How I type my reducers.

todos-actions.ts

// Regular old action constant literal.
export const ADD = 'todos/ADD'

// Action type.
export type ADD = Action<'todos/ADD', {readonly todo: Todo}>

export type AllActions =
  | ADD

todos-model.ts

export interface Todo {
  readonly id: string,
}

todos-reducer.ts

export interface TodosEntities {
  readonly [id: string]: Todo
}
type EntitiesReducer = Reducer<TodosEntities, ADD>

export const entitiesReducer: EntitiesReducer = (
  s = {},
  a,
) => {
  switch (a.type) {
    case ADD: return {...s, [a.payload.todo.id]: a.payload.todo}
    default: return s
  }
}

types.ts

export interface Action<T, P> {
  readonly type: T
  readonly payload: P
  readonly error?: boolean
  readonly meta?: any
}

export type Reducer<S, A> = (
  state: S,
  action: A,
) => S

The latest versions of TypeScript support enums. You can use those for the type keys.

export enum types {
  TOGGLE_USERS_TABLE = 'users/TOGGLE_USERS_TABLE',
  LOAD_USERS_REQUEST = 'users/LOAD_USERS_REQUEST',
  LOAD_USERS_RESPONSE = 'users/LOAD_USERS_RESPONSE',
  LOAD_USERS_FAIL = 'users/LOAD_USERS_FAIL',
  EDIT_USER_REQUEST = 'users/EDIT_USER_REQUEST',
  EDIT_USER_RESPONSE = 'users/EDIT_USER_RESPONSE',
  EDIT_USER_FAIL = 'users/EDIT_USER_FAIL',
  DELETE_USER_REQUEST = 'users/DELETE_USER_REQUEST',
  DELETE_USER_RESPONSE = 'users/DELETE_USER_RESPONSE',
  DELETE_USER_FAIL = 'users/DELETE_USER_FAIL',
  EDIT_UPDATE = 'users/EDIT_UPDATE',
  TOGGLE_EDIT_FORM = 'users/TOGGLE_EDIT_FORM',
  CREATE_NEW_USER = 'users/CREATE_NEW_USER',
  OTHER_ACTION = 'users/OTHER_ACTION',
}

Otherwise, the typing looks a little more complicated than what I have settled on with Idiomatic pattern.