// *** before: ***
// SwiftUI already stood on builders, but the attribute was underscored and
// officially did not exist: @_functionBuilder, with no promise to keep it

// *** in version 5.4: ***
@resultBuilder
struct StringBuilder {
    static func buildBlock(_ parts: String...) -> String {
        parts.joined(separator: "\n")
    }
    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.joined(separator: "\n")
    }
}

@StringBuilder
func page(admin: Bool) -> String {
    "header"
    if admin {
        "admin panel"
    } else {
        "user panel"
    }
    for index in 1...2 {
        "line \(index)"
    }
    "footer"
}

print(page(admin: true))

// the builder turns the statements of the body into calls of its own
// methods - this is the whole trick behind the body of a SwiftUI view