Eventful.Projection

Documentation

data Projection state event

A Projection is a piece of state that is constructed only from events. A Projection is how you reconstruct event sourced state from the ordered stream of events that constitute that state. The "seed" of a Projection is the initial state before any events are applied. The event handler for a projection is the function that actually modifies state based on the given event.

Constructors

Projection

Fields

• projectionSeed :: state

  Initial state of a projection

• projectionEventHandler :: state -> event -> state

  The function that applies and event to the current state, producing a new state.

Instances

Contravariant (Projection state)

Methods

contramap :: (a -> b) -> Projection state b -> Projection state a

(>$) :: b -> Projection state b -> Projection state a

latestProjection :: Foldable t => Projection state event -> t event -> state

Computes the latest state of a Projection from some events.

allProjections :: Projection state event -> [event] -> [state]

Given a list of events, produce all the Projections that were ever produced. Just a scanl using projectionEventHandler. This function is useful for testing Projections; you can easily assert that all the states of a Projection are valid given a list of events.

data StreamProjection state event

A StreamProjection is a Projection that has been constructed from events from a particular event stream. This is mostly useful so we can associate an EventVersion with some state.

Constructors

StreamProjection

Fields

• streamProjectionProjection :: Projection state event
• streamProjectionUuid :: !UUID
• streamProjectionVersion :: EventVersion
• streamProjectionState :: !state

streamProjection :: Projection state event -> UUID -> StreamProjection state event

Initialize a StreamProjection with a Projection.

getLatestProjection :: Monad m => EventStore event m -> StreamProjection state event -> m (StreamProjection state event)

Gets the latest projection from a store by using getEvents and then applying the events using the Projection's event handler.

data GloballyOrderedProjection state serialized

This is a combination of a Projection and the latest projection state with respect to some SequenceNumber. This is useful for in-memory read models, and for querying the latest state starting from some previous state at a lower SequenceNumber.

Constructors

GloballyOrderedProjection

Fields

• globallyOrderedProjectionProjection :: !(Projection state (GloballyOrderedEvent serialized))
• globallyOrderedProjectionSequenceNumber :: !SequenceNumber
• globallyOrderedProjectionState :: !state

globallyOrderedProjection :: Projection state (GloballyOrderedEvent serialized) -> GloballyOrderedProjection state serialized

Initialize a GloballyOrderedProjection at SequenceNumber 0 and with the projection's seed value.

globallyOrderedProjectionEventHandler :: GloballyOrderedProjection state serialized -> GloballyOrderedEvent serialized -> GloballyOrderedProjection state serialized

This applies an event to a GloballyOrderedProjection. NOTE: There is no guarantee that the SequenceNumber for the event is the previous SequenceNumber plus one (in fact, that isn't even a guarantee that some stores can provide). This function will update the GloballyOrderedProjetion to use the sequence number of the event.

getLatestGlobalProjection :: Monad m => GloballyOrderedEventStore serialized m -> GloballyOrderedProjection state serialized -> m (GloballyOrderedProjection state serialized)

Gets globally ordered events from the event store and builds a Projection based on ProjectionEvent. Optionally accepts the current projection state as an argument.

serializedProjection :: Projection state event -> Serializer event serialized -> Projection state serialized

Use a Serializer to wrap a Projection with event type event so it uses the serialized type.

projectionMapMaybe :: (eventB -> Maybe eventA) -> Projection state eventA -> Projection state eventB

Transform a Projection when you only have a partial relationship between the source event type and the target event type.