CollectionBuilder2.swift

@_functionBuilder
public enum CollectionBuilder<Component> {

    // MARK: - Building Expressions

    /// Basic uplift of an `Expression` into a `Component` (aka `[Expression]`)
    ///
    public static func buildExpression(_ expression: Component) -> [Component] {
        [expression]
    }

    /// Support for optional `Expression`s in your resut builder
    ///
    public static func buildExpression(_ expression: Component?) -> [Component] {
        []
    }

    // MARK: - Building Blocks

    /// Support for result builders that return no children or void
    ///
    public static func buildBlock() -> [Component] {
        []
    }

    /// Support for multiple lines of `Component`s
    ///
    public static func buildBlock(_ components: [Component]...) -> [Component] {
        components.flatMap { $0 }
    }

    // MARK: - Building Conditionals

    /// Support for single-branch conditional statements (eg an `if` statement without an `else` branch)
    ///
    public static func buildOptional(_ components: [Component]?) -> [Component] {
        components ?? []
    }

    /// Support for multi-branch conditional statements (eg an `if` statement with an `else` branch)
    ///
    public static func buildEither(first: [Component]) -> [Component] {
        first
    }

    /// Support for multi-branch conditional statements (eg an `if` statement with an `else` branch)
    ///
    public static func buildEither(second: [Component]) -> [Component] {
        second
    }

    /// Support for `if #available` statements, we don't really need this as we're not trying
    /// to type-erase whats inside the availability statement, but @bok is a completionist so ¯\_(ツ)_/¯
    ///
    public static func buildLimitedAvailability(_ components: [Component]) -> [Component] {
        components
    }

}

// MARK: - Array Support

public extension Array {

    /// Initialises an Array with the results returned by a function builder
    ///
    init(@CollectionBuilder<Element> collecting: () -> [Element]) {
        self = collecting()
    }

    /// Adds the results of the function builder to the Array
    ///
    mutating func collect(@CollectionBuilder<Element> _ builder: () -> [Element]) {
        append(contentsOf: builder())
    }
}

// MARK: - Set Support

public extension Set {

    /// Initialises a Set with the results returned by a function builder
    ///
    init(@CollectionBuilder<Element> collecting: () -> [Element]) {
        self.init(collecting())
    }

    /// Adds the results of the function builder to the Set
    ///
    mutating func collect(@CollectionBuilder<Element> _ builder: () -> [Element]) {
        for element in builder() {
            insert(element)
        }
    }
}