example.js

// @flow

import { createAction, createReducer, on, onType, Action } from "transdux";

const inc: Action<void> = createAction();
const add: Action<number> = createAction();

const DEC = "DEC";
const SUB = "SUB";

const dec = () => ({ type: DEC });
const sub = (value: number) => ({ type: SUB, value });

const count = createReducer(0, [
  on(inc, state => state + 1),
  onType(DEC, state => state - 1),
  on(add, (state, value) => state + value),
  onType(SUB, (state, { value }) => state - value)
]);

console.log(
  [inc(), inc(), add(10), inc(), dec(), sub(5)].reduce(count, undefined)
); // 7 (0 + 1 + 1 + 10 + 1 - 1 - 5)

transdux.js

// @flow

type Reducer<S> = (state: S | void, action: any) => S;
type Message<P> = { type: string, payload: P };
type Action<P> = (P => Message<P>) & { type: string };

let nextId = 0;
function createAction<P>(description?: string): Action<P> {
  const id = nextId++;
  const type = description == null ? `${id}` : `${description}-${id}`;
  const action = payload => ({ type, payload });
  action.type = type;

  return action;
}

function on<S, P>(
  action: Action<P>,
  handle: (state: S, payload: P) => S
): (Reducer<S>) => Reducer<S> {
  return reducer => (state, message: Message<P>) => {
    const newState = reducer(state, message);

    switch (message.type) {
      case action.type:
        return handle(newState, message.payload);
      default:
        return newState;
    }
  };
}

function onType<S, M: { type: string }>(
  type: string,
  handle: (state: S, message: M) => S
): (Reducer<S>) => Reducer<S> {
  return reducer => (state, message: M) => {
    const newState = reducer(state, message);

    switch (message.type) {
      case type:
        return handle(newState, message);
      default:
        return newState;
    }
  };
}

function createIdentityReducer<S>(defaultState: S): Reducer<S> {
  return (state: S = defaultState, action) => state;
}

function createReducer<S>(
  defaultState: S,
  transducers: ((Reducer<S>) => Reducer<S>)[]
): Reducer<S> {
  return transducers.reduce(
    (reducer, transducer) => transducer(reducer),
    createIdentityReducer(defaultState)
  );
}