redux-sac/src/node.js

import {cowValueModel} from './cow-value-model';

export const subReducer = (key, reducer, ...otherKeys) => {
    const {get: getValue, set: setValue} = cowValueModel(key),
        otherParts = otherKeys.map(each => cowValueModel(each).get);

    return (state, action) => {
        let newSubState = reducer(getValue(state), action, ...otherParts.map(getValue => getValue(state)));
        if (typeof newSubState === "undefined") {
            throw new Error(`The '${key}' reducer must not return undefined.`);
        }
        return setValue(state, newSubState);
    };
};

export const composeReducers = (...reducers) => (state, action) =>
    reducers.reduce((x, r) => r(x, action), state);