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Result builders are how SwiftUI's view DSL works:

VStack {
    Text("Hello")
    Image("icon")
    Button("Tap") { ... }
}

The block is processed by a result builder that combines each statement into a single result.

Defining a builder:

@resultBuilder
struct StringBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: " ")
    }
}

@StringBuilder
func greeting() -> String {
    "Hello,"
    "world!"
    "How are you?"
}

print(greeting())   // "Hello, world! How are you?"

The @StringBuilder annotation tells the compiler: pass each statement of the function body to buildBlock.

Optional and conditional support:

@resultBuilder
struct StringBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: " ")
    }
    static func buildOptional(_ component: String?) -> String {
        component ?? ""
    }
    static func buildEither(first: String) -> String { first }
    static func buildEither(second: String) -> String { second }
}

@StringBuilder
func message(formal: Bool) -> String {
    "Greetings,"
    if formal {
        "esteemed friend."
    } else {
        "dude."
    }
}

The builder's buildEither and buildOptional methods handle if / if-else and if let.

Result builder methods (implement only what you need):

• buildBlock(_:) — required; merges statements
• buildOptional(_:) — if (no else)
• buildEither(first:), buildEither(second:) — if/else
• buildArray(_:) — for loops
• buildExpression(_:) — pre-process individual expressions
• buildFinalResult(_:) — final transform

Where used:

• SwiftUI views
• Server-side Vapor's XCTAssert* builders
• Plot HTML/CSS/JS builders
• Regex Builders (Swift 5.7+)