Function discriminated

test/typescript/reducers.ts:66–175

* Reducer definition using discriminated unions. * * See https://basarat.gitbooks.io/typescript/content/docs/types/discriminated-unions.html#redux

()

 * See https://basarat.gitbooks.io/typescript/content/docs/types/discriminated-unions.html#redux
 */
function discriminated() {
  type State = number

  interface IncrementAction {
    type: 'INCREMENT'
    count?: number
  }

  interface DecrementAction {
    type: 'DECREMENT'
    count?: number
  }

  interface MultiplyAction {
    type: 'MULTIPLY'
    count?: number
  }

  interface DivideAction {
    type: 'DIVIDE'
    count?: number
  }

  // Union of all actions in the app.
  type MyAction0 = IncrementAction | DecrementAction
  type MyAction1 = MultiplyAction | DivideAction

  const reducer0: Reducer<State, MyAction0> = (state = 0, action) => {
    if (action.type === 'INCREMENT') {
      // Action shape is determined by `type` discriminator.
      // @ts-expect-error
      action.wrongField

      const { count = 1 } = action

      return state + count
    }

    if (action.type === 'DECREMENT') {
      // @ts-expect-error
      action.wrongField

      const { count = 1 } = action

      return state - count
    }

    return state
  }

  const reducer1: Reducer<State, MyAction1> = (state = 0, action) => {
    if (action.type === 'MULTIPLY') {
      // @ts-expect-error
      action.wrongField

      const { count = 1 } = action

      return state * count