ArrayBuilder

ArrayBuilder.swift

@resultBuilder
struct ArrayBuilder<Element> {
  enum Component {
    case atom(Element)
    case spread([Component])
  }
  
  static func buildBlock(_ elements: Component...) -> Component { 
    .spread(elements)
  }
  
  static func buildExpression(_ element: Element) -> Component {
    .atom(element)
  }
  
  static func buildExpression(_ component: Component) -> Component {
    component
  }
  
  static func buildOptional(_ component: Component?) -> Component {
    component ?? .spread([])
  }
  
  static func buildEither(first component: Component) -> Component {
    component
  }
  
  static func buildEither(second component: Component) -> Component {
    component
  }

  static func buildArray(_ components: [Component]) -> Component {
    .spread(components)
  }
  
  static func buildFinalResult(_ component: Component) -> [Element] {
    Array(component)
  }
}

extension Array {
  init(_ component: ArrayBuilder<Element>.Component) {
    switch component {
    case .atom(let element):
      self = [element]
    case .spread(let components):
      self = components.flatMap(Array.init)
    }
  }
  
  init(@ArrayBuilder<Element> build: () -> [Element]) {
    self = build()
  }
}

prefix func ...<S: Sequence>(_ sequence: S) -> ArrayBuilder<S.Element>.Component {
  .spread(sequence.map(ArrayBuilder.Component.atom))
}

EagerArrayBuilder.swift

@resultBuilder
struct ArrayBuilder<Element> {
  enum Component {
    case spread([Element])
  }
  
  static func buildBlock(_ components: Component...) -> Component { 
    ArrayBuilder.buildArray(components)
  }
  
  static func buildExpression(_ element: Element) -> Component {
    .spread([element])
  }
  
  static func buildExpression(_ component: Component) -> Component {
    component
  }
  
  static func buildOptional(_ component: Component?) -> Component {
    component ?? .spread([])
  }
  
  static func buildEither(first component: Component) -> Component {
    component
  }
  
  static func buildEither(second component: Component) -> Component {
    component
  }

  static func buildArray(_ components: [Component]) -> Component {
    .spread(components.flatMap(Array.init))
  }
  
  static func buildFinalResult(_ component: Component) -> [Element] {
    Array(component)
  }
}

extension Array {
  init(_ component: ArrayBuilder<Element>.Component) {
    switch component {
    case .spread(let elements):
      self = elements
    }
  }
  
  init(@ArrayBuilder<Element> build: () -> [Element]) {
    self = build()
  }
}