ReaderT, ExceptT, IO, composition

sample.hs

-- |
-- Application-wide service, which is merely a product of other services.
-- Shows how services can be de/composed and encapsulated.
module Services.Main where

  import qualified Services.Db as Db
  import qualified Services.Metrics as Metrics

  data Env = Env {
    db :: Db.Env,
    metrics :: Metrics.Err
  }

  data Err =
    DbErr Db.Err |
    MetricsErr Metrics.Err

  type Fx = ReaderT Env (ExceptT Err IO)

  selectUserEmailById :: Int -> Fx (Maybe Text)
  selectUserEmailById id = do
    result <- runDbFx $ Db.selectUserEmailById id
    runMetricsFx $ Metrics.bump "selectUserEmailById"
    return result

  runDbFx :: Db.Fx a -> Fx a
  runDbFx =
    withReaderT (.db) . mapReaderT (withExceptT DbErr)

  runMetricsFx :: Metrics.Fx a -> Fx a
  runMetricsFx =
    withReaderT (.metrics) . mapReaderT (withExceptT MetricsErr)

-- | Your application database.
module Services.Db where

  -- | Database-specific environment (usually a connection, or a pool of connections).
  data Env

  -- | Database-specific error.
  data Err

  type Fx = ReaderT Env (ExceptT Err IO)

  selectUserEmailById :: Int -> Fx (Maybe Text)


-- | Client to some metrics service.
module Services.Metrics where

  -- | Your grafana connection or something like that.
  data Env

  -- | Failure of your metrics service.
  data Err

  type Fx = ReaderT Env (ExceptT Err IO)

  -- | Increment metric by name.
  bump :: Text -> Fx ()