redux: Change composeStores to throw if undefined is returned

Right now it’s easy to forget a default case and have your reducer return nothing in some circumstances. There have been some WIP attempts to address it: #174. Here’s the issue that spawned that PR: #173.

But let’s take a moment to think about what composeStores really is. It’s a shortcut right? It’s a bit opinionated already: it assumes plain objects as the initial state. See #153.

Can we make it a bit more opinionated for convenience? We could make it return the previous state if the reducer returns undefined.

Edit: see this instead.

Pros:

• Solves issues like #173 nicely
• Slightly less boilerplate in the “vanilla Redux” code

Cons:

• Dilutes the contract of reducers by introducing some magic

Thoughts?