runState -package:basement -package:dunai package:cleff

Run the State effect.

Caveats

The runState interpreter is implemented with IORefs and there is no way to do arbitrary atomic transactions. The state operation is atomic though and it is implemented with atomicModifyIORefCAS, which can be faster than atomicModifyIORef in contention. For any more complicated cases of atomicity, please build your own effect that uses either MVars or TVars based on your need. Unlike mtl, in cleff the state will not revert when an error is thrown. runState will stop taking care of state operations done on forked threads as soon as the main thread finishes its computation. Any state operation done before main thread finishes is still taken into account.

Run a State effect where each thread has its thread-local state. This means that each thread will have an individual state that has the same initial value. Threfore, state operations on one thread will not change the state for any other thread. The returned final state is that of the current thread.

Caveats

Like runState, the state operation in this handler is atomic. Like runState, and unlike mtl, any errors will not revert the state changes. Be warned that if you use a thread pool, then when a thread is reused, it may read the state left from the last usage, therefore losing locality. If you use a thread pool, you will want to manually reset the state after each task.