Safe Haskell	Safe-Inferred
Language	Haskell2010

Control.Monad.Managed.Safe

Contents

Managed
Re-exports

Description

This module is a safer subset of Control.Monad.Managed that only lets you unwrap the Managed type using runManaged. This enforces that you never leak acquired resources from a Managed computation.

In general, you should strive to propagate the Managed type as much as possible and use runManaged when you are done with acquired resources. However, there are legitimate circumstances where you want to return a value other than acquired resource from the bracketed computation, which requires using with.

This module is not the default because you can also use the Managed type for callback-based code that is completely unrelated to resources.

Synopsis

data Managed a
class MonadIO m => MonadManaged m where
- using :: Managed a -> m a
managed :: MonadManaged m => (forall r. (a -> IO r) -> IO r) -> m a
managed_ :: MonadManaged m => (forall r. IO r -> IO r) -> m ()
defer :: MonadManaged m => IO r -> m ()
runManaged :: Managed () -> IO ()
class Monad m => MonadIO (m :: Type -> Type) where
- liftIO :: IO a -> m a

Managed

data Managed a Source #

A managed resource that you acquire using with

Instances

Instances details

MonadFail Managed Source #
Instance details Defined in Control.Monad.Managed Methods fail :: String -> Managed a #
MonadIO Managed Source #
Instance details Defined in Control.Monad.Managed Methods liftIO :: IO a -> Managed a #
Applicative Managed Source #
Instance details Defined in Control.Monad.Managed Methods pure :: a -> Managed a # (<>) :: Managed (a -> b) -> Managed a -> Managed b # liftA2 :: (a -> b -> c) -> Managed a -> Managed b -> Managed c # (>) :: Managed a -> Managed b -> Managed b # (<*) :: Managed a -> Managed b -> Managed a #
Functor Managed Source #
Instance details Defined in Control.Monad.Managed Methods fmap :: (a -> b) -> Managed a -> Managed b # (<$) :: a -> Managed b -> Managed a #
Monad Managed Source #
Instance details Defined in Control.Monad.Managed Methods (>>=) :: Managed a -> (a -> Managed b) -> Managed b # (>>) :: Managed a -> Managed b -> Managed b # return :: a -> Managed a #
MonadManaged Managed Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> Managed a Source #
Monoid a => Monoid (Managed a) Source #
Instance details Defined in Control.Monad.Managed Methods mempty :: Managed a # mappend :: Managed a -> Managed a -> Managed a # mconcat :: [Managed a] -> Managed a #
Semigroup a => Semigroup (Managed a) Source #
Instance details Defined in Control.Monad.Managed Methods (<>) :: Managed a -> Managed a -> Managed a # sconcat :: NonEmpty (Managed a) -> Managed a # stimes :: Integral b => b -> Managed a -> Managed a #
Floating a => Floating (Managed a) Source #
Instance details Defined in Control.Monad.Managed Methods pi :: Managed a # exp :: Managed a -> Managed a # log :: Managed a -> Managed a # sqrt :: Managed a -> Managed a # (**) :: Managed a -> Managed a -> Managed a # logBase :: Managed a -> Managed a -> Managed a # sin :: Managed a -> Managed a # cos :: Managed a -> Managed a # tan :: Managed a -> Managed a # asin :: Managed a -> Managed a # acos :: Managed a -> Managed a # atan :: Managed a -> Managed a # sinh :: Managed a -> Managed a # cosh :: Managed a -> Managed a # tanh :: Managed a -> Managed a # asinh :: Managed a -> Managed a # acosh :: Managed a -> Managed a # atanh :: Managed a -> Managed a # log1p :: Managed a -> Managed a # expm1 :: Managed a -> Managed a # log1pexp :: Managed a -> Managed a # log1mexp :: Managed a -> Managed a #
Num a => Num (Managed a) Source #
Instance details Defined in Control.Monad.Managed Methods (+) :: Managed a -> Managed a -> Managed a # (-) :: Managed a -> Managed a -> Managed a # (*) :: Managed a -> Managed a -> Managed a # negate :: Managed a -> Managed a # abs :: Managed a -> Managed a # signum :: Managed a -> Managed a # fromInteger :: Integer -> Managed a #
Fractional a => Fractional (Managed a) Source #
Instance details Defined in Control.Monad.Managed Methods (/) :: Managed a -> Managed a -> Managed a # recip :: Managed a -> Managed a # fromRational :: Rational -> Managed a #

class MonadIO m => MonadManaged m where Source #

You can embed a Managed action within any Monad that implements MonadManaged by using the using function

