Classy concurrency.

Concurrency is "lightweight", which means that both thread creation and context switching overheads are extremely low. Scheduling of Haskell threads is done internally in the Haskell runtime system, and doesn't make use of any operating system-supplied thread packages.

Haskell threads can communicate via MVars, a kind of synchronised mutable variable (see Control.Concurrent.Classy.MVar). Several common concurrency abstractions can be built from MVars, and these are provided by the Control.Concurrent.Classy library. Threads may also communicate via exceptions.