One large domain for applying functional programming is server-side programming. Server-side programming refers to web applications that constantly run on a server, and have the ability to communicate with the outer world. Such an application will typically listen on a port for incoming HTTP requests. After a request arrives, it will perform some work on the server, and reply back to the requesting client with the result of the computation.
Applications of such systems are wide. Everything from regular websites to mobile applications to Software as a Service (SaaS) systems are made as web applications. Also, once you have a web application that constantly runs on a server, communicates with the outer world via a well-defined protocol, and performs some computations, you can have a multitude of clients for such an application. For example, you may have...