Concurrent algorithms in Haskell using software transactional memory with the STM monad. The programs here implement the ideas in the 2005 paper Composable Memory Transactions. The two main gists are 1) no explicit locks or mutexes are used in the code, and 2) memory transactions are composable, freely and robustly so with no added risk of finicky lock management bugs.
The programs below are a mixture of classic concurrency problems from computer science, and problems from The Little Book of Semaphores. They are ordered roughly by complexity, starting with a simple example of 3 agents contending for shared resources.
| Problem | Description / Notes |
|---|---|
| Cigarette Smokers | Three smokers at a table contend for materials set down by an agent |
| Sleeping Barber | Customers contend for a single barber at a barbershop, with 3 chairs in the waiting area |
| Multiple Barbers | Like Sleeping Barber but there are now 3 barbers, with a couch for 4 waiting customers and standing room for 8 more |
| Sushi Bar 1 | Customers queue up to visit a 5-seat sushi bar, but sometimes have to wait for all customers to leave before sitting down |
| Sushi Bar 2 | Like Sushi Bar 1 but a more correct interpretation of the problem |
| Santa Claus | 9 reindeer and 10 elves contend for Santa's attention |