Package-level declarations

Similiar to React's useReducer hook, this contract defines a reducer that can be used to manage the state of a component. The reducer method is used to reduce an event to a new state and potentially produce a list of effects. This effect can be used to trigger side-effects, outside of the reducer, such as network requests, timers or updating the UI. A reducer should be used to manage the state of a component in a deterministic way (i.e., given the same sequence of events, the reducer should always produce the same state - pure function).