Extract Payload for enum cases having associated value

stephencelis · August 21, 2019, 6:12pm

To provide a more real-world example, we covered a compelling reason for enum key paths/properties this week on Point-Free:

Though this video is behind a paywall, our code is always open source:

github.com

pointfreeco/episode-code-samples/blob/master/0070-composable-state-management-action-pullbacks/ComposableArchitecture.playground/Contents.swift

import Combine
import SwiftUI

struct AppState {
  var count = 0
  var favoritePrimes: [Int] = []
  var loggedInUser: User? = nil
  var activityFeed: [Activity] = []

  struct Activity {
    let timestamp: Date
    let type: ActivityType

    enum ActivityType {
      case addedFavoritePrime(Int)
      case removedFavoritePrime(Int)

      var addedFavoritePrime: Int? {
        get {
          guard case let .addedFavoritePrime(value) = self else { return nil }

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In particular we show how reducer architecture can be modularized and reconstituted using a couple succinct operations:

An operation that converts Reducer<LocalState, LocalAction> to Reducer<GlobalState, GlobalAction>
An operation that combines reducers that work on the same state and action type together

The first operation cannot succinctly transform LocalActions into GlobalActions in a performant manner without compiler help and enum properties and key paths perfectly handle this today (with a little boilerplate that ideally the compiler handles for us).