Better Reducers with React and Typescript 3.4

useUserReducer.ts

import { useReducer } from 'react'

export function updateName(name: string) {
  return <const>{
    type: 'UPDATE_NAME',
    name
  }
}

export function addPoints(points: number) {
  return <const>{
    type: 'ADD_POINTS',
    points
  }
}

export function setLikesGames(value: boolean) {
  return <const>{
    type: 'SET_LIKES_GAMES',
    value
  }
}

export const initialState = {
  name: '',
  points: 0,
  likesGames: true
}

type State = typeof initialState
type Action = ReturnType<
  typeof updateName | typeof addPoints | typeof setLikesGames
>

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case 'UPDATE_NAME':
      return { ...state, name: action.name }
    case 'ADD_POINTS':
      return { ...state, points: action.points }
    case 'SET_LIKES_GAMES':
      return { ...state, likesGames: action.value }
    default:
      return state
  }
}

export default function useUserReducer(state = initialState) {
  return useReducer(reducer, state)
}

ReactTodoTypesafeState.ts

import uuid from "uuid/v4"

//
// Filter
//

export enum FilterType {
  ALL = "ALL",
  COMPLETE = "COMPLETE",
  INCOMPLETE = "INCOMPLETE",
}
export const FilterTypes = Object.values(FilterType)

export enum FilterActionType {
  SHOW_ALL = "SHOW_ALL",
  SHOW_COMPLETE = "SHOW_COMPLETE",
  SHOW_INCOMPLETE = "SHOW_INCOMPLETE",
}
export const FilterActionTypes = Object.keys(FilterActionType)

export function isOfFilterActionTypes({ type }: { type: string }): boolean {
  return FilterActionTypes.includes(type)
}

export function showAll() {
  return {
    type: FilterActionType.SHOW_ALL,
  } as const
}
export function showComplete() {
  return {
    type: FilterActionType.SHOW_COMPLETE,
  } as const
}
export function showIncomplete() {
  return {
    type: FilterActionType.SHOW_INCOMPLETE,
  } as const
}

// type FilterType = "ALL" | "COMPLETE" | "INCOMPLETE"
export type FilterState = FilterType
export type FilterAction = ReturnType<typeof showAll | typeof showComplete | typeof showIncomplete>

export const filterReducer = (state: FilterState, action: FilterAction): FilterState => {
  switch (action.type) {
    case FilterActionType.SHOW_ALL:
      return FilterType.ALL
    case FilterActionType.SHOW_COMPLETE:
      return FilterType.COMPLETE
    case FilterActionType.SHOW_INCOMPLETE:
      return FilterType.INCOMPLETE
    default:
      return state
  }
}

//
// Todo
//

export type Todo = {
  id: string
  task: string
  complete: boolean
}
export const todoInit: Todo = {
  id: "",
  task: "",
  complete: false,
}

export type TodosState = Todo[]
export const todosInit: Todo[] = [
  {
    id: uuid(),
    task: "Learn React",
    complete: true,
  },
  {
    id: uuid(),
    task: "Learn Firebase",
    complete: false,
  },
  {
    id: uuid(),
    task: "Learn GraphQL",
    complete: false,
  },
]

export enum TodoActionType {
  DO_TODO = "DO_TODO",
  UNDO_TODO = "UNDO_TODO",
  ADD_TODO = "ADD_TODO",
}
export const TodoActionTypes = Object.values(TodoActionType)
export function isOfTodoActionTypes({ type }: { type: string }): boolean {
  return TodoActionTypes.includes(type)
}

export function doTodo(id: string) {
  return {
    type: TodoActionType.DO_TODO,
    id,
  } as const
}
export function undoTodo(id: string) {
  return {
    type: TodoActionType.UNDO_TODO,
    id,
  } as const
}
export function addTodo(task: string) {
  return {
    type: TodoActionType.ADD_TODO,
    task,
  } as const
}

export type TodoAction = ReturnType<typeof doTodo | typeof undoTodo | typeof addTodo>

export function todoReducer(state: TodosState, action: TodoAction): TodosState {
  switch (action.type) {
    case TodoActionType.DO_TODO:
      return state.map(todo => {
        if (todo.id === action.id) {
          return { ...todo, complete: true }
        } else {
          return todo
        }
      })
    case TodoActionType.UNDO_TODO:
      return state.map(todo => {
        if (todo.id === action.id) {
          return { ...todo, complete: false }
        } else {
          return todo
        }
      })
    case TodoActionType.ADD_TODO:
      return state.concat({
        task: action.task,
        id: uuid(),
        complete: false,
      })
    default:
      return state
  }
}

//
// App
//

export type AppState = {
  todos: TodosState
  filter: FilterType
}
export const appStateInit: AppState = {
  todos: todosInit,
  filter: FilterType.ALL,
}
export const appStateEmpty: AppState = {
  todos: [],
  filter: FilterType.ALL,
}

export type AppAction = FilterAction | TodoAction

export function appReducer(state: AppState, action: AppAction): AppState {
  return {
    todos: isOfTodoActionTypes(action) ? todoReducer(state.todos, action as TodoAction) : state.todos,
    filter: isOfFilterActionTypes(action) ? filterReducer(state.filter, action as FilterAction) : state.filter,
  }
}

PureScript

data Action 
 = Increment Int 
 | Decrement Int 
 | Reset

reducer :: Action -> Int -> Int
reducer (Increment x) state = state + x
reducer (Decrement x) state = state - x
reducer Reset _ = 0

result :: Int
result = reducer (Decrement 5) 5

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