EventSourcing.elm

module EventSourcing exposing

type alias EventRecord a =
  { timestamp : Time
  , eventNumber : Int
  , event : a
  }

type alias EventLog a =
  { aggregateId : String
  , version : Int
  , events : List (EventRecord a)
  }

type alias Model a =
  { eventLog : EventLog a
  , currentTime : Maybe Time
  }

init : String -> Model a
init aggregateId =
  { eventLog =
      { aggregateId = aggregateId
      , version = 0
      , events = []
      }
  , currentTime = Nothing
  }

type Cmd a = CommitEvent a | UpdateTime Time

update cmd model =
  case cmd of
    UpdateTime time ->
      { model | currentTime = Just time }
      
    CommitEvent event ->
      { model | eventLog = commitEvent event model.currentTime model.eventLog }

commitEvent : a -> Time -> EventLog a -> EventLog a
commitEvent event timestamp eventLog =
  let
    eventNumber = eventLog.version + 1
    eventRecord =
      { eventNumber = eventNumber
      , timestamp = timestamp
      , event = event
      }
  in
    { eventLog
    | version = eventLog.version + 1
    , events = eventLog.events ++ [eventRecord]
    }
  
subscriptions model =
  [ Time.every Time.second UpdateTime
  ]