addFoodReducer.js

function addFood (state, action) {
  // favoriteFood :: {k: v} -> {k: v}
  const favoriteFood = favorite => Object.assign({}, {favoriteFood: []}, favorite)

  // food :: {k: v} -> {k: v}
  const food = favoriteFood(
    Object.assign({}, state[action.data.id], {
      [action.data.prop]: state[action.data.id].favoriteFood.concat(action.data.food)
    })
  )
  // assocPath :: [String] -> {k: v} -> {k: v} -> {k: v}
  return R.assocPath([action.data.id])(food)(state)
}

export default function reducer (state = people, action) {
  switch (action.type) {
    case 'ADD_FOOD':
      return addFood(state, action)
    default:
      return state
  }
}