Concurrent Haskell extends Haskell 98 with explicit concurrency. Its two main underlying concepts are: * A primitive type MVar α implementing a bounded/single-place asynchronous channel, which is either empty or holds a value of type α. * The ability to spawn a concurrent thread via the forkIO primitive. Built atop this is a collection of useful concurrency and synchronisation abstractions such as unbounded channels, semaphores and sample variables.