liftIO package:unliftio-core

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

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

The MonadUnliftIO typeclass for unlifting monads to IO Please see the documentation and README at https://www.stackage.org/package/unliftio-core

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.

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.

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.

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.