combineReducers to return Immutable.Record instead of Immutable.Map

[IlyaSemenov]

Currently combineReducers({sub1, sub2}) returns an Immutable.Map. However, Immutable.js provides a native Record type which is more natural fit for holding data with specific, unchanging set of keys.

Here's the breakdown:

Option	Immutable.Map	Immutable.Record
Accessing sub-states	state.get('sub1')	state.sub1 - much cleaner (but state.get('sub1') also supported)
Creating inexistent sub-states	Possible by mistake: state.set('foo', 5)	Not allowed

I believe redux-immutablejs must be improved to return a Record instead of a Map by default. This will be fully backwards-compatible in all cases besides creating inexistent sub-states (which is a clear misuse of the combined reducers pattern).

By the way, it is already possible to pass a Record instance to combineReducers, but it requires non-trivial boilerplate code (which is most probably suboptimal as well):

import Immutable from 'immutable'
import {combineReducers} from 'redux-immutablejs'

import sub1 from './sub1'
import sub2 from './sub2'

class State extends Immutable.Record({
    sub1: null,
    sub2: null
}) {

    // The two methods below allow to trick combineReducers to accept this class
    // instead of Immutable.Map or plain object

    filter() {
        return this;
    }

    map(k, v) {
        return new State(Immutable.Map(this).map(k, v));
    }
}

export default combineReducers(new State({sub1, sub2}));