EventBus.hs

module Eventium.EventBus
  ( synchronousEventBusWrapper,
    storeAndPublishEvents,
  )
where

import Eventium.Store.Class
import Eventium.UUID

-- | This function wraps an event store by sending events to event handlers
-- after running 'storeEvents'. This is useful to quickly wire up event
-- handlers in your application (like read models or process managers), and it
-- is also useful for integration testing along with an in-memory event store.
synchronousEventBusWrapper ::
  (Monad m) =>
  VersionedEventStoreWriter m event ->
  [VersionedEventStoreWriter m event -> UUID -> event -> m ()] ->
  VersionedEventStoreWriter m event
synchronousEventBusWrapper writer handlers = wrappedStore
  where
    -- NB: We need to use recursive let bindings so we can put wrappedStore
    -- inside the event handlers
    handlers' = map ($ wrappedStore) handlers
    wrappedStore = EventStoreWriter $ storeAndPublishEvents writer handlers'

-- | Stores events in the store and then publishes them to the event handlers.
-- This is used in the 'synchronousEventBusWrapper'.
storeAndPublishEvents ::
  (Monad m) =>
  VersionedEventStoreWriter m event ->
  [UUID -> event -> m ()] ->
  UUID ->
  ExpectedPosition EventVersion ->
  [event] ->
  m (Either (EventWriteError EventVersion) EventVersion)
storeAndPublishEvents store handlers uuid expectedVersion events = do
  result <- storeEvents store uuid expectedVersion events
  case result of
    Left err -> return $ Left err
    Right vers -> do
      -- NB: If a handler stores events, then its events will be published
      -- before the events of the next handler. That is, we will be storing
      -- events generated by handlers in depth-first order.
      mapM_ (\handler -> mapM_ (handler uuid) events) handlers
      return $ Right vers