A redux-observable framework for better locality of dependencies

Usage no npm install needed!

<script type="module">
  import nteractMyths from 'https://cdn.skypack.dev/@nteract/myths';


myths (currently: @nteract/myths, TBC)

This is a pre-alpha level package; interfaces are not stable yet!

The myths framework allows for integrating sets of closely related actions, reducers and epics. Myths allow close relationships where DRY and dependencies are minimized. Therefore, Myths provide for a structured way to avoid boilerplate.

Myths build on top of the Redux and RxJS libraries that are used elsewhere in the nteract core SDK. As a refresher, Redux helps you maintain application state. In Redux, actions and reducers provide predictable state management. The state may only be changed by dispatching an action to a reducer. In Redux-Observable, an epic is a function that takes in a stream of actions and returns a stream of actions.


$ yarn add myths
$ npm install --save myths



First, create a MythicPackage with a name, a type for its private state, and the initial state. As an example, the following creates a MythicPackage named "iCanAdd" which uses the number type for its private state sum and an initial state of sum as 0:

export const iCanAdd = createMythicPackage("iCanAdd")<
    sum: number;
  initialState: {
    sum: 0,


Next, you can the use the MythicPackage to create a Myth with a name, a type for its payload, and optionally a reducer operating on its package's private state. In this example, the MythicPackage named iCanAdd creates a Myth named "addToSum":

export const addToSum =
    reduce: (state, action) =>
      state.set("sum", state.get("sum") + action.payload),

A package can have any number of myths.


To create an action based on a myth, use its create function. You can then dispatch this action normally:



You get a store from a set of mythic packages, which has all the appropriate reducers and epics already in place:

type NonPrivateState = { foo: string };
const configureStore = makeConfigureStore<NonPrivateState>()({
  packages: [
export const store = configureStore({ foo: "bar" });

Epics: their definition

Epics can be defined using two different shorthand methods:

export const addToSum =

    reduce: (state, action) =>
      state.set("sum", state.get("sum") + action.payload),

    thenDispatch: [
      (action, state) =>
        state.get("sum") - action.payload < 100 && 100 <= state.get("sum")
          ? of(sendNotification.create({message: "Just passed 100!"}))
          : EMPTY,

    andAlso: [
        // Halve the sum every time an error action happens
        when: action => action.error ?? false,
        dispatch: (action, state, addToSum_) =>
          of(addToSum_.create(-state.get("sum") / 2)),


The first method uses thenDispatch: [] to define actions which should be dispatched when actions of the defined type are dispatched, and the second method uses andAlso: [] to generate actions based on a custom predicate. Since the type being defined is not available for reference yet, it is passed as third argument to the dispatch function.


To test the actions of a mythic package, you can use the testMarbles(...) method. Note that this only tests the epics, without evaluating reducers.


If you experience an issue while using this package or have a feature request, please file an issue on the issue board and add the pkg:myths label.