 26, 28, 30 | 31 (for) 100 + i // (E) 25, 27, 29 } } // 25] build expression: 101 -> [101] // 26] build block : [[101]] -> [101] // 27] build expression: 102 -> [102] // 28] build block : [[102]] -> [102] // 29] build expression: 103 -> [103] // 30] build block : [[103]] -> [103] // 31] build array : [[101], [102], [103]] -> [101, 102, 103] // 32] build block : [[101, 102, 103]] -> [101, 102, 103] print(c7) // [101, 102, 103]