ArrayBuilder.swift

@resultBuilder
struct ArrayBuilder<Element> {
    static func buildExpression(_ expression: Element) -> [Element] {
        [ expression ]
    }

    static func buildBlock(_ elements: [Element]...) -> [Element] {
        elements.flatMap { $0 }
    }

    static func buildOptional(_ elements: [Element]?) -> [Element] {
        elements ?? []
    }

    static func buildEither(first component: [Element]) -> [Element] {
        component
    }

    static func buildEither(second component: [Element]) -> [Element] {
        component
    }
}

@ArrayBuilder<Int> var bestNumbers: [Int] {
    16
    42
    9001

    if Bool.random() {
        69
    } else {
        68
    }

    if Bool.random() {
        101
    }
}

print(bestNumbers)