redux-sac/src/sac.js

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

export const subReducer = (key, reducer, ...otherKeys) => {
    const value = cowValueModel(key),
        otherParts = otherKeys.map(each => cowValueModel(each));

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

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

export const subMiddleware = (key, middleware) => {
    const cow = cowValueModel(key);
    return ({getState, ...rest}) => middleware({getState: () => cow(getState()), ...rest});
};

export const subEffex = (key, effects) =>
    effects.map(each => ({...each, effect: subMiddleware(key, each.effect)}));