lens usage

foo.js

import R from 'ramda'

const recordsLens = R.lensPath(['foo', 'records'])
const keysLens = R.lensPath(['bar', 'keys'])

const reducer = (state, {type, payload}) => {
  switch (type) {
    case 'SET_RECORDS': return R.set(recordsLens, payload, state)
    case 'SET_RECORD': {
      const exactRecordLens = R.lensPath([payload.id])
      const recordLens = R.compose(recordsLens, exactRecordLens)
      return R.set(recordLens, payload.value, state)
    }
    case 'ADD_KEY': return R.over(keysLens, R.append(payload), state)
    case 'CLEAR_KEYS': return R.set(keysLens, [], state)
    default: return state
  }
}

const s0 = {foo: {records: {}}, bar: {keys: []}}

const s1 = reducer(s0, {type: 'SET_RECORDS', payload: {a: 'Aye!', b: 'Boo!'}})//?$.foo
const s2 = reducer(s1, {type: 'SET_RECORD', payload: {id: 'a', value: 'Aight?'}})//?$.foo

const s3 = reducer(s2, {type: 'ADD_KEY', payload: 17})//?$.bar
const s4 = reducer(s3, {type:'ADD_KEY', payload: 42})//?$.bar

const s5 = reducer(s4, {type: 'CLEAR_KEYS'})