Haskell Legacy: Reversing writes

Previously, we have introduced events, however they are the second write, meaning that they are not actually used, they are "passive", even if we fail to write them, we let the execution continue.

A first step is to reverse writes, so we ensure that events are written.

As a reminder we have interceptors calling the persistent interpreters:

interceptBookingEffectEvents ::
  forall a m r.
  (Members '[EventStore TrainEvent, Error InternalApiError] r) =>
  Sem (BookingEffect ': r) a ->
  Sem (BookingEffect ': r) a
interceptBookingEffectEvents =
  intercept $ -- 1
    \case
      BookingCreate trainId travelerName -> do
        bookingId <- bookingCreate trainId travelerName -- 2
        let BookingKey bookingId' = bookingId
        events <- fetchEvents $ trainStreamId trainId -- 3
        unless (null events) $
          void $
            storeEvent -- 4
              (trainStreamId trainId)
              (EventNumber $ length events)
              BookingCreated {id = fromIntegral bookingId', travelerName = travelerName}
        return bookingId
      BookingDelete bookingId ->
        -- ...

In order to reverse writes we should:

• Make these interceptors interpreters (so they consume the effects), which implies
  • generate the unique id in them
  • have a dedicatedid type
  • change the legacy interpreters, so the take the id, which implies
    • change the schema so id is not auto incremented

Let's change the schema first:

share
  [mkPersist sqlSettings, mkMigrate "migrateAll"]
  [persistLowerCase|
Train
    trainId TrainId'
    departureDate T.Text
    departureStation T.Text
    arrivalStation T.Text
    deriving Show
    Primary trainId
|]

The last line tell persistent that the key is provided.

Then, let's create TrainId' (which cannot be Train as persistent generates one, which should be a configurable behavior IMO):

newtype TrainId' = TrainId' {unTrainId' :: Int64}
  deriving stock (Eq, Ord, Show)
  deriving newtype (Read, PathPiece, ToHttpApiData, FromHttpApiData, PersistField, PersistFieldSql, FromJSON, ToJSON)

Then we have to rework our effects, the first/external one (exposed to Handlers) should take TrainId':

data TrainEffect (m :: Type -> Type) (a :: Type) where
  TrainCreate :: DepartureDate -> DepartureStation -> ArrivalStation -> TrainEffect m TrainId'
  TrainFetch :: TrainId' -> TrainEffect m DisplayedTrain

makeSem ''TrainEffect

Which indeed forces us to change Handlers and Routes:

mkYesod
  "TrainMasterAPI"
  [parseRoutes|
/train CreateTrainR POST
/train/#TrainId' DisplayTrainR GET
/booking/#TrainId' CreateBookingR POST
/booking-admin/#BookingId' ManageBookingR DELETE
|]

getDisplayTrainR :: TrainId' -> HandlerFor TrainMasterAPI Value
getDisplayTrainR trainId = do
  train <- runEffect $ trainFetch trainId
  returnJson train

Note: that's what happen when you couple your persistence with your API.

Then we can create a second/internal effect for the legacy effect:

data TrainProjectionEffect (m :: Type -> Type) (a :: Type) where
  TrainProjectionCreate :: TrainId' -> DepartureDate -> DepartureStation -> ArrivalStation -> TrainProjectionEffect m ()
  TrainProjectionFetch :: TrainId' -> TrainProjectionEffect m DisplayedTrain

makeSem ''TrainProjectionEffect

At this point, we have to adapt our legacy interpreter to take the provided id:

interpretTrainProjectionEffectPersistent ::
  forall m r.
  (Members '[Embed (ReaderT SqlBackend m)] r, MonadIO m) =>
  InterpreterFor TrainProjectionEffect r
interpretTrainProjectionEffectPersistent =
  interpret $
    \case
      TrainProjectionCreate
        newTrainId
        (DepartureDate departureDate)
        (DepartureStation departureStation)
        (ArrivalStation arrivalStation) ->
          embed $
            insert_
              Train
                { trainTrainId = newTrainId,
                  trainDepartureDate = departureDate,
                  trainDepartureStation = departureStation,
                  trainArrivalStation = arrivalStation
                }
      TrainProjectionFetch trainId -> do
        -- ...

Then, we can rewrite our external interpreters:

interceptTrainEffectEvents ::
  forall r.
  (Members '[EventStore TrainEvent, Embed IO, Error InternalApiError, TrainProjectionEffect] r) =>
  InterpreterFor TrainEffect r
interceptTrainEffectEvents =
  interpret $
    \case
      TrainCreate
        departureDate'@(DepartureDate departureDate)
        departureStation'@(DepartureStation departureStation)
        arrivalStation'@(ArrivalStation arrivalStation) -> do
          newTrainId <- embed $ TrainId' <$> randomRIO (1000000, 9999999)
          eventStored <-
            storeEvent
              (StreamId newTrainId.unTrainId')
              (EventNumber 0)
              TrainCreated
                { departureDate = departureDate,
                  departureStation = departureStation,
                  arrivalStation = arrivalStation
                }
          case eventStored of
            Left e -> throw $ EventStoreIAE e
            Right x -> do
              trainProjectionCreate newTrainId departureDate' departureStation' arrivalStation'
              return newTrainId
      TrainFetch trainId ->
        trainProjectionFetch trainId

Note: at this point, we are still using the legacy view model (trainProjectionFetch)

Here, we force the persistence of the events before going on

Finally, we can add our new effects to the list:

type APIEffects =
  '[ TrainEffect,
     TrainProjectionEffect,
     BookingEffect,
     BookingProjectionEffect
     EventStore TrainEvent,
     Embed (ReaderT SqlBackend IO),
     Error InternalApiError,
     Embed IO
   ]

And everything works as previously.

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