[swift-evolution] [Review] SE-0155: Normalize Enum Case Representation

Karl Wagner razielim at gmail.com
Sat Feb 18 11:06:31 CST 2017

> - What are the memory layout optimizations described here? From a first glance this looks purely syntactic.

The compiler could “refactor” the payloads among all enum cases to maximise overlapping. If you know that an enum’s payload always contains a reference-counted or shared object, or that a element is always at a given position in the payload, you can operate on the payload’s elements without needing to switch.

(crude example)

enum EditorMode {
    case .textEditor(UIViewController)
    case .imageEditor(UIViewController)

// This would reduce to just returning the payload without switching.
// We could even think about generating a “EditorMode.0 : UIViewController” accessor

    var editorController: UIViewController {
      switch self { 
  case .textEditor(let t): return t
  case .imageEditor(let i): return i

- Karl

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