Crear mini-DSL con result builders

¿Por qué usar constructores de resultados?

Un constructor pequeño

@resultBuilder
struct StringListBuilder {
    static func buildBlock(_ parts: String...) -> [String] { parts }
    // Optional support for conditionals/arrays:
    static func buildEither(first: [String]) -> [String] { first }
    static func buildEither(second: [String]) -> [String] { second }
    static func buildArray(_ components: [[String]]) -> [String] { components.flatMap { $0 } }
}

// DSL entry function using the builder
func lines(@StringListBuilder _ content: () -> [String]) -> [String] {
    content()
}

// Usage: create an array of strings with a block
let list = lines {
    "Hello"
    "Swift"
    "Builders"
}
print(list)

Condicionales en los constructores

let showExtra = true
let conditional = lines {
    "Base"
    if showExtra {
        "Extra"
    } else {
        "Hidden"
    }
}
print(conditional)  // ["Base","Extra"]

Bucles en los constructores

let items = ["a","b","c"]
let looped = lines {
    for x in items { x.uppercased() } // contributes ["A","B","C"]
}
print(looped)

Constructor de un solo valor

@resultBuilder
struct TextBuilder {
    static func buildBlock(_ parts: String...) -> String {
        parts.joined(separator: " ")
    }
    static func buildEither(first: String) -> String { first }
    static func buildEither(second: String) -> String { second }
    static func buildArray(_ components: [String]) -> String {
        components.joined(separator: " ")
    }
}

func text(@TextBuilder _ content: () -> String) -> String { content() }

// Compose a small sentence with conditions
let loggedIn = false
let sentence = text {
    "[INFO]"
    "Welcome,"
    if loggedIn { "user!" } else { "guest." }
}
print(sentence) // "[INFO] Welcome, guest."

Consejos de diseño

Propósito de los constructores de resultados

Resumen