In today's world, the different FRP formulations are being used in several problem spaces by professionals and big organizations alike. Throughout this book, we'll look at several examples of how CES can be applied. Some of these are interrelated, as most modern programs have several cross-cutting concerns, but we will only highlight two main areas.

GUIs are a great example of asynchronous programming. Once you open a web or a desktop application, it simply sits there, idle, waiting for user input.

This state is often called the event or main event loop. It is simply waiting for external stimuli, such as a key press, a mouse button click, new data from the network, or even a simple timer.

Each of these stimuli is associated with an event handler that gets called when one of these events happen, hence the asynchronous nature of GUI systems.

This is a style of programming that we have been used to for many years, but as business and user needs grow, these applications grow in complexity as well, and better abstractions are needed to handle the dependencies between all the components of an application.

Another great example that deals with managing complexity around network traffic is Netflix, which uses CES to provide a reactive API for their backend services.

Games are, perhaps, the best example of complex user interfaces, as they have intricate requirements around user input and animations.

The Elm language we mentioned before is one of the most exciting efforts in building complex GUIs. Another example is Flapjax, also targeted at web applications, but it is provided as a JavaScript library that can be integrated with existing JavaScript code bases.