Make dependent reducer explanation more prominent

[gaearon]

This is a pattern we don’t encourage to use all the time, but sometimes it’s handy.
We should really include it in the FAQ, or better, write a recipe.

https://twitter.com/dan_abramov/status/723157303340326913

[gaearon]

Also I still think this shouldn’t be the default behavior for combineReducers(). The whole point is you do this explicitly and structure data dependencies any way you like:

export default function todos(state = {}, action) {
  return {
    byId: byId(state.byId, action, state),
    listedIds: listedIds(state.listedIds, action, state)
  }
}

or

export default function todos(state = {}, action) {
  var nextById = byId(state.byId, action, state.listedIds);
  var nextListedIds = listedIds(state.listedIds, action, nextById);
  return {
    listedIds: nextListedIds,
    byId: nextById
  }
}

or

export default function todos(state = {}, action) {
  var nextListedIds = listedIds(state.listedIds, action, state.byId);
  var nextById = byId(state.byId, action, nextListedIds);
  return {
    listedIds: nextListedIds,
    byId: nextById
  }
}

The order is up to you, depending on your needs.

[markerikson]

Writing a "Structuring Reducers" recipe page is still on my to-do list. Might have time to take a first crack at it this weekend. We'd then be able to add a link to it in the "sharing state" FAQ question.

[mattiamanzati]

Some month ago I wrote a micro-module to perform this task and combine reducers specifying their dependencies, turned out that this is mostly a rare case. Turns out it's handy but using side-effects handling could achieve the same result.
https://github.com/KodersLab/topologically-combine-reducers

[gaearon]

@mattiamanzati That’s a neat module 😄 .

[markerikson]

Covered in http://redux.js.org/docs/recipes/reducers/BeyondCombineReducers.html#sharing-data-between-slice-reducers , and also mentioned in http://blog.isquaredsoftware.com/2017/05/idiomatic-redux-tao-of-redux-part-1/#update-logic-and-data-flow-are-explicit .