What is a result builder?

Middle PRO

The idea

A result builder (@resultBuilder) is a type with static methods by which the compiler rewrites a closure body: every expression line becomes a call to buildExpression, their list becomes buildBlock, an if without else becomes buildOptional, if/else becomes buildEither(first:/second:), for becomes buildArray, if #available becomes buildLimitedAvailability. The result is a declarative DSL with ordinary Swift code inside. @ViewBuilder, RegexBuilder and @CommandsBuilder are built on it.

@resultBuilder
enum ListBuilder {
    static func buildExpression(_ s: String) -> [String] { [s] }
    static func buildBlock(_ parts: [String]...) -> [String] {
        parts.flatMap { $0 }
    }
    static func buildOptional(_ part: [String]?) -> [String] { part ?? [] }
    static func buildEither(first: [String]) -> [String] { first }
    static func buildEither(second: [String]) -> [String] { second }
    static func buildArray(_ parts: [[String]]) -> [String] {
        parts.flatMap { $0 }
    }
}

func menu(@ListBuilder _ content: () -> [String]) -> [String] { content() }

let isAdmin = true
let items = menu {
    "Home"
    if isAdmin { "Admin" }           // buildOptional
    for i in 1...2 { "Item \(i)" }   // buildArray
}
print(items)   // ["Home", "Admin", "Item 1", "Item 2"]