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 key position state event

A StreamProjection is a Projection that has been constructed from events from a particular event stream. This is useful when we want to cache the resulting state and also keep track of what part of the stream the state is caught up to.

Constructors

StreamProjection

Fields

• streamProjectionKey :: !key
• streamProjectionPosition :: !position
• streamProjectionProjection :: !(Projection state event)
• streamProjectionState :: !state

type VersionedStreamProjection = StreamProjection UUID EventVersion

type GlobalStreamProjection state event = StreamProjection () SequenceNumber state (VersionedStreamEvent event)

streamProjection :: key -> position -> Projection state event -> StreamProjection key position state event

Initialize a StreamProjection with a Projection, key, and order key.

versionedStreamProjection :: UUID -> Projection state event -> VersionedStreamProjection state event

Initialize a VersionedStreamProjection.

globalStreamProjection :: Projection state (VersionedStreamEvent event) -> GlobalStreamProjection state event

Initialize a GlobalStreamProjection.

getLatestStreamProjection :: (Monad m, Num position) => EventStoreReader key position m (StreamEvent key position event) -> StreamProjection key position state event -> m (StreamProjection key position state event)

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

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.