ArrayBuilder.swift

// THIS IS A UNIVERSAL SWIFT FUNCTION BIULDER
// FOR ARRAYS OF ANY TYPE
// tested on Swift 5.3.1

@_functionBuilder
public enum ArrayBuilder<Element> {
    public typealias Expression = Element

    public typealias Component = [Element]

    public static func buildExpression(_ expression: Expression) -> Component {
        [expression]
    }

    public static func buildExpression(_ expression: Expression?) -> Component {
        expression.map({ [$0] }) ?? []
    }

    public static func buildBlock(_ children: Component...) -> Component {
        children.flatMap({ $0 })
    }

    public static func buildOptional(_ children: Component?) -> Component {
        children ?? []
    }

    public static func buildBlock(_ component: Component) -> Component {
        component
    }

    public static func buildEither(first child: Component) -> Component {
        child
    }

    public static func buildEither(second child: Component) -> Component {
        child
    }
}

// USAGE EXAMPLE:

struct Schedule {
    var entries: [Entry]
    
    init(@ArrayBuilder<Entry> builder: () -> [Entry]) {
        self.entries = builder()
    }
}

// then use as you would expect:

let schedule = Schedule {
    Entry1()
    Entry2()
    if someCondition() {
        Entry3()
    } else {
        Entry4()
    }
}