According to#2,createHandlerMap should be capable to handle optimistic action creators.

Detailed Description

Yet, optimistic actions should be handled separately with multiplecreateHandlerMap call. So there would be a good point to reduce boilerplates.
Although, cancel is not part ofobserver protocol but this type practically is important and should be handled in many cases (isFetching entity, etc.) in the reducers.

Possible Implementation

createHandlerMap(addFoo,{next(state,action){},error(state,action){},complete(state,action){},cancel(state,action){}})