Control.Monad.IO.Unlift

Please see the README.md file for information on using this package.

Documentation

class MonadIO m => MonadUnliftIO m where

Monads which allow their actions to be run in IO.

While MonadIO allows an IO action to be lifted into another monad, this class captures the opposite concept: allowing you to capture the monadic context. Note that, in order to meet the laws given below, the intuition is that a monad must have no monadic state, but may have monadic context. This essentially limits MonadUnliftIO to ReaderT and IdentityT transformers on top of IO.

Laws. For any value u returned by askUnliftIO, it must meet the monad transformer laws as reformulated for MonadUnliftIO:

• unliftIO u . return = return

• unliftIO u (m >>= f) = unliftIO u m >>= unliftIO u . f

The third is a currently nameless law which ensures that the current context is preserved.

• askUnliftIO >>= (u -> liftIO (unliftIO u m)) = m

If you have a name for this, please submit it in a pull request for great glory.

Since: 0.1.0.0

Minimal complete definition

askUnliftIO

Methods

askUnliftIO :: m (UnliftIO m)

Capture the current monadic context, providing the ability to run monadic actions in IO.

See UnliftIO for an explanation of why we need a helper datatype here. @since 0.1.0.0

Instances

MonadUnliftIO IO

Methods

askUnliftIO :: IO (UnliftIO IO)

MonadUnliftIO m => MonadUnliftIO (IdentityT * m)

Methods

askUnliftIO :: IdentityT * m (UnliftIO (IdentityT * m))

MonadUnliftIO m => MonadUnliftIO (ReaderT * r m)

Methods

askUnliftIO :: ReaderT * r m (UnliftIO (ReaderT * r m))

newtype UnliftIO m

The ability to run any monadic action m a as IO a.

This is more precisely a natural transformation. We need to new datatype (instead of simply using a forall) due to lack of support in GHC for impredicative types.

Since: 0.1.0.0

Constructors

UnliftIO

Fields

• unliftIO :: forall a. m a -> IO a

askRunInIO :: MonadUnliftIO m => m (m a -> IO a)

Same ask askUnliftIO, but returns a monomorphic function instead of a polymorphic newtype wrapper. If you only need to apply the transformation on one concrete type, this function can be more convenient.

Since: 0.1.0.0

withUnliftIO :: MonadUnliftIO m => (UnliftIO m -> IO a) -> m a

Convenience function for capturing the monadic context and running an IO action.

Since: 0.1.0.0

withRunInIO :: MonadUnliftIO m => ((m a -> IO a) -> IO b) -> m b

Same as withUnliftIO, but uses a monomorphic function like askRunInIO.

Since: 0.1.0.0

toIO :: MonadUnliftIO m => m a -> m (IO a)

Convert an action in m to an action in IO.

Since: 0.1.0.0

class Monad m => MonadIO m 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)

Minimal complete definition

liftIO

Methods

liftIO :: IO a -> m a

Lift a computation from the IO monad.

Instances

MonadIO IO

Methods

liftIO :: IO a -> IO a

MonadIO m => MonadIO (IdentityT * m)

Methods

liftIO :: IO a -> IdentityT * m a

MonadIO m => MonadIO (ReaderT * r m)

Methods

liftIO :: IO a -> ReaderT * r m a