The effect capable of lifting and unlifting the IO monad, allowing you to use MonadIO, MonadUnliftIO, PrimMonad, MonadCatch, MonadThrow and MonadMask functionalities. This is the "final" effect that most effects eventually are interpreted into. For example, you can do:

log :: IOE :> es => Eff es ()
log = liftIO (putStrLn "Test logging")

It is not recommended to use this effect directly in application code, as it is too liberal and allows arbitrary IO, therefore making it harder to do proper effect management. Ideally, this is only used in interpreting more fine-grained effects.

Technical details

Note that this is not a real effect and cannot be interpreted in any way besides thisIsPureTrustMe and runIOE. This is mainly for performance concern, but also that there doesn't really exist reasonable interpretations other than the current one, given the underlying implementation of the Eff monad.

IOE can be a real effect though, and you can enable the dynamic-ioe build flag to have that. However it is only for reference purposes and should not be used in production code.

Lift an IO computation into Eff. This function is highly unsafe and should not be used directly; use liftIO instead, or if you're interpreting higher-order effects, use fromIO.

Give a runner function a way to run Eff actions as an IO computation. This function is highly unsafe and should not be used directly; use withRunInIO instead, or if you're interpreting higher-order effects, use withToIO.

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)

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: unliftio-core-0.1.0.0

Unsafely eliminate an IOE effect from the top of the effect stack. This is mainly for implementing effects that uses IO but does not do anything really impure (i.e. can be safely used unsafeDupablePerformIO on), such as a State effect.

Unwrap an Eff computation with side effects into an IO computation, given that all effects other than IOE are interpreted.

Unwrap a pure Eff computation into a pure value, given that all effects are interpreted.

Unwrap a pure Eff computation into an IO computation. You may occasionally need this.

The type of an IO effect handler, which is a function that transforms an effect e into IO computations. This is used for interpretIO.

Interpret an effect in terms of IO, by transforming an effect into IO computations.

interpretIO f = interpret (liftIO . f)

Temporarily gain the ability to unlift an Eff esSend computation into IO. This is analogous to withRunInIO, and is useful in dealing with higher-order effects that involves IO. For example, the Resource effect that supports bracketing:

data Resource m a where
  Bracket :: m a -> (a -> m ()) -> (a -> m b) -> Resource m b

can be interpreted into bracket actions in IO, by converting all effect computations into IO computations via withToIO:

runResource :: IOE :> es => Eff (Resource : es) a -> Eff es a
runResource = interpret \case
  Bracket alloc dealloc use -> withToIO $ \toIO ->
    bracket (toIO alloc) (toIO . dealloc) (toIO . use)

Lift an IO computation into Eff esSend. This is analogous to liftIO, and is only useful in dealing with effect operations with the monad type in the negative position, for example masking:

data Mask :: Effect where
  Mask :: ((m ~> m) -> m a) -> Mask m a
                 ^ this "m" is in negative position

See how the restore :: IO a -> IO a from mask is "wrapped" into Eff esSend a -> Eff esSend a:

runMask :: IOE :> es => Eff (Mask : es) a -> Eff es a
runMask = interpret \case
  Mask f -> withToIO $ \toIO -> mask $
    \restore -> f (fromIO . restore . toIO)

Here, toIO from withToIO takes an Eff esSend to IO, where it can be passed into the restore function, and the returned IO computation is recovered into Eff with fromIO.