Safe Haskell | Safe |
---|---|

Language | Haskell2010 |

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

- class MonadIO m => MonadUnliftIO m where
- newtype UnliftIO m = UnliftIO {}
- askRunInIO :: MonadUnliftIO m => m (m a -> IO a)
- withUnliftIO :: MonadUnliftIO m => (UnliftIO m -> IO a) -> m a
- toIO :: MonadUnliftIO m => m a -> m (IO a)
- class Monad m => MonadIO (m :: * -> *) where

# Documentation

class MonadIO m => MonadUnliftIO m where Source #

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*

askUnliftIO :: m (UnliftIO m) Source #

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

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

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*

MonadUnliftIO IO Source # | |

MonadUnliftIO m => MonadUnliftIO (IdentityT * m) Source # | |

MonadUnliftIO m => MonadUnliftIO (ReaderT * r m) Source # | |

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*

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

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 Source #

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) Source #

Convert an action in `m`

to an action in `IO`

.

*Since: 0.1.0.0*