Add the @@redux/ prefix to randomly generated action type in combineReducers

[gcanti]

Hi, I'm trying to add type safety to actions and state with the redux-tcomb project.
For what concerns the actions I need to detect the redux specific internal actions in order to skip the type checking (for example the redux's ActionTypes.INIT action). Currently I have an issue with the combineReducers function which probes all reducers to be combined with a randomly generated action type to ensure each reducer returns undefined for action types that should be unrecognized by reducers (relevant LOC: https://github.com/rackt/redux/blob/master/src/utils/combineReducers.js#L88).

Is it possible to add the @@redux/ prefix to those randomly generated action types?

[gaearon]

Can you give a relevant snippet of code? What gets wrapped into type checking, at which point does checking occur, etc?

The original motivation was to discourage people from trying to handle @@redux/ actions explicitly even by checking substring. We can add the prefix to the random action but then our check becomes weaker.

[gcanti]

Can you give a relevant snippet of code

Sure, I think it's a general issue for each library which wants to implement type-checked actions, so I'll try to keep the discussion as general as possible.

Let's start with a normal reducer (no type checking)

function todoReducer(state, action) {
  return state || [];
}

var reducer = combineReducers({
  todos: todoReducer
});

assert.deepEqual(reducer(undefined, {type: 'ADD_TODO', text: 'Use redux'}), {todos: []}); // ok

Sweet! Everything's fine. Now say I want to add type checking to the todoReducer:

function todoReducer(state, action) {
  // I want to type check the action argument here
  return state || [];
}

In redux-tcomb you define a type Action which is the union of all the user defined actions:

function todoReducer(state, action) {
  Action(action); // <= this must throw if the action argument contains an unknown action
  return state || [];
}

The first problem I encountered is that the todoReducer will throw when the @@redux/INIT action will be passed into, so I must add a check, say a isReduxAction predicate, which will skip the type checking in presence of an internal redux action:

function isReduxAction(action) {
  // rule out the @@redux/INIT action
  return action.type.indexOf('@@redux/') === 0;
}

If I add this check to my reducer, again everything will be fine:

function todoReducer(state, action) {
  if (!isReduxAction(action)) {
    Action(action); // <= type check only if the action is not an internal one 
  }
  return state || [];
}

Here comes the problem with combineReducers: at the moment there's no reliable way to rule out the randomly action type generated by combineReducers. For those actions isReduxAction will return false and the type checking system will throw.

[gaearon]

We can use something like @@redux/PROBE_UNKNOWN_ACTION_{randomString} instead. This clarifies the intent, keeps the randomizer, and is frightening enough to scare off people from trying to handle it directly.

Would you like to make a PR for this change?

[gcanti]

I'll try to push something asap 👍

[gaearon]

Released in 3.0.1.