This “closed loop” of messages and updates works well if all you need is an interactive document. Many applications will also need to talk to the outside world - whether that's fetching data from an API, setting up a WebSocket connection, or even just setting a timer. Let's address the first of these, fetching data.

First we need some data to fetch, and we're going to use an external API to pull in facts about cats.

The "fetch_fact" function does the work of calling the API and extracting the data it needs. Finally it calls dispatch with a message indicating "CatFactFetched" or in the unhappy case "FetchFailed". We'll return to this dispatch function later.

Effects

Lustre organises this kind of communication with effects. To use effects an application needs to be constructed using "lustre.application". Previously examples used lustre.simple but we are running full power these days.

Now the init and update functions need to return any effects that the app requires. This app has no effects when it starts, so the init function returns `effect.none`.

Lustre takes care of wiring up these effects and the state of you application. Any effect function is invoked with a dispatch function as it's argument. Calling the dispatch function sends messages back to the main application where your update function handles changing the model.

There is not much other data to get from this API, so we move on to the final lesson.