Example using database pool with `ReaderT`

Example.hs

#!/usr/bin/env stack
-- stack --resolver lts-12.0 --install-ghc runghc --package mtl --package resource-pool --package mysql-simple

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE FlexibleContexts #-}

import Control.Monad.Reader
import Data.Pool
import Database.MySQL.Simple


-- Or newtype, if you don't need anything else here
data Env = Env { dbPool :: Pool Connection }

main = IO ()
main = do
  pool <- createPool (connect connectionInfo) close 4 2 10

  runReaderT main_ (Env pool)

  -- flip runReaderT (Env pool) $ do
  --   tables <- runDb $ \conn -> do
  --     results :: [Only String] <- query_ conn "show tables"
  --     pure results
  --   liftIO $ print tables

  where
    connectionInfo = defaultConnectInfo
      { connectHost = "127.0.0.1"
      , connectDatabase = "mysql"
      , connectPassword = "root"
      }

main_ :: ReaderT Env IO ()
main_ = do
  tables :: [Only String] <- runDb $ \conn -> query_ conn "show tables"
  liftIO $ print tables

-- If you like flexibility (requires FlexibleContexts)
-- runDb
--   :: (MonadReader Env m, MonadIO m)
--   => (Connection -> IO b) -> m b

runDb :: (Connection -> IO a) -> ReaderT Env IO a
runDb action = do
  pool <- asks dbPool
  liftIO $ withResource pool action