ServantFreeEffect.hs

-- Make use of freer-effects, https://hackage.haskell.org/package/freer-effects
-- 
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE TypeOperators     #-}

import           Control.Monad.Freer            as FF
import           Control.Monad.Freer.Exception  as FF
import           Control.Monad.Freer.Reader     as FF
...
-- some other imports

-------------------------------------------------------------------------------------------

-- | The request handler functions, all of which operate in the `Effect` Monad.
serverT :: ServerT API Effect
serverT = 
       searchForCandidates 
  :<|> statusCnadidate

-- handlers implementation

-- | Conversion logic between our effect stack and the `Handler` Monad.
--   We can: 
--     - catch any errore thrown within the Effect stack
--     - write to the DB
--     - rethrow into the `Handler` Monad where `Handler` is just a type alias for `ExceptT`.
effToHandler :: Connection -> Effect a -> Handler a
effToHandler conn eff = 
  liftIO 
    (runEffect conn eff) >>=  either f pure
  where 
    f err = 
      liftIO (log conn) Fail err) >> throwE err404

-- | Log some event message via the `IO` Monad
-- 
logEvent :: Connection -> T.Text -> T.Text -> IO ()
logEvent conn category msg = do
  now <- getCurrentTime
  execute conn "INSERT INTO events (dt, cat, log) VALUES (?, ?, ?)" $ [T.pack $ show $ now, category,  msg]

---------------------------------------------------------------------------------

-- | Run full `Effect` stack. 
--   Useful for bringing actions into the `IO` monad and lifting via `liftIO`.
runEffect :: Connection -> Effect a -> IO (Either T.Text a)
runEffect conn eff = runM . FF.runError $ FF.runReader eff conn