All instances must obey the following two laws:

using (return x) = return x

using (m >>= f) = using m >>= \x -> using (f x)

Methods

using :: Managed a -> m a Source #

Instances

Instances details

MonadManaged Managed Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> Managed a Source #
MonadManaged m => MonadManaged (MaybeT m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> MaybeT m a Source #
MonadManaged m => MonadManaged (ExceptT e m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> ExceptT e m a Source #
MonadManaged m => MonadManaged (IdentityT m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> IdentityT m a Source #
MonadManaged m => MonadManaged (ReaderT r m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> ReaderT r m a Source #
MonadManaged m => MonadManaged (StateT s m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> StateT s m a Source #
MonadManaged m => MonadManaged (StateT s m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> StateT s m a Source #
(Monoid w, MonadManaged m) => MonadManaged (WriterT w m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> WriterT w m a Source #
(Monoid w, MonadManaged m) => MonadManaged (WriterT w m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> WriterT w m a Source #
MonadManaged m => MonadManaged (ContT r m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> ContT r m a Source #
(Monoid w, MonadManaged m) => MonadManaged (RWST r w s m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> RWST r w s m a Source #
(Monoid w, MonadManaged m) => MonadManaged (RWST r w s m) Source #
Instance details Defined in Control.Monad.Managed Methods using :: Managed a -> RWST r w s m a Source #

managed :: MonadManaged m => (forall r. (a -> IO r) -> IO r) -> m a Source #

Build a Managed value

managed_ :: MonadManaged m => (forall r. IO r -> IO r) -> m () Source #

Like managed but for resource-less operations.

defer :: MonadManaged m => IO r -> m () Source #

Defer running an action until exit (via runManaged).

For example, the following code will print "Hello" followed by "Goodbye":

runManaged $ do
  defer $ liftIO $ putStrLn "Goodbye"
  liftIO $ putStrLn "Hello"

runManaged :: Managed () -> IO () Source #

Run a Managed computation, enforcing that no acquired resources leak

Re-exports

Control.Monad.IO.Class re-exports MonadIO

class Monad m => MonadIO (m :: Type -> Type) where #

Monads in which IO computations may be embedded. Any monad built by applying a sequence of monad transformers to the IO monad will be an instance of this class.

Instances should satisfy the following laws, which state that liftIO is a transformer of monads:

```
liftIO . return = return
```

liftIO (m >>= f) = liftIO m >>= (liftIO . f)

Methods

liftIO :: IO a -> m a #

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

Expand

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3

Instances

Instances details

MonadIO IO	Since: base-4.9.0.0
Instance details Defined in Control.Monad.IO.Class Methods liftIO :: IO a -> IO a #
MonadIO Managed Source #
Instance details Defined in Control.Monad.Managed Methods liftIO :: IO a -> Managed a #
MonadIO m => MonadIO (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftIO :: IO a -> MaybeT m a #
MonadIO m => MonadIO (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftIO :: IO a -> ExceptT e m a #
MonadIO m => MonadIO (IdentityT m)
Instance details Defined in Control.Monad.Trans.Identity Methods liftIO :: IO a -> IdentityT m a #
MonadIO m => MonadIO (ReaderT r m)
Instance details Defined in Control.Monad.Trans.Reader Methods liftIO :: IO a -> ReaderT r m a #
MonadIO m => MonadIO (StateT s m)
Instance details Defined in Control.Monad.Trans.State.Lazy Methods liftIO :: IO a -> StateT s m a #
MonadIO m => MonadIO (StateT s m)
Instance details Defined in Control.Monad.Trans.State.Strict Methods liftIO :: IO a -> StateT s m a #
(Monoid w, MonadIO m) => MonadIO (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods liftIO :: IO a -> WriterT w m a #
(Monoid w, MonadIO m) => MonadIO (WriterT w m)
Instance details Defined in Control.Monad.Trans.Writer.Strict Methods liftIO :: IO a -> WriterT w m a #
MonadIO m => MonadIO (ContT r m)
Instance details Defined in Control.Monad.Trans.Cont Methods liftIO :: IO a -> ContT r m a #
(Monoid w, MonadIO m) => MonadIO (RWST r w s m)
Instance details Defined in Control.Monad.Trans.RWS.Lazy Methods liftIO :: IO a -> RWST r w s m a #
(Monoid w, MonadIO m) => MonadIO (RWST r w s m)
Instance details Defined in Control.Monad.Trans.RWS.Strict Methods liftIO :: IO a -> RWST r w s m a #