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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Moduleresourcet-1.3.0Haskell2010

Data.Acquire

This was previously known as the Resource monad. However, that term is confusing next to the ResourceT transformer, so it has been renamed.

  • 2 types
  • 5 values
  • Packageresourcet-1.3.0
  • Exports8
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceAcquire.hs
newtypenewtype Acquire a
#

A method for acquiring a scarce resource, providing the means of freeing it when no longer needed. This data type provides Functor/Applicative/Monad instances for composing different resources together. You can allocate these resources using either the bracket pattern (via with) or using ResourceT (via allocateAcquire).

This concept was originally introduced by Gabriel Gonzalez and described at: http://www.haskellforall.com/2013/06/the-resource-applicative.html. The implementation in this package is slightly different, due to taking a different approach to async exception safety.

Instances4Monad, Functor, Applicative, MonadIO

Example usage of Acquire for allocating a resource and freeing it up.

7 declarations

The code makes use of mkAcquire to create an Acquire and uses allocateAcquire to allocate the resource and register an action to free up the resource.

Reproducible Stack code snippet
#!/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
valuewith :: MonadUnliftIO m => Acquire a -> (a -> m b) -> m b
#

Allocate the given resource and provide it to the provided function. The resource will be freed as soon as the inner block is exited, whether normally or via an exception. This function is similar in function to bracket.

valuemkAcquire
  1. :: IO a

    acquire the resource

  2. -> (a -> IO ())

    free the resource

  3. -> Acquire a
#

Create an Acquire value using the given allocate and free functions.

To acquire and free the resource in an arbitrary monad with MonadUnliftIO, do the following:

acquire <- withRunInIO $ \runInIO ->
  return $ mkAcquire (runInIO create) (runInIO . free)

Note that this is only safe if the Acquire is run and freed within the same monadic scope it was created in.

valuemkAcquireType
  1. :: IO a

    acquire the resource

  2. -> (a -> ReleaseType -> IO ())

    free the resource

  3. -> Acquire a
#

Same as mkAcquire, but the cleanup function will be informed of how cleanup was initiated. This allows you to distinguish, for example, between normal and exceptional exits.

To acquire and free the resource in an arbitrary monad with MonadUnliftIO, do the following:

acquire <- withRunInIO $ \runInIO ->
  return $ mkAcquireType (runInIO create) (\a -> runInIO . free a)

Note that this is only safe if the Acquire is run and freed within the same monadic scope it was created in.