6.15. Parallel and Concurrent

GHC implements some major extensions to Haskell to support concurrent and parallel programming. Let us first establish terminology:

  • Parallelism means running a Haskell program on multiple processors, with the goal of improving performance. Ideally, this should be done invisibly, and with no semantic changes.
  • Concurrency means implementing a program by using multiple I/O-performing threads. While a concurrent Haskell program can run on a parallel machine, the primary goal of using concurrency is not to gain performance, but rather because that is the simplest and most direct way to write the program. Since the threads perform I/O, the semantics of the program is necessarily non-deterministic.

GHC supports both concurrency and parallelism.