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 function run provided by withRunInIO, it must meet the monad transformer laws as reformulated for MonadUnliftIO:

• run . return = return

• run (m >>= f) = run m >>= run . f

Instances of MonadUnliftIO must also satisfy the following laws:

Identity law

withRunInIO (\run -> run m) = m

Inverse law

withRunInIO (\_ -> m) = liftIO m

As an example of an invalid instance, a naive implementation of MonadUnliftIO (StateT s m) might be

withRunInIO inner =
  StateT $ \s ->
    withRunInIO $ \run ->
      inner (run . flip evalStateT s)

This breaks the identity law because the inner run m would throw away any state changes in m.

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.

Note: You can derive MonadUnliftIO for newtypes without this helper function in unliftio-core 0.2.0.0 and later.

Example

newtype AppT m a = AppT { unAppT :: ReaderT Int (ResourceT m) a }
  deriving (Functor, Applicative, Monad, MonadIO)

-- Same as `deriving newtype (MonadUnliftIO)`
instance MonadUnliftIO m => MonadUnliftIO (AppT m) where
  withRunInIO = wrappedWithRunInIO AppT unAppT

Since: 0.1.2.0

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

A helper function for lifting IO a -> IO b functions into any MonadUnliftIO.

Example

liftedTry :: (Exception e, MonadUnliftIO m) => m a -> m (Either e a)
liftedTry m = liftIOOp Control.Exception.try m

Since: 0.2.1.0

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. 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

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

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