An example Haskell program to copy data from one handle to another might look like this:

main =
    withFile "inFile.txt" ReadMode $ \inHandle ->
        withFile "outFile.txt" WriteMode $ \outHandle ->
            copy inHandle outHandle

-- A hypothetical function that copies data from one handle to another
copy :: Handle -> Handle -> IO ()

withFile is one of many functions that acquire some resource in an exception-safe way. These functions take a callback function as an argument and they invoke the callback on the resource when it becomes available, guaranteeing that the resource is properly disposed if the callback throws an exception.

These functions usually have a type that ends with the following pattern:

                   Callback
--                -----------
withXXX :: ... -> (a -> IO r) -> IO r

Here are some examples of this pattern from the base libraries:

withArray      :: Storable a => [a] -> (Ptr a   -> IO r) -> IO r
withBuffer     ::          Buffer e -> (Ptr e   -> IO r) -> IO r
withCAString   ::            String -> (CString -> IO r) -> IO r
withForeignPtr ::      ForeignPtr a -> (Ptr a   -> IO r) -> IO r
withMVar       ::            Mvar a -> (a       -> IO r) -> IO r
withPool       ::                      (Pool    -> IO r) -> IO r

Acquiring multiple resources in this way requires nesting callbacks. However, you can wrap anything of the form ((a -> IO r) -> IO r) in the Managed monad, which translates binds to callbacks for you:

import Control.Monad.Managed
import System.IO

inFile :: FilePath -> Managed Handle
inFile filePath = managed (withFile filePath ReadMode)

outFile :: FilePath -> Managed Handle
outFile filePath = managed (withFile filePath WriteMode)

main = runManaged $ do
    inHandle  <- inFile "inFile.txt"
    outHandle <- outFile "outFile.txt"
    liftIO (copy inHandle outHandle)

... or you can just wrap things inline:

main = runManaged $ do
    inHandle  <- managed (withFile "inFile.txt" ReadMode)
    outHandle <- managed (withFile "outFile.txt" WriteMode)
    liftIO (copy inHandle outHandle)

Additionally, since Managed is a Monad, you can take advantage of all your favorite combinators from Control.Monad. For example, the withMany function from Foreign.Marshal.Utils becomes a trivial wrapper around mapM:

withMany :: (a -> (b -> IO r) -> IO r) -> [a] -> ([b] -> IO r) -> IO r
withMany f = with . mapM (Managed . f)

Another reason to use Managed is that if you wrap a Monoid value in Managed you get back a new Monoid:

instance Monoid a => Monoid (Managed a)

This lets you combine managed resources transparently. You can also lift operations from some numeric type classes this way, too, such as the Num type class.

NOTE: Managed may leak space if used in an infinite loop like this example:

import Control.Monad
import Control.Monad.Managed

main = runManaged (forever (liftIO (print 1)))

If you need to acquire a resource for a long-lived loop, you can instead acquire the resource first and run the loop in IO, using either of the following two equivalent idioms:

with resource (\r -> forever (useThe r))

do r <- resource
   liftIO (forever (useThe r))

Control.Monad.IO.Class re-exports MonadIO