Control.Monad.IO.Unlift

Please see the README.md file for information on using this package at https://www.stackage.org/package/unliftio-core.

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

Instances of MonadUnliftIO must also satisfy the idempotency law:

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

This law showcases two properties. First, askUnliftIO doesn't change the monadic context, and second, liftIO . unliftIO u is equivalent to id IF called in the same monadic context as askUnliftIO.

Since: 0.1.0.0

Methods

withRunInIO :: ((forall a. m a -> IO a) -> IO b) -> m b

Convenience function for capturing the monadic context and running an IO action with a runner function. The runner function is used to run a monadic action m in IO.

Since: 0.1.0.0

Instances

MonadUnliftIO IO

Defined in Control.Monad.IO.Unlift

Methods

withRunInIO :: ((forall a. IO a -> IO a) -> IO b) -> IO b

MonadUnliftIO m => MonadUnliftIO (IdentityT m)

Defined in Control.Monad.IO.Unlift

Methods

withRunInIO :: ((forall a. IdentityT m a -> IO a) -> IO b) -> IdentityT m b

MonadUnliftIO m => MonadUnliftIO (ReaderT r m)

Defined in Control.Monad.IO.Unlift

Methods

withRunInIO :: ((forall a. ReaderT r m a -> IO a) -> IO b) -> ReaderT r m b

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

askUnliftIO :: MonadUnliftIO m => 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.

Prior to version 0.2.0.0 of this library, this was a method in the MonadUnliftIO type class. It was moved out due to https://github.com/fpco/unliftio/issues/55.

Since: 0.1.0.0

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

Same as 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. The UnliftIO newtype wrapper is rarely needed, so prefer withRunInIO to this function.

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

wrappedWithRunInIO

Arguments

:: MonadUnliftIO n

=> (n b -> m b)

The wrapper, for instance IdentityT.

-> (forall a. m a -> n a)

The inverse, for instance runIdentityT.

-> ((forall a. m a -> IO a) -> IO b)

The actual function to invoke withRunInIO with.

-> m b

A helper function for implementing MonadUnliftIO instances. Useful for the common case where you want to simply delegate to the underlying transformer.

Example

newtype AppT m a = AppT { unAppT :: ReaderT Int (ResourceT m) a }
  deriving (Functor, Applicative, Monad, MonadIO)
  -- Unfortunately, deriving MonadUnliftIO does not work.

instance MonadUnliftIO m => MonadUnliftIO (AppT m) where
  withRunInIO = wrappedWithRunInIO AppT unAppT

Since: 0.1.2.0

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.

Instances

MonadIO IO

Since: base-4.9.0.0

Defined in Control.Monad.IO.Class

Methods

liftIO :: IO a -> IO a

MonadIO m => MonadIO (IdentityT m)

Defined in Control.Monad.Trans.Identity

Methods

liftIO :: IO a -> IdentityT m a

MonadIO m => MonadIO (ReaderT r m)

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a