Parallel and Concurrent Programming in Haskell

Part II. Concurrent Haskell

Concurrent Haskell is the collective name for the facilities that Haskell provides for programming with multiple threads of control. Unlike parallel programming, where the goal is to make the program run faster by using more CPUs, the goal in concurrent programming is usually to write a program with multiple interactions. These interactions might be with the user via a user interface of some kind, with other systems, or indeed between different subsystems within the same program. Concurrency allows us to write a program in which each of these interactions is described separately but all happen at the same time. As we shall see, concurrency is a powerful tool for structuring programs with multiple interactions.

In many application areas today, some kind of concurrency is a necessity. A typical user-facing application will have an interface that must remain responsive while the application is downloading data from the network or calculating some results. Often these applications may be interacting with multiple servers over the network at the same time; a web browser, for example, will have many concurrent connections open to the sites that the user is browsing, while all the time maintaining a responsive user interface. Server-side applications also need concurrency in order to manage multiple client interactions simultaneously.

Haskell takes the view that concurrency is a useful abstraction because it allows each interaction to be programmed separately, ...