Concurrent Haskell

Concurrent Haskell extends^[1] Haskell 98 with explicit concurrency. The two main concepts underpinning Concurrent Haskell 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^[2] such as unbounded channels, semaphores and sample variables.

Default Haskell threads have very low overheads: creation, context-switching and scheduling are all internal to the Haskell runtime. Fully-fledged operating system processes are nevertheless available through the forkOS primitive on supported architecture and operating system combinations.

Software Transactional Memory

The recently introduced Software Transactional Memory (STM)^[3] extension^[4] to the Glasgow Haskell Compiler reuses the process forking primitives of Concurrent Haskell. STM however:

• eschews MVars in favour of TVars.
• introduces the retry and orElse primitives, allowing alternative atomic actions to be composed together.

References:

1. ^ Simon Peyton Jones, Andrew Gordon, and Sigbjorn Finne. Concurrent Haskell. ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (PoPL). 1996. (Some sections are out of date with respect to the current implementation.)
2. ^ The Haskell Hierarchical Libraries, Control.Concurrent
3. ^ Tim Harris, Simon Marlow, Simon Peyton Jones, and Maurice Herlihy. Composable Memory Transactions. ACM Conference on Principles and Practice of Parallel Programming 2005 (PPoPP'05). 2005.
4. ^ Control.Concurrent.STM

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