#!/usr/bin/env stack
{- stack
     --resolver lts-10.0
     --install-ghc
     runghc
     --package resourcet
-}

{-#LANGUAGE ScopedTypeVariables#-}

import Data.Acquire
import Control.Monad.Trans.Resource
import Control.Monad.IO.Class

main :: IO ()
main = runResourceT $ do
    let (ack :: Acquire Int) = mkAcquire (do
                          putStrLn "Enter some number"
                          readLn) (\i -> putStrLn $ "Freeing scarce resource: " ++ show i)
    (releaseKey, resource) <- allocateAcquire ack
    doSomethingDangerous resource
    liftIO $ putStrLn $ "Going to release resource immediately: " ++ show resource
    release releaseKey
    somethingElse

doSomethingDangerous :: Int -> ResourceT IO ()
doSomethingDangerous i =
    liftIO $ putStrLn $ "5 divided by " ++ show i ++ " is " ++ show (5 `div` i)

somethingElse :: ResourceT IO ()    
somethingElse = liftIO $ putStrLn
    "This could take a long time, don't delay releasing the resource!"

Execution output:

~ $ stack code.hs
Enter some number
3
5 divided by 3 is 1
Going to release resource immediately: 3
Freeing scarce resource: 3
This could take a long time, don't delay releasing the resource!

~ $ stack code.hs
Enter some number
0
5 divided by 0 is Freeing scarce resource: 0
code.hs: divide by zero