Retrieving pure values from acid state queries

Question

This seems like something that should be easy but how do I get a pure value out of a query if I am using AcidState's Data.Acid.Memory.Pure module. I guess I can generalize the question to "how do I get any value out of the Update monad?". You see, I'm trying to write a test that does the following run-of-the-mill tasks:

• Updates a pure AcidState with an object
• Queries that Object out of the state using IxSet
• Compares the Queried Object and the one returned by the Update for equivalence.

I need a pure "Bool" from this in order to make integration with test frameworks easy. At first I thought I'd simply use runState from Control.Monad.State but I was mistaken (or just didn't do it right). What should I do?

Answer 1

Since you are using Data.Acid.Memory.Pure, you can use the update, query, and update_ functions from that module (instead of the ones from Data.Acid) to look at the result of an event purely. As with regular, impure acid-state, you don't simply "run the Update and Query monads," you have to convert them to an event first. With Data.Acid.Memory.Pure, that means you simply wrap them with the constructors of Event.