One of my sub-reducers has a default state that needs to be calculated in a promise.
I could check for an empty state before creating the store, and calculate the default state, then create the store, but that's very brittle as I'd have to know a lot about state internal structure in a place where it doesn't belong.
Is there an good way to do this?
I wouldn't delay the creation of the store, because it does not depend on the data, it is a fundamental part of your app's mechanism (what happens if your promise is rejected?). I suggest dispatching an INIT_STATE action when the promise resolves.
There's also the option of lazy-loading reducers, please look here: How to dynamically load reducers for code splitting in a Redux application?