How to get the union of the specific fields of another union ?

I'm trying to have as much type safety as possible on my Redux app and I'm stuck with this problem. Basically I have these types:

type ActionA = { type: 'A' }
type ActionB = { type: 'B' }

type Actions = A | B

And I'd like to have a type `ActionType` that is the union of all the possible `type`s of my actions. i.e:

type ActionType = 'A' | 'B' // <-- How to get this from `Actions` ?

In other words, I would like to get the union of all the possible values of a field (`type`) in an union (`A | B`).

Is this even possible with flow ?

The use case is to be able to avoid misspelling actions types when I need to write them directly. For example, using `redux-saga`, I could write a definition of `take` that would be (over-simplified for the example):

define function take(action: ActionType, saga: Function)

Then I would be sure I can only call `take` with valid action types.