Documentation

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

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

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

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

Convert an action in m to an action in IO.

Since: 0.1.0.0

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

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)