Concurrency

Note: you may want to read Parallelism vs. Concurrency, as the terms have historically been conflated.

This page contains notes and information about how to write concurrent programs in Haskell.

For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.

Overview

GHC provides multi-scale support for concurrent programming, from very fine-grained, small tasks to coarse-grained explicit threads and locks, along with other models of concurrent programming including actors, CSP-style concurrency, and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar-based shared state or transactional memory.

Getting started

1. Manage simultaneous I/O actions (eg. multiple connections on a web server)

   Start with Concurrent Haskell (forkIO, MVar)

   Learn more about concurrency, then try using network protocol libraries like HTTP or zeromq.

2. Work with clusters or do distributed programming

   Look out for ongoing research into distributed Haskell.

Digging deeper

• Software Transactional Memory (STM) is a newer way to coordinate concurrent threads. There's a separate Wiki page devoted to STM.

STM was added to GHC 6.4, and is described in the paper Composable memory transactions. The paper Lock-free data structures using STM in Haskell gives further examples of concurrent programming using STM.

• Foreign function interface. If you are calling foreign functions in a concurrent program, you need to know about bound threads. They are described in a Haskell workshop paper, Extending the Haskell Foreign Function Interface with Concurrency. The GHC Commentary Supporting multi-threaded interoperation contains more detailed explanation of cooperation between FFI calls and multi-threaded runtime.

GHC concurrency specifics

You get access to concurrency operations by importing the library Control.Concurrent.

Community

• Ask questions on Haskell Cafe
• See what concurrent-haskell researchers and developers are working on
• Follow @concurrenthaskell on Twitter
• StackOverflow on Haskell: concurrency

Tools

• ThreadScope - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
• Various libraries, including those for concurrency.

Documentation

• Glossary
• Learning to use concurrency in Haskell
• Current research
• ... Concurrent Programming in Haskell (online book)

Alternative approaches

• CHP: CSP-style concurrency for Haskell.

See also

• Concurrency category
• Concurrency research