atomic-counter (hackage): This package defines Counter type that can be safely modified concurrently from multiple threads. The type supports only few operations, namely read, write, cas (compare and swap), add, subtract and a few bitwise ones like or, and xor.  Most common use case is having a shared counter that multiple threads increment. Another potential use case is lightweight locks.
Keywords: apache|concurrency|data|data-structures|library|haskell