Subscriptions are out

One approach to handling data that changes is to observe it. This means using some mechanism to attach a listener callback function that's called whenever the data changes. For example, you could have data that models a user interface component, and when that data changes, you would render the component so that it reflects the changed data.

To set up subscriptions like this will require data that can change, which we don't want. Since we're working with immutable data that never changes, subscriptions are a dead end. This means that you have to rethink your approach for notifying components about the state of your data. The rule of thumb with immutable architectures is that only new data is passed around when things change.