Data.Acid

State container offering full ACID (Atomicity, Consistency, Isolation and Durability) guarantees.

Atomicity

State changes are all-or-nothing. This is what you'd expect of any state variable in Haskell and AcidState doesn't change that.

Consistency

No event or set of events will break your data invariants.

Isolation

Transactions cannot interfere with each other even when issued in parallel.

Durability

Successful transaction are guaranteed to survive unexpected system shutdowns (both those caused by hardware and software).

Create an AcidState given an initial value.

This will create or resume a log found in the "state/[typeOf state]/" directory.

Create an AcidState given a log directory and an initial value.

This will create or resume a log found in directory. Running two AcidState's from the same directory is an error but will not result in dataloss.

Issue an Update event and wait for its result. Once this call returns, you are guaranteed that the changes to the state are durable. Events may be issued in parallel.

It's a run-time error to issue events that aren't supported by the AcidState.

Issue a Query event and wait for its result. Events may be issued in parallel.

Events return the same thing as Methods. The exact type of EventResult depends on the event.

All UpdateEvents are also Methods.

All QueryEvents are also Methods.

Context monad for Update events.

Context monad for Query events.

Create the control structures required for acid states using Template Haskell.

This code:

myUpdate :: Argument -> Update State Result
myUpdate arg = ...

myQuery :: Argument -> Query State Result
myQuery arg = ...

$(makeAcidic ''State ['myUpdate, 'myQuery])

will make State an instance of IsAcidic and provide the following events:

data MyUpdate = MyUpdate Argument
data MyQuery  = MyQuery Argument

Run a query in the Update Monad.