redux-sac/src/sac.js

import {atPath} from 'atpath';

export const subReducer = (key, reducer, ...otherKeys) => {
    const atKey = atPath(key),
        otherParts = otherKeys.map(atPath);

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

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

export const subMiddleware = (keyOrSelectorFn, middleware) => {
    const selector = typeof keyOrSelectorFn === "function" ? keyOrSelectorFn : atPath(keyOrSelectorFn).get;
    return (store, ...rest) => {
        const {getState} = store;
        return middleware({...store, getState: () => selector(getState())}, ...rest);
    };
};

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