Some Resource Management Experiments

resources.hs

import Control.Exception.Base (bracket_)
import Debug.Trace (trace)

-- These are dummy types for the purposes of demonstration
data Request = Request
data Response = Response
data Done = Done

-- a basic, non-CPS App example
type App = Request -> IO Response

-- We're gonna bracket the return value here
myApp :: App
myApp request =
  bracket_ (putStrLn "Normal: creating 'resource'")
           (putStrLn "Normal: destroying 'resource'")
           (case request of
              Request -> return (trace "Normal: Forcing response" Response))

runApp :: App -> IO ()
runApp app = do
  rs <- app (trace "Normal: Forcing request" Request)
  case rs of
    Response -> return ()

-- A CPS'ed App example
type App' = Request -> (Response -> IO Done) -> IO Done

myApp' :: App'
myApp' request f =
  bracket_ (putStrLn "CPS: creating 'resource'")
           (putStrLn "CPS: destroying 'resource'")
           (case request of
              Request -> f (trace "CPS: Forcing response" Response))

runApp' :: App' -> IO ()
runApp' app = do
  rs <- app (trace "CPS: Forcing request" Request)
            (\ r -> case r of
               Response -> return Done)
  case rs of
    Done -> return ()

-- And running both:
main :: IO ()
main = do
  putStrLn "Non-CPS'ed:"
  runApp myApp
  putStrLn ""
  putStrLn "CPS'ed:"
  runApp' myApp'