Transaction.hs

-- Assumptions:
-- Every query returns only one integer
-- The DB connection is somewhere global (should be hidden using a State
-- transformer in a real implementation)

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Transaction
    (
       Transaction
     , runTransaction
     , query
    ) where

newtype Transaction a = Transaction { runTransaction :: IO a }
    deriving (Monad)

-- Demonstrate we can do IO inside a transaction, but only from functions
-- we defined
query :: String -> Transaction Int
query q = Transaction ( putStrLn "Performing Query" >> return 123 )


main = runTransaction(do
                i <- query "SELECT * FROM foo"
                -- The following line is not allowed: compilation error
                -- putStrLn "Hello"
                query $ "UPDATE foo SET a = " ++ (show $ i + 1)
                return $ i + 2
                )